5. External lightning protection

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5. External lightning protection

5.1 Air-termination systemsThe function of the air-termination systems of alightning protection system is to prevent directlightning strikes from damaging the volume to beprotected. They must be designed to preventuncontrolled lightning strikes to the structure tobe protected.By correct dimensioning of the air-termination sys-tems, the effects of a lightning strike to a structurecan be reduced in a controlled way.

Air-termination systems can consist of the follow-ing components and can be combined with eachother as required:

⇒ Rods

⇒ Spanned wires and cables

⇒ Intermeshed conductors

When determining the siting of the air-termina-tion systems of the lightning protection system,special attention must be paid to the protection ofcorners and edges of the structure to be protected.This applies particularly to air-termination systemson the surfaces of roofs and the upper parts offacades. Most importantly, air-termination systemsmust be mounted at corners and edges.

Three methods can be used to determine thearrangement and the siting of the air-terminationsystems (Figure 5.1.1):

⇒ Rolling sphere method

⇒ Mesh method

⇒ Protective angle method

The rolling sphere method is the universal methodof design particularly recommended for geometri-cally complicated applications.The three different methods are described below.

5.1.1 Designing methods and types of air-termination systems

The rolling sphere method – “geometric-electricalmodel”For lightning flashes to earth, a downward leadergrows step-by-step in a series of jerks from thecloud towards the earth. When the leader has gotclose to the earth within a few tens, to a few hun-dreds of metres, the electrical insulating strengthof the air near the ground is exceeded. A further“leader” discharge similar to the downward leaderbegins to grow towards the head of the down-ward leader: the upward leader. This defines the

h 1

h 2

air-termination rod

α

protective angle

mesh size M

down conductor

r

rolling sphere

earth-termination system

I 20 m 5 x 5 mII 30 m 10 x 10 mIII 45 m 15 x 15 mIV 60 m 20 x 20 m

Class of LPS Radius of therolling sphere (r)

Mesh size(M)

Max. building height

Fig. 5.1.1 Method for designing of air-termination systems for high buildings

point of strike of the lightning strike (Figure5.1.1.1).

The starting point of the upward leader and hencethe subsequent point of strike is determined main-ly by the head of the downward leader. The headof the downward leader can only approach theearth within a certain distance. This distance isdefined by the continuously increasing electricalfield strength of the ground as the head of thedownward leader approaches. The smallest dis-tance between the head of the downward leaderand the starting point of the upward leader iscalled the final striking distance hB (corresponds tothe radius of the rolling sphere).

Immediately after the electrical insulating strengthis exceeded at one point, the upward leader whichleads to the final strike and manages to cross thefinal striking distance, is formed. Observations ofthe protective effect of guard wires and pylonswere used as the basis for the so-called“geometric-electrical model”.

This is based on the hypothesis that the head ofthe downward leader approaches the objects onthe ground, unaffected by anything, until it reach-es the final striking distance.

The point of strike is then determined by theobject closest to the head of the downward leader.The upward leader starting from this point “forcesits way through” (Figure 5.1.1.2).

Classification of the lightning protection systemand radius of the rolling sphereAs a first approximation, a proportionality existsbetween the peak value of the lightning currentand the electrical charge stored in the downwardleader. Furthermore, the electrical field strength ofthe ground as the downward leader approaches isalso linearly dependent on the charge stored inthe downward leader, to a first approximation.Thus there is a proportionality between the peakvalue I of the lightning current and the final strik-ing distance hB (= radius of the rolling sphere):

r in m

I in kA

The protection of structures against lightning isdescribed in IEC 62305-1 (EN 62305-1). Among other things, this standard defines the classifica-tion of the individual lightning protection systemand stipulates the resulting lightning protectionmeasures.

It differentiates between four classes of lightningprotection system. A Class I lightning protectionsystem provides the most protection and a Class IV,by comparison, the least. The interception effec-

r I= ⋅10 0 65 .

point afar from thehead of the down-ward leader

startingupward leader

downwardleader

head of thedownward leader

startingupward leader

closest point tothe head of thedownward leader

rolling sphere

final striking

distance hB

A rolling sphere can touch not only thesteeple, but also the nave of the church atseveral points. All points touched arepotential points of strike.

Fig. 5.1.1.1 Starting upward leader defining the point of strike Fig. 5.1.1.2 Model of a rolling sphere Ref: Prof. Dr. A. Kern, Aachen

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tiveness Ei of the air-termination systems is con-comitant with the class of lightning protection sys-tem, i.e. which percentage of the prospectivelightning strikes is safely controlled by the air-ter-mination systems. From this results the final strik-ing distance and hence the radius of the “rollingsphere”. The correlations between class of light-ning protection system, interception effectivenessEi of the air-termination systems, final striking dis-tance / radius of the “rolling sphere” and currentpeak value are shown in Table 5.1.1.1.

Taking as a basis the hypothesis of the “geometric-electrical model” that the head of the downwardleader approaches the objects on the earth in anarbitrary way, unaffected by anything, until itreaches the final striking distance, a generalmethod can be derived which allows the volume tobe protected of any arrangement to be inspected.Carrying out the rolling sphere method requires ascale model (e.g. on a scale of 1:100) of the build-ing / structure to be protected, which includes theexternal contours and, where applicable, the air-termination systems. Depending on the location ofthe object under investigation, it is also necessaryto include the surrounding structures and objects,since these could act as “natural protective mea-sures” for the object under examination.

Furthermore, a true-to-scale sphere is requiredaccording to the class of lightning protection sys-tem with a radius corresponding to the final strik-ing distance (depending on the class of lightningprotection system, the radius r of the “rollingsphere” must correspond true-to-scale to the radii20, 30, 45 or 60 m). The centre of the “rollingsphere” used corresponds to the head of thedownward leader towards which the respectiveupward leaders will approach.

The “rolling sphere” is now rolled around theobject under examination and the contact pointsrepresenting potential points of strike are markedin each case. The “rolling sphere” is then rolledover the object in all directions. All contact pointsare marked again. All potential points of strike arethus shown on the model; it is also possible todetermine the areas which can be hit by lateralstrikes. The naturally protected zones resultingfrom the geometry of the object to be protectedand its surroundings can also be clearly seen. Air-termination conductors are not required at thesepoints (Figure 5.1.1.3).

It must be borne in mind, however, that lightningfootprints have also been found on steeples inplaces not directly touched as the “rolling sphere”rolled over. This is traced to the fact that, amongother things, in the event of multiple lightningflashes, the base of the lightning flash movesbecause of the wind conditions. Consequently, anarea of approx. one metre can come up around the

Fig. 5.1.1.3 Schematic application of the “rolling sphere” method ata building with considerably structured surface

Table 5.1.1.1 Relations between lightning protection level, interception criterion Ei , final striking distance hB and min. peak value of current IRef.: Table 5, 6 and 7 of IEC 62305-1 (EN 62305-1)

Lightning protectionlevel LPL

Probabilities for the limit valuesof the lightning current parameters

Radius of the rolling sphere(final striking distance hB)

r in m

Min. peak valueof current

I in kA

IV

III

II

I

0.84

0.91

0.97

0.99

60

45

30

20

16

10

5

3

< Max. values acc. to Table 5IEC 62305-1 (EN 62305-1)

> Min. values acc. to Table 6IEC 62305-1 (EN 62305-1)

0.97

0.97

0.98

0.99

r

r

r

r

rr

building

rolling sphere

point of strike determined where lightning strikescan also occur.

Example 1: New administration building inMunichDuring the design phase of the new administra-tion building, the complex geometry led to thedecision to use the rolling sphere method to iden-tify the areas threatened by lightning strikes.This was possible because an architectural modelof the new building was available on a scale of1:100.It was determined that a lightning protection sys-tem Class I was required, i.e. the radius of therolling sphere in the model was 20 cm (Figure5.1.1.4).The points where the “rolling sphere” touchesparts of the building, can be hit by a direct light-ning strike with a corresponding minimum currentpeak value of 3 kA (Figure 5.1.1.5). Consequently,these points required adequate air-terminationsystems. If, in addition, electrical installations werelocalised at these points or in their immediatevicinity (e.g. on the roof of the building), addition-al air-termination measures were realised there.

The application of the rolling sphere methodmeant that air-termination systems were notinstalled where protection was not required. Onthe other hand, locations in need of more protec-tion could be equipped accordingly, where neces-sary (Figure 5.1.1.5).

Example 2: Aachen CathedralThe cathedral stands in the midst of the old townof Aachen surrounded by several high buildings.Adjacent to the cathedral there is a scale model(1:100) whose purpose is to make it easier for visi-tors to understand the geometry of the building.The buildings surrounding the Aachen Cathedralprovide a partial natural protection against light-ning strikes.Therefore, and to demonstrate the effectivenessof lightning protection measures, models of themost important elements of the surroundingbuildings were made according to the same scale(1:100) (Figure 5.1.1.6).

Figure 5.1.1.6 also shows “rolling spheres” forlightning protection systems Class II and III (i.e.with radii of 30 cm and 45 cm) on the model.

Fig. 5.1.1.4 Construction of a new administration building:Model with “rolling sphere” acc. to lightning protectionsystem Type I Ref.: WBG Wiesinger

Fig. 5.1.1.5 Construction of a DAS administration building:Top view (excerpt) on the zones threatened by lightningstrikes for lightning protection system Class I Ref.: WBG Wiesinger

Fig. 5.1.1.6 Aachen Cathedral: Model with environment and “rollingspheres” for lightning protection systems Class II and III Ref.: Prof. Dr. A. Kern, Aachen


The aim here was to demonstrate the increasingrequirements on the air-termination systems as theradius of the rolling sphere decreases, i.e. whichareas of Aachen Cathedral had additionally to beconsidered at risk of being hit by lightning strikes,if a lightning protection system Class II with a high-er degree of protection was used. The “rolling sphere” with the smaller radius(according to a class of lightning protection systemwith a higher lightning protection level) naturallytouches also the model at all points alreadytouched by the “rolling sphere” with the largerradius. Thus, it is only necessary to determine theadditional contact points.As demonstrated, when dimensioning the air-ter-mination system for a structure, or a structuremounted on the roof, the sag of the rolling sphereis decisive.The following formula can be used to calculate thepenetration depth p of the rolling sphere whenthe rolling sphere rolls “on rails”, for example. Thiscan be achieved by using two spanned wires, forexample.

r Radius of the rolling sphere

d Distance between two air-termination rods ortwo parallel air-termination conductors

Figure 5.1.1.7 illustrates this consideration.Air-termination rods are frequently used to pro-tect the surface of a roof, or installations mountedon the roof, against a direct lightning strike. Thesquare arrangement of the air-termination rods,over which no cable is normally spanned, meansthat the sphere does not “roll on rails” but “sitsdeeper” instead, thus increasing the penetrationdepth of the sphere (Figure 5.1.1.8).

The height of the air-termination rods Δh shouldalways be greater than the value of the penetra-tion depth p determined, and hence greater thanthe sag of the rolling sphere. This additionalheight of the air-termination rod ensures that therolling sphere does not touch the structure to beprotected.

p r r d= − − /( )⎡⎣

⎤⎦

2 21

2 2

Fig. 5.1.1.9 Calculation Δh for several air-termination rods accord-ing to rolling sphere method

domelightinstalled on the roof

d diagonal

Δh


Fig. 5.1.1.7 Penetration depth p of the rolling sphere

Δh

d

r air-terminationconductor

pene

trat

ion

dept

h p

Fig. 5.1.1.8 Air-termination system for installations mounted on theroof with their protective area

d

Δh

r

p

Class of LPSI II III IV

r 20 30 45 60

Cuboidal protective area bet-ween four air-termination rods

Another way of determining the height of the air-termination rods is using Table 5.1.1.2. The pene-tration depth of the rolling sphere is governed bythe largest distance of the air-termination rodsfrom each other. Using the greatest distance, thepenetration depth p (sag) can be taken from thetable. The air-termination rods must be dimen-sioned according to the height of the structuresmounted on the roof (in relation to the location ofthe air-termination rod) and also the penetrationdepth (Figure 5.1.1.9).

If, for example, a total height of an air-terminationrod of 1.15 m is either calculated or obtained fromthe table, an air-termination rod with a standardlength of 1.5 m is normally used.

Mesh method

A “meshed” air-termination system can be useduniversally regardless of the height of the struc-ture and shape of the roof. A reticulated air-termi-nation network with a mesh size according to theclass of lightning protection system is arranged onthe roofing (Table 5.1.1.3).

To simplify matters, the sag of the rolling sphere isassumed to be zero for a meshed air-terminationsystem.

By using the ridge and the outer edges of thestructure, as well as the metal natural parts of thestructure serving as an air-termination system, theindividual cells can be sited as desired.

The air-termination conductors on the outer edgesof the structure must be laid as close to the edgesas possible.A metal attic can serve as an air-termination con-ductor and / or a down-conductor system if therequired minimum dimensions for natural compo-nents of the air-termination system are compliedwith (Figure 5.1.1.10).

Protective angle methodThe protective angle method is derived from theelectric-geometrical lightning model. The protec-tive angle is determined by the radius of therolling sphere. The comparable protective anglewith the radius of the rolling sphere is given whena slope intersects the rolling sphere in such a waythat the resulting areas have the same size (Figure5.1.1.11).This method must be used for structures with sym-metrical dimensions (e.g. steep roof) or roof-mounted structures (e.g. antennas, ventilationpipes).The protective angle depends on the class of light-ning protection system and the height of the air-


e.g. gutter

Class of LPS

I

II

III

IV

Mesh size

5 x 5 m

10 x 10 m

15 x 15 m

20 x 20 m

Table 5.1.1.3 Mesh size

Fig. 5.1.1.10 Meshed air-termination system

I (20 m) II (30 m) III (45 m) IV (60 m)

Class of LPS with rolling sphere radius in meters

Sag of the rolling sphere [m] (rounded up)d

Distancebetween air-termniation

rods [m]

2 0.03 0.02 0.01 0.014 0.10 0.07 0.04 0.036 0.23 0.15 0.10 0.088 0.40 0.27 0.18 0.1310 0.64 0.42 0.28 0.2112 0.92 0.61 0.40 0.3014 1.27 0.83 0.55 0.4116 1.67 1.09 0.72 0.5418 2.14 1.38 0.91 0.6820 2.68 1.72 1.13 0.8423 3.64 2.29 1.49 1.1126 4.80 2.96 1.92 1.4329 6.23 3.74 2.40 1.7832 8.00 4.62 2.94 2.1735 10.32 5.63 3.54 2.61

Table 5.1.1.2 Sag of the rolling sphere over two air-terminationrods or two parallel air-termination conductors

α1

α 2

h 2

Hh 1h 1

Note:Protective angle α1 refers to the height of the air-termination systemh1 above the roof surface to be protected (reference plane);Protective α2 refers to the height h2 = h1 + H, while the earthsurface is the reference plane.

h1: Physical height of the air-termination rod

Fig. 5.1.1.16 External lightning protection system, volume protectedby a vertical air-termination rod

Fig. 5.1.1.14 Example of air-termination systems with protectiveangle α

angle α

angle α

Fig. 5.1.1.13 Cone-shaped protection zone

h 1

α° α°


Fig. 5.1.1.11 Protective angle and comparable radius of the rollingsphere

Fig. 5.1.1.12 Protective angle α as a function of height h dependingon the class of lightning protection system

Fig. 5.1.1.15 Area protected by an air-termination conductor

base

equal surface areasair-termi-nation rod

r

α°

rolling sphere

protectiveangle

h[m]

α°

I II III

80

70

60

50

40

30

20

10

00 2 10 20 30 40 50 60

IV

α° h1

air-terminationconductor

Angle α depends on the class of lightning protection systemand the height of the air-termination conductor above ground

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Height of the air-termination rod

h in m

Class of LPS IAngle Distance

α a in m

Class of LPS IIAngle Distance

α a in m

Class of LPS IIIAngle Distance

α a in m

Class of LPS IVAngle Distance

α a in m

αangle

height hof the air-

termination rod

distance a

Table 5.1.1.4 Protective angle α depending on the class of lighting protection system


termination system above the reference plane(Figure 5.1.1.12).

Air-termination conductors, air-termination rods,masts and wires should be arranged to ensure thatall parts of the building to be protected are situa-

ted within the volume of protection of the air-ter-mination system.The protection zone can be “cone-shaped” or“tent-shaped”, if a cable, for example, is spannedover it (Figures. 5.1.1.13 to 5.1.1.15).If air-termination rods are installed on the surfaceof the roof to protect structures mounted thereon,the protective angle α can be different. In Figure5.1.1.16, the roof surface is the reference plane forprotective angle α1. The ground is the referenceplane for the protective angle α2. Therefore theangle α2 according to Figure 5.1.1.12 and Table5.1.1.4 is less than α1.Table 5.1.1.4 provides the corresponding protec-tive angle for each class of lightning protectionsystem and the corresponding distance (zone ofprotection).

Protective angle method for isolated air-termina-tion systems on roof-mounted structuresSpecial problems may occur when roof-mountedstructures, which are often installed at a later date,protrude from zones of protection, e.g. the mesh.If, in addition, these roof-mounted structures con-tain electrical or electronic equipment, such asroof-mounted fans, antennas, measuring systemsor TV cameras, additional protective measures arerequired.

If such equipment is connected directly to theexternal lightning protection system, then, in theevent of a lightning strike, partial currents are con-ducted into the structure. This could result in thedestruction of surge sensitive equipment. Directlightning strikes to such structures protrudingabove the roof can be prevented by having isolat-ed air-termination systems.Air-termination rods as shown in Figure 5.1.1.17are suitable for protecting smaller roof-mountedstructures (with electrical equipment).They form a “cone-shaped” zone of protectionand thus prevent a direct lightning strike to thestructure mounted on the roof.

The separation distance s must be taken intoaccount when dimensioning the height of the air-termination rod (see Chapter 5.6).

Isolated and non-isolated air-termination systemsWhen designing the external lightning protectionsystem of a structure, we distinguish between twotypes of air-termination system:

Fig. 5.1.1.18 Gable roof with conductor holder

Fig. 5.1.1.19 Flat roof with conductor holders: Protection of thedomelights


Fig. 5.1.1.17 Protection of small-sized installations on roofs againstdirect lightning strikes by means of air-terminationrods

⇒ isolated

⇒ non-isolated

The two types can be combined.

The air-termination systems of a non-isolatedexternal lightning protection system of a structurecan be installed in the following ways:

If the roof is made of non-flammable material, theconductors of the air-termination system can beinstalled on the surface of the structure (e.g. gableor flat roof). Normally non-flammable buildingmaterials are used. The components of the exter-nal lightning protection system can therefore bemounted directly on the structure (Figures 5.1.1.18and 5.1.1.19).

If the roof is made of easily inflammable materiale.g. thatched roofs, then the distance between theflammable parts of the roof and the air-termina-tion rods, air-termination conductors or air-termi-nation meshes of the air-termination system mustnot be less than 0.4 m.Easily inflammable parts of the structure to be pro-tected must not be in direct contact with parts ofthe external lightning protection system. Neithermay they be located under the roofing, which canbe punctured in the event of a lightning strike (seealso Chapter 5.1.5 Thatched roofs).

With isolated air-termination systems, the com-plete structure is protected against a direct light-ning strike via air-termination rods, air-termina-tion masts or masts with cables spanned overthem. When installing the air-termination systems,the separation distance s to the structure must bekept (Figures 5.1.1.20 and 5.1.1.21).

The separation distance s between the air-termina-tion system and the structure must be kept.

Air-termination systems isolated from the struc-ture are frequently used, when the roof is coveredwith inflammable material, e.g. thatch or also forex-installations, e.g. tank installations.

See also Chapter 5.1.5 “Air-termination system forstructures with thatched roofs”.

A further method of designing isolated air-termi-nation systems consists in securing the air-termina-tion systems (air-termination rods, conductors orcables) with electrically insulating materials such asGRP (glass fibre-reinforced plastic).This form of isolation can be limited to local use orapplied to whole parts of the installation. It isoften used for roof-mounted structures such as fansystems or heat exchangers with an electricallyconductive connection into the structure (see alsoChapter 5.1.8).

s s

α α

reference plane

protectedstructure

air-termi-nation mast


s separation distance acc. to IEC 62305-3 (EN 62305-3)α protective angle acc. to Table 5.1.1.4

s2

s 1

s2

reference plane

protectedstructureair-termi-

nation mast

s1, s2 separation distance acc. to IEC 62305-3 (EN 62305-3)

horizontal air-termination conductor


Fig. 5.1.1.20 Isolated external lightning protection system with twoseparate air-termination masts according to the pro-tective angle method: Projection on a vertical area

Fig. 5.1.1.21 Isolated external lightning protection system, consist-ing of two separate air-termination masts, connectedby a horizontal air-termination conductor: Projectionon a vertical surface via the two masts (vertical sec-tion)



Natural components of air-termination systemsMetal structural parts such as attics, guttering, rail-ings or cladding can be used as natural compo-nents of an air-termination system.

If a structure has a steel skeleton construction witha metal roof and facade made of conductive mate-rial, these can be used for the external lightningprotection system, under certain circumstances.

Sheet metal cladding on the walls or roof of thestructure to be protected can be used if the electri-cal connection between the different parts is per-manent. These permanent electrical connectionscan be made by e.g. brazing, welding, pressing,screwing or riveting, for example.If there is no electrical connection, a supplemen-tary connection must be made for these elementse.g. with bridging braids or bridging cables.

If the thickness of the sheet metal is not less thanthe value t' in Table 5.1.1.5, and if there is norequirement to take account of a through-meltingof the sheets at the point of strike or the ignition offlammable material under the cladding, then suchsheets can be used as an air-termination system.

The material thicknesses are not distinguished ac-cording to the class of lightning protection system.

If it is, however, necessary to take precautionarymeasures against through-melting or intolerableheating-up at the point of strike, if the thickness ofthe sheet metal shall not be less than value t inTable 5.1.1.5.

The required thicknesses t of the materials cangenerally not be complied with, for example, formetal roofs.For pipes or containers, however, it is possible tomeet the requirements for these minimum thick-nesses (wall thickness). If, though, the temperaturerise (heating-up) on the inside of the pipe or tankrepresents a hazard for the medium containedtherein (risk of fire or explosion), then these mustnot be used as air-termination systems (see alsoChapter 5.1.4).

If the requirements on the appropriate minimumthickness are not met, the components, e.g. con-duits or containers, must be situated in an areaprotected from direct lightning strikes.

A thin coat of paint, 1 mm bitumen or 0.5 mm PVCcannot be regarded as insulation in the event of adirect lightning strike. Such coatings break downwhen subjected to the high energies depositedduring a direct lightning strike.There must be no coatings on the joints of the na-tural components of the down-conductor systems.

If conductive parts are located on the surface ofthe roof, they can be used as a natural air-termina-tion system if there is no conductive connectioninto the structure.By connecting, e.g. pipes or electrical conductorsinto the structure, partial lightning currents canenter the structure and affect or even destroy sen-sitive electrical / electronic equipment.In order to prevent these partial lightning currentsfrom penetrating, isolated air-termination systemsshall be installed for the aforementioned roof-mounted structures.The isolated air-termination system can bedesigned using the rolling sphere or protectiveangle method. An air-termination system with amesh size according to the class of lightning pro-tection system used can be installed if the wholearrangement is isolated (elevated) from the struc-ture to be protected by at least the required sepa-ration distance s.

Table 5.1.1.5 Min. thickness of metal plates

Material

-

4

4

5

7

-

2.0

0.5

0.5

0.5

0.65

0.7

Lead

Steel (stainless,galvanised)

Titanium

Copper

Aluminium

Zinc

Thick-nessa t

mm

Thick-nessb t`

mm

Class of LPS

I to IV

a t prevents from puncturing, overheating, and inflamming

b t` only for metal plates, if the prevention of puncturing, overheating, and inflamming is not important

A universal system of components for the installa-tion of isolated air-termination systems is descri-bed in Chapter 5.1.8.

5.1.2 Air-termination systems for buildingswith gable roofs

Air-termination systems on roofs are the metalcomponents in their entirety, e.g. air-terminationconductors, air-termination rods, air-terminationtips.The parts of the structure usually hit by lightningstrikes, such as the top of the gable, chimneys,ridges and arrises, the edges of gables and eaves,parapets and antennas and other protruding struc-tures mounted on the roof, must be equipped withair-termination systems.Normally, a reticulated air-termination network is installed on the surface of gabled roofs, said network corresponding to the mesh size of theappropriate class of lightning protection system(e.g. 15 m x 15 m for a lightning protection systemClass III) (Figure 5.1.2.1).By using the ridge and the outer edges of thestructure, as well as the metal parts of the struc-ture serving as an air-termination system, the indi-vidual meshes can be sited as preferred. The air-termination conductors on the outer edges of thestructure must be installed as close to the edges aspossible.

Generally, the metal gutter is used for closing the“mesh” of the air-termination system on the roofsurface. If the gutter itself is connected so as to beelectrically conductive, a gutter clamp is mounted

at the crossover of the air-termination system andthe gutter.

Roof-mounted structures made of electrically non-conductive material (e.g. PVC vent pipes) are con-sidered to be sufficiently protected if they do notprotrude more than h = 0.5 m from the plane ofthe mesh (Figure 5.1.2.2).

If the protrusion is h > 0.5 m, the structure must beequipped with an air-termination system (e.g.interception tip) and connected to the nearest air-termination conductor. One way of doing thiswould be to use a wire with a diameter of 8 mm up to a maximum free length of 0.5 m, as shown inFigure 5.1.2.3.

Metal structures mounted on the roof withoutconductive connection into the structure do notneed to be connected to the air-termination sys-tem if all the following conditions are met:

⇒ Structures mounted on the roof may protrudea maximum distance of 0.3 m from the planeof the mesh

⇒ Structures mounted on the roof may have amaximum enclosed area of 1 m2 (e.g. dormerwindows)

⇒ Structures mounted on the roof may have amaximum length of 2 m (e.g. sheet metal roof-ing parts)

Only if all three conditions are met, no terminal isrequired.

h

Fig. 5.1.2.1 Air-termination system on agable roof

Fig. 5.1.2.2 Height of a roof superstructuremade of electrically non-conduc-tive material (e.g. PVC), h ≤ 0.5 m

Fig. 5.1.2.3 Additional air-termination systemfor ventilation pipes



Furthermore, with the conditions stated above,the separation distance to the air-termination con-ductors and down-conductor systems must bemaintained (Figure 5.1.2.4).

Air-termination rods for chimneys must be erectedto ensure that the whole chimney is in the zone ofprotection. The protective angle method is appliedwhen dimensioning the air-termination rods.

If the stack is brick-built or constructed with pre-formed sections, the air-termination rod can bemounted directly on the stack.

If there is a metal insert pipe in the interior of thestack, e.g. as found when redeveloping old build-ings, the separation distance to this conductivecomponent must be kept. This is an examplewhere isolated air-termination systems are usedand the air-termination rods are erected with dis-tance holders. The inserted metal pipe must beconnected to the equipotential bonding.Theassembly to protect parabolic antennas in particu-lar is similar to that to protect stacks with an inter-nal stainless steel pipe.

In the event of a direct lightning strike toantennas, partial lightning currents can enter thestructure to be protected via the shields of thecoaxial cables and cause the effects and destruc-tion previously described. To prevent this, anten-nas are equipped with isolated air-termination sys-tems (e.g. air-termination rods) (Figure 5.1.2.5).

Air-termination systems on the ridge have a tent-shaped zone of protection (according to the pro-tective angle method). The angle depends on theheight above the reference plane (e.g. surface of

the earth) and the class of lightning protection sys-tem chosen.

