SURGE AND SURGE PROTECTION DEVICES

SURGE AND SPDs VER 1.0 1

(Part-1)

SURGE AND

SURGE PROTECTION DEVICES

JAYARAJAN.D

INSTRUCTOR (TELE)

What is Surge?

• Surges are fast, short duration electrical transients in

voltage (voltage spikes), current (current spike), or

transferred energy (energy spikes) in an electrical circuit.

• A surge is an electrical transient causing total or partial

damage to electrical and electronic equipment.

Surges are caused by:

• Lightning Discharges.

• Switching on/off of inductive loads (for example

transformers, coils, motors).

• Ignition and interruption of electric arcs (for example

welding process).

• Tripping of fuses.

• Short circuits, etc.

Losses (Cost of Damages) due To Various Reasons

Cause Loss as % of Total Loss

Negligence 36.1%

Surges 27.4%

Burglaries 12.9%

Floods/Storms 6.9%

Others 16.7%

Lightning Surges

Lightning is the most prominent and damaging

among surges.

• Lightning takes place due to accumulation of electric

charges in cloud mass in atmospheric conditions.

• Violent up-draught of air through centre of cloud cell

causes the following:

1. Ice crystals are positively charged

2. Water droplets negatively charged

• Positive charge centre lies at about 10000 m &

negative charge centre at about 5000 m in tropical

regions.

• Positive charge centre lies at about 6000 m &

negative charge centre at about 2500 m in temperate

regions.

• The negatively charged part of the cloud electro-statically

induces positive charge on the ground directly below it.

• The negative charge will be accelerated towards the

ground and it is called as the ‘Stepped leader’.

• When the step leader comes close to the ground a strong

electric field is created which drives the positive charge on

the ground to neutralize the negative charge.

• This is called the returning stroke which is also

called as the ‘Streamer’.

• This returning stroke is much brighter than the

step leader.

• The returning stroke is the origin of intense light,

heat and sound in lightning.

Physical Effects of Lightning

Lightning has the following physical effects:

• Heating of air up to 30000 0 K

• Creation of pressure shock wave

• Flow of current of magnitudes 10 kA to 200 kA

• Heavy potential difference of the order of 1 to 10

Million Volts

The above effects are transients and are to be

discharged through suitable protection mechanisms to

safeguard electrical and electronic installations.

Surge Currents

Surge current due to lightning is expressed in 3 parameters:

• Surge amplitude

• Time taken by the surge to reach it’s maximum value.

• Time taken by the surge to fall to it’s half max. value.

Normally, we encounter surges of

• 200kA 10/350 micro sec.

• 50 kA 10/350 micro sec., and

• 15 kA 8/20 micro sec., as per severity of lightning.

Surge Pulse

Concept of Lightning Protection Zones (LPZ)

Lightning strike can be -

1. Direct strike on equipment room.

2. Indirect strike

• Galvanic coupling, through metallic conductor coming

in contact with surge.

• Capacitance coupling due to capacitive effect caused by

parallel surface.

• Inductive coupling due to conductors, parallel to surge

movement / discharge path.

ZoneConcept Details

LPZ 0A Direct strikes; Full lightning current; Full

magnetic field

LPZ 0B No direct strikes; Partial lightning or

induced current; full magnetic field

LPZ1 No direct strikes; Partial lightning or

induced current; Damped magnetic field

LPZ2 No direct strikes; Induced current;

Further damped magnetic field

Classification of SPDs

• Classification of SPDs are depend on the LPZ

(Lightning Protection Zone) under consideration.

Classes of SPDs as per IEC 61643 are:

• Class-A, Class-B, Class-C and Class-D

Class - A

Class-A protection is essentially external lightning

conductor on top of building connected to ground

through a down conductor.

• Surroundings outside the building fall in LPZ0 (Lightning

Protection Zone-0).

• In this zone, 50% of lightning energy is transferred to

ground through class-A SPD.

• Balance 50% enters the building through power service

cables, Telecom conductors, etc.

• Lightning conductor can be single spike / multiple spike /

• Lightning conductor shall not touch the structure.

• Down conductor shall be a cable rated for HV (50 sq.mm)

for structures of 80m or high.

• For other places, 40 mm X 6 mm MS flat strip insulated

from building is to be used.

• Availability of class-A protection for the buildings is to be

ensured by coordinating with electrical counter parts.

• It cannot be assumed to be present.

Class - A

Class - B

Class-B protection is the first stage protection, i.e.

before the equipment, at mains distribution panel.

These SPDs operate on arc chopping principle.

Lightning currents handled are:

• 10/350 micro sec. pulses

• 100 kA amplitude between N & E

• 50 kA amplitude between R/Y/B & N

Class-B SPDs are to be provided between each phase &

neutral and neutral & earth.

Parameter Value / Limit

Nominal Voltage (U0) 230 V

Maximum Continuous Operating Voltage (Uc) >253 V

Temporary Over Voltage (Ut) 300V

Lightning Impulse Current Between R/Y/B & N > 50 kA, 10/350 micro sec.

for each phase

Lightning Impulse Current Between N & E > 100 kA, 10/350 micro sec.

Response Time (Tr) <200 n sec.

Voltage Protection Level (Up) <1.5 kV

Short Circuit Withstand & Follow Up Current

Extinguishing Capacity Without Back-Up Fuse (Isc)

>10 kA

Operating Temperature / RH 70 o C / 95% RH

PARAMETERS OF SPDs

1. Nominal Voltage (Uo): It corresponds to the nominal

voltage of the system to be protected. The nominal

voltage is indicated in case of surge protective devices

for IT installations for type designation purposes. For

AC voltages it is indicated as RMS value.

2. Rated Voltage (Uc): It is the RMS value of maximum

voltage which may be applied to the correspondingly

marked terminals of the surge protective device during

operation. It is the maximum voltage on the SPD in the

defined non-conductive state which ensures that this

state is regained after response and discharge.

3. Temporary Over Voltage (Ut): Voltage which can be

withstood by SPD (or safely disconnect) when applied for

a specific duration (5 m sec. or 200 m sec.)

4. Voltage Protection Level (Up): The voltage protection

level of a surge protective device is the max.