5.1.3 Air-termination systems for flat-roofedstructures

An air-termination system for structures with flatroofs (Figures 5.1.3.1 and 5.1.3.2) is designed usingthe mesh method. A mesh-type air-terminationsystem with a mesh size corresponding to the classof lightning protection system is installed on theroof (Table 5.1.1.3).

Figure 5.1.3.3 illustrates the practical applicationof the meshed air-termination system in combina-tion with air-termination rods to protect the struc-tures mounted on the roof, e.g. domelights, pho-tovoltaic cells or fans. Chapter 5.1.8 shows how todeal with these roof-mounted structures.

Roof conductor holders on flat roofs are laid atintervals of approx. 1 m. The air-termination con-ductors are connected with the attic, this being anatural component of the air-termination system.As the temperature changes, so does the length ofthe materials used for the attic, and hence theindividual segments must be equipped with “slideplates”.If the attic is used as an air-termination system,these individual segments must be permanentlyinterconnected so as to be electrically conductivewithout restricting their ability to expand. This can

Fig. 5.1.2.5 Antenna with air-termination rodFig. 5.1.2.4 Building with photovoltaic system Ref.: Wettingfeld Lightning Protection, Krefeld, Germany

be achieved by means of bridging braids, straps orcables (Figure 5.1.3.4).

The changes in length caused by changes in tem-perature must also be taken into account with air-termination conductors and down-conductor sys-tems (see Chapter 5.4).A lightning strike to the attic can cause the materi-als used to melt through. If this is unacceptable, a

expansion piece

distance between theroof conductor holdersapprox. 1 m

flexible connection

Bridging braidPart No. 377 015

Roof conductor holder Type FB2Part No. 253 050

Roof conductor holder Type FBPart No. 253 015

Fig. 5.1.3.1 Air-termination system

Fig. 5.1.3.2 Air-termination system on a flat roof

Fig. 5.1.3.5 Example how to protect a metal roof attic, if meltingthrough is unacceptable (front view)

Fig. 5.1.3.3 Use of air-termination rods

Fig. 5.1.3.4 Bridged attic

bridging braid

air-termination tip

rolling sphere

parapet

metal attic


supplementary air-termination system, e.g. withair-termination tips, must be installed, its locationbeing determined by using the rolling spheremethod (Figure 5.1.3.5).

Conductor holders for flat roofs, homogeneouslywelded

In the wind, roof sheetings can move across theroof surface horizontally, if they are only fixedmechanically / laid on the surface. A special posi-tion fixing is required for the air-termination con-ductor for preventing the conductor holders forair-termination systems from being displaced onthe smooth surface. Conventional roof conductorholders cannot be permanently bonded to roofsheetings since the latter do not usually permit theapplication of adhesives.

A simple and safe way of fixing the position is touse roof conductor holders Type KF in combinationwith straps (cut the strips to fit) made of the roofsheeting material. The strap is clamped into theplastic holder and both sides are welded onto theseal. Holder and strap should be positioned imme-diately next to a roof sheeting joint at a distanceof approx. 1 m. The strip of foil is welded to theroof sheeting according to the manufacturer ofthe roof sheeting. This prevents air-terminationconductors on flat roofs from being displaced.

If the slope of the roof is greater than 5 °, eachroof conductor holder must be equipped with aposition fixing element. If the synthetic roof sheet-ings are secured by mechanical means, the roofconductor holders must be arranged in the imme-diate vicinity of the mechanical fixing elements.

When carrying out this work, it must be consideredthat welding and bonding work on the seal affectthe guarantee provided by the roofer.The work to be carried out must therefore only bedone with the agreement of the roofer responsi-ble for the particular roof, or be carried out by himhimself (Figure 5.1.3.6).

5.1.4 Air-termination systems on metalroofs

Modern industrial and commercial purpose-builtstructures often have metal roofs and facades. Themetal sheets and plates on the roofs are usually 0.7 – 1.2 mm thick.

Figure 5.1.4.1 shows an example of the construc-tion of a metal roof.When the roof is hit by a direct lightning strike,melting through or vaporisation can cause a holeformed at the point of strike. The size of the holedepends on the energy of the lightning strike and


Fig. 5.1.3.6 Synthetic flat roof sheetings – Roof conductor holder Type KF / KF2

~300~ 300

~90

~70

distance between theroof conductor holdersapprox. 1 m

flexible connection

the characteristics of the material, (e.g. thickness).The biggest problem here is the subsequent dam-age, e.g. water entering at this point. Days orweeks can pass before this damage is noticed. The

roof insulation becomes damp and / or the ceilingbecomes wet and is no longer rainproof.One example of damage, assessed using BLIDS(Blitz-Informations Dienst von Siemens – SiemensLightning Information Service) illustrates thisproblem (Figure 5.1.4.2). A current of approx.20,000 A struck the sheet metal and made a hole(Figure 5.1.4.2: Detail A). Since the sheet metal wasnot earthed by a down-conductor system, flash-overs to natural metal components in the walloccurred in the area around the fascia (Figure5.1.4.2: Detail B), which also caused a hole.To prevent such kind of damage, a suitable exter-nal lightning protection system with wires andclamps capable of carrying lightning currents mustbe installed even on a “thin” metal roof. The IEC62305-3 (EN 62305-3) lightning protection stan-dard clearly illustrates the risk of damage to metalroofs. Where an external lightning protection sys-tem is required, the metal sheets must have theminimum values stated in Table 5.1.1.5.

The thicknesses t are not relevant for roofingmaterials. Metal sheets with a thickness t’ may onlybe used as a natural air-termination system ifpuncturing, overheating and melting is tolerated.The owner of the structure must agree to toleratethis type of roof damage, since there is no longerany guarantee that the roof will offer protectionfrom the rain. Also the Rules of the German Roof-ing Trade concerning lightning protection on andattached to roofs require the agreement of theowner.

If the owner is not prepared to tolerate damage tothe roof in the event of a lightning strike, then aseparate air-termination system must be installed

Distance of thehorizontal conductors

3 m

4 m

5 m

6 m

Height of theair-termination tip*)

0.15 m

0.25 m

0.35 m

0.45 m

Suitable for all classes of lightning protection system

*) recommended values

rolling sphere with a radiusacc. to class of LPS

air-termination tip

Evaluation: BLIDS – SIEMENSI = 20400 A residential building

Detail B

Detail A

Fig. 5.1.4.2 Example of damage: Metal plate cover

Table 5.1.4.1 Lightning protection for metal roofs – Height of theair-termination tips

Fig. 5.1.4.3 Air-termination system on a metal roof – Protectionagainst holing

Fig. 5.1.4.1 Types of metal roofs, e.g. roofs withround standing seam



on a metal roof. The air-termination system mustbe installed to ensure that the rolling sphere(radius r which corresponds to the class of light-ning protection system chosen) does not touch themetal roof (Figure 5.1.4.3).

When mounting the air-termination system it isrecommended to install a so-called “hedgehogroof” with longitudinal conductors and air-termi-nation tips.

In practice, the heights of air-termination tipsaccording to Table 5.1.4.1 are tried and tested,regardless of the class of lightning protection sys-tem involved.

Holes must not be drilled into the metal roof whenfixing the conductors and air-termination tips. Var-ious conductor holders are available for the differ-ent types of metal roofs (round standing seam,standing seam, trapezoidal). Figure 5.1.4.4a showsone possible design for a metal roof with roundstanding seam.

When installing the conductors, care must be tak-en that the conductor holder located at the high-est point of the roof must be designed with a fixedconductor leading, whereas all other conductorholders must be designed with a loose conductorleading because of the linear compensation

2

1 3

Parallel conectorSt/tZn Part No. 307 000

Roof conductor holderfor metal roofs, loose con-ductor leading, DEHNgripconductor holderStSt Part No. 223 011Al Part No. 223 041

Roof conductor holderfor metal roofs fixed con-ductor leading with clampingframeStSt Part No. 223 010Al Part No. 223 040

1

2

3

roof connection

bridging braid

conductor holder withloose conductor leading

bridging cable

KS connector

air-termination tip

Fig. 5.1.4.4a Conductor holders for metal roofs – Round standing seam Fig. 5.1.4.4b Conductor holder for metal roofs –Round standing seam

Fig. 5.1.4.5 Model construction of a trape-zoidal sheet roof, conductorholder with clamping frame

Fig. 5.1.4.6 Model construction of a roofwith standing seam

Fig. 5.1.4.7 Air-termination rod for a dome-light on a roof with round stand-ing seam

caused by changes in temperature (Figure5.1.4.4b).

The conductor holder with fixed conductor lead-ing is illustrated in Figure 5.1.4.5 using the exam-ple of a trapezoidal sheet roof.

Figure 5.1.4.5 also shows an air-termination tipnext to the conductor holder. The conductor hold-er must be hooked into the fixing screw above thecovering plate for the drill hole to prevent anyentering of water.

Figure 5.1.4.6 uses the example of a round stand-ing seam roof to illustrate the loose conductorleading.Figure 5.1.4.6 also shows the connection to theroof with round standing seam at the roof edge,which is capable of carrying currents.Unprotected installations projecting above theroof, e.g. domelights and chimney covers, areexposed points of strike for a lightning discharge.In order to prevent these installations from beingstruck by a direct lightning strike, air-terminationrods must be installed adjacent to the installationsprojecting above the roof. The height of the air-termination rod results from the protective angle α(Figure 5.1.4.7).

5.1.5 Principle of an air-termination systemfor structures with thatched roof

The design of lightning protection systems Class IIIgenerally meets the requirements of such a struc-ture. In particular individual cases, a risk analysisbased on IEC 62305-2 (EN 62305-2) can be carriedout.

The air-termination conductors on such roofs(made of thatch, straw or rushes) must be fastenedacross isolating supports to be free to move. Cer-tain distances must also be maintained around theeaves.

In case of subsequent installation of a lightningprotection system on a roof, the distances must beincreased. This allows to maintain the necessaryminimum distances when re-roofing is carried out.For a lightning protection system Class III, the typi-cal distance of the down-conductor system is 15 m.

The exact distance of the down-conductor systemsfrom each other results from calculating the sepa-ration distance s in accordance with IEC 62305-3(EN 62305-3).

Chapter 5.6 explains how to calculate the separa-tion distance.

Ideally, ridge conductors shouldhave spans up to around 15 m, anddown-conductor systems up toaround 10 m without additionalsupports.Fastening posts must be tightlyconnected to the roof structure(rafters and rails) by means of boltsand washers (Figures 5.1.5.1 to5.1.5.3).

Metal components situated abovethe roof surface (such as weathervanes, irrigation systems, anten-nas, metal plates, conductors) mustbe entirely in the protected vol-ume of isolated air-terminationsystems.In such cases, effective protectionagainst lightning can only beachieved with an isolated externallightning protection system with


Fig. 5.1.5.1 Air-termination system for buildings with thatched roofs

Signs and symbolsAir-termination conductorConnecting pointIsolating point /Measuring pointEarth conductorDown conductor

Important distances (min. values)a 0.6 m Air-term. conductor / Gableb 0.4 m Air-term. conductor/Roofingc 0.15 m Eaves / Eaves supportd 2.0 m Air-termination conductor /

Branches of trees

air-termination rods near the structure, or air-ter-mination conductors or interconnected air-termi-nation masts adjacent to the structure.

If a thatched roof borders onto metal roofingmaterial, and if the structure has to be equippedwith an external lightning protection system, thenan electrically non-conductive roofing material atleast 1 m wide, e.g. in plastic, must be insertedbetween the thatched roof and the other roof.

Tree branches must be kept at least 2 m away froma thatched roof. If trees are very close to, and high-er than, a structure, then an air-termination con-ductor must be mounted on the edge of the rooffacing the trees (edge of the eaves, gable) and

This method can be found in Chapter 5.1.8 isolatedair-termination system (steel telescopic lightningprotection masts).A new and architecturally very attractive possibili-ty of isolated lightning protection is the use of iso-lated down conductor systems.Example for the installation of isolated down con-ductor systems: Redevelopment of the roof of ahistorical farmhouse in Lower Saxony (Figure5.1.5.4).

Referring to the building regulations (LBO) of therespective federal state as well as to the modelbuilding regulations (MBO), the competent build-ing authority decides about the necessity of alightning protection system.

connected to thelightning protectionsystem. The necessarydistances must bemaintained.

A further way of pro-tecting structureswith thatched roofsagainst a strike oflightning is to erectair-termination mastsso that the wholestructure is in the pro-tected volume.


6

3

4

5

Pos Description DIN Part No.1 Clamping cap with 48811 A 145 309

air-termination rod2 Wood pile 48812 145 2413 Support for roof conductors − 240 0004 Eaves support 48827 239 0005 Tensioning block 48827 B 241 0026 Air-term. conductor, e.g. Al cable − 840 050

1

2

Fig. 5.1.5.2 Components for thatched roofs

Fig. 5.1.5.3 Thatched roof Fig. 5.1.5.4 Historical farmhouse with external lightning protection(Ref. Photo: Hans Thormählen GmbH & Co.KG)

The building regulations of LowerSaxony (NBauO) for example stipu-late in § 20 (3) that:“Buildings or structures which dueto the location, type of constructionor use are particularly susceptibleto lightning strikes, or where such astrike can have serious conse-quences, must be equipped withpermanently effective lightningprotection systems.”

With regard to the increasing dam-age events caused by lightningstrikes and surges, property insurersrequire that measures of lightningand surge protection are taken pri-or to the conclusion of new, oradjustment of existing insurancecontracts. Basis for the risk assess-ment is a risk analysis according toIEC 62305-2 (EN 62305-2).At the historical farmhouse a light-ning protection system Class III hasbeen installed, which meets thestandard requirements for buildingswith thatched roofs IEC 62305-3 (EN62305-3).


Fig. 5.1.5.5 Sectioning at the central building

rolling sphere with r = 45 m

2 m

10 m

1.5

m1

m

13 m

GRP/Al insulating pipe ∅ 50 mm

Legend:

Down conductor

HVI® conductor(under roof)

Earth conductor

Isolating point

Thatched roof

Fig. 5.1.5.6 Schematic diagram and diagram of the down conductor installation at the rafter

insulating pipe withinterior HVI® conductor

heather or divot-cladded ridge

boltedwooden traverse HVI® conductor led under

the roof to the eaves

mast sealing film

cornice plank

EBB

MEBB

Legend:

Down conductor

HVI® conductor(under roof)

Earth conductor

Isolating point

Thatched roof

HVI® conductorinside

(EN 62305-3). The isolated HVI conductor is speci-fied with an equivalent separation distance in airof s = 0.75 m or s = 1.50 m for solid building mate-rials. Figure 5.1.5.6 shows how the down conduc-tor system is arranged.

The HVI conductor is run in an insulating pipe. Theconstruction requires a down leading of the HVIconductor via a central earthing busbar, theequipotential bonding measures being performedby a flexible conductor H07V-K 1 x 16 mm2. Theinsulating pipe is fixed at a special construction(wooden traverse) and further down, the downconductors are routed along the rafters of the roofconstruction underneath the battens (Figure5.1.5.6).At the eaves, the HVI conductors are led throughthe cornice plank (Figure 5.1.5.7).For architectural reasons aluminium down conduc-tors are installed further down. Like for the wholeinstallation, the crossover of the HVI conductor tothe uninsulated, bare down conductor near theearthing system is effected on the basis of themounting instructions of the DEHNconductor sys-tem. A sealing unit was not necessary.

5.1.6 Walkable and trafficable roofs

It is not possible to mount air-termination conduc-tors (e.g. with concrete blocks) on trafficable roofs.One possible solution is to install the air-termina-tion conductors in either concrete or in the jointsbetween the sections of the roadway. If the air-ter-mination conductor is installed in these joints,mushroom head collectors are installed at theintersections of the mesh as defined points ofstrike.

The mesh size must not exceed the value accordingto the class of lightning protection system (seeChapter 5.1.1, Table 5.1.1.3).

If it can be guaranteed that no persons will be onthis area during a thunderstorm, then it is suffi-cient to install the measures described above.Persons who can go onto this storey of the car parkmust be informed by means of a sign that theymust immediately clear this storey when a thun-derstorm occurs, and not return for the durationof the storm (Figure 5.1.6.1).

The heather-cladded ridge of the object is protect-ed by a reticulated plastic cover to avoid abrasionby birds.

Before designing of the air-termination system,the protected volumes are to be determined bythe rolling sphere method. A rolling sphere radiusof 45 m is applicable in case of a lightning protec-tion system Class III according to the standard spec-ifications. The height of the air-termination systemwas ascertained to be 2.30 m, thus the two stacksat the ridge and the three new dormers at the oneside of the roof are within the protected volume(Figure 5.1.5.5).

An insulating pipe (Glass Fibre Reinforced Unsatu-rated Plastic) was chosen to keep the air-termina-tion system correspondingly elevated and to sup-port the isolated down-conductor system. The low-er part of the insulating pipe is aluminium toensure the mechanical stability. Due to the induc-tion of neighbouring components unwantedsparking is possible in this section. To avoid this,there are no earthed parts or electrical equipmentwithin a distance of 1 m from the air-terminationsystem.The electrical isolation of air-termination systemsand down-conductor systems on the one hand andof the metal installations to be protected and thesystems of power supply and information technology of the building or structure to be protectedon the other hand, can be achieved by the separa-tion distance s between these conductive parts.This must be determined according to IEC 62305-3


HVI® conductor

leading through the cornice

Fig. 5.1.5.7 HVI conductor led through the cornice plank

If it is also possible that persons are on the roofduring a thunderstorm, then the air-terminationsystem must be designed to protect these persons,assuming they have a height of 2.5 m (with out-stretched arm) from direct lightning strikes.

The air-termination system can be dimensionedusing the rolling sphere or the protective anglemethod according to the class of lightning protec-tion system (Figure 5.1.6.2).

These air-termination systems can also be con-structed from spanned cables or air-terminationrods. These air-termination rods are secured tostructural elements such as parapets or the like, forexample. Furthermore, lightning masts, for example, canalso act as air-termination rods to prevent life haz-ard. With this version, however, attention must bepaid to the partial lightning currents which can beconducted into the structure via the power lines. Itis imperative to have lightning equipotentialbonding measures for these lines.

5.1.7 Air-termination system for green andflat roofs

A planted roof can make economic and ecologicalsense. This is because it provides noise insulation,

protects the roof skin, suppresses dust from theambient air, provides additional heat insulation,filters and retains rainwater and is a natural way ofimproving the living and working conditions.Moreover, in many regions it is possible to obtaingrants from public funds for cultivating plants onthe roof. A distinction is made between so-calledextensive and intensive cultivation. An extensiveplanted area requires little care, in contrast to anintensive planted area which requires fertiliser,irrigation and cutting. For both types of plantedarea, either earth substrate or granulate must belaid on the roof.It is even more expensive if the granulate or sub-strate has to be removed because of a direct light-ning strike.

If there is no external lightning protection system,the roof seal can be damaged at the point ofstrike.

Experience has shown that, regardless of the typeof care required, the air-termination system of anexternal lightning protection system can, andshould, also be installed on the surface of a greenroof.

For a meshed air-termination system, the IEC62305-3 (EN 62305-3) lightning protection stan-


Fig. 5.1.6.1 Lightning protection for car park roofs – Building protec-tion

Fig. 5.1.6.2 Lightning protection for car park roofs – Building andlife protection IEC 62305-3 (EN 62305-3); Annex E

Down conducting via steel reinforcement

Conductors installed withinconcrete or in the joints ofthe roadway (plates)

Warning!Keep off the car parkduring thunderstorms

Mushroom headcollectorArt.-Nr. 108 001

Mushroom headcollector after asphalting

hh

r

Height of the air-termination roddimensioned according to therequired protective angle

Additionalair-terminationcable

h = 2.5 m + s

dard prescribes a mesh size which depends on theclass of lightning protection system chosen (seeChapter 5.1.1, Table 5.1.1.3). An air-terminationconductor installed inside the covering layer is dif-ficult to inspect after a number of years becausethe air-termination tips or mushroom head collec-tors are overgrown and no longer recognisable,and frequently damaged by maintenance work.Moreover, air-termination conductors installedinside the covering layer are more susceptible tocorrosion. Conductors of air-termination meshesinstalled uniformlyon top of the cover-ing layer are easier toinspect even if theybecome overgrown,and the height of theinterception systemcan be lifted up bymeans of air-termi-nation tips and rodsand “grown” withthe plants on theroof. Air-terminationsystems can be de-signed in differentways. The usual wayis to install a meshedair-termination netwith a mesh size of 5 m x 5 m (lightningprotection systemClass I) up to a max.mesh size of 15 m x15 m (lightning pro-tection system ClassIII) on the roof sur-face, regardless of

the height of the structure. It is preferable todetermine the installation site of the mesh consid-ering the external edges of the roof and any met-al structures acting as an air-termination system.

Stainless steel (Material No. 1.4571) has proven tobe a good material for the conductors of air-termi-nation systems on planted roofs.Aluminium wire must not be used for installingconductors in the covering layer (in the earth sub-strate or granulate), (Figures 5.1.7.1 to 5.1.7.3).


Fig. 5.1.7.1 Green roof

Fig. 5.1.8.1 Connection of roof-mounted structures

Fig. 5.1.7.3 Conductor leading on the cover-ing layer

Fig. 5.1.7.2 Air-termination system on agreen roof

roof

1st floor

ground floor

basement

connection viaisolating spark gapdirect connection

EBB

data lines

5.1.8 Isolated air-termination systems

Roof-mounted structures such as air conditioningand cooling systems, e.g. for mainframes, arenowadays used on the roofs of larger office blocksand industrial structures. Antennas, electricallycontrolled domelights, advertising signs with inte-grated lightning and all other protruding roof-mounted structures having a conductive connec-tion, e.g. via electrical cables or ducts, into thestructure, must be treated in a similar way.According to the state of the art for lightning pro-tection, such roof-mounted structures are protect-ed against direct lightning strikes by means of sep-arately mounted air-termination systems. This pre-vents partial lightning currents from entering thestructure, where they would affect or even destroythe sensitive electrical /electronic installations.In the past, these roof-mounted structures wereconnected directly. This direct connection meant that parts of thelightning current were conducted into the struc-ture. Later, “indirect connection” via a spark gapwas introduced. This meant that direct lightningstrikes to the roof-mounted structure could alsoflow away via the “internal conductors” to someextent, and in the event of a more distant light-ning strike to the structure, the spark gap shouldnot operate. The operating voltage of approx. 4 kV was almost always attained and hence partial

lightning current was also carried into the struc-ture via the electrical cable, for example. This canaffect or even destroy electrical or electronicinstallations inside the structure.

The only way of preventing these currents to becarried in is to use isolated air-termination systemswhich maintain the separation distances.

Figure 5.1.8.1 shows a partial lightning currentpenetrating the inside of the structure.

These widely different roof-mounted structurescan be protected by various designs of isolated air-termination systems.

Air-termination rods

For smaller roof-mounted structures (e.g. smallfans) the protection can be achieved by using indi-vidual, or a combination of several, air-termina-tion rods. Air-termination rods up to a height of2.0 m can be fixed with one or two concrete basespiled on top of each other (e.g. Part No. 102 010)as self supporting installation (Figure 5.1.8.2).

If air-termination rods are higher than 2.5 m or 3.0 m, they must be fixed at the object to be pro-tected by distance holders made of electricallyinsulating material (e.g. DEHNiso distance holder)(Figure 5.1.8.3).

Angled supports are a practical solution when air-termination rods also have to be secured against


Fig. 5.1.8.2 Isolated air-termination system, protection provided byan air-termination rod

Fig. 5.1.8.3 Air-termination rod with distance holder

the effects of side winds (Figures 5.1.8.4 and5.1.8.5).If higher air-termination rods are required, e.g. forlarger roof-mounted structures, which nothing canbe secured to, the air-termination rods can beinstalled by using special supports.Self-supporting air-termination rods up to a heightof 8.5 m can be installed by using a tripod. Thesesupports are fixed to the floor with standard con-crete bases (one on top of another). Additionalguy lines are required above a free height of 6 m inorder to withstand the stresses caused by the wind.

These self-supporting air-termination rods can beused for a wide variety of applications (e.g. anten-nas, PV installations). The special feature of thistype of air-termination system is its short installa-tion time as no holes need to be drilled and onlyfew elements need to be screwed together (Fig-ures 5.1.8.6 to 5.1.8.7).For protecting complete structures or installations(e.g. PV installations, ammunition depots) with air-termination rods, lightning protection masts areused. These masts are installed in a concrete foun-

dation. Free heights of 19 m above ground levelcan be achieved, even higher, if custom-made onesare used. It is also possible to span a cable betweenthese masts if they are especially designed for thispurpose. The standard lengths of the steel tele-scopic lightning protection masts are supplied insections, offering enormous advantages for trans-portation.

Further information (e.g. installation, assembly)about these steel telescopic lightning protectionmasts can be found in Installation Instructions No. 1574 (Figures 5.1.8.8 and 5.1.8.9).

Spanned over by cables or conductors

According to IEC 62305-3 (EN 62305-3), air-termi-nation conductors can be installed above the struc-ture to be protected.

The air-termination conductors generate a tent-shaped protective space at the sides, and a cone-shaped one at the ends. The protective angle αdepends on the class of lightning protection sys-tem and the height of the air-termination systemabove the reference plane.


Fig. 5.1.8.4 Angled support for air-termina-tion rods

Fig. 5.1.8.6 Isolated air-termination systemfor photovoltaic system

Fig. 5.1.8.5 Supporting element for the air-termination rod

Fig. 5.1.8.7 Isolated air-termination systemfor roof-mounted structures

Fig. 5.1.8.9 Installation of a steel telescopiclightning protection mast

Fig. 5.1.8.8 Additional protection in the tran-sition area by anticorrosive bandfor underground application

The rolling sphere method with its correspondingradius (according to the class of lightning protec-tion system) can also be used to dimension theconductors or cables.

The mesh type of air-termination system can alsobe used if an appropriate separation distance sbetween the components of the installation andthe air-termination system must be maintained. Insuch cases, isolating distance holders in concretebases are installed vertically, for example, for guid-ing the mesh on an elevated level (Figure 5.1.8.10).

DEHNiso-CombiA user-friendly way of installing conductors orcables in accordance with the three differentdesign methods for air-termination systems(rolling sphere, protective angle, mesh) is providedby the DEHNiso-Combi programme of products.

The aluminium insulating pipes with “isolating dis-tance” (GRP – Glass-fibre Reinforced Plastic) whichare fixed to the object to be protected, provide away of guiding the cables. By means of the GRPdistance holder, a subsequently separate guidingto the down-conductor system or supplementaryair-termination systems (e.g. mesh) is realised.

Further information about the application is con-tained in the brochures DS 123E, DS 111E and inthe set of installation instructions No. 1475.

The types of design described can be combinedwith each other as desired to adapt the isolatedair-termination systems to the local conditions(Figures 5.1.8.11 to 5.1.8.14).


Fig. 5.1.8.10 Installed air-termination system Ref.: Blitzschutz Wettingfeld , Krefeld. Germany

Fig. 5.1.8.12 Isolated air-termination systems with DEHNiso-Combi

Fig. 5.1.8.11 Tripod support for self-supporting insulating pipes

5.1.9 Air-termination system for steeplesand churches

External lightning protection systemAccording to the German standard DIN EN 62305-3,Supplement 2, lightning protection systems ClassIII meet the normal requirements for churches andsteeples. In particular individual cases, for example

in the case of culturally significant structures, aspecial risk analysis in accordance with IEC 62305-2(EN 62305-2) must be carried out.

NaveAccording to the German standard DIN EN 62305-3,Supplement 2, the nave must have its own light-ning protection system and, if a steeple isattached, this system must be connected by theshortest route with a down-conductor system ofthe steeple. In the transept, the air-terminationconductor along the transverse ridge must beequipped with a down-conductor system at eachend.

SteepleSteeples up to a height of 20 m must be equippedwith a down-conductor system. If steeple and naveare joined, then this down-conductor system mustbe connected to the external lightning protectionsystem of the nave by the shortest route (Figure5.1.9.1). If the down-conductor system of thesteeple coincides with a down-conductor system ofthe nave, then a common down-conductor systemcan be used at this location. According to the Ger-man standard DIN EN 62305-3, Supplement 2,steeples above 20 m in height must be provided


Fig. 5.1.8.13 Detail picture of DEHNiso-Combi

Fig. 5.1.9.1 Installing the down-conductor system at a steeple

Fig. 5.1.8.14 Isolated air-termination system with DEHNiso-Combi

with at least two down conductors. At least one ofthese down conductors must be connected withthe external lightning protection system of thenave via the shortest route.

Down-conductor systems on steeples must alwaysbe guided to the ground on the outside of thesteeple. The installation inside the steeple is notallowed (DIN EN 62305-3 Supplement 2). Further,the separation distance s to metal components andelectrical installations in the steeple (e.g. clockmechanisms, belfry) and under the roof (e.g. airconditioning, ventilation and heating systems)must be maintained by suitable arrangement ofthe external lightning protection system. Therequired separation distance can become a prob-lem especially at the clock. In this case, the conduc-tive connection into the structure can be replacedwith an isolating connector (e.g. a GRP pipe) toprevent hazardous sparking in parts of the exter-nal lightning protection system.

In more modern churches built with reinforcedconcrete, the reinforcement steels can be used asdown-conductor systems if it can be ensured thatthey provide a continuous conductive connection.If pre-cast reinforced concrete parts are used, thereinforcement may be used as a down-conductorsystem if terminals to connect the reinforcementcontinuously are provided on the pre-cast concreteparts.

In Germany the lightning equipotential bondingwith the electronic equipment (power system,telephone and public address system) shall beeffected at the entrance to the building and forthe bells control and timing system in the steepleand at the control and timing system, in accor-dance with Supplement 2 of DIN EN 62305-3.

5.1.10 Air-termination systems for wind tur-bines (WT)

Requirement for protection against lightningIEC 61400-24 describes measures required to pro-tect wind turbines against lightning. In the certifi-cation directives of the German Lloyd, a lightningprotection system Class III is required for WT hubsin a height of 60 m and Class II if the hub is in aheight of more than 60 m. In case of offshoreplants a lightning protection system Class I is

required. This can control lightning strikes withcurrents measuring up to 200,000 A. This require-ments are based on the experience made at theoperation of WT and on the assessment of the riskof damage according to IEC 62305-2 (EN 62305-2).

Principle of an external lightning protection sys-tem for wind turbinesThe external lightning protection system compri-ses air-termination systems, down-conductor sys-tems and an earth termination system and protectsagainst mechanical destruction and fire. Lightningstrikes to wind turbines usually affect the rotorblades. Hence, receptors, for example, are inte-grated to determine defined points of strike (Fig-ure 5.1.10.1).

In order to allow the coupled lightning currents toflow to earth in a controlled way, the receptors inthe rotor blades are connected to the hub with ametal interconnecting conductor (solid tape con-ductor St/tZn 30 mm x 3.5 mm or copper cable 50 mm2). Carbon fibre brushes or air spark gapsthen, in turn, bridge the ball-bearings in the headof the nacelle in order to avoid the welding of therevolving parts of the structure.


Fig. 5.1.10.1 WT with integrated receptors in the rotor blades

receptor

wire meshwork

In order to protect structures on the nacelle, suchas anemometers in the event of a lightning strike,air-termination rods or “air-termination cages”are installed (Figure 5.1.10.2).The metal tower or, in case of a prestressed con-crete version, the down-conductor systems embed-ded in the concrete (round conductor St/tZn Ø 8 ...10 mm or tape conductor St/tZn 30 mm x 3.5 mm) is used as the down-conductor system. Thewind turbine is earthed by a foundation earthelectrode in the base of the tower and the meshedconnection with the foundation earth electrode ofthe operation building. This creates an “equipo-tential surface” which prevents potential differ-ences in the event of a lightning strike.

5.1.11 Wind load stresses on lightning pro-tection air-termination rods

Roofs are used more and more as areas for techni-cal installations. Especially when extending thetechnical equipment in the structure, extensiveinstallations are sited more than ever on the roofsof larger office blocks and industrial structures. It isessential to protect roof-mounted structures suchas air conditioning and cooling systems, transmit-ters for cell sites on host buildings, lamps, flue gasvents and other apparatus connected to the elec-trical low voltage system (Figure 5.1.11.1).

In accordance with the relevant lightning protec-tion standards contained in the IEC 62305 (EN62305) series, these roof-mounted structures canbe protected from direct lightning strikes with iso-lated air-termination systems. This requires an iso-

lation of both the air-termination systems, such asair-termination rods, air-termination tips or air-ter-mination meshes, and the down-conductor sys-tems, i.e. to be installed with sufficient separationdistance from the roof-mounted structures withinthe zone of protection. The construction of an iso-lated lightning protection system creates a zone ofprotection in which direct lightning strikes cannotoccur. It also prevents partial lightning currentsfrom entering the low voltage system and hencethe structure. This is important as the entering ofpartial lightning currents into the building canaffect or destroy sensitive electrical /electronicinstallations.Extended roof-mounted structures are alsoequipped with a system of isolated air-terminationsystems. These are connected with each other andalso with the earth-termination system. Amongother things the magnitude of the zone of protec-tion created depends on the number and theheight of the air-termination systems installed.A single air-termination rod is sufficient to providethe protection required by smaller roof-mountedstructures. The procedure involves the applicationof the rolling sphere method in accordance withIEC 62305-3 (EN 62305-3) (Figure 5.1.11.2).With the rolling sphere method, a rolling spherewhose radius depends on the class of lightning


Fig. 5.1.10.2 Lightning protection for wind speed indicators at WT

Fig. 5.1.11.1 Protection against direct lightning strikes by self-sup-porting air-termination rods

protection system chosen is rolled in all possibledirections on and over the structure to be protect-ed. During this procedure, the rolling sphere musttouch the ground (reference plane) and/or the air-termination system only.This method produces a protection volume wheredirect lightning strikes are not possible.To achieve the largest possible volume of protec-tion, and also to be able to protect larger roof-mounted structures against direct lightningstrikes, the individual air-termination rods shouldideally be erected with a corresponding height. To prevent self-supporting air-termination rodsfrom tilting and breaking a suitably designed baseand supplementary braces are required (Figure5.1.11.3).The requirement for the self-supporting air-termi-nation rods to be built as high as possible must bebalanced against the higher stress exerted by theactive wind loads. A 40 % increase in wind speed,for example, doubles the active tilting moment. Atthe same time, from the application point of view,

users demand a lightweight system of “self-sup-porting air-termination rods”, which are easier totransport and install. To ensure that it is safe to useair-termination rods on roofs, their mechanical sta-bility must be proven.

Stress caused by wind loadsSince self-supporting air-termination rods areinstalled at exposed sites (e.g. on roofs), mechani-cal stresses arise which, owing to the comparablelocation and the upcoming wind speeds, corre-spond to the stresses suffered by antenna frames.Self-supporting air-termination rods must there-fore basically meet the same requirements con-cerning their mechanical stability as set out in theGerman standard DIN 4131 for antenna frames.DIN 4131 divides Germany up into 4 wind zoneswith zone-dependent wind speeds (Figure5.1.11.4).When calculating the prospective actual wind loadstresses, apart from the zone-dependent windload, the height of the structure and the local con-


Fig. 5.1.11.3 Self-supporting air-termina-tion rod with variable tripod

Fig. 5.1.11.2 Procedure for installation of air-termination systems according to IEC 62305-3 (EN 62305-3)

h 1h 2

air-termination rod

α

protective angle

mesh size M

downconductor

r

rollingsphere

earth-termination system

I 20 m 5 x 5 mII 30 m 10 x 10 mIII 45 m 15 x 15 mIV 60 m 20 x 20 m

Class of LPS Radius of therolling sphere (r)

Mesh size(M)

Max. height of the building

air-terminationrod with air-termination tip

bracing

variabletripod

When designing self-supporting air-terminationrods, the following requirements must be met forthe wind load stress:

⇒ Tilt resistance of the air-termination rods

⇒ Fracture resistance of the rods

⇒ Maintaining the required separation distanceto the object to be protected even under windloads (prevention of intolerable deflections)

Determination of the tilt resistanceThe dynamic pressure arising (depends on thewind speed), the resistance coefficient cw and thecontact surface of the wind on the air-terminationrod, generate a uniform load q‘ on the surfacewhich generates a corresponding tilting momentMT on the self-supporting air-termination rod. To

ensure that the self-supporting air-termination rod is stable, the tilt-ing moment MT must be opposedby a load torque MO , which is gen-erated by the post. The magnitudeof the load torque MO depends onthe standing weight and the radiusof the post. If the tilting moment isgreater than the load torque, thewind load pushes the air-termina-tion rod over.The proof of the stability of self-supporting air-termination rods isalso obtained from static calcula-tions. Besides the mechanical char-acteristics of the materials used,the following information is in-cluded in the calculation:

⇒ Wind contact surface of theair-termination rod: deter-mined by length and diameterof the individual sections ofthe air-termination rod.

⇒ Wind contact surface of thebracing: Very high self-sup-porting air-termination rodsare anchored with 3 bracesmounted equidistantly aroundthe circumference. The windcontact surface of these bracescorresponds to the area pro-jected by these braces onto aplane in a right angle to thedirection of the wind, i.e. the

ditions (structure standing alone in open terrain orembedded in other buildings) must also be includ-ed. From Figure 5.1.11.4 it can be seen that around95 % of Germany´s surface area lies within WindZones I and II. Air-termination rods are thereforegenerally designed for Wind Zone II. The use ofself-supporting air-termination rods in Wind ZoneIII and Wind Zone IV must be assessed for eachindividual case taking the arising stresses intoaccount.According to DIN 4131 a constant dynamic pres-sure over the height of a structure can be expectedfor structures up to a height of 50 m. For the calcu-lations, the maximum height of the structure wasconsidered 40 m, so that a total height (height ofthe structure plus length of the air-terminationrods) is kept below the 50 m mark.


Zone IV

Zone III Zone II

Zone I

München

Augsburg

Regensburg

Nürnberg

Würzburg

Stuttgart

Freiburg

Saarbrücken Mannheim

FrankfurtWiesbaden

Köln

Düsseldorf

Bonn

EssenDortmund

Erfurt Chemnitz

DresdenLeipzig

Halle

Magedburg

Berlin

PotsdamHannover

Bremen

HamburgSchwerin

RostockKiel

I

I

II

II

IV

Zone Dynamicpressure

2]q [kN/m

0.8

1.05

1.4

1.7

Windvelocityv [km/h]

126.7

145.1

161.5

184.7

Windstrength

12 - 17

Fig. 5.1.11.4 Division of Germany into wind load zones and corresponding values ofdynamic pressure and max. wind speed Ref.: DIN 4131:1991-11: Steel antenna frames, Berlin: Beuth-Verlag, GmbH

brace lengths are shortened accordingly whenconsidered in the calculation.

⇒ Weight of the air-termination rod and thebracing: The dead weight of the air-termina-tion rod and the braces is taken into account inthe calculation of the load torque.

⇒ Weight of the post: The post is a tripodweighted down with concrete blocks. Theweight of this post is made up of the deadweight of the tripod and the individualweights of the concrete blocks used.

⇒ Tilting lever of the post: The tilting leverdenotes the shortest distance between thecentre of the tripod and the line or pointaround which the whole system would tilt.

The proof of stability is obtained by comparing thefollowing moments:

⇒ Tilting moment formed from the wind-load-dependent force on the air-termination rod orthe braces and the lever arm of the air-termi-nation rod.

⇒ Load torque formed from the weight of thepost, the weight of the air-termination rodand the braces, and the length of the tilt leverthrough the tripod.

Stability is achieved when the ratio of load torqueto the tilting moment assumes a value >1.Basically: the greater the ratio of load torque totilting moment, the greater the stability.The required stability can be achieved in the fol-lowing ways:

⇒ In order to keep the wind contact surface ofthe air-termination rod small, the cross sec-tions used have to be as small as possible. Theload on the air-termination rod is reduced,but, at the same time, the mechanical strengthof the air-termination rod decreases (risk ofbreaking). It is therefore crucial to make acompromise between a smallest possible crosssection to reduce the wind load and a largestpossible cross section to achieve the requiredstrength.

⇒ The stability can be increased by using largerbase weights and /or larger post radii. Thisoften conflicts with the limited areas for erec-tion and the general requirement for lowweight and easy transport.

ImplementationIn order to provide the smallest possible wind con-tact surface, the cross sections of the air-termina-tion rods were optimised in accordance with theresults of the calculation. For easier transportationand installation, the air-termination rod comprisesan aluminium tube (in sections, if so desired) andan aluminium air-termination rod. The post tohold the air-termination rod is hinged and is avail-able in two versions. Roof pitches up to 10 ° can becompensated..

Determination of the fracture resistanceNot only the stability of the air-termination rodmust be proven, but also the fracture resistance,since the occurring wind load exerts bendingstresses on the self-supporting air-termination rod.The bending stress in such cases must not exceedthe max. permissible stress. The bending stressoccurring is higher for longer air-termination rods.The air-termination rods must be designed toensure that wind loads as can arise in Wind Zone IIcannot cause permanent deformation of the rods.Since both the exact geometry of the air-termina-tion rod and the non-linear performance of thematerials used must be taken into account, theproof of the fracture resistance of self-supportingair-termination rods is obtained using an FEM cal-culation model. The finite elements method, FEMfor short, is a numerical method for calculation ofstresses and deformations of complex geometricalstructures. The structure under examination is bro-ken down into so-called “finite elements” usingimaginary surfaces and lines which are intercon-nected via nodes.The calculation requires the following informa-tion:

⇒ FEM-calculation model

The FEM calculation model corresponds to thesimplified geometry of the self-supporting air-termination rod.

⇒ Material characteristics

The performance of the material is represent-ed by the details of cross-sectional values,modulus of elasticity, density and lateral con-traction.

⇒ Loads

The wind load is applied to the geometricmodel as a pressure load.


The fracture resistance is determined by compar-ing the permissible bending stress (materialparameter) and the max. bending stress which canoccur (calculated from the bending moment andthe effective cross section at the point of maxi-mum stress).

Fracture resistance is achieved if the ratio of per-missible to actual bending stress is >1. Basically, thesame principle also applies here: the greater theratio of permissible to actual bending stress, thegreater the fracture resistance.

Using the FEM calculation model, the actual bend-ing moments for two air-termination rods (length= 8.5 m) were calculated as a function of theirheight with and without braces (Figure 5.1.11.5).This clearly illustrates the effect of a possible braceon the course of the moments. Whereas the max.bending moment of the air-termination rod with-out a brace in the fixed-end point is around 1270 Nm, the brace reduces the bending momentto around 460 Nm. This brace makes it possible toreduce the stresses in the air-termination rod tosuch an extent that, for the max. expected windloads, the strength of the materials used is not

exceeded and the air-termination rod is notdestroyed.

ImplementationBraces create an additional “bearing point” whichsignificantly reduces the bending stresses occur-ring in the air-termination rod. Without supple-mentary bracing, the air-termination rods wouldnot cope with the stresses of Wind Zone II. There-fore, air-termination rods higher than 6 m areequipped with braces.

In addition to the bending moments, the FEM cal-culation also provides the tensile forces occurringin the bracing, whose strength must also beproven.

Determination of the wind-load-dependent de-flection of the air-termination rodA further important value calculated with the FEMmodel is the deflection of the tip of the air-termi-nation rod. Wind loads cause the air-terminationrods to bend. The bending of the rod results in achange to the volume to be protected. Objects tobe protected are no longer situated in the zone of


Fig. 5.1.11.5 Comparison of bending moment courses at self-supporting air-termination rods with and without braces (length = 8.5 m)

protection and /or proximities can no longer bemaintained.The application of the calculation model on a self-supporting air-termination rod without and withbraces produces the following results (Figures5.1.11.6 and 5.1.11.7).For the example chosen, the calculation gives a dis-placement of the tip of the air-termination rodwith bracing of around 1150 mm. Without bracingthere would be a deflection of around 3740 mm, atheoretical value which exceeds the breakingpoint of the air-termination rod under considera-tion.

ImplementationAbove a certain rod height, supplementary bracesreduce this defection significantly. Furthermore,this also reduces the bending load on the rod.

ConclusionTilting resistance, fracture resistance and deflec-tion are the decisive factors when designing air-termination rods. Base and air-termination rodmust be coordinated to ensure that the loadsoccurring as a result of the wind speeds of Zone IIdo not cause a tilting of the rod, nor damage it.It must still be borne in mind that large deflectionsof the air-termination rod reduce the separationdistance and thus intolerable proximities can arise.

Higher air-termination rods require a supplemen-tary bracing to prevent such intolerable deflec-tions of the tips of the air-termination rods.The measures described ensure that self-support-ing air-termination rods can cope with Zone IIwind speeds according to DIN 4131 (German stan-dard).

5.2 Down-conductor systemThe down-conductor system is the electrically con-ductive connection between the air-terminationsystem and the earth-termination system. Thefunction of down-conductor systems is to conductthe intercepted lightning current to the earth-ter-mination system without intolerable temperaturerises, for example, to damage the structure.To avoid damage caused during the lightning cur-rent discharge to the earth-termination system,the down-conductor systems must be mounted toensure that from the point of strike to the earth,

⇒ several parallel current paths exist,

⇒ the length of the current paths is kept as shortas possible (straight, vertical, no loops),

⇒ the connections to conductive components ofthe structure are made wherever required (dis-tance < s; s = separation distance).


Fig. 5.1.11.6 FEM model of a self-supporting air-termination rodwithout bracing (length = 8.5 m)

Fig. 5.1.11.7 FEM model of a self-supporting air-termination rodwith bracing (length = 8.5 m)

5.2.1 Determination of the number of downconductors

The number of down conductors depends on theperimeter of the external edges of the roof(perimeter of the projection on the ground sur-face).The down conductors must be arranged to ensurethat, starting at the corners of the structure, theyare distributed as uniformly as possible to theperimeter.Depending on the structural features (e.g. gates,precast components), the distances between thevarious down conductors can be different. In eachcase, there must be at least the total number ofdown conductor required for the respective classof lightning protection system.The IEC 62305-3 (EN 62305-3) standard gives typi-cal distances between down conductors and ringconductors for each class of lightning protectionsystem (Table 5.2.1.1).The exact number of down conductors can only bedetermined by calculating the separation distances. If the calculated separation distance cannot bemaintained for the intended number of down con-ductors of a structure, then one way of meetingthis requirement is to increase the number ofdown conductors. The parallel current pathsimprove the current splitting coefficient kc. Thismeasure reduces the current in the down conduc-tors, and the required separation distance can bemaintained.Natural components of the structure (e.g. rein-forced concrete supports, steel skeleton) can alsobe used as supplementary down conductors if con-tinuous electrical conductivity can be ensured.By interconnecting the down conductors atground level (base conductor) and using ring con-ductors for higher structures, it is possible to bal-

ance the distribution of the lightning currentwhich, in turn, reduces the separation distance s.The latest IEC 62305 (EN 62305) series of standardsattaches great significance to the separation dis-tance. The measures specified can change the sep-aration distance positively for structures and thusthe lightning current can be safely discharged.If these measures are not sufficient to maintain therequired separation distance, it is also possible touse a new type of high voltage-resistant isolatedconductors (HVI). These are described in Chapter5.2.4.Chapter 5.6 describes how the exact separationdistance can be determined.

5.2.2 Down-conductor system for a non-iso-lated lightning protection system

The down-conductor systems are primarily mount-ed directly onto the structure (with no distance).The criterion for installing them directly on thestructure is the temperature rise in the event oflightning striking the lightning protection system.If the wall is made of flame-resistant material ormaterial with a normal level of flammability, thedown-conductor systems may be installed directlyon or in the wall.


Class of LPS

I

II

III

IV

Typical distance

10 m

10 m

15 m

20 m

Table 5.2.1.1 Distance between down conductors according to IEC 62305-3 (EN 62305-3)

Table 5.2.2.1 Max. temperature rise ΔT in K of different conductor materials

16

50

78

8 mm

10 mm

qmm2

III + IV II IIII + IV II IIII + IV II IIII + IV II I

* * *

190 460 940

78 174 310

56 143 309

5 12 22

3 5 9

1120 * *

37 96 211

15 34 66

146 454 *

12 28 52

4 9 17

Stainless steelCopperIronAluminium

Type of lightning protection system

* melting / vaporising

Owing to the specifications in the building regula-tions of the German federal states, highly flamma-ble materials are generally not used. This meansthat the down-conductor systems can usually bemounted directly on the structure.Wood with a bulk density greater than 400 kg/m2

and a thickness greater than 2 mm is considered tohave a normal level of flammability. Hence thedown-conductor system can be mounted on wood-en poles, for example.If the wall is made of highly flammable material,the down conductors can be installed directly onthe surface of the wall, provided that the temper-ature rise when lightning currents flow is not haz-ardous.The maximum temperature rise ΔT in K of the var-ious conductors for each class of lightning protec-tion system are stated in Table 5.2.2.1. These valuesmean that, generally, it is even permissible toinstall down conductors underneath heat insula-tion because these temperature rises present nofire risk to the insulation materials.This ensures that the fire retardation measure isalso provided.When installing the down-conductor system in orunderneath heat insulation, the temperature rise(on the surface) is reduced if an additional PVCsheath is used. Aluminium wire sheathed in PVCcan also be used.If the wall is made of highly flammable material,and the temperature rise of the down-conductorsystems presents a hazard, then the down conduc-tors must be mounted to ensure that the distancebetween the down-conductor systems and thewall is greater than 0.1 m. The mounting elementsmay touch the wall. The erector of the structuremust state whether the wall, where a down-con-ductor system is to be installed, is made of flamma-ble material.

In Germany the precise definition of the termsflame-resistant, normal level of flammability andhighly flammable can be taken from Supplement 1of DIN EN 62305-3 (VDE 0185-305-3).

5.2.2.1 Installation of down-conductor sys-tems

The down conductors must be arranged to be thedirect continuation of the air-termination conduc-tors. They must be installed straight and vertically

so as to represent the shortest possible direct con-nection to the earth.Loops, e.g. projecting eaves or structures, must beavoided. If this is not possible, the distance meas-ured where two points of a down-conductor sys-tem are closest, and the length I of the down-con-ductor system between these points, must fulfillthe requirements on the separation distance s (Fig-ure 5.2.2.1.1).The separation distance s is calculated using thetotal length l = l1 + l2 + l3.

Down-conductor systems must not be installed ingutters and downpipes, even if they are sheathedin an insulating material. The damp in the gutterswould badly corrode the down-conductor systems.

If aluminium is used as a down conductor, it mustnot be installed directly (with no distance) on, in orunder plaster, mortar, concrete, neither should itbe installed in the ground. If it is equipped with aPVC sheath, then aluminium can be installed inmortar, plaster or concrete, if it is possible toensure that the sheath will not be mechanicallydamaged nor will the insulation fracture at lowtemperatures.It is recommended to mount down conductors tomaintain the required separation distance s to alldoors and windows (Figure 5.2.2.1.2).

Metal gutters must be connected with the downconductors at the points where they intersect (Fig-ure 5.2.2.1.3).The base of metal downpipes must be connectedto the equipotential bonding or the earth-termi-nation system, even if the pipe is not used as adown conductor. Since it is connected to the eavesgutter, through which the lightning current flows,the downpipe also takes a part of the lightning


Fig. 5.2.2.1.1 Loop in the down conductor

l 2

l1

l3

s

current which must be conducted into the earth-termination system. Figure 5.2.2.1.4 illustrates onepossible design.

5.2.2.2 Natural components of a down-con-ductor system

When using natural components of the structureas a down-conductor system, the number of downconductors to be installed separately can bereduced or, in some cases, they can be dispensedwith altogether.

The following parts of a structure can be used as“natural components” of the down-conductor sys-tem:

⇒ Metal installations, provided that the safe con-nection between the various parts is perma-nent and their dimensions conform to theminimum requirements for down conductors.These metal installations may also be sheathedin insulating material. The use of conduits con-taining flammable or explosive materials asdown conductors is not permitted if the sealsin the flanges /couplings are non-metallic orthe flanges/couplings of the connected pipes

are not otherwise connected so as to be elec-trically conductive.

⇒ The metal skeleton of the structure

If the metal frame of structures with a steelskeleton or the interconnected reinforcedsteel of the structure is used as a down-con-ductor system, then ring conductors are notrequired since additional ring conductorswould not improve the splitting of the current.

⇒ Safe interconnected reinforcement of thestructure

The reinforcement of existing structures can-not be used as a natural component of thedown-conductor system unless it can beensured that the reinforcement is safely inter-connected. Separate external down conduc-tors must be installed.

⇒ Precast parts

Precast parts must be designed to provide ter-minal connections for the reinforcement. Pre-cast parts must have an electrically conductiveconnection between all terminal connections.The individual components must be intercon-nected on site during installation (Figure5.2.2.2.1).


Downpipes mayonly be used asdown conductor, ifthey are soldered orriveted

The connectionmust be asshort as pos-sible, straightand installedvertically StSt wire

Ø 10 mm

Fig. 5.2.2.1.2Down-conductor system Fig. 5.2.2.1.4 Earthed downpipe

Fig. 5.2.2.1.3 Air-termination system withconnection to the gutter

Note:In the case of prestressed concrete, attention mustbe paid to the particular risk of possible intolera-ble mechanical effects arising from lightning cur-rent and resulting from the connection to thelightning protection system.For prestressed concrete, connections to tension-ing rods or cables must only be effected outsidethe stressed area. The permission of the personresponsible for erecting the structure must be giv-en before using tensioning rods or cables as adown conductor.If the reinforcement of existing structures is notsafely interconnected, it cannot be used as adown-conductor system. In this case, externaldown conductors must be installed.

Furthermore, facade elements, mounting channelsand the metal substructures of facades can be usedas a natural down-conductor system, providedthat:

⇒ the dimensions meet the minimum require-ments of down-conductor systems. For sheetmetal, the thickness must not be less than 0.5 mm. Their electrical conductivity in verticaldirection must be ensured. If metal facades areused as a down-conductor system, they mustbe interconnected to ensure that the individ-ual plates are safely interconnected with each

other by means of screws, rivets, or bridgingconnections. There must be a safe connectioncapable of carrying currents to the air-termi-nation system and also to the earth-termina-tion system.

⇒ If metal plates are not interconnected in accor-dance with the above requirement, but thesubstructure ensures that they are continuous-ly conductive form the connection on the air-termination system to the connection on theearth-termination system, then they can beused as a down-conductor system (Figures5.2.2.2.2 and 5.2.2.2.3).


Fig. 5.2.2.2.1 Use of natural components – new buildings made ofready-mix concrete

Fig. 5.2.2.2.2 Metal subconstruction, conductively bridged

Fig. 5.2.2.2.3 Earth connection of a metal facade

expansion joint

expansion joint


Fixed earthing terminalPart No. 478 200

vertical box section

wall fixing

horizontal support


Metal downpipes can be used as natural downconductors, as long as they are safely interconnect-ed (brazed or riveted joints) and comply with theminimum wall thickness of the pipe of 0.5 mm.If a downpipe is not safely interconnected, it canserve as a holder for the supplementary down con-ductor. This type of application is illustrated in Fig-ure 5.2.2.2.4. The connection of the downpipe tothe earth-termination system must be capable ofcarrying lightning currents since the conductor isheld only along the pipe.