Instantaneous value of the voltage on the terminals of a

surge protective device.

5. Voltage Withstand (Uw): Insulation withstand level (4

levels 1.5/2/4/6 KV)

6. Lightning Impulse Current (Iimp): It is the standardized

impulse current curve, with a waveform 10/350 milli-sec.

its parameters (peak value, charge, specific power)

simulate the loads of natural lightning currents and

combined lightning current and surge protectors must

be capable of discharging such lightning impulse

currents several times without consequential damage to

the equipment.

7. Nominal Discharge Current (In): It is the peak value of

an impulse current, waveform 8/20 milli-sec, which the

surge protective device rated for, according to a certain

test programme.

8. Follow-Up Current (If): Current delivered by the

distribution system which can be safely extinguished by

9. Response Time (Tr): Response times generally

characterize the response performance of the individual

protection elements used in surge protective devices.

Class - C

Class-C protection comprises of fast acting MOV to

provide effective surge protection with low let through

voltage.

It is provided between phase and neutral.

Surge rating taken care is 50 kA 8/20 micro sec pulses.

Class-C SPD shall be a single compact device.

Class-C SPD shall have following additional features:

• Indication (shows red) when device failed.

• Thermal disconnection of device when it starts

having heavy leakage current due to ageing/

handling several surges.

• Potential free contact for remote monitoring.

Parameter Value / Limit

Nominal Voltage (U0) 230 V

Maximum Continuous

Operating Voltage (Uc)

Lightning Impulse Current

Between R/Y/B & N

> 10 kA, 8/20 micro sec.

for each phase

Response Time (Tr) <25 n sec.

Voltage Protection Level (Up) <1.6 kV

Operating Temperature / RH 70 o C / 95% RH

Class B SPD

Class C SPD

Class (BC) SPD

Class - D

All external data/power supply (AC/DC) lines connected

to electronic equipment are to be provided with class-D

SPDs at both ends of the conductors.

Class-D SPDs comprise of MOVs and GDs and

combinations. Their ratings are given voltage (supply)

Class D SPD

These SPDs shall have following additional features:

• Indication (shows red) when device failed.

• Thermal disconnection of device when it starts

having heavy leakage current due to ageing/

handling several surges.

• Potential free contact for remote monitoring.

Some of the Class D surge protection devices

• Spark gaps (air gaps)

• Gas discharge tubes (GDTs)

• Zener diodes (avalanche diodes)

• Metal oxide varistors (MOVs)

• Transobers

• Relays

• Fuses

• PTCR (Positive Temperature Coefficient Resistor)

• TBU (Transient Blocking Unit)

Gas Discharge Tubes

Two electrodes, close together, enclosed in a

tube filled with gas.

When the voltage rises, a low impedance arc is

formed between the two electrodes.

Symbol:

Also called gas-filled surge arrester.

Gas Discharge Tube

Silicon Avalanche Diodes

Semiconductor devices with similar characteristics

to varistors.

Symbol:

Also called “transorbs” and “clamping diodes”.

Silicon Avalanche Diodes

SAD Properties

Surge rating depends on size.

Surge ratings are much lower than MOVs, 1A 1kA

SADs lifespan also depends on:

Magnitude of surges

Duration of surges

Surge rating of SAD

A prolonged over voltage will cause a SAD to fail.

Generally fail short-circuit, so they must be fused.

Capacitance is relatively low, less than 200pF.

Variable resistor – resistance depends on voltage.

Symbol:

The most common type of varistor is the Metal

Oxide Varistor, or MOV.

Metal Oxide Varistors (MOVs)

Surge rating depends on disc size:

5mm MOV rating 100A

40mm MOV rating 100kA

MOVs lifespan depends on:

Number of surges

Magnitude of surges

Duration of surges

Surge rating of varistor

A prolonged over voltage will cause a varistor to

They may become extremely hot and can catch

fire or explode!

Generally fail short-circuit, so they must be fused.

Capacitance is relatively high, between 1nF and

Lightning Protection Systems

A lightning protection system is a system designed to

protect a structure from damage due to lightning strikes by

intercepting such strikes and safely passing their extremely

high voltage currents to "ground".

Most lightning protection systems include a network of

lightning rods, metal conductors, and ground electrodes

designed to provide a low resistance path to ground for

potential strikes.

Principle of Power Protection

• A protector performs like a switch controlled by

voltage.

• If the voltage is higher than the rated voltage of the

electrical line to be protected, then the protector

changes its state to low impedance and derives current

to earth.

• The usual state of the protector is being in high

impedance, so that the protector is transparent for the

installation.

IEC-62305 STANDARD VER 1.0 55

(Part-2)

IEC 62305 STANDARD FOR

LIGHTNING PROTECTION

JAYARAJAN.D

INSTRUCTOR (TELE)

Structure of BS EN/IEC 62305

The BS EN/IEC 62305 series consists of four parts, all of

which need to be taken into consideration. These four

parts are:

Part 1: General principles,

Part 2: Risk management,

Part 3: Physical damage to structures and life hazard, and

Part 4: Electrical and electronic systems within structures

Part-1: General principles

• This part serves as an introduction to the further parts of

the standard. It classifies the sources and types of

damage to be evaluated and introduces the risks or types

of loss to be anticipated as a result of lightning.

• It defines the relationships between damage and loss that

form the basis for the risk assessment calculations in part

2 of the standard.

• Lightning current parameters defined in this part are

used as the basis for the selection and implementation

of the appropriate protection measures detailed in

parts 3 and 4 of the standard.

• This part also introduces new concepts for

consideration when preparing a lightning protection

scheme, such as Lightning Protection Zones (LPZs) and

separation distance.