5.2.2.3 Measuring pointsThere must be a measuring point at every connec-tion of a down conductor with the earth-termina-tion system (above the lead-in, if possible).

Measuring points are required to allow the inspec-tion of the following characteristics of the light-ning protection system:

⇒ Connections of the down conductors via theair-termination systems to the next down con-ductor

⇒ Interconnections of the terminal lugs via theearth-termination system, e.g. in the case ofring or foundation earth electrodes (earthelectrode Type B)

⇒ Earth electrode resistance of single earth elec-trodes (earth electrode Type A)

Measuring points are not required if the structuraldesign (e.g. reinforced concrete structure or steelskeleton) allows no “electrical” disconnection ofthe “natural” down-conductor system to theearth-termination system (e.g. foundation earthelectrode).

The measuring point may only be opened with thehelp of a tool for the purpose of taking measure-ments, otherwise it must be closed.Each measuring point must be able to be clearlyassigned to the design of the lightning protectionsystem. Generally, all measuring points are markedwith numbers (Figure 5.2.2.3.1).


Fig. 5.2.2.2.4 Down conductorinstalled along adownpipe

Fig. 5.2.2.3.1 Measuring pointwith number plate

roofingheat insulation

wood insulation

metal construction

internal downconductor

roof bushing

If the separation distance is too short, the conductive parts of the buildingconstruction have to be connected to the air-termination system. The effectsfrom the currents have to be taken into account.

separationdistance s

Courtyards with circumference of morethan 30 m. Typical distances accordingto class of LPS

15 m

7.5

m

30 m

45 m

metal attic

courtyardcircumference> 30 m

Fig. 5.2.2.4.1 Air-termination system installed on large roofs – Internal down-conduc-tor system

Fig. 5.2.2.5.1 Down-conductor systems for court-yards

5.2.2.4 Internal down-conductor systems

If the edges of the structure (length and width) arefour times as large as the distance of the downconductors which corresponds to the class of light-ning protection system, then supplementary inter-nal down conductors must be installed (Figure5.2.2.4.1).The grid dimension for the internal down-conduc-tor systems is around 40 m x 40 m.Large structures with flat roofs, such as large pro-duction halls or also distribution centres, frequent-ly require internal down conductors. In such cases,the ducts through the surface of the roof shouldbe installed by a roofer because he is responsiblefor ensuring that the roof provides protectionagainst rain.The consequences of the partial lightning currentsthrough internal down-conductor systems withinthe structure must be taken into account. Theresulting electromagnetic field in the vicinity ofthe down conductor must be taken into considera-tion when designing the internal lightning protec-tion system (pay attention to inputs to electrical /electronic systems.)

5.2.2.5 Courtyards

Structures with enclosed courtyards having aperimeter greater than 30 m (Figure 5.2.2.5.1)must have down-conductor systems installed withthe distances shown in Table 5.2.1.1.

5.2.3 Down conductors of an isolated exter-nal lightning protection system

If an air-termination system comprises air-termina-tion rods on isolated masts (or one mast), then this

is both air-termination system and down-conduc-tor system at the same time (Figure 5.2.3.1).Each individual mast requires at least one downconductor. Steel masts or mast with an intercon-nected steel reinforcement require no supplemen-tary down-conductor system.

For optical reasons, a metal flag pole, for examplecan also be used as an air-termination system.The separation distance s between the air-termina-tion and down-conductor systems and the struc-ture must be maintained.If the air-termination system consists of one ormore spanned wires or cables, each end of thecable which the conductors are attached torequires at least one down conductor (Figure5.2.3.2).

If the air-termination system forms an intermeshednetwork of conductors, i.e. the individual spannedwires or cables are interconnected to form a mesh(being cross-linked), there must be at least onedown conductor at the end of each cable the con-ductors are attached to (Figure 5.2.3.3).

5.2.4 High voltage-resistant, isolated down-conductor system – HVI conductor

A multitude of structures is used in order to createan exhaustive network of cell sites. Some of thesestructures have lightning protection systems. Inorder to design and implement the mast infra-structure in accordance with the standards, theactual situation must be taken into account duringthe design phase while the relevant standardshave to be strictly differentiated.

For the operator of a mobile phone network thereare basically three different situations:


ss

s

mechanical fixing

downconductor

Fig. 5.2.3.1 Air-termination masts isolatedfrom the building

Fig. 5.2.3.2 Air-termination masts spannedwith cables

Fig. 5.2.3.3 Air-termination masts spannedwith cables with cross connection(meshing)

⇒ Structure has no lightning protection system

⇒ Structure is equipped with a lightning protec-tion system which is no longer capable of func-tioning

⇒ Structure is equipped with a functioning light-ning protection system

Structure has no lightning protection systemIn Germany cell sites are constructed in accordancewith DIN VDE 0855-300. This deals with the earth-ing of the cell site. In accordance with the conceptfor protection against surges of the mobile phonenetwork operators, supplementary protectionagainst surges is integrated into the meter section.

Structure is equipped with a lightning protectionsystem which is no longer capable of functioningIn Germany cell sites are connected to the externallightning protection system as required by theclass of lightning protection system (LPS) deter-mined. The lightning current paths required forthe cell site are investigated and assessed. Thisinvolves replacing non-functional components ofthe existing installation which are required to dis-charge the lightning current, such as air-termina-tion conductor, down-conductor system and con-nection to the earth-termination system. Anyobserved defects to parts of the installation whichare not required must be notified in writing to theowner of the structure.

Structure is equipped with a functioning lightningprotection systemExperience has shown that most lightning protec-tion systems are designed according to LPS Class III.Regular inspections are prescribed for certainstructures. It must be planned to integrate the cellsite installation in accordance with the class oflightning protection system (LPS) determined. Forinstallations with LPS Class I and II, the surround-ings of the installation must be recorded photo-graphically to ensure that, if problems subsequent-ly arise with proximities, the situation at the timeof construction can be proven. If a cell site is erect-ed on a structure with a functional external light-ning protection system, its erection is governed bythe latest lightning protection standard (IEC 62305(EN 62305)). In this case for example, in Germanythe DIN VDE 0855-300 can only be used for theequipotential bonding of the antenna cable. Prox-imities must be calculated as appropriate to the

class of LPS. All mechanical components used mustbe able to cope with the prospective partial light-ning currents. For reasons of standardisation, allthe steel fixing elements and structures for hold-ing antennas of many mobile phone networkoperators must be designed for LPS Class I. Theconnection should be done via the shortest route,which is not a problem, however, as the air-termi-nation conductors on flat roofs are usuallydesigned to be meshed. If there is a functionallightning protection system on the host building,this has a higher priority than an antenna earthinginstallation.

Because of how it is designed, the class of light-ning protection system to be effected must be laiddown at the discussion stage of the project:

⇒ If the system technology components are alsosituated on the roof, it is preferable to installthe electrical cable on the exterior side of thestructure.

⇒ If the system technology components are situ-ated on the roof, and if it is intended to erecta central mast, the installation must beequipped with an isolated lightning protec-tion system.

⇒ If the system technology components arelocated within the structure, it is preferable tohave an isolated lightning protection embed-ment. Care must be taken that the cell siteinfrastructure is designed to be geometricallysmall so that the costs of the isolated lightningprotection system are economically viable.

Experience has shown that, in many cases, existinglightning protection systems have old defectswhich adversely affect the effectiveness of theinstallation. These defects mean that even if thecell site is correctly “tied-in” to the external light-ning protection system, damage can still be causedwithin the structure.

In order to enable a designer of mobile phone net-works to erect antenna installations in accordancewith the standards even in difficult situations, theonly thing available to him used to be the isolatedlightning protection system with horizontal dis-tance holders. In such cases, however, the design ofthe antenna installation, could really not be con-sidered architecturally aesthetic (Figure 5.2.4.1).


Air-termination systems as shown in Figure 5.2.4.1are not applicable for locations where the anten-nas have to be pleasing to look at.

The isolated HVI conductor is an innovative solu-tion which provides the installer of lightning pro-tection systems with novel possibilities for designand for easy maintaining of the separation dis-tance (Figure 5.2.4.2).

5.2.4.1 Installation and performance of theisolated down-conductor system HVI

Basic conception of the isolated down-conductorsystem is to cover the lightning current carryingconductor with an insulating material, allowingthe necessary separation distance s to other con-ductive parts of the structure, to electrical conduc-tors and conduits to be kept. Incorrect proximitiesmust be avoided. Basically the following require-ments to the isolated down-conductor system haveto be met, if insulating materials are used to avoidinadmissible proximities:

⇒ Possibility of a lightning current proof connec-tion of the down-conductor system with theair-termination system (air-termination rod,air-termination conductor, air-termination tip,etc.) by terminals.

⇒ Compliance with the required separation dis-tance s by sufficient dielectric strength of thedown-conductor system in the range of theinput point as well as in the course of thedown-conductor system.

⇒ Sufficient current carrying capability becauseof an adequate cross section of the down-con-ductor system.

⇒ Possibility of connection to the earth-termina-tion system or of equipotential bonding.

Sheathing of the down conductors with insulatingmaterials of high dielectric strength basicallyallows to reduce the separation distance. Certainhigh voltage technological requirements, howev-er, have to be met. This being necessary as thedielectric strength of the isolated down-conductorsystem depends on its positioning and on theoccurrence of creeping discharges.

The use of unshielded, isolated down-conductorsystems is a fundamental solution to be independ-ent with regard to positioning and laying. A con-ductor, however, which has only a sheathing ofinsulating material does not solve the problem.Already relatively low induced impulse voltageswill release creeping discharges in the range of theproximities (e.g. between metal, earthed conduc-tor holders and the feeding point), which canresult in a total flashover at the surface of longconductor sections. Ranges of insulating material,metal (at high voltage potential or earthed) get-ting in contact with the air are critical with regardto creeping discharges. This range is subject to ahigh voltage stressing because of the potentialarising of creeping discharges, resulting in a con-siderably reduced voltage resistance. Creeping dis-charges have to be taken into account, wheneverusual (vertical to the surface of the insulatingmaterial) components of electrical field strength E,lead to the tripping voltage of the creeping dis-charge being exceeded and, field components tan-gentially enforce the increase of creeping dis-charges (Figure 5.2.4.1.1).

The creeping discharge release-voltage determinesthe resistance of the whole insulation, being in themagnitude of 250 – 300 kV lightning impulse volt-age.


5.2.4.25.2.4.1

Fig. 5.2.4.1 Isolated air-termination system with distance holderFig. 5.2.4.2 Isolated air-termination system for cell sites –

Application of DEHNconductor system

By the coaxial single conductor cable – HVI conduc-tor – shown in Figure 5.2.4.1.2 the occurrence ofthe creeping discharge is avoided and the light-ning current is safely conducted to the earth.

Isolated down-conductor systemswith field control and semi-con-ductive shield prevent from creep-ing discharges by a targeted influ-encing of the electric field in therange of the input point. Theyallow the lightning current to beconducted into the special cable,the safe discharge of the lightningcurrent and the required separa-tion distance s to be kept. Thesemi-conductive shield of the coax-ial input cable insulates from theelectric field. It has to be minded,however, that the magnetic fieldsurrounding the current carryinginner conductor is not affected.Optimisation of the field controlallows an adjusted cable sealingunit length of 1.50 m to realise the

required equivalent separation distance in air of s ≤ 0.75 m and in case of solid construction materi-al of s ≤ 1.50 m (Figure 5.2.4.1.3).This special cable sealing unit is realised by anadjusted connection element to the air-termina-tion system (supply point) and the equipotentialbonding terminal in a fixed distance. Comparedwith a coaxial cable with metal shield, the wholesemi-conductive coating of the cable has a clearlyhigher resistance. Even by a multiple equipotentialbonding connection of the cable coating onlyinsignificant partial lightning currents will bedragged into the building.

Apart from the required separation distance s , themaximum conductor length Lmax of such an isolat-ed down-conductor system is calculated with

5.2.4.2 Installation examplesApplication for cell sitesCell site installations are frequently erected onhost structures. There is usually an agreementbetween the operator of the cell site installationand the owner of the structure that the erection ofthe cell site installation must not increase the risk

Lk s

k km

i cmax =

⋅

⋅


innerconductor

insulation

proximity

Fig. 5.2.4.1.1 Basic development of a creeping discharge at an isolated down conductorwithout special coating

coupling ofthe lightningimpulse current

air-terminationbonding element

inner conductor

high voltage-resistant insulation equipotential

bonding element

semi-conductivesheath

sealing unit range

Fig. 5.2.4.1.2 Components of HVI Conductor

Fig. 5.2.4.1.3 HVI conductor I and components of the DEHNconduc-tor system

to the structure. For protection against lightning,this particularly means that no partial lightningcurrents must enter the structure if there is a light-ning strike to the frame structure. A partial light-ning current within the structure would especiallyput the electrical and electronicapparatus at risk.Figure 5.2.4.2.1 shows one possi-ble solution for the “isolated air-termination system” on theframe structure of an antenna.The air-termination tip must befixed to the frame structure ofthe antenna by means of aninsulating pipe in non conduc-tive material so that it is isolated.The height of the air-termina-tion tip is governed by therequirement that the structureof the frame and any electricaldevices which are part of the cellsite installation (BTS – BaseTransceiver Station) must bearranged in the zone of protec-tion of the air-termination tip.

Structures with several antenna systems must beequipped with multiple “isolated air-terminationsystems”.Figures 5.2.4.2.2a and b illustrate the installationon an antenna post.


Fig. 5.2.4.2.2b Connection to the antennaframe structure for directingpotential

Fig. 5.2.4.2.2a Insulating pipe within theantenna area

Fig. 5.2.4.2.1 Integration of a new 2G/3G antenna into the existing lightning protection system by using the HVI conductor

earth connectionfeeding point

HVI® conductor

insulating pipe

earthingclamp

air-termination tip

feeding point

HVI® conductor

insulatingpipe

earthconnection

α α

antenna cableearthing acc. to VDE 0855-300

HVI® conductor II

insulating pipe GRP/AL

air-termination tip

sealing unitrange

BTS

LV-supply

equipotential bonding line

sealing unit

bare down conductor

Isolated lightning protection

Note: Clarify existing state of protection

air-termination system

Roof-mounted structuresMetal and electrical roof-mounted structures pro-trude above roof level and are exposed points forlightning strikes. The risk of partial lightning cur-rents flowing within the structure is also existingbecause of conductive connections with conduitsand electrical conductors leading into the struc-ture. To prevent this and to set up the necessaryseparation distance for the complete structure easily, the air-termination system must be instal-led with a terminal to the isolated down-conduc-tor system, as shown in Figure 5.2.4.2.3a and5.2.4.2.3b.

Hence all metal and electrical roof-mounted struc-tures protruding above roof level are within thearea protected against lightning strikes. The light-ning current will be “channeled” along the struc-ture and distributed by the earth-termination sys-tem.

If several structures are mounted on the roof then,according to the basic illustration in Figure5.2.4.2.4, several isolated air-termination systemsmust be installed. This must be done to ensure thatall structures protruding above the roof must bearranged in an area protected from lightningstrikes (lightning protection zone LPZ 0B).

Down-conductor systemEspecially problematical from the optical point ofview often is the integration of a down-conductorsystem, taking into account the required separa-tion distance s.The HVI conductor e.g. can be installed or evenintegrated in the facade (Figure 5.2.4.2.5). Thisnew kind of isolated down-conductor system con-


α

cable duct

foundation earth electrode

metal earthed roof-mounted structure

reinforcementcable duct

HVI® conductor I

EB terminal

separationdistance s

sealing unit

isolated air-termination system

metal attic cover within the protective area of an isolated air-termination system

ring conductor

HVI® conductor

Fig. 5.2.4.2.3a Fan with air-termination rod and spanned cable

Fig. 5.2.4.2.3b Air-termination rod, elevated ring conductor connect-ed to the isolated down-conductor system

Fig. 5.2.4.2.4 Keeping the required separation distance with volt-age-controlled isolated down conductor (HVI)

Fig. 5.2.4.2.5 Air termination system with spanned cable and isolated down-conductor system

tributes to an architectural more pleasing struc-ture. Functionality and design can be an entity.Therefore this innovative technology is an impor-tant feature of modern architecture.

5.2.4.3 Project example: Training and resi-dential building

StructureThe structure in Figure 5.2.4.3.1 was built conven-tionally from the ground floor to the 6th floor. Ata later date, the 7th floor was attached to theexisting roof surface.The external facade of the 7th floor consists ofmetal sheets.The media centre is situated on the 3rd floor, theground floor is used for administration. All otherfloors up to the 7th floor are used for apartments.The roof surface of the 6th and 7th floor was fin-ished off with a metal attic whose components areinterconnected so as to be non-conductive.The complete structure is 25.80 m high (withoutattic) up to the roof level.Subsequently, five antenna systems for mobilephone systems and microwaves were installed bydifferent operators of mobile phone networks onthe roof surface of the 7th floor. The antennaswere erected both in the corners and in the middleof the roof surface.The cable (coax cables) from the four antennas inthe corners of the roof surface were installed inthe vicinity of the attic to the south-west corner.From this point, the cables are ledthrough a metal cable duct which isconnected to the attic of the roof sur-faces of the 7th and 6th floors to theBTS room on the 6th floor.The cables from the antenna in themiddle are also installed by means of ametal cable duct directly to the 2ndBTS room on the north-east side of thestructure to the 6th floor. This cableduct is also connected to the surround-ing attics.The structure was equipped with alightning protection system. The newinstallation of the external lightningprotection system to protect againstdamage to the structure and life haz-ards was designed in accordance withthe national lightning protection stan-

dard DIN V VDE V 0185-3, which was applicablewhen the building was erected.

During the installation of the antennas, theequipotential bonding and earthing measures ofthe system were carried out in accordance with theGerman standard DIN VDE 0855 Part 300.

The earthing of the systems, however, was not iso-lated from the existing external lightning protec-tion system at the earth-termination system atground level, but directly at the air-terminationsystem.

Hence, in the event of a lightning discharge, par-tial lightning currents are conducted directly intothe structure via the coax cable shields. These par-tial lightning currents do not only present a lifehazard, they also present a hazard to the existingtechnical equipment of the structure.

New concept

A lightning protection system was required, whichprevents partial lightning currents from being con-ducted directly into the structure via the antennacomponents (frame structures, cable shields andinstallation systems). At the same time, therequired separation distance s between the framestructures of the antennas and the air-terminationsystem on the roof surface of the 7th floor must berealised.

This cannot be effected with a lightning protec-tion system of a conventional design.


Fig. 5.2.4.3.1 Total view

54

3 cable tray1 2

Antennas of the cell site operators (1 - 5)

By installing the HVI conductor,a lightning protection systemwas constructed with an isolat-ed air-termination system. Thisrequired the following compo-nents:

⇒ Air-termination tips oninsulating pipes in GRPmaterial, secured directlyto the antenna pole (Fig-ure 5.2.4.2.2a).

⇒ Down conductor from theair-termination tip bymeans of an HVI conductorwith connection to the iso-lated ring conductor (Fig-ure 5.2.4.3.2).

⇒ Sealing end feeding pointto ensure the resistanceagainst creeping flashoversat the input (Figures5.2.4.2.2a and 5.2.4.2.2b).

⇒ Isolated ring conductor oninsulating supports madeof GRP, supports as high asaccording to the calcula-tion of the required sepa-ration distance

⇒ Down conductors installedseparately from the isolat-ed ring conductor via therespective metal attics andmetal facade to the baremetal down conductors onthe 6th floor with therequired separation dis-tance s to the lower attic(Figure 5.2.4.3.3).

⇒ Supplementary ring con-ductor, all down-conductorsystems interconnected ata height of approx. 15 m toreduce the required sepa-ration distance s of theinterception and down-conductor system (Figures5.2.4.3.4 and 5.2.4.4.1).

The various implementationstages explained in detail aresummarised in Figure 5.2.4.3.4.


air-termination tip

HVI® conductor

isolated ring conductor

bare down conductorcable duct

attic

ring conductor

bare down conductor

isolatedring conductor

HVI® conductorconnection toequipotential bonding

isolated ring conductor

cable tray

HVI®

conductor

Fig. 5.2.4.3.2 Isolated air-terminationsystem and isolated ringconductorRef.: H. Bartels GmbH,Oldenburg, Germany

Fig. 5.2.4.3.3 Down conductor of isolated ring con-ductor

Fig. 5.2.4.3.4 Total view on a newly installed external lightning protection system

It is also important to note that the proposeddesign concept was discussed in detail with the sys-tem erector in order to avoid mistakes when carry-ing out the work.When designing the external lightning protectionsystem, care was taken that the deck on the 6thfloor (Figure 5.2.4.3.1) and the lower attachments(Figure 5.2.4.3.4) were also arranged in the zone ofprotection/protective angle of the air-terminationsystem.

5.2.4.4 Separation distanceWhen calculating the required separation distances, not only the height of the structure but also theheights of the individual antennas with the isolat-ed air-termination system had to be taken intoconsideration.Each of the four corner antennas protrudes 3.6 mabove the surface of the roof. The antenna in themiddle protrudes 6.6 m above the roof surface.

Considering the height of the structure, result thefollowing total heights to be taken into accountwhen calculating the installation:

⇒ 4 corner antennas to the base of the air-termi-nation tip + 29.40 m

⇒ 1 antenna in the middle of the roof surface tothe base of the air-termination tip + 32.40 m

⇒ Three further, isolated separate air-termina-tion rods on the west side of the roof surfaceand two isolated air-termination masts on thebalcony 6th floor, south side, realise the zoneof protection of the complete roof surface.

A special cable, DEHNconductor, Type HVI, wasused as the isolated down conductor, allowing anequivalent separation distance of s = 0.75 m (air) /1.5 m (solid building materials) to be maintained.

The calculation of the required separation dis-tances was done as shown in Figure 5.2.4.4.1 forthree partial areas:

1. Partial area at a level of + 32.4 m and a level of+ 29.4 m (antennas) to + 27.3 m (isolated ringconductor) on the roof.

2. Partial area at + 27.3 m to + 15.0 m (isolatedring conductor on roof up to lower supple-mentary ring conductor).

3. Partial area at + 15.0 to ± 0 m (lower ring con-ductor to ground level).

The complete down-conductor system comprisessix down conductors from the isolated ring con-ductor at a height of + 27.3 m to the supplemen-tary ring conductor at a level of + 15.0 m. The ringconductor at a level of + 15.0 m is connected withthe earthing ring conductor via the six down con-ductors of the residential structure and four fur-ther down conductors on attached parts of thestructure.This produces a different splitting of the current inthe individual partial areas which had to be takeninto consideration for the design of the lightningprotection system.The equipotential bonding required and theearthing of the antenna components on the roofsurface (including the cable ducts, metal fa-cades and the attics on both roof levels) was done using two supplementary earthing cablesNYY 1 x 25 mm2 connected to the equipotentialbonding of the individual BTS stations.The erection of this isolated air-termination systemon the surface of the roof and on the antenna sys-tems, as well as the isolated down conductorsaround metal parts of the structure, prevent par-tial lightning currents from entering the structure.


Fig. 5.2.4.4.1Calculation of the required separation distance

ring conductor

EB c

ondu

ctor

dow

nco

nduc

tor

kc1

kc2

kc3

L 1L 2

L 3 1st floor

2nd floor

3rd floor

4th floor

5th floor

7th floor

ground floor

6th floor


Remarks10)Min. cross-section mm2

Material Configuration

Copper solid flat materialsolid round material7)

cablesolid round material3), 4)

508)

508)

508)

2008)

min. thickness 2 mmdiameter 8 mmmin. diameter each wire 1.7 mmdiameter 16 mm

Tin platedcopper1)

solid flat materialsolid round material7)

cable

508)

508)

508)

min. thickness 2 mmdiameter 8 mmmin. diameter each wire 1.7 mm

Aluminium solid flat materialsolid round materialcable

70508)

508)

min. thickness 3 mmdiameter 8 mmmin. diameter each wire 1.7 mm

Aluminiumalloy

solid flat materialsolid round materialcablesolid round material3)

508)

50508)

2008)

min. thickness 2.5 mmdiameter 8 mmmin. diameter each wire 1.7 mmdiameter 16 mm

Hot dippedgalvanisedsteel2)

solid flat materialsolid round material9)

cablesolid round material3), 4), 9)

508)

50508)

2008)

min. thickness 2.5 mmdiameter 8 mmmin. diameter each wire 1.7 mmdiameter 16 mm

Stainlesssteel5)

solid flat material6)

solid round material6)

cablesolid round material3), 4)

508)

50708)

2008)

min. thickness 2 mmmin. thickness 8 mmmin. diameter each wire 1.7 mmdiameter 16 mm

1) Hot dipped or electroplated, minimum thickniss of the coating 1 μm.

2) The coating should be smooth, continuous and free of residual flux, minimum thickness 50 μm.

3) For air-termination rods. For applications where mechanical loads, like wind loads are not critical, a max.1 m long air-termination rod with a diameter of 10 mm with an additional fixing may be used.

4) For lead-in earth rods.

5) Chromium 16 %, nickel 8 %, carbon 0.03 %

6) For stainless steel in concrete and/or in direct contact with flammable material, the min. cross sectionfor solid round material has to be increased to 78 mm2 (10 mm diameter) and for solid flat material to75 mm2 (3 mm thickness).

7) For certain applications where the mechnical strength is not important, 28 mm2 (6 mm diameter) materialmay be used instead of 50 mm2 (8 mm diameter). Then distance of the fixing elements has to be reduced.

8) If thermal and mechanical requirements are important, the min. cross section for solid flat material canbe increased to 60 mm2 and for solid round material to 78 mm2.

9) At a specific energy of 10,000 kJ/Ω the min. cross section to prevent from melting is 16 mm2 (copper),25 mm2 (aluminium), 50 mm2 (steel) and 50 mm2 (stainless steel). For further information see Annex E.

10) Thickness, width and diameter are defined at a tolerance of ± 10 %.

Table 5.3.1 Material, configuration and min. cross sections of air-termination conductors, air-termination rods and down conductors accordingto IEC 62305-3 (EN 62305-3) Table 6

5.3 Materials and minimum dimen-sions for air-termination conduc-tors and down conductors

Table 5.3.1 gives the minimum cross sections, formand material of air-termination systems.

This requirements arise from the electrical conduc-tivity of the materials to carry lightning currents(temperature rise) and the mechanical stresseswhen in use.

When using a round conductor Ø 8 mm as an air-termination tip, the max. free height permitted is0.5 m. The height limit for a round conductor Ø 10mm is 1 m in free length.

Note:According to IEC 62305-3 (EN 62305-3) Clause 1,Table 8, the minimum cross section for an intercon-necting conductor between two equipotentialbonding bars is 14 mm2 Cu.

Tests with a PVC-insulated copper conductor andan impulse current of 100 kA (10/350 μs) deter-mined a temperature rise of around 56 K. Thus, acable NYY 1 x 16 mm2 Cu can be used as a downconductor or as a surface and underground inter-connecting cable, for example.

5.4 Assembly dimensions for air-ter-mination and down-conductorsystems

The following dimensions (Figure 5.4.1) have beentried and tested in practice and are primarilydetermined by the mechanical forces acting on thecomponents of the external lightning protectionsystem.These mechanical forces arise not so much as aresult of the electrodynamic forces generated bythe lightning currents, but more as a result of thecompression forces and the tensile forces, e.g. dueto temperature-dependent changes in length,wind loads or the weight of snow.The information concerning the max. distances of1.2 m between the conductor holders primarilyrelates to St/tZn (relatively rigid). For using alu-minium, distances of 1 m have become the stan-dard in practice.IEC 62305-3 (EN 62305-3) gives the followingassembly dimensions for an external lightning pro-tection system (Figures 5.4.1 and 5.4.2).

Figure 5.4.3 illustrates the application on a flatroof.


0.3 m

1.0

m

0.3

m1.

5 m

0.5

m

0.05 m

α

e

e = 0.2 mdistanceappropriate1.0 m 0.

15 m

1.0 m

as closeas possible

to the edge

Fig. 5.4.1 Detail examples of an external lightning protection system at a building with a slopedtiled roof

Fig. 5.4.2 Air-termination rod for chim-neys

1 m

Fig. 5.4.3 Application on a flat roof

If possible, the separation dis-tance to windows, doors andother openings should be main-tained when installing downconductors.Further important assemblydimensions are illustrated in Fig-ures 5.4.3 – 5.4.5.

Installation of surface earth elec-trodes (e.g. ring earth electrodes)around the structure at a depthof > 0.5 m and a distance ofapprox. 1 m from the structure(Figure 5.4.4).

For the earth entries or terminalson the foundation earth elec-trode (ring earth electrodes), cor-rosion protection must be consid-ered. Measures such as anticorrosive bands orwires with PVC sheath at a min. of 0.3 m above andbelow the turf (earth entry) must be employed(Figure 5.4.5) for protection.

An optically acceptable and noncorrosive connec-tion possibility is provided by a stainless steel fixedearthing terminal set to be laid in concrete.Moreover, there must also be corrosion protectionfor the terminal lug for equipotential bondinginside the building in damp and wet rooms.

The material combinations below (within air-ter-mination systems, down conductors and with partsof the structure) have been tried and tested, pro-vided that no particularly corrosive environmentalconditions must be taken into consideration. Theseare values obtained from experience (Table 5.4.1).

5.4.1 Change in length of metal wires

In practice, the temperature-dependent changesin length of air-termination and down conductors are often underestimated.

The older regulations and stipulations recom-mended an expansion piece about every 20 m as ageneral rule in many cases. This stipulation wasbased on the use of steel wires, which used to bethe usual and sole material employed. The highervalues for the coefficients of linear expansion ofstainless steel, copper and especially aluminiummaterials were not taken into account.

In the course of the year, temperature changes of100 K must be expected on and around the roof.The resulting changes in length for different metalwire materials are shown in Table 5.4.1.1. It isnoticeable that, for steel and aluminium, the tem-


Steel (tZn)

Aluminium

Copper

StSt

Titanium

Tin

Steel (tZn)

yes

yes

no

yes

yes

yes

Aluminium

yes

yes

no

yes

yes

yes

Copper

no

no

yes

yes

no

yes

StSt

yes

yes

yes

yes

yes

yes

Titanium

yes

yes

no

yes

yes

yes

Tin

yes

yes

yes

yes

yes

yes

building

≥ 0.

5 m

≈ 1 m

0.3 m

protectionagainst

corrosion0.3 m

Fig. 5.4.4 Dimensions for ring earth elec-trodes

Table 5.4.1 Material combinations

Fig. 5.4.5 Points threatened by corrosion

perature-dependent changes in length differ by afactor of 2.The stipulations governing the use of expansionparts in practice are thus as shown in Table 5.4.1.2.When using pieces, care must be taken that theyprovide flexible length equalisation. It is not suffi-cient to bend the metal wires into an S shape sincethese “expansion pieces“, handmade on site, arenot sufficiently flexible.When connecting air-termination systems, forexample to metal attics surrounding the edges ofroofs, care should be taken that there is a flexibleconnection to suitable components or measures. Ifthis flexible connection is not made, there is a riskthat the metal attic cover will be damaged by thetemperature-dependent change in length.To compensate for the temperature-dependentchanges in length of the air-termination conduc-tors, expansion pieces must be used to equalise theexpansion (Figure 5.4.1.1).

5.4.2 External lightning protection systemfor an industrial structure and a resi-dential house

Figure 5.4.2.1a illustrates the design of the exter-nal lightning protection system for a residentialhouse with attached garage and Figure 5.4.2.1bthat for an industrial structure.Figures 5.4.2.1a and 5.4.2.1b and Tables 5.4.2.1aand b show examples of the components in usetoday.No account is taken of the measures required foran internal lightning protection system such aslightning equipotential bonding and surge protec-tion (see also Chapter 6).

Particular attention is drawn to DEHN´s DEHNsnapand DEHNgrip programme of holders.The DEHNsnap generation of synthetic holders(Figure 5.4.2.2) is suitable as a basic component


Δ ΔL L T= ⋅ ⋅α

Material Coefficientof linear expansion α

Assumed temperature change on the roof: ΔT = 100 K

Steel

StSt

Copper

Aluminium

11.5

16

17

23.5

ΔL = 11.5 • 10-6 • 100 cm • 100 = 0.115 cm • 1.1 mm/m

ΔL = 16 • 10-6 • 100 cm • 100 = 0.16 cm = 1.6 mm/m

ΔL = 17 • 10-6 • 100 cm • 100 = 0.17 cm = 1.7 mm/m

ΔL = 23.5 • 10-6 • 100 cm • 100 = 0.235 cm • 2.3 mm/m

1

106

1

K

ΔL

Calculation formula

X

X

X

X

X

X

15

20

10

15

10

Material Surface under the fixing of the air-terminationsystem or down conductor

Distance ofexpansion pieces

in msoft,

e. g. flat roof with bitumen-or synthetic roof sheetings

hard,e. g. pantilesor brickwork

Steel

StSt/Copper

Aluminium

Use of expansion pieces, if no other length compensation is provided

Table 5.4.1.2 Expansion pieces in lightning protection – Recommended application

Table 5.4.1.1 Calculation of the temperature-related change in length ΔL of metal wires in light-ning protection

Fig. 5.4.1.1 Air-termination system –Compensation of expansionwith bridging braid


EBB

3

14

13

15

210

9

7

8

6

1

4

511

Pos. Part description Part No.1 Round conductor 8 mm - DEHNALU,

medium hard soft- twistable840 008840 018

2 Steel strip 30 x 3.5 mm St/tZnRound conductor 10 mm StSt V4A

810 335860 010

3 Roof conductor holders St/tZnfor ridge and hip tiles StSt

StStStStStStStSt

202 020204 109204 249204 269206 109206 239

4 Roof conductor holders StStfor conductors within roof surfaces StSt

St/tZnSt/tZnSt/tZn

StStSt/tZn

204 149204 179202 010202 050202 080206 209206 309

5 DEHNsnapDEHNgripconductor holder with cleat and flangeconductor holder for heat insulation

204 006207 009275 160273 740

6 Gutter clamp for beads St/tZnStSt

Single-screw gutter clamp St/tZnStSt

339 050339 059339 100339 109

7 MV clamp St/tZnMV clamp StSt

390 050390 059

8 Gutter board clamp St/tZn 343 0009 Downpipe clamp adjustable for 60 - 150 mm

Downpipe clamp for any cross sectionsKS connector for connecting conductorsKS connector StSt

423 020423 200301 000301 009

Pos. Part description Part No.

10 MV clamp 390 05111 Bridging bracket Aluminium

Bridging braid Aluminium377 006377 015

12 Lead-in earthing rod 16 mmcomplete

480 150480 175

13

14

Parallel connector

Cross unitSV clamps St/tZnSV clamps StSt

305 000306 020319 201308 220308 229

15

Rod holder with cleat and flangeRod holder for heat insulation

275 260273 730

Number plate for marking isolating points 480 006480 005

16 Air-termination rod with forged tabAir-termination rod with rounded endsRod clamp

100 075483 075380 020

Fig. 5.4.2.1a External lightning protection of a residential building

Table 5.4.2.1a Components for external lightning protection of a residential building

16

12

(roof and wall). The cap simply snaps in to fix theconductor in the holder while still being looselyguided. The special snap-in technique exerts nomechanical load on the fastening.

DEHNgrip (Figure 5.4.2.2) is a stainless steel systemof holders without screws which was put into the

programme to supplement the DEHNsnap systemof synthetic holders.

This system of holders without screws can also beused as both a roof and a wall conductor holderfor Ø 8 mm conductors.

Simply press in the conductors and the conductor isfixed in DEHNgrip (Figure 5.4.2.2).


1

2

3

4

5

6

8

9

7

10

11

Pos. Part description Part No.1234567

Stainless steel conductor 10 mm StStSet of lead-in earthing rods St/tZnCross unit StStDEHNALU-DRAHT® AlMgSiConductor holder DEHNsnap®

Bridging braid AlAir-termination rod AlMgSiwith concrete base with adapted flat washer

860 010480 150319 209840 008204 120377 015104 200120 340

Pos. Part description Part No.8910

11

Roof conductor holder for flat roofsDEHNiso distance holder ZDC-St/tZnElevated ring conductorwith concrete base with adapted flat washerand distance holder StStIsolated air-termination rod

253 050106 100

102 340106 160105 500

Fig. 5.4.2.1b External lightning protection of an industrial structure

Table 5.4.2.1b Components for external lightning protection of a residential structure

5.4.3 Application tips for mounting roofconductors holders

Ridge and hip tiles:Adjust roof conductor holders with adjustingscrew to suit the dimension of the ridge tile (Figure5.4.3.1).The conductor leading can, in addition, be gradu-ally adjusted by means of conductor holders fromthe top centre to the bottom side.

(Conductor holder can be loosened by either turn-ing the holder or opening the fixing screw.)

⇒ SPANNsnap roof conductor holder with DEHN-snap synthetic conductor holder or DEHNgripstainless steel conductor holder (Figure5.4.3.2).Permanent tension due to stainless steel ten-sion spring. Universal tension range from


1

2

basic component

cap

Conductor holderDEHNgrip

Conductor holderDEHNsnap

Fig. 5.4.2.2 DEHNsnap and DEHNgrip conductor holders

Fig. 5.4.3.1 Conductor holder with DEHNsnap for ridge tiles

Fig. 5.4.3.2 SPANNsnap with plasticDEHNsnap conductor holder

Fig. 5.4.3.3 FIRSTsnap for mounting on existing ridge clamp

180 – 280 mm with laterally adjustable conduc-tor leading for Rd 8 mm conductors.

⇒ FIRSTsnap conductor holder with DEHNsnapsynthetic conductor holder for putting onexisting ridge clamps for dry ridges.

For dry ridges, the DEHNsnap conductor holder (1)(Figure 5.4.3.3) is put on the ridge clamp alreadyon the structure (2) and tightened manually (onlyturn DEHNsnap).

Grooved pantiles:UNIsnap roof conductor holder with preformedstruts is used for the roof surfaces. The conductorholder is bent by hand before being hooked intothe battens. Additionally, it can also be securedwith nails (Figure 5.4.3.4).

Smooth tiles (Figure 5.4.3.5)

Slate roofs:When using it on slate roofs, the internal hook sys-tem is bent (Figure 5.4.3.6) or equipped with a sup-plementary clamp (Part No. 204 089).

Grooved tiles:⇒ FLEXIsnap roof conductor holder for grooved

tiles, for direct fitting on the groove (Figure5.4.3.7).

The flexible stainless steel strut is pushedbetween the grooved tiles.By pressing on the top grooved tile, the stain-less steel strut is deformed and adapts itself tothe shape of the groove.Thus it is fixed tightly under the tile.This application with an aluminium strutmakes it easy to adapt to the shape of thegroove.A notch is provided for an eventually existingwindow hook. The strut of the holder can alsobe nailed down (holes in the strut).

⇒ Roof conductor holders with preformed strut,for hooking into the bottom grove for pantileroofs (Figure 5.4.3.8).

Flat tiles or slabs:DEHNsnap conductor holder (1) (Figure 5.4.3.9)and its clamping device (2) is pushed in betweenthe flat tiles (3) (e.g. plain tile) or slabs and tight-ened manually (only turn DEHNsnap).

Overlapped constructions:In case of overlapped constructions (3) (e.g. slabsand natural slates), DEHNsnap conductor holder(1) (Figure 5.4.3.10) with clamping terminals (2) ispushed on from the side and secured with a screwdriver when the holder is open.For slabs laid on a slat, DEHNsnap can also beturned to allow a plumb conductor leading.


angled by hand

angle the inner latchingfor use on slate roofs

Fig. 5.4.3.4 UNIsnap roof conductor holderwith preformed strut – Used ongrooved pantiles

Fig. 5.4.3.5 UNIsnap roof conductor holderwith preformed strut – Used onsmooth tiles, e.g. plain tiles

Fig. 5.4.3.6 UNIsnap roof conductor holderwith preformed strut – Used onslate roofs


insert the holderunderneath

lift tile

press tileon it

insert the holderunderneath

lift tile

press tileon it

DEHNsnap

1

2

1

4

3

DEHNsnap

1

2

3

1

3

Fig. 5.4.3.7 Conductor holder for direct fitting on the seams Fig. 5.4.3.8 Roof conductor holder for hanging into the bottomseam of pantile roofs

Fig. 5.4.3.9 ZIEGELsnap, for fixing between flat tiles or plates Fig. 5.4.3.10 PLATTENsnap roof conductor holder for overlapped construction

5.5 Earth-termination systems

A detailed explanation of the terms used in earth-termination technology is contained in IEC 62305-3 (EN 62305-3) “Lightning protection –physical damage to structures and life hazard”, HD 637 S1 “Power installations exceeding 1 kV”,IEC 60050-826 “International electrotechnicalvocabulary Part 826: Electrical installations” andIEC 60364-5-54 “Electrical installations of buildings– Part 5-54”. In Germany DIN 18014 is additionallyapplicable for foundation of earth electrodes.Below, we repeat only the terminology which isrequired to understand the following designs.

Terminology

Earthis the conductive ground whose electrical poten-tial at each point is set equal to zero as agreed. Theword “earth” also the designation for both theearth as a place as well as earth as a material, e.g.

the type of soil: humus, loam, sand, gravel androck.Reference earth(neutral earth) is the part of the earth, especiallythe surface of the earth outside the sphere ofinfluence of an earth electrode or an earth-termi-nation system, in which, between two arbitrarypoints, no perceptible voltages arising from theearthing current occur (Figure 5.5.1).

Earth electrodeis a conductive component or several conductivecomponents in electrical contact with the earthand forming an electrical connection with it(includes also foundation earth electrodes).

Earth-termination systemis a localised entirety of interconnected conductiveearth electrodes or metal components acting assuch, (e.g. reinforcements of concrete founda-tions, cable metal sheaths in contact with theearth, etc.).


Fig. 5.5.1 Earth surface potential and voltages at a foundation earth electrode FE and control earth electrode CE flown through by currents

1 m

UB2

ϕFE

US

FE

ϕ

UB1

ϕFE + SE

UE

UE Earth potentialUB Touch voltageUB1 Touch voltage without potential control (at the

foundation earth electrode)UB2 Touch voltage with potential control (foundation

and control earth electrode)US Step voltageϕ Earth surface potentialFE Foundation earth electrodeCE Control earth electrode (ring earth electrode)

reference earth

CE

Earthing conductoris a conductor connecting a system component tobe earthed to an earth electrode and which isinstalled above the ground or insulated in theground.

Lightning protection earthingis the earthing installation of a lightning protec-tion system to discharge lightning currents intothe earth.

Below some types of earth electrodes and theirclassification are described according to location,form and profile.

Classification according to location

Surface earth electrodeis an earth electrode generally driven in at a shal-low depth down to 1 m. It can consist of roundmaterial or flat strips and be designed as a star-type, ring or meshed earth electrode or a combina-tion thereof.

Earth rodis an earth rod generally driven in plumb down togreater depths. It can consist of round material ormaterial with another profile, for example.

Foundation earth electrodecomprises one or more conductors embedded inconcrete which is in contact with the earth over awide area.

Control earth electrodeis an earth electrode whose form and arrangementserves more to control the potential than to main-tain a certain earth electrode resistance.

Ring earth electrodeis an an earth electrode underneath or on the sur-face of the earth, leading as closed ring around thestructure.

Natural earth electrodeis a metal component in contact with the earth orwith water either directly or via concrete, whoseoriginal function is not as an earth electrode butwhich acts as an earth electrode (reinforcementsof concrete foundations, conduits, etc.).

Classification according to form and profileOne distinguishes between:flat strip earth electrodes, cruciform earth elec-trodes and earth rods.

Types of resistanceSpecific earth resistanceρE is the specific electrical resistance of the earth. Itis given in Ωm and represents the resistancebetween two opposite sides of a cube of earthwith edges of 1 m in length.

Earth electrode resistanceRA of an earth electrode is the resistance of theearth between the earth electrode and referenceearth. RA is practically a resistance.

Impulse earth resistanceRst is the resistance as lightning currents traversefrom one point of an earth-termination system tothe reference earth.

Voltages at current carrying earth-termina-tion systems, control of potentialEarth potentialUE is the voltage arising between an earth-termi-nation system and reference earth (Figure 5.5.1).

Potential of the earth´s surfaceϕ is the voltage between one point of the earth´ssurface and reference earth (Figure 5.5.1).

Touch voltageUB is the part of the potential of the earth´s surfacewhich can be bridged by humans (Figure 5.5.1), thecurrent path via the human body running fromhand to foot (horizontal distance from touchablepart around 1 m) or from one hand to the other.

Step voltageUS is the part of the potential of the earth´s surfacewhich can be bridged by humans taking one step 1m long, the current path via the human body run-ning from one foot to the other (Figure 5.5.1).

Potential controlis the effect of the earth electrodes on the earthpotential, particularly the potential of the earth´ssurface (Figure 5.5.1).


Equipotential bondingfor lightning protection system is the connectionof metal installations and electrical systems to thelightning protection system via conductors, light-ning current arresters or isolating spark gaps.

Earth electrode resistance/Specific earthresistance

Earth electrode resistance RA

The conduction of the lightning current via theearth electrode into the ground does not happenat one point but rather energises a particular areaaround the earth electrode.The type of earth electrode and the way it isinstalled must now be chosen to ensure that thevoltages affecting the surface of the earth (touchand step voltages) do not assume hazardous val-ues.The earth electrode resistance RA of an earth elec-trode can best be explained with the help of ametal sphere buried in the ground.If the sphere is buried deep enough, the currentdischarges radially to be equally distributed overthe surface of the sphere. Figure 5.5.2a illustratesthis case; as a comparison, Figure 5.5.2b illustratesthe case of a sphere buried just under the earth´ssurface.The concentric circles around the surface of thesphere represent surface of equal voltage. Theearth electrode resistance RA is composed of thepartial resistances of individual layers of thesphere connected in series.The resistance of such alayer of the sphere is calculated using

where ρE is the specific earth resistance of theground, assuming it is homogeneous,

l the thickness of an imaginary layer of thesphere

and

q the medial surface of this layer of the sphere.

To illustrate this, we assume a metal sphere 20 cmin diameter buried at a depth of 3 m at a specificearth resistance of 200 Ωm.

If now the increase in earth electrode resistancefor the different layers of the sphere is calculated,then as a function of the distance from the centreof the sphere, a curve as shown in Figure 5.5.3. isobtained.

The earth electrode resistance RA for the sphericalelectrode is calculated using:

ρE Specific earth resistance in Ωm

Rr

r

tA

E

K

K

=⋅

⋅⋅

+ρπ

100

2

12

2

Rl

qE= ⋅ρ


equipotential lines

a) Spherical earthelectrode deep inthe ground

b) Spherical earthelectrode close to the earth surface

1 2 3 4 5

160

140

120

100

80

60

40

20

RA = 161 Ω

Eart

h el

ectr

ode

resi

stan

ce R

A (Ω

)

approx. 90%

Distance x (m)

Fig. 5.5.2 Current distribution from the spherical earth electrode

Fig. 5.5.3 Earth electrode resistance RA of a spherical earth elec-trode with Ø 20 cm, 3 m deep, at ρE = 200 Ωm as a func-tion of the distance x from the centre of the sphere

t Burial depth in cm

rK Radius of the spherical earth electrode in cm

This formula gives a earth electrode resistance ofRA = 161 Ω for the spherical earth electrode.

The trace of the curve in Figure 5.5.3 shows thatthe largest fraction of the total earth electroderesistance occurs in the immediate vicinity of theearth electrode. Thus, for example, at a distance of5 m from the centre of the sphere, 90 % of thetotal earth electrode resistance RA has alreadybeen achieved.

Specific earth resistance ρEThe specific earth resistance ρE which determinesthe magnitude of the earth electrode resistance RAof an earth electrode, is a function of the composi-

tion of the soil, the amount ofmoisture in the soil and thetemperature. It can fluctuatebetween wide limits.

Values for various types ofsoilFigure 5.5.4 gives the fluctua-tion ranges of the specificearth resistance ρE for varioustypes of soil.

Seasonal fluctuationsExtensive measurements (lit-erature) have shown that thespecific earth resistance varies

greatly according to the burial depth of the earthelectrode. Owing to the negative temperaturecoefficient of the ground (α = 0.02 ... 0.004), thespecific earth resistance attain a maximum in win-ter and a minimum in summer. It is therefore advis-able to convert the measured values obtainedfrom earth electrodes to the maximum prospectivevalues, since even under unfavourable conditions(very low temperatures), permissible values mustnot be exceeded. The curve of the specific earthresistance ρE as a function of the season (groundtemperature) can be represented to a very goodapproximation by a sinus curve having its maxi-mum around the middle of February and its mini-mum around the middle of August. Investigationshave further shown that, for earth electrodesburied not deeper than around 1.5 m, the maxi-mum deviation of the specific earth resistancefrom the average is around ± 30 % (Figure 5.5.5).


0.1 1 10 100 1000 10000 ρE

Concrete

Boggy soil, turf

Farmland, loam

Humid sandy soil

Dry sandy soil

Rocky soil

Gravel

Lime

River and lake water

Sea water

in Ωm

e e e

a M a’

measuringdevice

30

20

10

0

10

20

30

burial depth < 1.5 m+ ρE in %

burial depth > 1.5 m

− ρE in %

June July Aug. Sept. Oct. Nov.

Jan. Feb. March April May Dec.

Fig. 5.5.4 Specific earth resistance ρE of different ground types

Fig. 5.5.5 Specific earth resistance ρE as a function of the seasonswithout influencing of rainfall (burial depth of the earthelectrode < 1.5 m)

Fig. 5.5.6 Determination of the specific earth resistance ρE with afour-terminal measuring bridge acc. to the WENNERmethod

For earth electrodes buried deeper (particularly forearth rods), the fluctuation is merely ± 10 %. Fromthe sineshaped curve of the specific earth resist-ance in Figure 5.5.5, the earthing electrode resist-ance RA of an earth-termination system measuredon a particular day can be converted to the maxi-mum prospective value.

MeasurementThe specific earth resistance ρE is determined usingan earthing measuring bridge with 4 clamps whichoperates according to the null method.Figure 5.5.6 illustrates the measuring arrangementof this measuring method named after WENNER.The measurement is carried out from a fixed cen-tral point M which is retained for all subsequentmeasurements. Four measuring probes (earthing

spikes 30 ... 50 cm long) are driven into the soilalong a line a – a' pegged out in the ground. Fromthe measured resistance R one can determine thespecific earth resistance ρE of the ground:

R measured resistance in Ω

e probe distance in m

ρE average specific earth resistance in Ωm downto a depth corresponding to the probe dis-tance e

By increasing the probe distance e and re-tuningthe earthing measuring bridge, the curve of the

ρ πE e R= ⋅ ⋅2


Table 5.5.1 Formulae for calculating the earth electrode resistance RA for different earth electrodes

Earth electrode Rough estimate Auxiliary

Surface earth electrode(star-type earth electrode)

Earth rod

Ring earth electrode

Meshed earth electrode

Earth plate

Hemispherical earth electrode

RA Earth electrode resistance (Ω)ρE Specific earth resistance (Ωm)l Length of earth electrode (m)d Diameter of a ring earth electrode, of the area of the equivalent circuit or of a hemispherical earth electrode (m)A Area (m2) of the enclosed area of a ring or meshed earth electrodea Edge length (m) of a square earth plate, for rectangular plates value: , while b and c are the

two sides of the rectangleV Content (m3) of a single foundation element

−

−

−

b c⋅

RlA

E=⋅2 ρ

RlAE=

ρ

RdAE=

⋅⋅

2

3

ρd A= ⋅1 13 2.

d A= ⋅1 13 2. RdA

E=⋅

ρ2

RaA

E=⋅

ρ4 5.

RdA

E=⋅

ρπ d V= ⋅1 57. 3

specific earth resistance can be determined ρE as afunction of the depth.

Calculation of earth electrode resistancesTable 5.5.1 gives the formulae for calculating theearth electrode resistances of the most commontypes of earth electrode. In practice, these approx-imate formulae are quite sufficient. The preciseformulae for the calculations must be taken fromthe following sections.

Straight surface earth electrodeSurface earth electrodes are generally embeddedhorizontally in the ground at a depth of 0.5 ... 1 m.Since the layer of soil covering the earth electrodedries out in summer and freezes in winter, theearth electrode resistance RA of such a surfaceearth electrode is calculated as if it lays on the sur-face of the ground:

RA Earth electrode resistance of a stretched sur-face earth electrode in Ω


l Length of the surface earth electrode in m

r Quarter width of steel strip in m or diameterof the round wire in m

The earth electrode resistance RA as a function ofthe length of the earth electrode can be takenfrom Figure 5.5.7.

Rl

l

rAE=⋅

⋅ρ

π

ln


50 100

100

50

ρE = 100 Ωm

ρE = 200 Ωm

ρE = 500 Ωm

Earth electrode resistance RA (Ω)

Length I of the stretched surface earth electrode (m)

UE

100

80

60

40

20

a

UE

100

80

60

40

20

a

V

a

t

V

V

t

V

a

100 cm

t = 0 cm50 cm

t = 0 cm

50 cm100 cm

LONGITUDINAL DIRECTION

TRANSVERSE DIRECTION

Eart

h po

tent

ial U

E (%

)Ea

rth

pote

ntia

l UE (

%)

Distance a (m) from earth electrode

Distance a (m) from earth electrode

10080604020

0.5 1 1.5 2 m

%

Max

. ste

p vo

ltage

in %

of th

e to

tal v

olta

ge

Burial depth

Fig. 5.5.7 Earth electrode resistance RA as a function of length I ofthe surface earth electrode at different specific earthresistance ρE

Fig. 5.5.9 Max. step voltage US as a function of the burial depth fora stretched earth strip

Fig. 5.5.8 Earth potential UE between supply conductor and earthsurface as a function of the distance from the earth elec-trode, at an earth strip (8 m long) in different depths

Figure 5.5.8 shows the transverse and longitudinalearthing potential UE for an 8 m long flat stripearth electrode.The effect of the burial depth on the earthingpotential can be clearly seen.

Figure 5.5.9 illustrates the step voltage US as afunction of the burial depth.

In practice, the calculation is done using theapproximate formula in Table 5.5.1:

Earth rodThe earth electrode resistance RA of a earth rod iscalculated using:

RA earth electrode resistance in Ω


l Length of the earth rod in m

r Radius of the earth rod in m

As an approximation, the earth electrode resist-ance RA can be calculated using the approximateformula given in Table 5.5.1:

Figure 5.5.10 shows the earth electrode resistanceRA as a function of the rod length I and the specif-ic earth resistance ρE.