BS EN/IEC 62305 identifies four main sources of

damage:

S1 Flashes to the structure

S2 Flashes near to the structure

S3 Flashes to a service

S4 Flashes near to a service

Each source of damage may result in one or more of

three types of damage:

D1 Injury of living beings due to step and touch

voltages

D2 Physical damage (fire, explosion, mechanical

destruction, chemical release) due to lightning

current effects including sparking

D3 Failure of internal systems due to Lightning

Electromagnetic Impulse (LEMP)

The following types of loss may result from damage due

to lightning:

L1 Loss of human life

L2 Loss of service to the public

L3 Loss of cultural heritage

L4 Loss of economic value

Point of strikeSource of

damage

Type of

damageType of loss

Structure S1

L1, L4

L1, L2, L3, L4

L1, L2, L4

Near a structure S2 D3 L1, L2, L4

Service connected to

the structureS3

L1, L4

L1, L2, L3, L4

L1, L2, L4

Near a service S4 D3 L1, L2, L4

Lightning Protection Levels (LPL)

• Four protection levels have been determined and

each level has a fixed set of maximum and minimum

lightning current parameters.

• Lightning current for each LPL based on 10/350 µs

waveform:

LPL I II III IV

Maximum Current (kA) 200 150 100 100

Minimum Current (kA) 3 5 10 16

The types of damage and loss

Air termination networkRolling sphere radius

LPZ 0A

Direct flash, full lightning current, full

magnetic fieldSPD 0B/1

LPZ 0B

No direct flash, partial

lightning

or induced current, full

magnetic field

LPZ 1No direct flash, partial lightning or

induced current, damped SPD 0B/1

magnetic fieldLPZ 2No direct flash, induced

currents, further damped

magnetic field

LPZ 3SPD 0B/1

Down conductor network

SPD 0B/1LPZ 0B

Equipotential bonding by means

of SPD

Earth termination network

The LPZ Concept

Part-2: Risk management

• This part specifically deals with making a risk assessment,

the results of which define the level of Lightning

Protection System (LPS) required.

• The first stage of the risk assessment is to identify which of

the four types of loss as identified in part-1, the structure

and its contents can incur.

• The ultimate aim of the risk assessment is to quantify and

if necessary reduce the relevant primary risks according to

the level of damages; i.e.

R1 risk of loss of human life

R2 risk of loss of service to the public

R3 risk of loss of cultural heritage

R4 risk of loss of economic value

• Each primary risk (Rn) is determined through a long series

of calculations as defined within the standard.

• If the actual risk (Rn) is less than or equal to the tolerable

risk (RT), then no protection measures are needed. If the

actual risk (Rn) is greater than its corresponding tolerable

risk (RT), then protection measures must be instigated.

Lightning Protection System (LPS)

IEC 62305-1 has defined four Lightning Protection Levels

(LPLs) based on probable minimum and maximum lightning

currents. These LPLs equate directly to classes of Lightning

Protection System (LPS).

The correlation between the four levels of LPL and LPS is

identified in Table 7. In essence, the greater the LPL, the

higher class of LPS is required.

LPL Class of LPS

III III

Relation between Lightning Protection Level (LPL) and Class of LPS

6 Point protection Plan

Point 1: Capture the lightning strike.

Point 2: Conduct the lightning current to ground safely

Point 3: Dissipate the energy into the ground.

Point 4: Eliminate ground loops and differentials.

Point 5: Protect equipment from surges and transients

on power lines.

Point 6: Protect equipment from surges and transients

on communication lines.

• Depending upon the consequences the designer may

choose either of the following types of external LPS:

1. Isolated

2. Non-isolated

• An Isolated LPS is typically chosen when the structure

is constructed of combustible materials or presents a

risk of explosion.

• Conversely a non-isolated system may be fitted where

no such danger exists.

An external LPS consists of:

1. Air termination system

2. Down conductor system

3. Earth termination system

• These individual elements of an LPS should be

connected together using appropriate lightning

protection components (LPC).

• This will ensure that in the event of a lightning current

discharge to the structure, the correct design and

choice of components will minimize any potential

damage.

1. Air Termination System

• The role of an air termination system is to capture the

lightning discharge current and dissipate it harmlessly

to earth via the down conductor and earth

termination system.

• Therefore it is vitally important to use a correctly

designed air termination system.

• IEC 62305-3 advocates the following, in any

combination, for the design of the air termination:

• Air rods (or finials) whether they are free standing

masts or linked with conductors to form a mesh on the

• Catenary (or suspended) conductors, whether they are

supported by free standing masts or linked with

conductors to form a mesh on the roof.

• Meshed conductor network that may lie in direct

contact with the roof or be suspended above it (in the

event that it is of paramount importance that the roof

is not exposed to a direct lightning discharge)

• It highlights that the air termination components

should be installed on corners, exposed points and

edges of the structure.

• The three basic methods recommended for

determining the position of the air termination

systems are:

1. The rolling sphere method

2. The protective angle method

3. The mesh method

The rolling sphere method

• The rolling sphere method is a simple means of

identifying areas of a structure that need protection,

taking into account the possibility of side strikes to the

structure.

• The basic concept of applying the rolling sphere to a

structure is illustrated in Figure.

• The rolling sphere method was used in BS 6651, the

only difference being that in IEC 62305 there are

different radii of the rolling sphere that correspond to

the relevant class of LPS.

Class of LPSRolling sphere

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

The protective angle method

• The protective angle is determined by the radius of the

rolling sphere.

• The protective angle, which is comparable with the

radius of the rolling sphere, is given when a slope

intersects the rolling sphere in such a way that the

resulting areas have the same size.

• This method must be used for buildings with

symmetrical dimensions (e.g. steep roof) or roof-

mounted structures (e.g. antennas, ventilation pipes).

• The protective angle (α) is the angle created between

the tip (A) of the vertical rod and a line projected down

to the surface on which the rod sits.

• The protective angle differs with varying height of the air

rod and class of LPS. The protective angle afforded by an

air rod is given in the table.

• The protective angle depends on the class of LPS and the

height of the air-termination system above the reference

plane.

Air-termination systems withprotective angle α

• Air-termination conductors, air-termination rods,

masts and wires should be arranged in such a way that

all parts of the structure to be protected are situated

within the protected volume of the air-termination

system.

Mesh method

• A “meshed” air-termination system can be used

universally regardless of the height of the building and

shape of the roof. A meshed air-termination network

with a mesh size according to the class of LPS is arranged

on the roofing.

• By using the ridge and the outer edges of the building as

well as the metal natural parts of the building serving as

an air-termination system, the individual meshes can be

positioned as desired.