Combination of earth electrodesIf the soil conditions require several earth rods, thedriving down depth of the earth rods is applicablefor the corresponding minimum distance of thedifferent earth rods which have to be intercon-nected.The earth electrode resistance calculated using theformulae and the measurement results given inthe diagrams apply to low frequency d.c. current

and a.c. current provided that the expansion of theearth electrode is relatively small (a few hundredmetres). For longer lengths, e.g. for surface earthelectrodes, the a.c. current also has an inductivepart. Furthermore, the calculated earth electrode resist-ances do not apply to lightning currents. This iswhere the inductive part plays a role, which canlead to higher values of the impulse earthingresistance for larger expansion of the earth-termi-nation system. Increasing the length of the surface earth elec-trodes or earth rods above 30 m reduces theimpulse earth electrode resistance by only aninsignificant amount. It is therefore expedient tocombine several shorter earth electrodes. In suchcases, because of their interaction, care must betaken that the actual total earth electrode resist-ance is greater than the value calculated from theindividual resistances connected in parallel.

Star-type earth electrodesStar-type earth electrodes in the form of cruciformsurface earth electrodes are important when rela-tively low earth electrode resistances shall be cre-ated in poorly conducting ground at an affordableprice.

RlAE=

ρ

Rl

l

rAE=⋅

⋅ρ

π2

ln

RlA

E=⋅2 ρ


2 4 6 8 10 12 14 16 18 20

100

80

60

40

20

Earth electrode resistance RA

Drive-in depth l of the earth rod

ρE = 100 Ωm

ρE = 500 Ωm

ρE = 200 Ωm

Fig. 5.5.10 Earth electrode resistance RA of earth rods as a functionof their length I at different specific earth resistances ρE

The earth electrode resistance RA of a cruciformsurface earth electrode whose sides are at 90 ° toeach other is calculated using:

RA Earth electrode resistance of the cruciform sur-face earth electrode in Ω


l Side length in m

d Half a bandwidth in m or diameter of theround wire in m

As a rough approximation, for longer lengths ofthe star arrangement (l > 10 m), the earth elec-trode resistance RA can be determined using thetotal length of the star obtained from the equa-tions in Table 5.5.1.

Figure 5.5.11 shows the curve of the earth elec-trode resistance RA of cruciform surface earth elec-trodes as a function of the burial depth;

Figure 5.5.12 shows the curve of the earthing volt-age.

For star-type earth electrodes, the angle betweenthe individual arms should be greater than 60 °.According to Figure 5.5.12 the earth electroderesistance of a meshed earth electrode is given bythe formula:

Where d is the diameter of the analogous circlehaving the same area as the meshed earth elec-trode, which is determined as follows:For rectangular or polygonal dimensions of themeshed earth electrode:

A Area of the meshed earth electrode

dA

=⋅4

π

RdA

E=⋅

ρ2

Rl

l

rAE=⋅

⋅ +ρ

π41 75

ln .


l

Earth electrode resistance RA (Ω)

Burial depth (m)

l = side length

ρE = 200 Ωm

l = 10 m

l = 25 m

%

14

12

10

8

6

4

2

0.5 1 1.5

%

100

80

60

40

20

10 20 30 m

45°

II

I

Voltage

Distance from the centre of the intersection

direction ofmeasurement II

direct

ion of

measu

remen

t I

side length 25 m

Fig. 5.5.11 Earth electrode resistance RA of crossed surface earthelectrode (90 °) as a function of the burial depth

Fig. 5.5.12 Earth potential UE between the supply conductor of theearth electrode and earth surface of crossed surfaceearth electrode (90 °) as a function of the distance fromthe cross centre point (burial depth 0.5 m)

For square dimensions (edge length b):

Figure 5.5.13 illustrates the curve of the impulseearth electrode resistance of surface earth elec-trodes with single and multiple star for square-wave voltages.

As can be seen from this diagram, for a givenlength, it is more expedient to install a radial earthelectrode than one single arm.

Foundation earth electrode

The earth electrode resistance of a metal conduc-tor in a concrete foundation can be calculated asan approximation using the formula for hemi-spherical earth electrodes:

Where d is the diameter of the analogous hemi-sphere having the same volume as the foundation:

V Volume of the foundation

When calculating the earth electrode resistance,one must be aware that the foundation earth elec-trode can only be effective if the concrete bodyhas a large contact area with the surroundingground. Water repellent, isolating shielding signif-icantly increases the earth electrode resistance, orisolate the foundation earth electrode (see 5.5.2).

Earth rods connected in parallelTo keep the interactions within acceptable limits,the distances between the individual earth elec-trodes and earth rods connected in parallel shouldnot be less than the pile depth, if possible.If the individual earth electrodes are arrangedroughly in a circle and if they all have about thesame length, then the earth electrode resistancecan be calculated as follows:

d V= ⋅1 57. 3

RdA

E=⋅

ρπ

d b= ⋅1 1.


0 1 2 3 4 5 6

Ω

160

140

120

100

80

60

40

20

0

Impu

lse

eart

h re

sist

ance

Rst

Time μs

n = 12

3

4

RA = 10 Ωl

n = 4Z = 150 ΩRA = 10 Ωn = 1 ... 4n · l = 300 m

Z Surge impedance of the earth conductorRA Earth electrode resistancen Quantity of the parallel connected earth electrodesl Mean length of the earth electrodes

al

p

n = 20

10

5

3

2

p Reduction factorn Quantity of the parallel connected earth electrodesa Mean distance of the earth electrodesl Mean length of the earth electrodes

0.5 1 2 5 10

20

10

5

3

2

1

Fig. 5.5.13 Impulse earth resistance Rst of single or multiple star-type earth electrodes with equal length

Fig. 5.5.14 Reduction factor p for calculating the total earth elec-trode resistance RA of earth rods connected in parallel

Where RA' is the average earth electrode resistanceof the individual earth electrodes. The reductionfactor p as a function of the length of the earthelectrode, the distance of the individual earth elec-trodes and the number of earth electrodes can betaken from Figure 5.5.14.

Combination of flat strip earth electrodes andearth rodsIf sufficient earth electrode resistance is providedby earth rods, for example from deep water carry-ing layers in sandy soil, then the earth rod shall beas close as possible to the object to be protected. Ifa long feed is required, it is expedient to install aradial multiple star-type earth electrode in parallelto this in order to reduce the resistance as the cur-rent rises.

As an approximation, the earth electrode resist-ance of a flat strip earth electrode with earth rodcan be calculated as if the flat strip earth electrodewere extended by the drive-in depth of the earthrod.

Ring earth electrodeFor circular ring earth electrodes with large diame-ters (d > 30 m), the earth electrode resistance is cal-culated as an approximation using the formula forthe flat strip earth electrode (where the circumfer-ence π ⋅ d is used for the length of the earth elec-trode):

r Radius or the round conductor or quarterwidth of the flat strip earth electrode in m

For non-circular ring earth electrodes, the earthelectrode resistance is calculated by using thediameter d of an analogous circle with the samearea:

A Area enclosed by the ring earth electrode

ImplementationAccording to the standards, each installation to beprotected must have its own earth-terminationsystem which must be fully functional in itselfwithout requiring metal water pipes or earthedconductors of the electrical installation.The magnitude of the earth electrode resistanceRA is of only secondary importance for protecting astructure or installation against physical damage.It is important that the equipotential bonding atground level is carried out systematically and thelightning current is safely distributed in theground.The lightning current i raises the structure to beprotected to the earthing potential UE

with respect to the reference earth.

The potential of the earth´s surface decreases withincreasing distance from the earth electrode (Fig-ure 5.5.1).The inductive voltage drop across the earth elec-trode during the lightning current rise must onlybe taken into account for extended earth-termina-tion systems (e.g. as required for long surfaceearth electrodes in poorly conducting soils withbedrock). In general, the earth electrode resistanceis determined only by the ohmic part.

If isolated conductors are led into the structure,the earthing potential UE has its full value withrespect to the conductor.In order to avoid the risk of punctures andflashovers here, such conductors are connected viaisolating spark gaps or with live conductors viasurge protective devices (see DEHN main cataloguefor Surge Protection) to the earth-termination sys-

U i R Ldi

dtE A= ⋅ + ⋅ ⋅ 1

2

dA

=⋅ 4

π

RdA

E=⋅

⋅

2

3

ρ

Rd

d

rAE=⋅

⋅⋅ρ

ππ

2

ln

Rl lA

E

flat strip eath rod

≈+

ρ

RR

pAA= '


tem as part of the lightning equipotential bond-ing.

In order to keep touch and step voltages as low aspossible, the magnitude of the earth electroderesistance must be limited.The earth-termination system can be designed as afoundation earth electrode, a ring earth electrodeand, for structures with large surface areas, as ameshed earth electrode and, in special cases, alsoas an individual earth electrode.In Germany foundation earth electrodes must bedesigned in accordance with DIN 18014.The foundation earth electrode must be designedas a closed ring and arranged in the foundations ofthe external walls of the structure, or in the foun-dation slab, in accordance with DIN 18014. Forlarger structures, the foundation earth electrodeshould contain interconnections to prevent anexceeding of the max. mesh size 20 m x 20 m.The foundation earth electrode must be arrangedto be enclosed by concrete on all sides. For steelstrips in non-reinforced concrete, the earth elec-trode must be installed on edge.In the service entrance room, a connection must beestablished between foundation earth electrodeand equipotential bonding bar. According to IEC62305-3 (EN 62305-3), a foundation earth elec-trode must be equipped with terminal lugs forconnection of the down-conductor systems of theexternal lightning protection system to the earth-termination system.Due to the risk of corrosion at the point where aterminal lug comes out of the concrete, supple-mentary corrosion protection should be consid-ered (with PVC sheath or by using stainless steelwith Material No. 1.4571).The reinforcement of plate and strip foundationscan be used as a foundation earth electrode if therequired terminal lugs are connected to the rein-forcement and the reinforcements are intercon-nected via the joints.Surface earth electrodes must be installed in adepth of at least 0.5 m.

The impulse earthing resistance of earth elec-trodes is a function of the maximum value of thelightning current and of the specific earth resist-ance. See also Figure 5.5.13. The effective lengthof the earth electrode for the lightning current iscalculated as an approximation as follows:

Surface earth electrode:

Earth rod:

Ieff Effective length of the earth electrode in m

î Peak value of the lightning current in kA

ρE Specific earth resistance Ωm

The impulse earth resistance Rst can be calculatedusing the formulae in (Table 5.5.1), where theeffective length of the earth electrode Ieff is usedfor the length I.

Surface earth electrodes are always advantageouswhen the upper soil layers have less specific resist-ance than the subsoil.If the ground is relatively homogeneous (i.e. if thespecific earth resistance at the surface is roughlythe same as it is deep down) then, for a given earthelectrode resistance, the construction costs of sur-face earth electrodes and earth rods are roughlythe same.

According to Figure 5.5.15, an earth rod must haveonly around half the length of a surface earth elec-trode.

l îeff E= ⋅0 2. ρ

l îeff E= ⋅0 28. ρ


0 5 10 15 20 30 40 50 60 70 80 90 100

90

80

70

60

50

40

30

201510

50

Length of the earth electrode l (m)

surface earth electrode

earth rod

ρE = 400 Ωm

ρE = 100 Ωm

Eart

h el

ectr

ode

resi

stan

ce R

A(Ω

)

Fig. 5.5.15 Earth electrode resistance RA of surface and earth rodsas a function of the length of the earth electrode I

If the conductivity of the ground is better deepdown than it is on the surface, e.g. because ofground water, then an earth rod is generally morecost-effective than the surface earth electrode.The issue of whether earth rods or surface earthelectrodes are more cost-effective in a particularcase, can often only be decided by measuring thespecific earth resistance as a function of the depth.Since earth rods are easy to assemble and achieveexcellent constant earth electrode resistanceswithout the need to dig a trench and withoutdamaging the ground, these earth electrodes arealso suitable for improving existing earth-termina-tion system.

5.5.1 Earth-termination systems in accor-dance with IEC 62305-3 (EN 62305-3)

Earth-termination systems are the continuation ofair-termination and down-conductor systems todischarge the lightning current into the earth. Fur-ther functions of the earth-termination system areto create equipotential bonding between thedown conductors and a potential control in thevicinity of the walls of the structure.It must be borne in mind that a common earth-ter-mination system for the various electrical systems(lightning protection, low voltage systems andtelecommunications systems) is preferable. Thisearth-termination system must be connected tothe equipotential bonding (MEBB – main equipo-tential bonding bar).Since IEC 62305-3 (EN 62305-3) assumes a systemat-ic lightning equipotential bonding, no particularvalue is required for the earth electrode resistance.

Generally, however, a low earth resistance (lessthan 10 Ω, measured with low frequency) is recom-mended.The standard classifies earth electrode arrange-ments into Type A and Type B.

For both Type A and B earth electrode arrange-ments, the minimum earth electrode length I1 ofthe earthing conductor is a function of the class oflightning protection system (Figure 5.5.1.1)The exact specific earth resistance can only bedetermined by on-site measurements using the“WENNER method“ (four-conductor measure-ment).

Earth electrode Type AEarth electrode arrangement Type A describesindividually arranged horizontal star-type earthelectrodes (surface earth electrodes) or verticalearth electrodes (earth rods), each of which mustbe connected to a down-conductor system.There must be at least 2 earth electrodes Type A.Lightning protection systems Class III and IVrequire a minimum length of 5 m for earth elec-trodes. For lightning protection systems, Class Iand II the length of the earth electrode is deter-mined as a function of the specific ground resist-ance. The minimum length for earth electrodes I1can be taken from Figure 5.5.1.1.

Minimum length of each earth electrode is:

I1 x 0.5 for vertical or slanted earth electrodes

I1 for star-type earth electrodes

The values determined apply to each individualearth electrode.

For combinations of the various earth electrodes(vertical and horizontal) the equivalent totallength should be taken into account.The minimum length for the earth electrode canbe disregarded if an earth electrode resistance ofless than 10 Ω is achieved.

Earth rods are generally driven vertically down togreater depths into natural soil which is generallyinitially encountered below the foundations. Earthelectrode lengths of 9 m have provided the advan-tage of lying at greater depths in soil layers whosespecific resistance is generally lower than in theareas closer to the surface.


80

70

60

50

40

30

20

10

00 500 1000 1500 2000 2500 3000

l1 (m)

ρE (Ωm)

class of LPS III-IV

class of LP

S I

class of LPS II

Fig. 5.5.1.1 Minimum lengths of earth electrodes

In frosty conditions, it is recommended to considerthe first 100 cm of a vertical earth electrode asineffective.

Earth electrodes Type A do not fulfill the equipo-tential bonding requirements between the downconductors and the potential control.Earth electrodes Type A must be interconnected tosplit the current equally. This is important for cal-culating the separation distance s. Earth electrodesType A can be interconnected underground or onsurface. When upgrading existing installations theinterconnection of the individual earth electrodescan also be realised by laying a conductor in thebuilding or structure.

Earth electrode Type BEarth electrodes of the Type B arrangement arering earth electrodes around the structure to beprotected, or foundation earth electrodes. In Ger-many the requirements on these earth electrodesare described in DIN 18014.If it is not possible to have a closed ring outsidearound the structure, the ring must be completedusing conductors inside the structure. Conduits orother metal components which are permanentlyelectrically conductive can also be used for thispurpose. At least 80 % of the length of the earthelectrode must be in contact with the earth toensure that, when calculating the separation dis-tance, the earth electrode Type B can be used asthe base.

The minimum lengths of the earth electrodes cor-responding to the Type B arrangement are a func-tion of the class of lightning protection system. Forlightning protection systems Class I and II, the min-imum length for earth electrodes is also deter-mined as a function of the specific ground resist-ance (see also Figure 5.5.4).For earth electrodes Type B, the average radius r ofthe area enclosed by the earth electrode must benot less than the given minimum length l1.To determine the average radius r, the area underconsideration is transferred into an equivalent cir-cular area and the radius is determined as shownin Figures 5.5.1.2 and 5.5.1.3.

Below a calculation example:

If the required value of l1 is greater than the valuer corresponding to the structure, supplementarystar-type earth electrodes or vertical earth elec-trodes (or slanted earth electrodes) must beadded, their respective lengths lr (radial/horizon-tal) and lv (vertical) being given by the followingequations:

ll r

v =−1

2

l l rr = −1


Fig. 5.5.1.2 Earth electrode Type B – Determination of the meanradius – example calculation

Fig. 5.5.1.3 Earth electrode Type B – Determination of the meanradius

r

area A1 to beconsidered

circular area A2,mean radius r

A = A1 = A2

r =

r l1

Aπ

With respect to ring or foundationearth electrodes, the mean radiusr of the area enclosed by the earthelectrode must not be shorterthan l1.

12 m

12 m

5 m

5 m

7 m

7 m

r

area A1to be considered

Example: Residential building,LPS Class III, l1 = 5 m

A1 = 109 m2

r =

r = 5.89 m

109 m2

3.14

circular area A2mean radius r

A = A1 = A2

r =

r l1

Aπ

No furtherearthelectrodesrequired!

The number of supplementary earth electrodesmust not be less than the number of down conduc-tors, but a minimum of 2. These supplementaryearth electrodes shall be connected to the ringearth electrode so as to be equidistant around thecircumference.

If supplementary earth electrodes have to be con-nected to the foundation earth electrode, caremust be taken with the materials of the earth elec-trode and the connection to the foundation earthelectrode. It is preferable to use stainless steel withMaterial No. 1.4571 (Figure 5.5.2.1).

The following systems can make additionaldemands on the earth-termination system, forexample:

⇒ Electrical systems – conditions of disconnectionfrom supply with respect to the type of net-work (TN-, TT-, IT systems) in accordance withIEC 60364-4-41: 2005, mod and HD 60364-4-41:2007

⇒ Equipotential bonding in accordance with IEC60364-5-54: 2002 and HD 60364-5-54: 2007

⇒ Electronic systems – data information techno-logy

⇒ Antenna earthing installation in accordancewith VDE 0855 (German standard)

⇒ Electromagnetic compatibility

⇒ Substation in or near the structure in accor-dance with HD 637 S1 and En 50341-1

5.5.2 Earth-termination systems, foundationearth electrodes and foundation earthelectrodes for special structural mea-sures

Foundation earth electrodes – Earth electrodesType BDIN 18014 (German standard) specifies the re-quirements on foundation earth electrodes.Many national and international standards specifyfoundation earth electrodes as a preferred earthelectrode because, when professionally installed, itis enclosed in concrete on all sides and hence cor-rosion-resistant. The hygroscopic characteristics ofconcrete generally produce a sufficiently low earthearth electrode resistance.The foundation earth electrode must be installedas a closed ring in the strip foundation or the bed-plate (Figure 5.5.2.1) and thus also acts primarily asthe equipotential bonding. The division into mesh-es ≤ 20 m x 20 m and the terminal lugs to the out-side required to connect the down conductors ofthe external lightning protection system, and tothe inside for equipotential bonding, must be con-sidered (Figure 5.5.2.2).According to DIN 18014, the installation of thefoundation earth electrode is an electrical engi-neering measure to be carried out or monitored bya recognised specialist electrical engineer.The question of how to install the foundationearth electrode must be decided according to themeasure required to ensure that the foundation


Terminal lugmin. 1.5 m long, noticeably marked− steel strip 30 mm x 3.5 mm− StSt round steel bar 10 mm− round steel bar 10 mm with PVC coating− fixed earthing point

Foundation earth electrode− steel strip 30 mm x 3.5 mm− round steel bar 10 mm

20 m

≤ 20

m

Recommendation:Several terminal lugs e.g. in every technical centre

terminal lug

additional terminal conductorfor forming meshes ≤ 20 m x 20 m

Fig. 5.5.2.1 Foundation earth electrode with terminal lug Fig. 5.5.2.2 Mesh of a foundation earth electrode

earth electrode is enclosed on all sides as the con-crete is being poured in.

Installation in non-reinforced concreteNon-reinforced foundations, e.g. strip foundationsof residential structures (Figure 5.5.2.3), requirethe use of spacers.Only by using the spacers at distances of approx. 2 m, is it possible to ensure that the foundationearth electrode is “lifted up” and can be enclosedon all sides by concrete.

Installation in reinforced concreteWhen using steel mats, reinforcement cages orreinforcement irons in foundations, it is not onlypossible to connect the foundation earth electrodeto these natural iron components, but this shouldbe done. The function of the foundation earthelectrode is thus made even more favourable.There is no need to use spacers. The modern meth-ods of laying concrete and then vibrating it, ensurethat the concrete also “flows” under the founda-tion earth electrode enclosing it on all sides.Figure 5.5.2.4 illustrates one possible applicationfor the horizontal installation of a flat strip as afoundation earth electrode. The intersections ofthe foundation earth electrode must be connectedso as to be capable of carrying currents. Galvanisedsteel is sufficient as material of the foundationearth electrode.Terminal lugs to the outside into the ground musthave supplementary corrosion protection at theoutlet point. Suitable materials are, for example,plastic sheathed steel wire (owing to the risk offracture of the plastic sheath at low temperatures,special care must be taken during the installation),

high-alloy stainless steel, Material No. 1.4571, orfixed earthing terminals.If professionally installed, the earth electrode isenclosed on all sides by concrete and hence corro-sion-resistant.When designing the foundation earth electrode,meshes no bigger than 20 m x 20 m shall berealised. This mesh size bears no relation to theclass of lightning protection system of the externallightning protection system.Modern building techniques employ various typesof foundations in a wide variety of designs andsealing versions.The terminal insulation regulations have also influ-enced the design of the strip foundations andfoundation slabs. For foundation earth electrodesinstalled in new structures in accordance with DIN18014, the insulation affects their installation andarrangement.

Perimeter insulation/Base insulation“Perimeter” is the earth-touching area of the walland base of a structure. The perimeter insulation isthe external heat insulation around the structure.The perimeter insulation seated on the externalsealing layer encloses the structure so that there isno heat bridge and protects the sealing additional-ly against mechanical damage.

The magnitude of the specific resistance of theperimeter insulating plates is a decisive factorwhen considering the effect of perimeter insu-lation on the earth electrode resistance of foun-dation earth electrodes in conventional arrange-ments in the foundation (strip foundation, foun-dation slab). Thus, for a polyurethane rigid foam


Fig. 5.5.2.3 Foundation earth electrode Fig. 5.5.2.4 Foundation earth electrode in use


granular sub-grade course

foundation slab

concrete

basement floor

drainage

moisture barrier

insulation

soil

perimeter /base insulation


terminal lug

Ref.: Acc. to DIN 18014: 2007-09; VDE series 35, Schmolke, H.; Vogt, D., “Der Fundamenterder”; HEA Elektro+: 2004


foundation slab

concrete

basement floor

drainage

moisture barrier

insulation

soil



terminal lug

insulating layer


MV TerminalPart No. 390 050

Distance holderPart No. 290 001

Cross unitPart No. 318 201

Fixed earthing terminal for EBBPart No. 478 800


Distance holderPart No. 290 001



Fig. 5.5.2.5 Arrangement of a foundation earth electrode in a strip foundation (insulated basement wall)

Fig. 5.5.2.6 Arrangement of a foundation earth electrode in a strip foundation (insulated basement wall and foundation slab)

with bulk density 30 kg/m2, for example, a specificresistance of 5.4 ⋅ 1012 Ωm is given. In contrast, thespecific resistance of concrete lies between 150 Ωmand 500 Ωm. This alone shows that, in the case ofcontinuous perimeter insulation, a conventionalfoundation earth electrode arranged in the foun-dations has practically no effect. The perimeterinsulation also acts as an electrical insulator.The diagrams below illustrate the various ways ofinsulating the foundations and walls for structureswith perimeter and base insulation.Figures 5.5.2.5 to 5.5.2.7 show the arrangement ofthe foundation earth electrodes at structures withperimeter and base insulation.The arrangement of the earth electrode in thestrip foundation with insulated sides towards theoutside and the bedplate is not regarded as critical(Figure 5.5.2.5 and 5.5.2.6).

If the foundation slab is completely insulated, theearth electrode must be installed below the bed-plate. Material V4A (Material No. 1.4571) shouldbe used (Figure 5.5.2.7).

It is efficient to install fixed earthing terminals,especially for reinforced structures. In such cases,care must be taken that the installation during theconstruction phase is carried out professionally(Figure 5.5.2.8).


concrete

basement floor

moisture barrier

insulation

soil


ring earth electrode Mat. No. 1.4571

terminal lugMat. No. 1.4571

foundation slab

reinforcement granular sub-grad course





Fig. 5.5.2.7 Arrangement of a foundation earth electrode in case of a closed floor slab (fully insulated)

Fig. 5.5.2.8 Fixed earthing terminal

Black tank, white tank

In structures erected in regions with a highgroundwater level, or in locations, e.g. on hillsides,with “pressing” water, the cellars are equippedwith special measures to prevent moisture pene-trating. The outer walls surrounded by earth, andthe foundation slab are sealed against the pene-tration of water to ensure that no troublesomemoisture can form on the inside of the wall.

Modern building techniques apply both abovementioned processes for sealing against penetrat-ing water.

One particular issue in this context is whether theefficiency of a foundation earth electrode is stillprovided for maintaining the measures to protectagainst life hazards in accordance with IEC 60364-4-41, and as a lightning protection earth electrodein accordance with IEC 62305-3 (EN 62305-3).

Foundation earth electrodes for structures withwhite tankThe name “white tank” is used to express theopposite of “black tank”: a “white tank” receivesno additional treatment on the side facing theearth, hence it is “white”.

The “white tank” is manufactured from a specialtype of concrete. Due to the aggregates used atmanufacturing of the concrete the concrete bodyis absolutely waterproof. In contrast to formeryears there is no risk of humidity penetrating a fewcentimeters into the tank. Therefore an earth elec-trode is laid outside of structures with white tank.

Figure 5.5.2.8 shows the designing of an earth con-nection by a fixed earthing terminal.

Figure 5.5.2.9 illustrates the arrangement of thefoundation earth electrode in a white tank.


concrete

basement floor

drainage

moisture barrier

insulation

soil

terminal lug



foil

foundation plate

MEBB


ring earth electrode corrossion-resistant e.g. StSt V4A (Material No. 1.4571) reinforcement

sealing tape


Connecting clampPart No. 308 025



Fig. 5.5.2.9 Arrangement of the foundation earth electrode in case of a closed tank “white tank”

Earth electrodes for structures with black tankThe name “black tank” derives from the multi-lay-ered strip of black bitumen applied to the sectionsof the structure which are outside in the ground.The body of the structure is coated withbitumen/tar which is then covered by generally upto 3 layers of bitumen strips.A ring conductor set into the foundation slababove the seal can act as the potential control inthe structure. Due to the high-impedance insula-tion to the outside, however, the earth electrode isineffective.In order to comply with the earthing requirementsstipulated in the various standards, an earth elec-trode, e.g. a ring earth electrode, must be installedexternally around the structure or below all sealsin the granular sub-grade course.Wherever possible, the external earth electrodeshould be led into the structure above the seal ofthe structure (Figure 5.5.2.10), in order to ensurethe tightness of the tank also in the long term. Awaterproof penetration of the “black tank” is only

possible using a special bushing for the earth elec-trodes.

Fibre concrete foundation slabsFibre concrete is a type of concrete which forms aheavy-duty concrete slab with steel fibres added tothe liquid concrete before hardening.The steel fibres are approx. 6 cm long and have adiameter of 1 – 2 mm. The steel fibres are slightlywavy and are admixed equally to the liquid con-crete. The proportion of steel fibres is around 20 – 30 kg/m3 concrete.The admixture gives the concrete slab both a highcompression strength and also a high tensilestrength and, compared to a conventional con-crete slab with reinforcement, it also provides aconsiderably higher elasticity. The liquid concrete is poured on site. This allows tocreate large areas with a smooth surface and nojoints. It is used for bedplates in the foundations oflarge halls, for example.