• The air-termination conductors on the outer edges of

the structure must be laid as close to the edges as

possible.

• The metal capping of the roof parapet can serve as an

air-termination conductor and / or a down conductor if

the required minimum dimensions for natural

components of the air-termination system are

complied with

Meshed air-termination system

Class of LPS Mesh size

I 5 x 5 m

II 10 x 10 m

III 15 x 15 m

IV 20 x 20 m

MESH SIZE

Air terminations at roof edges, ridges, roof overhangs.

No metal installation protrudes above Air termination system.

Down Conductor System

• The down conductor is the electrically conductive

connection between the air-termination system and the

earth-termination system.

• The function of a down conductor is to conduct the

intercepted lightning current to the earth-termination

system without damaging the building e.g. due to

intolerable temperature rises.

Down Conductor System

• The number of down conductors depends on the

perimeter of the external edges of the roof (perimeter

of the projection onto the ground surface).

• The down conductors must be arranged to ensure that,

starting at the corners of the structure, they are

distributed as uniformly as possible to the perimeter.

• Depending on the structural conditions (e.g. gates,

precast components), the distances between the

various down conductors can be different.

• By interconnecting the down conductors at ground

level and using ring conductors for higher structures,

it is possible to balance the lightning current

distribution which reduces the separation distance ‘s’.

• There should always be a minimum of two down

conductors distributed around the perimeter of the

structure. Down conductors should wherever possible

be installed at each exposed corner of the structure

as research has shown these to carry the major part

of the lightning current.

Class of LPS Typical distance

I 10 m

II 10 m

III 15 m

IV 20 m

Distance between down conductors

• When internal reinforcing bars are required to be

connected to external down conductors or earthing

network either of the arrangements shown in the figure

is suitable.

• If the reinforcing bars (or structural steel frames) are to

be used as down conductors then electrical continuity

should be ascertained from the air termination system

to the earthing system.

• For new build structures this can be decided at the

early construction stage by using dedicated reinforcing

bars or alternatively to run a dedicated copper

conductor from the top of the structure to the

foundation prior to the pouring of the concrete.

• This dedicated copper conductor should be bonded to

the adjoining/adjacent reinforcing bars periodically.

Earth termination system

• The earth-termination system is the continuation of the

air-termination systems and down conductors to

discharge the lightning current to the earth.

• Other functions of the earth-termination system are to

establish equipotential bonding between the down

conductors and to control the potential in the vicinity of

the building walls.

• It must be observed that a common earth-termination

system is to be preferred for the different electrical

systems (lightning protection systems, low-voltage

systems and telecommunication systems).

• This earth-termination system must be connected to the

equipotential bonding system via the main earthing

busbar (MEB).

The standard distinguishes two types of earth electrode

arrangements, type A and type B.

Type A arrangement

• Type A earth electrode arrangements describe

individually arranged vertical earth electrodes or

horizontal radial earth electrodes, which must be

connected to a down conductor.

• A type A earth electrode arrangement require at least

two earth electrodes. A single earth electrode is

sufficient for individually positioned air-termination rods

or masts.

• A minimum earth electrode length of 5 m is required for

class of LPS III and IV.

• For class of LPS I and II the length of the earth

electrode is defined as a function of the soil resistivity.

• If different earth electrodes (vertical and horizontal) are

combined, the equivalent total length should be taken

into account.

• Single earth electrodes of type A must be

interconnected to ensure that the current is evenly

split. This is important for calculating the separation

distance s.

Type B arrangement

• Type B earth electrodes are ring earth electrodes

encircling the object to be protected or foundation earth

electrodes. In Germany, the requirements for earth-

termination systems of new buildings are described in

DIN 18014.

• If it is not possible to encircle the structure by means of a

closed ring, the ring must be complemented by means of

conductors inside the structure. Pipework or other

permanently conductive metal components can also be

used for this purpose.

• The earth electrode must be in contact with the soil

for at least 80 % of its total length to ensure that a

type B earth electrode can be used as a base for

calculating the separation distance.

• The minimum lengths of type B earth electrodes

depend on the class of LPS. In case of classes of

LPS I and II, the minimum earth electrode length

also depends on the soil resistivity.

Foundation earth electrodes

• This is essentially a type B earthing arrangement. It

comprises conductors that are installed in the concrete

foundation of the structure.

• If any additional lengths of electrodes are required

they need to meet the same criteria as those for type

B arrangement.

• Foundation earth electrodes can be used to augment

the steel reinforcing foundation mesh.

Step and touch voltage

IEC 62305-3 points out that, in special cases, touch or

step voltage outside a building in the vicinity of the down

conductors can present a life hazard even though the

lightning protection system was designed according to

the latest standards.

Touch voltage

• Touch voltage is a voltage acting on a person between its

standing surface on earth and when touching the down

conductor. The current path leads from the hand via the

body to the feet.

• A downpipe, even if it is not defined as a down

conductor, can present a risk for persons touching it. In

this a case, the metal pipe must be replaced by a PVC

Step Voltage

• Step voltage is a part of the earth potential which can

be bridged by a person taking a step of 1 m. The current

path runs via the human body from one foot to the

other.

• The step voltage depends on the shape of the potential

gradient area. As shown in the figure, the step voltage

decreases as the distance from the building increases.

The risk to persons is therefore reduced the further they

are from the structure.

STEP VOLTAGE AND TOUCH VOLTAGE

Internal LPS design considerations

• The fundamental role of the internal LPS is to ensure the

avoidance of dangerous sparking occurring within the

structure to be protected.

• This could be due to lightning current flowing in the

external LPS or indeed other conductive parts of the

structure and attempting to flash or spark over to

internal metallic installations.

• Carrying out appropriate equipotential bonding

measures or ensuring there is a sufficient electrical

insulation distance between the metallic parts can

avoid dangerous sparking between different metallic

parts.

Lightning equipotential bonding

• Equipotential bonding is simply the electrical

interconnection of all appropriate metallic

installations/parts, such that in the event of lightning

currents flowing, no metallic part is at a different

voltage potential with respect to one another.

• If the metallic parts are essentially at the same potential

then the risk of sparking or flashover is nullified.