Fig. 5.5.2.10 Arrangement of the earth electrode in case of a closed tank “black tank”


concrete

soil

foundation plate

Max. ground water leveltank seal

terminal luge.g. StSt V4A(Mat. No. 1.4571)

MEBB

ring earth electrode corrosion-resistant e.g. StSt V4A (Material No. 1.4571)mesh size of the ring earth electrode max. 10 m x 10 m


soilfoundation earth electrode


Connection clampPart No. 308 025

Bushing for walls and earth electrodesPart No. 478 320

Fibre concrete has no reinforcement. This requiresa supplementary ring conductor or a meshed net-work to be constructed for installing earthingmeasures. The earthing conductor can be set in theconcrete and, if it is made of galvanised material, itmust be enclosed on all sides. This is very difficultto do on site.It is therefore recommended to install a corrosion-resistant high-alloy stainless steel, Material No.1.4571, below the subsequent concrete bedplate.The corresponding terminal lugs have to be con-sidered.Note:A specialist must install the earthing conductorsand connecting components in concrete. If this isnot possible, the building contractor can under-take the work only if it is supervised by a specialist.

5.5.3 Ring earth electrode – Earth electrodeType B

In Germany the national standard DIN 18014 stipu-lates that all new structures must have foundationearth electrodes. The earth-termination system ofexisting structures can be designed in the form ofa ring earth electrode (Figure 5.5.3.1).This earth electrode must be installed in a closedring around the structure or, if this is not possible,a connection to close the ring must be made insidethe structure.

80 % of the conductors of the earth electrode shallbe installed so as to be in contact with the earth. Ifthis 80 % cannot be achieved, it has to be checkedif supplementary earth electrodes Type A arerequired.The requirements on the minimum length of earthelectrodes according to the class of lightning pro-tection system must be taken into account (seeChapter 5.5.1).When installing the ring earth electrode, care mustbe taken that it is installed at a depth > 0.5 m anda distance of 1 m from the structure.If the earth electrode is driven in as previouslydescribed, it reduces the step voltage and thus actsas a potential control around the structure.

This earth electrode should be installed in naturalsoil. Setting it in gravel or ground filled with con-struction waste worsens the earth electrode resist-ance.When choosing the material of the earth electrodewith regard to corrosion, the local conditions mustbe taken into consideration. It is advantageous touse stainless steel. This earth electrode materialdoes not corrode nor does it subsequently requirethe earth-termination system to be refurbishedwith time-consuming and expensive measures suchas removal of paving, tar coatings or even steps,for installing a new flat strip.In addition, the terminal lugs must be particularlyprotected against corrosion.


EBB

Type S Type Z Type AZ

Fig. 5.5.3.1 Ring earth electrode around a residential building Fig. 5.5.4.1 Couplings of DEHN earth rods

5.5.4 Earth rod – Earth electrode Type AThe sectional earth rods, System DEHN, are manu-factured from special steel and hot-dip galvanised,or they consist of high-alloy stainless steel withMaterial No. 1.4571 (the high-alloy stainless steelearth electrode is used in areas especially at riskfrom corrosion). The particular feature of theseearth rods is their coupling point, which allows theearth rods to be connected without increasingtheir diameter.Each rod has a bore at its lower end, while the oth-er end of the rod has a corresponding spigot (Fig-ure 5.5.4.1).

With DEHN earth electrode Type “S”, the soft metal insert deforms as it is driven into the bore,creating an excellent electrical and mechanicalconnection.With DEHN earth electrode Type “Z”, the high coupling quality is achieved with a multiplyknurled spigot.With DEHN earth electrode Type “AZ”, the highcoupling quality is achieved with a multiplyknurled and shouldered spigot.

The advantages of the DEHN earth rods are:.

⇒ Special coupling:

⇒ no increase in diameter so that the earth rod isin close contact with the ground along thewhole of its length

⇒ Self-closing when driving in the rods

⇒ Simple to drive in with vibration hammers (Fig-ure 5.5.4.2) or mallets

⇒ Constant resistance values are achieved sincethe earth rods penetrate through the soil lay-ers which are unaffected by seasonal changesin moisture and temperature

⇒ High corrosion resistance as a result of hot-dipgalvanising (zinc coating 70 μm thick)

⇒ Galvanised earth rods also provide hot-gal-vanised coupling points

⇒ Easy to store and transport since individualrods are 1.5 or 1 m long.

5.5.5 Earth electrodes in rocky groundIn bedrock or stony ground, surface earth elec-trodes such as ring earth electrodes or star-typeearth electrodes are often the only way of creatingan earth-termination system.When installing the earth electrodes, the flat stripor round material is laid on the stony ground or onthe rock. The earth electrode should be coveredwith gravel, wet-mix slag aggregate or similar.It is advantageous to use stainless steel MaterialNo. 1.4571 as earth electrode material. Theclamped points should be installed with particularcare and be protected against corrosion (anticorro-sive band).

5.5.6 Intermeshing of earth-termination sys-tems

An earth-termination system can serve a wide vari-ety of purposes.The purpose of protective earthing is to safely con-nect electrical installations and equipment toearth potential and to prevent life hazard andphysical damage to property in the event of anelectrical fault.The lightning protection earthing system takesover the current from the down conductors anddischarges it into the ground.


Fig. 5.5.4.2 Driving the earth rod in with a work scaffolding and avibrating hammer

The functional earthing installation serves toensure that the electrical and electronic installa-tions operate safely and trouble-free.The earth-termination system of a structure mustbe used for all earthing tasks together, i.e. theearth-termination system deals with all earthingtasks. If this were not the case, potential differ-ences could arise between the installations earth-ed on different earth-termination systems.Previously, a “clean earth” was sometimes appliedin practice for functional earthing of the electronicequipment, separately from the lightning protec-tion and the protective earth. This is extremely disadvantageous and can even be dangerous. Inthe event of lightning effects, great potential dif-ferences up to a few 100 kV occur in the earth-ter-mination system. This can lead to destruction ofelectronic installations and also to life hazards.Therefore, IEC 62305-3 and -4 (EN 62305-3 and -4)require continuous equipotential bonding withina structure.The earthing of the electronic systems can be con-structed to have a radial, central or intermeshed 2-dimensional design within a structure, (Figure5.5.6.1). This depends both on the electromagnet-ic environment and also on the characteristics ofthe electronic installation. If a larger structure

comprises more than one building, and if these areconnected by electrical and electronic conductors,then combining the individual earthing systemscan reduce the (total) earth resistance. In addition,the potential differences between the structuresare also reduced considerably. This diminishesnoticeably the voltage load of the electrical andelectronic connecting cables. The interconnectionof the individual earth-termination systems of thestructure should produce a meshed network. Themeshed earthing network should be constructedto contact the earth-termination systems at thepoint where the vertical down conductors are alsoconnected. The smaller the mesh size of the net-work of the earthing installation, the smaller thepotential differences between the structures in theevent of a lightning strike. This depends on thetotal area of the structure. Mesh sizes from 20 m x 20 m up to 40 m x 40 m have proved to becost-effective. If, for example, high vent stacks(preferred points of strike) are existing, then theconnections around this part of the plant shouldbe made closer, and, if possible, radial with circularinterconnections (potential control) When choos-ing the material for the conductors of the meshedearthing network, the corrosion and material com-patibility must be taken into account.


administrationworkshop stock

gate

production

production

production

power centre

Fig. 5.5.6.1 Intermeshed earth-termination system of an industrial facility

5.5.7 Corrosion of earth electrodes

5.5.7.1 Earth-termination systems with par-ticular consideration of corrosion

Metals in immediate contact with soil or water(electrolytes) can be corroded by stray currents,corrosive soils and the formation of voltaic cells. Itis not possible to protect earth electrodes fromcorrosion by completely enclosing them, i.e. byseparating the metals from the soil, since all theusual sheaths employed until now have had a highelectrical resistance and therefore negate theeffect of the earth electrodes.Earth electrodes made of a uniform material canbe threatened by corrosion from corrosive soilsand the formation of concentration cells. The riskof corrosion depends on the material and the typeand composition of the soil.Corrosion damage due to the formation of voltaiccells is being increasingly observed. This cell forma-tion between different metals with widely differ-ent metal /electrolyte potentials has been knownfor many years. What is not widely realised, how-ever, is that the reinforcements of concrete foun-dations can also become the cathode of a cell andhence cause corrosion to other installations.With the changes to the way buildings are con-structed – larger reinforced concrete structuresand smaller free metal areas in the ground –anode / cathode surface ratio is becoming moreand more unfavourable, and the risk of corrosionof the more base metals is inevitably increasing.An electrical isolation of installations acting asanodes to prevent this cell formation is only pos-sible in exceptional cases. The aim nowadays is tointegrate all earth electrodes including thosemetal installations connected to the earth in orderto achieve equipotential bonding and hence maxi-mum safety against touch voltages at faults orlightning strikes.In high voltage installations, high voltage protec-tive earth electrodes are increasingly connected tolow voltage operating earth electrodes in accor-dance with HD 637 S1. Furthermore IEC 60364-4-41, mod and HD 60364-4-41 requires the integra-tion of conduits and other installations into theshock hazard protective measures. Thus, the onlyway of preventing or at least reducing the risk ofcorrosion for earth electrodes and other installa-tions in contact with them is choosing suitablematerials for the earth electrodes.

In Germany, the national standard DIN VDE 0151“Material and minimum dimensions of earth elec-trodes with respect to corrosion” has been avail-able since June 1986 as a white paper. Apart fromdecades of experience in the field of earthing tech-nology, the results of extensive preliminary exami-nations have also been embodied in this standard.Many interesting results are available which areimportant for the earth electrodes, including thoseof lightning protection systems.The fundamental processes leading to corrosionare explained below.Practical anticorrosion measures especially forlightning protection earth electrodes shall bederived from this and from the wealth of materialalready acquired by the VDE task force on “Earthelectrode materials”.

Terms used in corrosion protection and corrosionprotection measurements

Corrosionis the reaction of a metal material to its environ-ment which leads to impairment of the character-istics of the metal material and/or its environment.The reaction is usually of electrochemical charac-ter.

Electrochemical corrosionis corrosion during which electrochemical process-es occur. They take place exclusively in the pres-ence of an electrolyte.

Electrolyteis an ion-conducting corrosive medium (e.g. soil,water, fused salts).

Electrodeis an electron-conducting material in an elec-trolyte. The system of electrode and electrolyteforms a half-cell.

Anodeis an electrode from which a d.c. current enters theelectrolyte.

Cathodeis an electrode from which a d.c. current leaves theelectrolyte.


Reference electrodeis a measuring electrode for determining thepotential of a metal in the electrolyte.

Copper sulphate electrodeis a reference electrode which can hardly bepolarised, made of copper in saturated copper sul-phate solution.The copper sulphate electrode is the most commonform of reference electrode for measuring thepotential of subterranean metal objects (Figure5.5.7.1.1).

Corrosion cellis a voltaic cell with different local partial currentdensities for dissolving the metal. Anodes andcathodes of the corrosion cell can be formed

⇒ on the materialdue to different metals (contact corrosion) ordifferent structural components (selective orintercrystalline corrosion).

⇒ on the electrolytecaused by different concentrations of certainmaterials having stimulatory or inhibitorycharacteristics for dissolving the metal.

PotentialsReference potentialPotential of a reference electrode with respect tothe standard hydrogen electrode.

Electropotentialis the electrical potential of a metal or an electron-conducting solid in an electrolyte.

5.5.7.2 Formation of voltaic cells, corrosionThe corrosion processes can be clearly explainedwith the help of a voltaic cell. If, for example, ametal rod is dipped into an electrolyte, positivelycharged ions pass into the electrolyte and con-versely, positive ions are absorbed from the elec-trolyte from the metal band. In this context onespeaks of the “solution pressure” of the metal andthe “osmotic pressure” of the solution. Dependingon the magnitude of these two pressures, eithermore of the metal ions from the rod pass into thesolution (the rod therefore becomes negative com-pared to the solution) or the ions of the electrolytecollect in large numbers on the rod (the rodbecomes positive compared to the electrolyte). Avoltage is thus created between two metal rods inthe electrolyte.In practice, the potentials of the metals in theground are measured with the help of a coppersulphate electrode. This consists of a copper roddipped into a saturated copper sulphate solution(the reference potential of this reference electroderemains constant).Consider the case of two rods made of differentmetals dipping into the same electrolyte. A volt-age of a certain magnitude is now created on eachrod in the electrolyte. A voltmeter can be used to


i

i

electrolyte

electrode IICu

electrode IFe

i

i

electrolyte I

permeable to ions

electrode IIelectrode I

electrolyte II

12

34

5

6

1 Electrolyte copper bar with hole formeasurements

2 Rubber plug3 Ceramic cylinder with porous base4 Glaze5 Saturated Cu/CuSO4 solution6 Cu/CuSO4 crystals

Fig. 5.5.7.1.1 Application example of a non-polarisable measuringelectrode (copper /copper sulphate electrode) for tap-ping a potential within the electrolyte (cross-sectionalview)

Fig. 5.5.7.2.1 Galvanic cell: iron/copper

Fig. 5.5.7.2.2 Concentration cell

measure the voltage between the rods (elec-trodes); this is the difference between the poten-tials of the individual electrodes compared withthe electrolyte.

How does it now come that current flows in theelectrolyte and hence that material is transported,i.e. corrosion occurs?If, as shown here, the copper and iron electrodesare connected via an ammeter outside the elec-trolyte, for example, the following (Figure5.5.7.2.1) is ascertained: in the outer circuit, thecurrent i flows from + to –, i.e. from the “nobler”copper electrode according to Table 5.5.7.2.1 tothe iron electrode.

In the electrolyte, on the other hand, the current imust therefore flow from the “more negative”iron electrode to the copper electrode to close thecircuit. As a generalisation, this means that themore negative pole passes positive ions to the elec-trolyte and hence becomes the anode of the volta-ic cell, i.e. it dissolves. The dissolution of the metaloccurs at those points where the current enters theelectrolyte.A corrosion current can also arise from a concen-tration cell (Figure 5.5.7.2.2). In this case, two elec-trodes made of the same metal dip into differentelectrolytes. The electrode in electrolyte II with thehigher concentration of metal ions becomes elec-trically more positive than the other. Connecting


Km

I t=

Δ

Ws

tlin =Δ

ZincDefinition−0.9 to −1.15)

Free corrosion potential in the soil1) [V]

1Iron−0.5 to −0.83)

Tin−0.4 to −0.62)

Lead−0.5 to −0.6

Copper0 to −0.1

Symbol(s)UM-Cu/CuSO4

−1.25)Cathodic protective potential in the soil1) [V]

2 −0.854)−0.652)−0.65−0.2UM-Cu/CuSO4

10.7Electrochemical equivalent[kg/(A • year)]

3 9.119.433.910.4

0.15Linear corrosion rate [mm/year]at J = 1 mA/dm2

4 0.120.270.30.12

1) Measured to saturated copper/copper sulphate electrode (Cu/Cu So4).

2) Values are verified in presently performed tests. The potential of tin-coated copper depends on the thickness of the tin coating. Common tin coatings up to now have amounted up to a few μm and are thus between the values of tin and copper in the soil.

3) These values do also apply to lower alloyed types of iron. The potential of steel in concrete (reinforcing iron of foundations) depends considerably on external influences. Measured to a saturated copper/copper sulphate elctrode it generally amounts −0.1 to −0.4 V. In case of metal conductive connections with wide underground installations made of metal with more negative potential, it is cathodically polarised and thus reaches values up to approximately −0.5 V.

4) In anaerobic soils the protective potential should be −0.95 V.

5) Hot-dip galvanised steel, with a zinc coating according to the above mentioned table, has a closed external pure zinc layer. The potential of hot-dip galvanised steel in the soil corresponds therefore to approximately the stated value of zinc in the soil. In case of a loss of the zinc layer, the potential gets more positive. With its complete corresion it can reach the value of steel.

The potential of hot-dip galvanised steel in concrete has approximately the same initial values. In the course of time, the potential can get more positive. Values more positive than approx. −0.75 V, however, have not been found yet.

Heavily hot-dip galvanised copper with a zinc layer of min. 70 μm has also a closed external pure zinc layer. The potential of hot-dip galvanised copper in soil corresponds therefore to approx. the stated value of zinc in soil. In case of a thinner zinc layer or a corrosion of the zinc layer, the potential gets more positive. Limit values have still not been defined yet.

Table 5.5.7.2.1 Potential values and corrosion rates of common metal materials

the two electrodes enables the current i to flowand the electrode, which is electrochemically morenegative, dissolves.A concentration cell of this type can be formed, forexample, by two iron electrodes, one of which isfixed in concrete while the other lies in the ground(Figure 5.5.7.2.3).Connecting these electrodes, the iron in the con-crete becomes the cathode of the concentrationcell and the one in the ground becomes the anode;the latter is therefore destroyed by ion loss.For electrochemical corrosion it is generally thecase that, the larger the ions and the lower theircharge, the greater the transport of metal associ-ated with the current flow i, (i.e. i is proportionalto the atomic mass of the metal).In practice, the calculations are carried out withcurrents flowing over a certain period of time, e.g.over one year. Table 5.5.7.2.1 gives values whichexpress the effect of the corrosion current (currentdensity) in terms of the quantity of metal dis-solved. Corrosion current measurements thusmake it possible to calculate in advance how manygrammes of a metal will be eroded over a specificperiod.Of more practical interest, however, is the predic-tion if, and over which period of time, corrosionwill cause holes or pitting in earth electrodes, steeltanks, pipes etc. So it is important whether theprospective current attack will take place in a dif-fuse or punctiform way.For the corrosive attack, it is not solely the magni-tude of the corrosion current which is decisive, butalso, in particular, its density, i.e. the current perunit of area of the discharge area.It is often not possible to determine this currentdensity directly. In such cases, this is managed withpotential measurements the extent of the avail-

able “polarisation” can be taken from. The polari-sation behaviour of electrodes is discussed onlybriefly here.Let us consider the case of a galvanised steel stripsituated in the ground and connected to the(black) steel reinforcement of a concrete founda-tion (Figure 5.5.7.2.4). According to our measure-ments, the following potential differences occurhere with respect to the copper sulphate elec-trode:steel, (bare) in concrete: – 200 mVsteel, galvanised, in sand: – 800 mV

Thus there is a potential difference of 600 mVbetween these two metals. If they are now con-nected above ground, a current i flows in the out-er circuit from reinforced concrete to the steel inthe sand, and in the ground from the steel in thesand to the steel in the reinforcement.The magnitude of the current i is now a functionof the voltage difference, the conductance of theground and the polarisation of the two metals.Generally, it is found that the current i in theground is generated by changes in the material.But a change to the material also means that thevoltage of the individual metals changes withrespect to the ground. This potential drift causedby the corrosion current i is called polarisation. Thestrength of the polarisation is directly proportion-al to the current density. Polarisation phenomenanow occur at the negative and positive electrodes.However, the current densities at both electrodesare mostly different.

For illustration, we consider the following exam-ple:A well-insulated steel gas pipe in the ground isconnected to copper earth electrodes.


i

i

soil

electrode IISt

electrode ISt/tZn

concretesoil

i

electrode IIFe

electrode IFe

i

concrete

Fig. 5.5.7.2.3 Concentration cell: Iron in soil / iron in concrete Fig. 5.5.7.2.4 Concentration cell: Galvanised steel in soil / steel(black) in concrete

If the insulated pipe has only a few small spotswhere material is missing, there is a higher currentdensity at these spots resulting in rapid corrosionof the steel.In contrast, the current density is low over themuch larger area of the copper earth electrodeswhere the current enters. Thus the polarisation is greater at the more nega-tive insulated steel conductor than at the positivecopper earth electrodes. The potential of the steelconductor is shifted to more positive values. Thus,the potential difference across the electrodesdecreases as well. The magnitude of the corrosioncurrent is therefore also a function of the polarisa-tion characteristics of the electrodes. The strength of the polarisation can be estimatedby measuring the electrode potentials for a splitcircuit. The circuit is split in order to avoid the volt-age drop in the electrolyte. Recording instrumentsare usually used for such measurements since thereis frequently a rapid depolarisation immediatelyafter the corrosion current is interrupted.If strong polarisation is now measured at theanode (the more negative electrode), i.e. if there isan obvious shift to more positive potentials, thenthere is a high risk that the anode will corrode.

Let us now return to our corrosion cell-steel (bare)in concrete / steel, galvanised in the sand (Figure5.5.7.2.4). With respect to a distant copper sulphate electrode, it is possible to measure apotential of the interconnected cells of between –200 mV and –800 mV. The exact value dependson the ratio of the anodic to cathodic area and thepolarisability of the electrodes. If, for example, the area of the reinforced concretefoundation is very large compared to the surfaceof the galvanised steel wire, then a high anodiccurrent density occurs at the latter, so that it ispolarised to almost the potential of the reinforce-ment steel and destroyed in a relatively short time.High positive polarisation thus always indicates anincreased risk of corrosion.In practice it is, of course, now important to knowthe limit above which a positive potential shiftingmeans an acute risk of corrosion. Unfortunately, itis not possible to give a definite value, whichapplies in every case; the effects of the soil condi-tions alone are too various. It is, however, possibleto stipulate fields of potential shifting for naturalsoils.

Summary:A polarisation below +20 mV is generally non-haz-ardous. Potential shifts exceeding +100 mV aredefinitely hazardous. Between 20 and 100 mVthere will always be cases where the polarisationcauses considerable corrosion phenomena.

To summarise, one can stipulate:The precondition for the formation of corrosioncells (voltaic cells) is always the presence of metaland electrolytic anodes and cathodes connected tobe conductive.

Anodes and cathodes are formed from:

⇒ Materials

• different metals or different surface condi-tions of a metal (contact corrosion),

• different structural components (selective orintercrystalline corrosion),

⇒ Electrolytes

• different concentration (e.g. salinity, ventila-tion).

In corrosion cells, the anodic fields always have amore negative metal /electrolyte potential thanthe cathodic fields.The metal /electrolyte potentials are measuredusing a saturated copper sulphate electrodemounted in the immediate vicinity of the metal inor on the ground. If there is a metal conductiveconnection between anode and cathode, then thepotential difference gives rise to a d.c. current inthe electrolyte which passes from the anode intothe electrolyte by dissolving metal before enteringagain the cathode.

The “area rule” is often applied to estimate theaverage anodic current density JA:

JA Average anodic current density

UA,UC Anode or cathode potentials in V

ϕC Specific polarisation resistance of the cathode in Ωm2

AA,AC Anode or cathode surface m2

JU U A

AAC A

C

C

A

=−

⋅ϕ

in A/m2


The polarisation resistance is the ratio of the polar-isation voltage and the total current of a mixedelectrode (an electrode where more than one elec-trode reaction takes place).In practice, it is indeed possible to determine thedriving cell voltages UC – UA and the size of theareas AC and AA as an approximation for estimat-ing the rate of corrosion. The values for ϕA (speci-fic polarisation resistance of the anode) and ϕC ,however, are not available to a sufficient degree ofaccuracy. They depend on the electrode materials,the electrolytes and the anodic and cathodic cur-rent densities.The results of examinations available until nowallow the conclusion that ϕA is much smaller thanϕC .To ϕC applies:steel in the ground approx. 1 Ωm2

copper in the ground approx. 5 Ωm2

steel in concrete approx. 30 Ωm2

From the area rule, however, it is clear, that power-ful corrosion phenomena occur both on enclosedsteel conductors and tanks with small spots in thesheath where material is missing, connected tocopper earth electrodes, and also on earthing con-ductors made of galvanised steel connected toextended copper earth-termination systems orextremely large reinforced concrete foundations.By choosing suitable materials it is possible toavoid or reduce the risk of corrosion for earth elec-trodes. To achieve a satisfactory service life, mate-rial minimum dimensions must be maintained(Table 5.5.8.1).

5.5.7.3 Choice of earth electrode materialsTable 5.5.8.1 is a compilation of the earth elec-trode materials and minimum dimensions usuallyused today.

Hot-dip galvanised steelHot-dip galvanised steel is also suitable for embed-ding in concrete. Foundation earth electrodes,earth electrodes and equipotential bonding con-ductors made of galvanised steel in concrete maybe connected with reinforcement iron.

Steel with copper sheathIn the case of steel with copper sheath, the com-ments for bare copper apply to the sheath mater-

ial. Damage to the copper sheath, however, cre-ates a high risk of corrosion for the steel core,hence a complete closed copper layer must alwaysbe present.

Bare copperBare copper is very resistant due to its position inthe electrolytic insulation rating. Moreover, incombination with earth electrodes or other instal-lations in the ground made of more “base” mate-rials (e.g. steel), it has additional cathodic protec-tion, albeit at the expense of the more “base”metals.

Stainless steelsCertain high-alloy stainless steels according to EN 10088 are inert and corrosion-resistant in theground. The free corrosion potential of high-alloystainless steels in normally aerated soils is mostlyclose to the value of copper.The surface of stainless steel earth electrode mate-rials passivating within a few weeks, they are neu-tral to other (more inert and base) materials.Stainless steels shall contain at least 16 % chrome,5 % nickel and 2 % molybdenum.Extensive measurements have shown that only ahigh-alloy stainless steel with the Material No.1.4571, for example, is sufficiently corrosion-resist-ant in the ground.

Other materialsOther materials can be used if they are particularlycorrosion-resistant in certain environments or areat least equally as good as the materials listed inTable 5.5.8.1.

5.5.7.4 Combination of earth electrodesmade of different materials

The cell current density resulting from the combi-nation of two different metals installed in theearth to be electrically conductive, leads to the cor-rosion of the metal acting as the anode (Table5.5.7.4.1). This essentially depends on the ratio ofthe magnitude of the cathodic area AC to the mag-nitude of the anodic area AA.The “Corrosion behaviour of earth electrode mate-rials” research project has found the followingwith respect to the choice of earth electrode mate-rials, particularly regarding the combination of dif-ferent materials:


A higher degree of corrosion is only to be expect-ed if the ratio of the areas is

Generally, it can be assumed that the material withthe more positive potential will become the cath-ode. The anode of a corrosion cell actually presentcan be recognised by the fact that it has the morenegative potential when opening the metal con-ductive connection.Connecting steel installations in the ground, thefollowing earth electrode materials always behaveas cathodes in (covering) soils:

– bare copper,

– tin-coated copper,

– high-alloy stainless steel.

Steel reinforcement of concrete foundationsThe steel reinforcement of concrete foundationscan have a very positive potential (similar to cop-per). Earth electrodes and earthing conductorsconnected directly to the reinforcement of largereinforced concrete foundations should thereforebe made of stainless steel or copper.This also applies particularly to short connectingcables in the immediate vicinity of the founda-tions.

Installation of isolating spark gapsAs already explained, it is possible to interrupt theconductive connection between systems with verydifferent potentials installed in the ground byintegrating isolating spark gaps. Normally, then it

is no longer possible for corrosion currents to flow.At upcoming surges, the isolating spark gap oper-ates and interconnects the installations for theduration of the surges. However, isolating sparkgaps must not be installed for protective and oper-ating earth electrodes, since these earth electrodesmust always be connected to the plant.

5.5.7.5 Other anticorrosion measures

Galvanised steel connecting cables from founda-tion earth electrodes to down conductorsGalvanised steel connecting cables from founda-tion earth electrodes to down conductors shall belaid in concrete or masonry up to above the sur-face of the earth.If the connecting cables are led through theground, galvanised steel must be equipped withconcrete or synthetic sheathing or, alternatively,terminal lugs with NYY cable, stainless steel orfixed earthing terminals must be used.Within the masonry, the earth conductors can alsobe led upwards without corrosion protection.