Equipotential Bonding Arrangement

• The gas, water and central heating system are all bonded

directly to the equipotential bonding bar located inside

but close to an outer wall near ground level.

• The power cable is bonded via a suitable SPD, upstream

from the electric meter, to the equipotential bonding bar.

• This bonding bar should be located close to the main

distribution board (MDB) and also closely connected to

the earth termination system with short length

conductors.

• In larger or extended structures several bonding bars

may be required but they should all be interconnected

with each other.

• The screen of any antenna cable along with any shielded

power supply to electronic appliances being routed into

the structure should also be bonded at the equipotential

• Indeed, as per IEC 62305-3, an LPS system can no longer

be fitted without lightning current or equipotential

bonding SPDs to incoming metallic services that have

live power and telecoms cables, which cannot be directly

bonded to earth.

• Such SPDs are required to protect against the risk of loss

of human life by preventing dangerous sparking that

could present fire or electric shock hazards.

• The use of these SPDs alone “provides no effective

protection against failure of sensitive electrical or

electronic systems”, to quote IEC 62305 part 4, which

is specifically dedicated to the protection of electrical

and electronic systems within structures.

Lightning equipotential bonding requires that all metal

conductive parts such as cable cores and shields at the

entrance point into the building be integrated in the

equipotential bonding system so as to cause as little

impedance as possible.

Examples of such parts include antenna lines,

telecommunication lines with metal conductors and also

optical fibre installations with metal elements.

• The lines are connected with the help of lightning

current carrying elements (arresters and shield

terminals).

• An adequate place of installation is the point where

the cabling extending beyond the building transfers

to cabling inside the building.

• Both the arresters and the shield terminals must be

chosen according to the lightning current parameters

to be expected.

Earthing and Earth Measurement

VER 1.0 123

(Part-3)

EARTHING AND

EARTH MEASUREMENT

JAYARAJAN.D

INSTRUCTOR (TELE)

VER 1.0 124

Earthing is required for the following purposes

1. To provide protection against lightning strikes & power

surges.

2. To protect equipment from surge voltages & currents and

human beings from electric shocks.

3. As one conductor in earth-return circuits where the

earth is part of the circuit.

4. Discharging induced voltages, in communication systems.

5. To minimize RF radiation and EM interference.

VER 1.0 125

Properties of Earth

• Earth is not a good conductor.

• But it is an ideal Equipotential surface.

• It can SINK any amount of charge without any

appreciable rise in its potential.

• Effect of injected currents due to a fault or lightning is

negligible, if not felt only locally.

VER 1.0 126

What is System Earth and Equipment Earth?

System Earth:

System earth means the connection of earth to the

neutral points of current carrying conductors such as the

neutral point of a circuit; a transformer, rotating

machinery, or a system, either solidly or with a current-

limiting device.

VER 1.0 127

VER 1.0 128

Equipment Earth:

Equipment Earth means the connection of earth to non-

current carrying conductive materials such as conduit,

cable trays, junction boxes, enclosures, and motor

frames.

VER 1.0 129

Equipment earthing based on IS:3043-1987

Standards

VER 1.0 130

There are two differences

1. In terms of connection

1. In system earthing, earth is connected to the current

carrying parts.

2. In equipment earthing, earth is connected to non

current carrying part or the chassis (the external body

of the equipment).

2. In terms of purpose

1. System earthing is used to protect the equipment.

2. Equipment earthing is used to protect the

personnel.

VER 1.0 131

Characteristics of Good Earthing System

(Equipment Earth)

A good earthing systems will feature the following 7

characteristics:

1. Good electrical conductivity

2. Conductors capable of withstanding high fault currents

3. Long life – at least 40 years

4. Low ground resistance and impedance

5. Equipotential Bonding

6. Good corrosion Resistance

7. Electrically and mechanically robust and reliable

VER 1.0 132

The following locations are suitable for locating earth

• Low lying areas close to the building or equipment

are good for locating Earth Electrodes.

• The location can be close to any existing water

bodies or water points but not naturally well-

drained.

VER 1.0 133

The following locations are not suitable for locating earth

1. Dry sand, gravel chalk, lime stone, granite and any

stony ground locations should be avoided.

2. All locations where virgin rock is very close to the

surface.

3. Earthing electrode should not be installed on high

bank or made-up soil.

VER 1.0 134

Soils suitable for making earth pit in the order of

preference:

1. Wet marshy ground

2. Clayey soil, loamy soil, arable land, clay or loam

mixed with small quantities of sand

3. Clay and loam mixed with varying proportions of

sand, gravel and stones

4. Damp & wet sand or peat.

VER 1.0 135

Earth Resistance

• It is the resistance of soil offered to the passage of

electric current.

• Earth Resistance value of an earth pit depends solely

on soil resistivity at the location.

• Hence, it varies from soil to soil.

VER 1.0 136

Soil Resistivity

Soil Resistivity mainly depends on:

• Soil composition

• Moisture content

• Dissolved salts

• Grain size and its distribution

• Temperature and

• Magnitude of current that flows.

VER 1.0 137

Material Resistivity

Copper 1.7 X 10 -8 Ohm. Meter

GI 10 -7 Ohm. Meter

Wet soil 10 Ohm. Meter

Moist soil 100 Ohm. Meter

Dry soil 1000 Ohm. Meter

Bed rock 10000 Ohm. Meter

VER 1.0 138

Required Earth Resistance Value

• Ideally, earth resistance has to be 0 Ω.

• But practically it should be 1 or 2 ohms

• Value of Earth Resistance depends on two factors:

1. Soil Resistance ( Electrode to Soil Resistance)

2. Electrode Resistance

VER 1.0 139

Soil Resistance

The resistance offered by soil to the spread of electric

current/field is called Earth/Soil Resistance.

1. It depends mainly on Soil Resistivity .

And also on

2. Shape and size of earth electrode.

VER 1.0 140

Electrode Resistance

• Electrode Resistance is the resistance of the

material of which the electrode is made of. It

also depends on shape and size of the electrode.

• Electrodes of GI, Copper and copper welded steel

are used to keep electrode resistance negligible

or less than 1 Ω.

VER 1.0 141

Earth Electrode Types

1. Rod Electrodes

2. Strip Electrodes

3. Pipe Electrodes

4. Plate Electrodes

• A pipe, rod or strip has a much lower resistance than

a plate of equal surface area.

• The resistance is not inversely proportional to the

surface area of the electrode.

VER 1.0 142

Plate Electrode: The approximate resistance to earth

of a plate can be calculated from:

R = ρ x L/A

Where,

ρ = resistivity of the soil (in Ω.m)

A = cross-sectional area of plate (in m2)

L = the length of the conducting path

VER 1.0 143

Strip Electrode: In case of strip electrode earth

resistance is given by :

R = ρ/2πL [ln (8L/T) + ln (L/h)-2+(2h/L)-(h/L)2]

L is length,

h is depth of laying,

T is thickness

L has major influence than T

VER 1.0 144

Pipe Electrode: In case of pipe electrode earth

resistance is given by:

R = (ρ / 2πL) [ln 8L / 2.7183 D]

L is length of electrode,

D is diameter,

ρ is soil resistivity

VER 1.0 145

Better earth Resistance can be achieved by:

1. Increasing the surface area of the electrode in contact

with the earth,

2. Increasing the depth to which it is driven,

3. Using several connected ground rods,

4. Increasing the moisture of the soil,

5. Improving the conductive mineral content of the soil,

6. Increasing the land area covered by the ground

system

VER 1.0 146

Types of earthing

1. Conventional earth

2. Maintenance free earth

1. Conventional Earthing Practice

Earth Electrode:

VER 1.0 147

Procedure of Installation

Earth pit of 600 mm dia and 2.5 m depth is made by

manual trenching or by using “Earth Auger”.

The electrode is placed at the center.

Top of the electrode is kept 30 cm above the ground.

After inserting the electrode, the hole is filled with

earth properly and water is spread to ensure good

contact between electrode and filling.

VER 1.0 148

Earth Auger

VER 1.0 149

If the soil is of high resistivity, it is treated with salt and

charcoal in appropriate proportion.

The pit is filled alternately with layers of common salt

and charcoal, each layer of about 2.5 cm thick up to a

depth of about 150 cm from the bottom of the

electrode.

A brick wall of 400 mm height below the ground level

is constructed in rectangular fashion and walls are

plastered and then filled with sand.

VER 1.0 150

The surroundings of the earth electrode are kept

moist by periodically pouring water through the

pipe in order to keep the resistance below specified

value.

Coke treated electrodes shall not be situated within

6 meters of other metal structure. (This results in

rapid corrosion)

VER 1.0 151

Problems of Conventional Earthing

• The salt poured in pit causes severe corrosion of GI

pipe and makes the earthing ineffective.

• The earth resistance value depends on “soil

resistivity” which depends on strata so the effect of

earthing is dependent on property of soil, season.

• The earth resistance value is high, fluctuating &

increases with time.

VER 1.0 152

VER 1.0 153

Conventional / Traditional Earthing

VER 1.0 154

• Bolt nut mechanism is not a permanent solution

• It is corrosive

• It can not offer consistent resistance

• It is not Maintenance Free

VER 1.0 155

Maintenance of conventional Earth

1) Check earth and its connections periodically at

interval of not more than one month, to ensure that

all connections are intact and soldered joints are in

proper condition.

2) Measure the earth resistance once in a year. Enter

the value, date of last test on the inspection pit

cover and in a register.

VER 1.0 156

3) Water to be added every day to the earth electrode in

summer and once in two days in other seasons.

4) If earth resistance is more than the nominal value,

either renew the old earth or provide a new earth.

VER 1.0 157

Maintenance free Earth

Earth Electrode:

Steel circular rods, bonded with copper on outer surface

to meet the requirements of Underwriters Laboratories

(UL) latest specifications.

(strength, corrosion resistance, low resistance path to

earth and cost effectiveness).

VER 1.0 158

Copper Bonded Rods:

• Steel rods (minimum 17 mm dia, 3 m

• Copper bonded

• Coating thickness - 250 microns min.

• UL marking

Service Life - 40 years in most soil types

VER 1.0 159

Earth Enhancement material

• A superior conductive material that improves

conductivity of the electrode and ground contact area.

It improves earth’s absorbing power and humidity

retention capability.

• Non-corrosive, low water solubility, highly hygroscopic.

• Resistivity of less than 0.2 ohms-meter.

• Suitable for installation in dry/slurry form.

VER 1.0 160

• Does not depend on continuous presence of water

to maintain its conductivity.

• Permanent & maintenance free and maintain

constant earth resistance.

VER 1.0 161

Construction of Earth Pit

• A hole of 100mm to 125mm dia is augured/dug to a

depth of about 2.8 meters.

• Earth electrode is placed into this hole.

• It is penetrated into the soil by gently driving on the

top of the rod. Here natural soil is assumed to be

available at the bottom of the electrode so that

min. 150 mm of the electrode shall be inserted in

the natural soil.

VER 1.0 162

Earth enhancement material (30-35 kg) is filled into

the hole in slurry form and allowed to set.

Remaining portion of the hole is covered by backfill

soil, which is taken out during digging.

A copper strip of 150mmX25mmX6mm is welded to

earth electrode for taking connection to the main

equipotential earth bus-bar in the equipment room

and to other earth pits, if any.

VER 1.0 163

• Exothermic weld material should be UL listed.

• The main earth pit is located as near to the MEEB

in the equipment room as possible.

VER 1.0 164

Earth enhancement powder with backfill soil

Exothermic weld

25X2 mm copper tape

PVC insulated multistrand 35 Sqmm Cu cable as per IS: 694 (Duplicated)

Augured Hole 100 mm to 125 mm Dia

50 50 200

100 mm

Copper bonded steel earth electrode, minimum 3.0 mtr. long & minimum 17.0 mm dia.