Earth entries made of galvanised steelEarth entries made of galvanised steel must beprotected against corrosion for a distance of atleast 0.3 m above and below the surface of theearth.Generally, bitumen coatings are not sufficient.Sheathing not absorbing moisture offers protec-tion, e.g. butyl rubber strips or heat-shrinkablesleeves.

Underground terminals and connectionsCut surfaces and connection points in the groundmust be designed to ensure that the corrosionresistance of the corrosion protection layer of theearth electrode material is the same for both. Con-nection points in the ground must therefore beequipped with a suitable coating, e.g. sheathedwith an anticorrosive band.

Corrosive wasteWhen filling ditches and pits to install earth elec-trodes, pieces of slag and coal must not come intoimmediate contact with the earth electrode mate-rial; the same applies to construction waste.

A

AC

A

> 100


Material with great areaMaterial with Galvanised Steel Steel in Coppersmall area steel concrete

Galvanised steel + + − −

Steel + + − −

Steel in concrete + + + +

Steel with Cu coating + + + +

Copper/StSt + + + ++ combinable − not combinable

zinc removal

Table 5.5.7.4.1 Material combinations of earth-termination systems for different area ratios (AC > 100 x AA)


NotesEarth rodØ mm

Material Configuration

Copper stranded3)

solid round material3)

solid flat material3)

solid round material

pipe

solid plate

grid-type plate

158)

20

min. diameterof each strand 1.7 mm

diameter 8 mm

min. thickness 2 mm

min. wall thickness 2 mm

min. thickness 2 mm

section 25 mm x 2 mm,min. length of gridconstruction: 4.8 m

1) The coating must be smooth, continuous and free of residual flux, mean value 50 μm for round and70 μm for flat material.

2) Threads must be tapped before galvanising.3) Can also be tin-coated.4) The copper must be connected unresolvably with the steel.5) Only permitted, if embedded completely in concrete.6) Only permitted for the part of the foundation in contact with the earth, if connected safely with the

reinforcement every 5 m.7) Chrome 16 %, nickel 5 %, molybdenum 2 %, carbon 0,08 %.8) In some countries 12 mm are permitted.9) Some countries require earth lead-in rods to connect down conductor and earth electrode.

Earthconductor50 mm2

50 mm2

50 mm2

Earth platemm

500 x 500

600 x 600

Steel galvanised solid roundmaterial1), 2)

galvanised pipe1), 2)

galvanised solid flatmaterial1)

galvanised solid plate1)

galvanised grid-type plate1)

copper-plated solid roundmaterial4)

bare solid roundmaterial5)

bare or galvanised solidflat material5), 6)

galvanised cable5), 6)

169)

25

14

min. wall thickness 2 mm

min. thickness 3 mm

min. thickness 3 mm

section 30 mm x 3 mm

min. 250 μmcoating with 99.9 %copper

min. thickness 3 mm

min. diameter of everywire 1.7 mm

diameter10 mm

90 mm2

diameter10 mm

75 mm2

70 mm2

500 x 500

600 x 600

Min. dimensions

StainlessSteel7)

solid round material

solid flat material

15

min. thickness 2 mm

diameter10 mm

100 mm2

Table 5.5.8.1 Material, configuration and min. dimensions of earth electrodes according to IEC 62305-3 (EN 62305-3) Table 7

5.5.8 Materials and minimum dimensionsfor earth electrodes

Table 5.5.8.1 illustrates the minimum cross sec-tions, shape and material of earth electrodes.

5.6 Electrical isolation of the exter-nal lightning protection system– Separation distance

There is a risk of uncontrolled flashovers betweencomponents of the external lightning protectionsystem and metal and electrical installations with-in the structure, if there is insufficient distancebetween the air-termination or down-conductorsystem on one hand, and metal and electricalinstallations within the structure to be protected,on the other.

Metal installations such as water and air condition-ing pipes and electric power lines, produce induc-tion loops in the structure which are induced byimpulse voltages due to the rapidly changing mag-netic lightning field. These impulse voltages mustbe prevented from causing uncontrolled flash-overs which can also possibly cause a fire.Flashovers on electric power lines, for example,can cause enormous damage to the installationand the connected consumers. Figure 5.6.1 illus-trates the principle of separation distance.The formula for calculating the separation dis-tance is difficult for the practitioner to apply.

The formula is:

ki is a function of the class of lightning protec-tion system chosen (induction factor),

kc is a function of the geometric arrangement(current splitting coefficient),

km is a function of the material in the point ofproximity (material factor) and

l (m) is the length of the air-termination systemor down-conductor system from the pointat which the separation distance shall bedetermined to the next point of equipoten-tial bonding.

The coefficient ki (induction factor) of the corres-ponding class of lightning protection system repre-sents the threat from the steepness of the current.

Factor kc takes into consideration the splitting ofthe current in the down-conductor system of theexternal lightning protection system. The standardgives different formulae for determining kc. Inorder to achieve the separation distances whichstill can be realised in practice, particularly forhigher structures, it is recommended to install ringconductors, i.e. to intermesh the down conductors.This intermeshing balances the current flow, whichreduces the required separation distance.The material factor km takes into consideration theinsulating characteristics of the surroundings. Thiscalculation assumes the electrical insulating char-acteristics of air to be a factor of 1. All other solidmaterials used in the construction industry (e.g.masonry, wood, etc.) insulate only half as well asair.

Further material factors are not given. Deviatingvalues must be proved by technical tests. A factorof 0.7 is specified for the GRP material (glass-fibrereinforced plastic) used in the products of the iso-lated air-termination systems from DEHN + SÖHNE(DEHNiso distance holder, DEHNiso Combi). This

s kk

kl mi

c

m

= ⋅ ( )


l

s

s

soil

EBB

MDB


electrical installation

metal installation

downconductor

s separation distanceMDB Main Distribution Board

Fig. 5.6.1 Illustration – Separation distance

factor can be used for calculation in the same wayas the other material factors.

Length l is the actual length along the air-termina-tion system or down-conductor system from thepoint at which the separation distance to the nextpoint of equipotential bonding or the next light-ning equipotential bonding level shall be deter-mined.

Each structure with lightning equipotential bond-ing has an equipotential surface of the foundationearth electrode or earth electrode near the surfaceof the earth. This surface is the reference plane fordetermining the distance l.

If a lightning equipotential bonding level is to becreated for high structures, then for a height of 20 m, for example, the lightning equipotentialbonding must be carried out for all electrical andelectronic conductors and all metal installations.The lightning equipotential bonding must berealised by using surge protective devices Type I.

Otherwise, even for high structures, the equipo-tential surface of the foundation earth elec-trode/earth electrode shall be used as referencepoint and basis for the length l. Higher structures

are making it more and more difficult to maintainthe required separation distances.

The potential difference between the structure’sinstallations and the down conductors is equal tozero near the earth’s surface. The potential differ-ence increases with increasing height. This can beimagined as a cone standing on its tip (Figure5.6.2).

Hence, the separation distance to be maintained isgreatest at the tip of the building or on the surfaceof the roof and becomes less towards the earth-termination system.This requires a multiple calculation of the distancefrom the down conductors with a different dis-tance l.

The calculation of the current splitting coefficientkc is often difficult because of the different struc-tures.If a single air-termination rod is erected next to thestructure, for example, the total lightning currentflows in this one air-termination conductor anddown conductor. Factor kc is therefore equal to 1.The lightning current cannot split here. Thereforeit is often difficult to maintain the separation dis-


s

s

soil

down conductor

earth electrode

α

protective angle

s

I

Fig. 5.6.2 Potential difference with increasing height Fig. 5.6.3 Air-termination mast with kc = 1

tance. In Figure 5.6.3, this can be achieved byerecting the mast further away from the structure.Almost the same situation occurs for air-termina-tion rods e.g. for roof-mounted structures. Until itreaches the next connection of the air-terminationrod to the air-termination or down conductor. Thisdefined path carries 100 % (kc = 1) of the lightningcurrent (Figure 5.6.4).

If two air-termination rods or air-terminationmasts have a cable spanned between them, thelightning current can split between two paths (Fig-ure 5.6.5). Owing to the different impedances,however, the splitting is not always 50 % to 50 %,since the lightning flash does not always strike theexact centre of the arrangement but can also strikealong the length of the air-termination system. The most unfavourable case is taken into accountby calculating the factor kc in the formula.This calculation assumes an earth-termination sys-tem Type B. If single earth electrodes Type A areexisting, these must be interconnected.

h length of the down conductor

c mutual distance of the air-termination rods orair-termination masts

The following example illustrates the calculationof the coefficient for a gable roof with two downconductors (Figure 5.6.6). An earth-terminationsystem Type B (ring or foundation earth electrode)is existing.

kc =+

⋅ +=

9 12

2 9 120 7

.

kh c

h cc =+

+2


s

soil

kc = 1

M

h

c

h

c

Fig. 5.6.4 Flat roof with air-termination rod and ventilation outlet

Fig. 5.6.5 Determination of kc with two masts with overspannedcable and an earth electrode Type B

Fig. 5.6.6 Determination of kc for a gable roof with 2 down conduc-tors

The arrangement of the down-conductor systemshown in Figure 5.6.6 should no longer beinstalled, not even on a detached house either. Thecurrent splitting coefficient is significantlyimproved by using two further down conductors,i.e. a total of 4 (Figure 5.6.7). The following formu-la is used in the calculation:

h length of the down conductor up to the eavesgutter of the building as worst point for alightning input

c mutual distance of the down conductors

n is the total number of down conductors

Result: kc ≈ 0.51

For structures with flat roofs, the current splittingcoefficient is calculated as follows. In this case, anearth electrode arrangement Type B is a precondi-tion (Figure 5.6.8).

kc =⋅

+ +1

2 40 1 0 2

12

43

. .

kn

c

hc = + +1

20 1 0 2 3. .

h plumb distance, height of the building

c mutual distance of the down conductors

n the total number of down conductors

The distances of the down conductors are assumedto be equal. If not, c is the greatest distance.

If electrical structures or domelights are located onthe flat roof (Figure 5.6.9), then two current split-ting coefficients must be taken into account whencalculating the separation distance. For the air-ter-mination rod, kc = 1 to the next air-termina-tion/down conductor.The calculation of the current splitting coefficientkc for the subsequent course of the air-terminationsystem and down conductors is performed asexplained above. For illustration, the separationdistance s for a flat roof with roof-mounted struc-tures is determined below.

Example:Domelights were installed on a structure with alightning protection system Class III. They are con-trolled electrically.

Structure data:

⇒ Length 40 mWidth 30 m

Height 14 m

⇒ Earth-termination system, foundation earthelectrode Type B

⇒ Number of down conductors: 12

⇒ Distance of the down conductors: min. 10 mmax. 15 m

⇒ Height of the electrically controlled dome-lights: 1.5 m

The calculation of the current splitting coefficientkc for the structure is:

Result: kc ≈ 0.35

kc =⋅

+ +1

2 120 1 0 2

15

143. .

kn

c

hc = + +1

20 1 0 2 3. .


l

c

h

Fig. 5.6.7 Gable roof with 4 down conductors

It is not necessary to calculate the factor kc for theair-termination rod kc = 1.

For the calculation of the current splitting the air-termination rod is assumed to be positioned at theedge of the roof and not within the mesh of theair-termination system. If the air-termination rod iswithin the mesh, the current splitting and theshortest length in the mesh has to be consideredadditionally.

Calculation of the separation distance for the topedge of the roof of the structure:

The material factor km is set as for solid buildingmaterial km = 0.5.

Result: s ≈ 0.39 m

Calculation of the separation distance for the air-termination rod:

The material factor is km = 0.5 because of the posi-tion of the air-termination rod on the flat roof.

Result: s = 0.12 m

This calculated separation distance would be cor-rect if the air-termination rod were erected on thesurface of the earth (lightning equipotential bond-ing level).In order to obtain the separation distance com-pletely and correctly, the separation distance ofthe structure must be added.

Stot = sstructure + sair-termination rod

= 0.39 m + 0.12 m

Stot = 0.51 m

This calculation states that a separation distance of0.51 m must be maintained at the uppermost pointof the domelight. This separation distance wasdetermined using the material factor 0.5 for solidmaterials.Erecting the air-termination rod with a concretebase, the “full insulating characteristics” of the airare not available at the foot of the air-terminationrod (Figure 5.6.9). At the foot of the concrete basea separation distance of sstructure = 0.39 (solid mate-rial) is sufficient.

If lightning equipotential bonding levels are creat-ed for high structures at different heights by inte-grating all metal installations and all electrical andelectronic conductors by means of lightning cur-rent arresters (SPD Type I), then the following cal-culation can be carried out. This involves calculat-ing distances to conductors installed on only onelightning equipotential bonding level, and also tothose installed over several levels.

s m= 0 041

0 5.

.( ) 1.5

s m= 0 040 35

0 5.

.

.( ) 14


c

h

s

km = 0.5

km = 1

Fig. 5.6.8 Value of coefficient kc in case of a meshed network of air-termination conductors and an earthing Type B

Fig. 5.6.9 Material factors of an air-termination rod on a flat roof

This assumes an earth-termination system in formof a foundation or ring earth electrode (Type B) ora meshed network (Figure 5.6.10).

As previously explained, supplementary ring con-ductors can be installed around the structure(truss) to balance the lightning current. This has apositive effect on the separation distance. Figure5.6.10 illustrates the principle of ring conductorsaround the structure, without installing a light-ning equipotential bonding level by using light-ning current arresters at the height of the ringconductors.

The individual segments are assigned differentcurrent splitting coefficients kc. If the separationdistance for a roof-mounted structure shall now bedetermined, the total length from the equipoten-tial surface of the earth electrode to the upper-most tip of the roof-mounted structure must beused as the base (sum of the partial lengths). If the

total separation distance stot is to be determined,the following formula must be used for the calcu-lation:

With this design of supplementary ring conductorsaround the structure, it is still the case that no par-tial lightning currents whatsoever are conductedinto the structure.Even if the numerous down conductors and sup-plementary ring conductors do not allow a main-taining of the separation distance for the com-plete installation, it is possible to define the upperedge of the structure as the lightning equipoten-tial bonding surface (+/–0). This roof-level light-ning equipotential bonding surface is generallyimplemented for extremely high structures whereit is physically impossible to maintain the separa-tion distance.

This requires the integration of all metal installa-tions and all electrical and electronic conductorsinto the equipotential bonding by means of light-ning current arresters (SPD Type I). This equipoten-tial bonding is also directly connected to the exter-nal lightning protection system. These previouslydescribed measures allow to set the separation dis-tances on the upper edge of the structure to 0. Thedisadvantage of this type of design is that all con-ductors, metal installations, e.g. reinforcements,lift rails and the down conductors as well, carrylightning currents. The effect of these currents onelectrical and electronic systems must be takeninto account when designing the internal light-ning protection system (surge protection).It is advantageous to split the lightning currentover a large area.

5.7 Step and touch voltagesIEC 62305-3 (EN 62305-3) draws attention to thefact that, in special cases, touch or step voltagesoutside a structure in the vicinity of the down con-ductors can present a life hazard even though thelightning protection system was designed accord-ing to the latest standards.Special cases are, for example, the entrances orcanopies of structures frequented by large num-

sk

kk l k l k ltot

i

ml tot c c= ⋅ + ⋅ + ⋅( ) 3 3 4 4


h 1h 2

h 3h 4

h n

I a

I gI f

I bI c

I d

c c

sa

sb

sc

sd

sf

sg

(A)

Fig. 5.6.10 Value of coefficient kc in case of an intermeshed net-work of air-termination, ring conductors interconnectingthe down conductors and an earthing Type B

bers of people such as theatres, cinemas, shoppingcentres, where bare down conductors and earthelectrodes are present in the immediate vicinity.

Structures which are particularly exposed (at riskof lightning strikes) and freely accessible to mem-bers of the public may also be required to havemeasures preventing intolerably high step andtouch voltages.These measures (e.g. potential control) are prima-rily applied to steeples, observation towers, moun-tain huts, floodlight masts in sports grounds andbridges.

Gatherings of people can vary from place to place(e.g. in shopping centre entrances or in the stair-case of observation towers). Measures to reducestep and touch voltages are therefore only re-quired in the areas particularly at risk.Possible measures are potential control, isolationof the site or the additional measures describedbelow. The individual measures can also be com-bined with each other.

Definition of touch voltageTouch voltage is a voltage acting upon a personbetween his position on the earth and whentouching the down conductor.The current path leads from the handvia the body to the feet (Figure 5.7.1).

For a structure built with a steel skele-ton or reinforced concrete, there is norisk of intolerably high touch voltagesprovided that the reinforcement is safe-ly interconnected or the down conduc-tors are installed in concrete. Moreover, the touch voltage can be dis-regarded for metal facades if they areintegrated into the equipotential bond-ing and/or used as natural componentsof the down conductor.

If there is a reinforced concrete with asafe tying of the reinforcement to thefoundation earth electrode under thesurface of the earth in the areas outsidethe structure which is at risk, then thismeasure already improves the curve ofthe gradient area and acts as a poten-tial control. Hence step voltage can beleft out of the considerations.

The following measures can reduce the risk ofsomeone being injured by touching the down con-ductor:

⇒ The down conductor is sheathed in insulatingmaterial (min. 3 mm crosslinked polyethylenewith an impulse withstand voltage of 100 kV1.2/50 μs).

⇒ The position of the down conductors can bechanged, e.g. not in the entrance of the struc-ture.

⇒ The probability of people accumulating can bereduced with information or prohibition signs;barriers can also be used.

⇒ The specific resistance of the surface layer of the earth at a distance of up to 3 m aroundthe down conductor must be not less than 5000 Ωm.

A layer of asphalt with a thickness of 5 cm,generally meets this requirement.

⇒ Compression of the meshed network of theearth-termination system by means of poten-tial control.

NoteA downpipe, even if it is not defined as a downconductor, can present a hazard to persons touch-


Fig. 5.7.1 Illustration of touch voltage and step voltage

1 m

ϕFE

US

FE

ϕ

UE

Ut

UE Earth potentialUt Touch voltageUS Step voltageϕ Potential of earth surfaceFE Foundation earth electrode

reference earth

ing it. In such a case, one possibility is to replacethe metal pipe with a PVC one (height: 3 m).

Definition of step voltageStep voltage is a part of the earthing potentialwhich can be bridged by a person taking a stepover 1 m. The current path runs via the humanbody from one foot to the other (Figure 5.7.1).

The step voltage is a function of the form of thegradient area.As is evident from the illustration, the step voltagedecreases as the distance from the structureincreases. The risk to persons therefore decreasesthe more they are away from the structure.

The following measures can be taken to reducethe step voltage:

⇒ Persons can be prevented from accessing thehazardous areas (e.g. by barriers of fences)

⇒ Reducing the mesh size of the earthing instal-lation network – potential control

⇒ The specific resistance of the surface layer ofthe earth at a distance of up to 3 m around thedown-conductor system must be not less than5000 Ωm.

A layer of asphalt with a thickness of 5 cm, ora 15 cm thick bed of gravel generally meetsthis requirement


symbolic course

refe

renc

e e

arth

0.5

m

1 m

1.5

m

1 m 3 m 3 m2

m3 m

Fig. 5.7.2 Potential control – Illustration and symbolic course of the gradient area

If a large number of people frequently congregatein a hazardous area near to the structure to beprotected, then a potential control must be pro-vided to protect them.

The potential control is sufficient if the resistancegradient on the surface of the earth in the field tobe protected does not exceed 1 Ω/m.To achieve this, an existing foundation earth elec-trode should be supplemented by a ring earthelectrode installed at a distance of 1 m and a depthof 0.5 m. If the structure already has an earth-ter-mination system in form of a ring earth electrode,this is already “the first ring” of the potential con-trol.

Additional ring earth electrodes should beinstalled at a distance of 3 m from the first one and

the subsequent ones. The depth of the ring earthelectrode shall be increased (in steps of 0.5 m) themore it is away from the structure (see Table 5.7.1).

If a potential control is implemented for a struc-ture, it must be installed as follows (Figure 5.7.2and 5.7.3):The down conductors must be connected to all therings of the potential control. The individual rings must be connected at leasttwice, however (Figure 5.7.4).

If ring earth electrodes (control earth electrodes)cannot be designed to be circular, their ends mustbe connected to the other ends of the ring earthelectrodes. There should be at least two connec-tions within the individual rings (Figure 5.7.5).

When choosing the materials for the ring earthelectrodes, attention must be paid to the possiblecorrosion load (Chapter 5.5.7). Stainless steel V4A (Material No. 1.4571) hasproved to be a good choice for taking the forma-tion of voltaic cells between foundation and ringearth electrodes into account.Cables Ø 10 mm or flat strips 30 mm x 3.5 mm canbe installed as ring earth electrodes.


1m 3m 3m 3m

mast

clamped points

1m3m 3m 3m

conn

ectio

n to

e.g.

exi

stin

g fo

unda

tion

(reifo

rced

con

cret

e)

mast

Distance fromthe building

Depth

1st ring

2nd ring

3rd ring

4th ring

1 m

4 m

7 m

10 m

0.5 m

1.0 m

1.5 m

2.0 m

Table 5.7.1 Ring distances and depths of the potential control

Fig. 5.7.3 Possible potentialcontrol in entrancearea of the building

Fig. 5.7.4 Potential control performance for a flood light orcell site mast

Fig. 5.7.5 Connection control at the ring/ foun-dation earth electrode

5.7.1 Control of the touch voltage at downconductors of lightning protection sys-tems

The hazardous area of touch and step voltages forpersons outside of a building is within the distanceof 3 m to the building and up to a height of 3 m.This height of the area to be protected corres-ponds to the level which a person can reach withhis hand plus an additional separation distance s(Figure 5.7.1.1).

Special measures of protection are required, forexample, for the entrances or canopies of struc-tures highly frequented such as theatres, cinemas,shopping centres, kindergartens where non-insu-lated down conductors and earth electrodes arenearby.

Structures which are particularly exposed (at riskof lightning strikes) and freely accessible to mem-bers of the public, for example mountain huts,may also be required to have measures preventingintolerably high touch voltages. Moreover life haz-ard is considered as parameter L1 (injury or deathof persons) in the risk analyse of a structureaccording to IEC 62305-2 (EN 62305-2).

The following measures can reduce the risk oftouch voltage:

⇒ The down conductor is sheathed in insulatingmaterial (min. 3 mm polymerised polyethylene

with an impulse withstand voltage of 100 kV(1.2/50 μs).

⇒ The position of the down conductors ischanged, (e.g. down conductors are notinstalled in the entrance of the structure).

⇒ The specific resistance of the surface layer ofthe earth at a distance of up to 3 m around thedown conductor is at least 5 kΩm.

⇒ The probability of people accumulating can bereduced by information or prohibition signs;barriers can also be used.

The measures of protection against touch voltagemay be insufficient with regard to an effective pro-tection of people. The required high-voltageresistant coating of an exposed down conductor,for example is not enough if there are no addition-al measures of protection against creep-flashoversat the surface of the insulation. This is particularlyimportant if environmental influences such as rain(humidity) are to be considered.Just like at a bare down conductor, high voltagesoccurs at an insulated down conductor in case of alightning strike. This voltage, however, is separat-ed from people by the insulation. The human bodybeing a very good conductor compared with theinsulator, the insulating layer is stressed by almostthe whole touch voltage. If the insulation does notcope with the voltage, part of the lightning cur-rent might flow to the earth via the human bodyas in case of the bare down conductor. Safe protec-tion against life hazard due to touch voltagerequires to prevent from flashover through theinsulation and from creep-flashovers along theinsulation.A balanced system solution as provided by the CUIconductor meets these requirements of electric


s

2.50 m

copper conductor

PEX insulation

PE coating

Fig. 5.7.1.1 Area to be protected for a person Fig. 5.7.1.2 Structure of the CUI conductor

strength and creep-flashover insulation to protectagainst touch voltage.

Structure of the CUI conductorA copper conductor with a cross section of 50 mm2

is coated with an insulating layer of surge proofcross-linked polyethylene (PEX) of approx. 6 mmthickness (Figure 5.7.1.2).

The insulated conductor has an additional thinpolyethylene (PE) layer for protection againstexternal influences. The insulated down conductorCUI is installed vertically in the whole hazard area,

i.e. from the earth surface level up to a height of 3 m. The upper end of the conductor is connectedto the down conductor coming from the air-termi-nation system, the lower end to the earth-termina-tion system.Not only the electric strength of the insulation butalso the risk of creep-flashovers between the ter-minal point at the bare down conductor and thehand of the touching person has to be considered.This problem of creeping discharges, well-knownin high voltage engineering, is getting worse incase of rain, for example. Tests have shown thatunder sprinkling the flashover distance can bemore than 1 m at an insulated down conductorwithout additional measures. A suitable shield onthe insulated down conductor keeps the CUI con-ductor dry enough to avoid a creep-flashoveralong the insulating surface. The operating safetyof the CUI conductor with regard to the electricstrength and the resistance against creep-flash-overs at impulse voltages up to 100 kV (1.2/50 μs)has been tried and tested in withstand voltagetests under sprinkling conditions according to IEC60060-1. At these sprinkling tests water of a cer-tain conductivity and quantity is sprinkled on theconductor in an angle of approx. 45 ° (Figure5.7.1.3).

The CUI conductor is prefabricated with connec-tion element to be connected to the down conduc-tor (inspection joint) and can be shortened on siteif necessary for being connected to the earth-ter-mination system. The product is available inlengths of 3.5 m or 5 m and with the necessaryplastic or metal conductor holders (Figure 5.7.1.4).By the special CUI conductor the touch voltage atdown conductors can be controlled with easymeasures and little installation work. Hence thedamage risk for persons in special areas will beconsiderably reduced.

Inductive coupling at a very great steepness ofcurrentRegarding the damage risk for persons also themagnetic field of the arrangement with its influ-ence on the closer surrounding of the down con-ductor has to be considered. In extended installa-tion loops, for example, voltages of several 100 kVcan occur near the down conductor which canresult in high economic losses. Also the humanbody, due to its conductivity, together with thedown conductor and the conductive earth, forms a


connectionelement

shield

conductorholder

Fig. 5.7.1.4 CUI conductor

Fig. 5.7.1.3 Withstand voltage test under sprinkling

loop having a mutual inductanceof M where high voltages Ui canbe induced (Figures 5.7.1.5a and5.7.1.5b). In this case the systemarrester-person has the effect of atransformer.

This coupled voltage arises at theinsulation, the human body andthe earth being primarily consid-ered as conductive. The voltageload becoming too high it resultsin a puncture or creeping flash-over. The induced voltage thendrives a current through this loop,the magnitude of which dependson the resistances and the self-inductance of the loop and meanslife hazard for the person con-cerned. Hence the insulation mustwithstand this voltage load. Thenormative specification of 100 kVat 1.2/50 μs includes the high butvery short voltage impulses whichare only applied as long as the cur-rent rises (0.25 μs in case of a neg-ative subsequent lightning strike).The deeper the insulated downconductors are buried, the greateris the loop and thus the mutualinductance. Hence the inducedvoltage and the loading of theinsulation increases correspond-ingly which also has to be takeninto account with regard to theinductive coupling.


h

a

∆i/∆t

a)

∆i/∆t

b)

M

Ui

U Mi

ti = ⋅ΔΔ

M ha

rconductor= ⋅ ⋅

⎛

⎝⎜

⎞

⎠⎟0 2. ln

Fig. 5.7.1.5 (a) Loop formed by conductor and person (b) Mutual inductance M and induced voltage Ui

5. External lightning protection

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