Concrete lid cover, approx. 50 mm thick with pulling hook

Cement concrete

Inspection Chamber

MEBB 6X25X300 mm Cu bus bar

To other earth pit

6X25X150 mm Cu bus bar

150 mm (Minimum) Natural soil/backfill compound

VER 1.0 165

Comparison with Conventional System

Corrosion Proof

Space Savings

Maintenance Free

Steady Performance

Very Long Life

All over Fit and forget Technology

VER 1.0 166

Construction of Loop/ring Earth by

Providing Multiple Earth Pits

• At certain locations, it may not be possible to

achieve earth resistance of ≤1 ohm with one earth

electrode/pit due to higher soil resistivity. In such

cases, provision of loop/ring earth consisting of

more than one earth pit is done.

• The distance between two successive earth

electrodes should be between 3 to 6 m.

VER 1.0 167

• At certain locations, it may not be possible to

achieve earth resistance of ≤1 ohm with one earth

electrode/pit due to higher soil resistivity. In such

cases, provision of loop/ring earth consisting of

more than one earth pit is done.

VER 1.0 168

Horizontal Earthing and Ring Earthing

VER 1.0 169

VER 1.0 170

VER 1.0 171

VER 1.0 172

Equipotential Earth Busbar

• There is one Sub-Equipotential Earth Bus-bar for each of

the equipment room (SEEB).

• The equipotential earth bus-bar located in the IPS /Battery

charger room and directly connected to Class ‘B’ SPDs and

main earth pit is termed as Main Equipotential Earth

Busbar (MEEB).

• The EEBs have pre-drilled holes of suitable size for

termination of bonding conductors.

VER 1.0 173

• EEBs are insulated from the walls and are installed on

the wall with insulator spacers of height 60mm.

• EEBs are installed at the height of 0.5m from the

room floor surface for ease of installation &

maintenance. All terminations on the EEBs are done

using copper lugs with spring washers.

VER 1.0 174

Bonding Connections

• Each of the SEEBs are directly connected to MEEB. Also,

equipment/racks in the room are directly connected to

its SEEB. The bonding conductors are bonded to the

lugs by exothermic welding.

• Routing of bonding conductors from equipments to

SEEB & from SEEBs to MEEB should be as short and

direct as possible with min. bends. Connection from

SPD to MEEB should be as short as possible and

preferably without bend.

VER 1.0 175

Component/Bonding Material Size

Main equipotential earth busbar

(MEEB)

Copper 300X25X6 mm

(min.)

Sub equipotential earth busbar

(SEEB)

Copper 150X25X6 mm

(min.)

Individual equipments to SEEB

using copper lugs with stainless

steel nut and bolts.

Multi-strand single core

PVC insulated copper

10 sq.mm

SEEB to MEEB using copper lugs

with stainless steel nut and bolts.

16 sq.mm

Surge protection devices (SPD)

to MEEB using copper lugs with

stainless steel nut and bolts.

16 sq.mm

MEEB to main earth electrode Multi-strand single core

cable (Duplicated)

35 sq.mm

Main earth pit to other earth pit in

case of loop earth

Copper tape 25mm X 2mm

VER 1.0 176

Code of Practice for Earthing and Bonding

System for Signalling and Telecom

Equipments

RDSO/SPN/197/2008

VER 1.0 177

Earth enhancement powder with backfill soil

Exothermic weld

25X2 mm copper tape

PVC insulated multistrand 35 Sqmm Cu cable as per IS: 694 (Duplicated)

Augured Hole 100 mm to 125 mm Dia

50 50 200

100 mm

Copper bonded steel earth electrode, minimum 3.0 mtr. long & minimum 17.0 mm dia.

Concrete lid cover, approx. 50 mm thick with pulling hook

Cement concrete

Inspection Chamber

MEBB 6X25X300 mm Cu bus bar

To other earth pit

6X25X150 mm Cu bus bar

150 mm (Minimum) Natural soil/backfill compound

VER 1.0 178

EARTHING COMPONENTS

Copper Clad Earth Electrodes

Inspection joint

Backfill Compound

Copper Strip

Earth Pit Cover

VER 1.0 179

Earthing of Signalling Equipments

It covers earthing & bonding for signalling equipments

with solid state components which are more susceptible

to damage due to surges/over voltages.

Suitable for EI, IPS, Digital Axle counters, Data logger etc.

Acceptable Earth Resistance at earth busbar - 1 ohm or

VER 1.0 180

Components of Earthing & Bonding system

• Earth electrode,

• Earth enhancement material,

• Earth pit,

• Equipotential earth bus-bar,

• Connecting cable & tape/strip

• Other accessories.

VER 1.0 181

Earth Electrode

• Steel circular rods, bonded with copper on outer

surface to meet the requirements of Underwriters

Laboratories (UL) latest specifications.

• (strength, corrosion resistance and cost effectiveness).

VER 1.0 182

• Steel circular rods, bonded with copper on

outer surface to meet the requirements of

Underwriters Laboratories (UL) latest

specifications.

• Steel rods (minimum 17 mm dia, 3 m long)

• Molecularly bonded copper

• Coating thickness - 250 microns

• UL marking

• CPRI tested – 23 kA for 1 second

• Service Life - 40 years in most soil types

VER 1.0 183

Earth Enhancement Material

A superior conductive material that improves conductivity

of the electrode and ground contact area. It improves

earth’s absorbing power and humidity retention capability.

Mainly consist of Graphite , Portland cement with less

Bentonite content

Suitable for installation in dry/slurry form, does not

depend on continuous presence of water to maintain its

conductivity.

Permanent & maintenance free and in its “set form”,

maintain constant earth resistance.

VER 1.0 184

• Highly Conductive cementious

compound

• Homogeneous and stable

• Doesn't leach into the ground

• Enhances the life of copper bonding

• Environmentally Friendly

• Low Resistivity – 0.2 Ω-m

• Suitable for all terrain

• Can be used in dry or slurry form

• Conducrete 10 kg – 3 bags

VER 1.0 185

• A hole of 100mm to 125mm dia is augured/dug to a

depth of about 2.8 meters.

• Earth electrode is placed into this hole.

• It is penetrated into the soil by gently driving on the top

of the rod. Here natural soil is available at the bottom of

the electrode so that min. 150 mm of the electrode is

inserted in the natural soil.

Earth enhancement material (30-35 kg) is filled into the

hole in slurry form and allowed to set.

VER 1.0 186

• Remaining portion of the hole is covered by backfill soil,

which is taken out during digging.

• A copper strip of 150mm X 25mm X 6mm is welded to

earth electrode for taking connection to the main

equipotential earth bus-bar in the equipment room and

to other earth pits, if any.

• Exothermic weld material is UL listed and as per RDSO

VER 1.0 187

• The main earth pit is located as near to the MEEB in

the equipment room as possible.

- At certain locations, it may not be possible to achieve

earth resistance of ≤1ohm with one earth

electrode/pit due to higher soil resistivity.

- Solution : Ring earth by provision of loop earth

consisting of more than one earth pit.

VER 1.0 188

- These earth pits are inter-linked using 25mm X 2mm

copper tape to form a loop using exothermic welding.

- The interconnecting tape is buried at depth of 50cm

below ground level. This tape is also covered with

earth enhancing compound.

VER 1.0 189

Measurement of Earth Electrode resistance

VER 1.0 190

The earth tester generates an a.c. signal which is fed into the

system under test. Having checked that the conditions for

the test are met, the instrument automatically steps through

its measurement ranges to find the optimum signal to apply.

Measuring the current flowing and the voltage generated by

the instrument calculates and displays the system resistance

in the range of 0.001 to 20,000 ohms, depending on the

model chosen.

VER 1.0 191

There are three basic test methods for measuring the earth

resistance:

1. Fall-of-potential method, or three-terminal test.

2. Dead Earth method (two-point test).

3. Clamp-on test method

Out of the above three methods, the first one is most

commonly used method of measuring the earth resistance

of an earth electrode.

VER 1.0 192

1. Fall of potential method:

• For this type of test, special instruments such as Megger earth

testers are recommended for making tests to avoid the effect of

back e.m.f. and stray currents.

• In this method two auxiliary earth electrodes named as “Current

Electrode” (C) and “Potential Electrode (P) are placed at suitable

distance from the Test Electrode (E).

• All the electrodes shall be so placed that they are independent of

the resistance area of each other.

VER 1.0 193

VER 1.0 194

Set up for Measuring Earth Resistance

VER 1.0 195

• An alternating current (I) is passed through the outer electrode C

and the voltage is measured, by means of an inner electrode P, at

some intermediary point between them.

• When performing a measurement, the aim is to position the

auxiliary test electrode C far enough away from the earth

electrode under test so that the auxiliary test electrode P will lie

outside the effective resistance areas of both the earth system and

the other test electrode.

• The electrode C shall be placed at least 30 meters away from the

test electrode E and the auxiliary potential electrode P shall be at

62% away from the test electrode.

VER 1.0 196

• All the three must be in same straight line.

• A measured current is passed between the electrode E to

be tested and the auxiliary current electrode C.

• The potential difference between the electrode E and the

auxiliary potential electrode P is measured.

• The resistance of the test electrode E is then given by

R=V/I where;

R= resistance of test electrode in ohms,

V= reading of the voltmeter in volts,

I = reading of the ammeter in ampere.

VER 1.0 197

• If the test is made at power frequency that is 50 Hz, the resistance

of the voltmeter should be high compared to that of the auxiliary

potential electrode P and in no case should be less than 20,000

• The sources of current shall be isolated from the supply by a

double wound transformer.

• At the time of test, when possible, the test electrode shall be

separated from the earthing system.

• The auxiliary electrodes usually consist of 12.5 mm diameter mild

steel rod driven up to 1 metre in the ground.

VER 1.0 198

• Unless three consecutive readings of test electrode resistance

with different spacing of electrode agree the test shall be

repeated by increasing the distance between the electrodes E

and C upto 50 metres and each time placing the electrode P

between them.

VER 1.0 199

2. Dead Earth Method: This technique is quite popular

because of its simplicity. Only two leads are used, one

hooked to the test ground and one to a reference

ground.

• With this method, the resistance of two electrodes in a

series is measured by connecting the P1 and

C1 terminals to the ground electrode under test; P2 and

C2 connect to a separate all-metallic grounding

point (like a water pipe or building steel).

VER 1.0 200

• The dead earth method is the simplest way to obtain a

ground resistance reading but is not as accurate as the

three-point method and should only be used as a last

resort, it is most effective for quickly testing the

connections and conductors between connection points.

VER 1.0 201

VER 1.0 202

3. Clamp on Method: The clamp on method is unique in

that it offers the ability to measure resistance without

disconnecting the ground system.

• It is quick, easy, and also includes the bond to ground

and overall grounding connection resistances in its

measurement.

• Measurements are made by "clamping" the tester

around the grounding electrode under test, similar to

how you would measure current with a multi-meter

current clamp.

VER 1.0 203

VER 1.0 204

• The tester applies a known voltage without a direct

electrical connection via a transmit coil and measures

the current via a receive coil. The test is carried out at a

high frequency to enable the transformers to be as small

and practical as possible.

• For the clamp-on method to be effective, there must be

a complete grounding circuit in place. The tester

measures the complete resistance path (loop) that the

signal is taking. All elements of the loop are measured in

series.

VER 1.0 205

Some limitations of the clamp-on method include:

1.effective only in situations with multiple grounds in

parallel.

2.cannot be used on isolated grounds, not applicable

for installation checks or commissioning new sites.

3.cannot be used if an alternate lower resistance

return exists not involving the soil, such as with

cellular towers or substations.

4.results must be accepted on "faith."

VER 1.0 206

Measurement of Earth resistivity

• Earth tester normally used for these tests comprise the current

sources and metres in a single instrument and directly read the

resistance.

• The most frequently used earth tester is the four terminal

megger as shown in the figure.

• Four earth ground stakes are positioned in the soil in a straight

line, equidistant from one another.

• The distance between earth ground stakes should be at least

three times greater than the stake depth.

VER 1.0 207

• The earth tester generates a known current through the two

outer ground stakes and the drop in voltage potential is

measured between the two inner ground stakes.

• The tester automatically calculates the soil resistance using

Ohm’s Law (V=IR).

• The depth of electrodes in the ground shall be of the order of 10

to 15 cm.

VER 1.0 208

VER 1.0 209

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