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Questions relating to therequirements forsupplementaryequipotential bonding arefrequently asked; a verycommon one is whereshould it be installed.This article looks at the

requirements forsupplementary equipotentialbonding in BS 7671:2008,where supplementaryequipotential bondingshould be installed andoffers help on the process ofevaluation.

TERMINOLOGYFirstly, it is important thatthe terminology is correct.Earthing and bonding aretwo different concepts yet theterms are often used together.Once we have establishedthat "earth-bonding"* isa nonsensical expression andshould never be used, wecan look at the requirementsof supplementaryequipotential bonding inBS 7671:2008.

*Earthing and bondingare two separate concepts

EarthingConnection of the exposed-conductive-parts of aninstallation to the mainearthing terminal of thatinstallation.

An example of earthing iswhere the metallic outer-caseof a class I appliance isconnected by the circuitprotective conductor to themeans of earthingproviding a safe pathfor fault or highleakage/high protectiveconductor currents.

EquipotentialbondingElectrical connectionmaintaining variousexposed-conductive-partsand extraneous-conductive-parts at substantiallythe same potential.

There are two types ofequipotential bondingconductor:

Main protective bondingconductor Used to connectextraneous-conductive-parts,such as a metallic waterpipe, to the main earthingterminal.

Supplementaryequipotential bondingconductors Used tosupplement to faultprotection by maintainingvarious exposed conductive-parts and extraneous-conductive-parts atsubstantially the samepotential, such as theconnection of all exposed-conductive-parts andextraneous-conductive-partsthat can be touched bylivestock in an agriculturalinstallation.

HISTORYThe change from the 14th tothe 15th Edition of the IEEWiring Regulations in 1981created a big upheaval in theelectrical installationindustry as many new

concepts were introducedand many existing practiceswere expanded or enhanced -one of those beingsupplementary equipotentialbonding. To a great extent,the 15th Edition was basedon CENELEC harmoniseddocuments (HDs) (more sonow with the introduction ofthe 17th Edition).

HD 384.4.41 was the basis forChapter 41 of the 15thEdition - Protection againstelectric shock, which hadrequirements forsupplementary equipotentialbonding in Regulation 413-7,reproduced here:

413-7 Within the zone formedby the main equipotentialbonding, localsupplementary equipotentialbonding connections shall bemade to metal parts, tomaintain the equipotentialzone, where those parts -(i) are extraneous conductiveparts, and(ii) are simultaneouslyaccessible with exposedconductive parts or otherextraneous conductive parts,and(iii) are not electricallyconnected to the mainequipotential bonding bypermanent and reliablemetal-to-metal joints ofnegligible impedance.NOTE - Where localequipotential bonding isprovided in accordance withRegulation 413-7, metalworkwhich may be required to bebonded includes baths andexposed metal pipes, sinks,taps, tanks, and radiatorsand, where practicable,accessible structuralmetalwork

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Those far reaching changes,issued on 31st March 1981,are still being felt todaywith designers andspecifiers stillimplementing therequirements forsupplementaryequipotential bonding fromthe 15th Edition. Much ofthe confusion can beattributed to the note ofRegulation 413-7 whichrequired the bonding of allmetallic items, essentially,those within the designatedequipotential zone. This ledto the installation ofsupplementaryequipotential bonding ofgeneral metallic items suchas baths, ceiling grids, handrails, kitchen sinks,radiators, pipework atboilers, etc. Thankfully, wehave moved on from thisgeneral concept.

THE REQUIREMENTS FORSUPPLEMENTARYEQUIPOTENTIAL BONDINGWe'll look at therequirements forsupplementary equipotentialbonding then at instanceswhere it would be required.Initially, the scene is set

by Regulation 410.3.7 whichrequires that if theconditions of a protectivemeasure cannot be met,supplementary provisionsshall be applied to achievethe same degree of safety.

What is a protectivemeasure?There are four protectivemeasures generally permittedby BS 7671:2008, given inRegulation 410.3.3:(i) Automatic disconnectionof supply (Section 411)

(ii) Double or reinforcedinsulation (Section 412)(iii) Electrical separation forthe supply to one item ofcurrent-using equipment(Section 413)(iv) Extra-low voltage (SELVand PELV) (Section 414).

A note at the end of thisRegulation acknowledgesthat, in electricalinstallations, the mostcommonly used protectivemeasure is automaticdisconnection ofsupply.

DISCONNECTION TIMESRegulation 411.3.2.6 statesthat where automaticdisconnection cannot beachieved in the requiredtime, supplementaryequipotential bonding shallbe provided.In this instance, if

disconnection will not occurin the required 0.4 s, forexample, supplementaryequipotential bonding isused to hold various exposed-conductive-parts andextraneous-conductive-partsat substantially the samepotential to limit the risk ofa dangerous electric shock -this clears the confusioncreated by Regulation 413-7of the 15th Edition.Do bear in mind that

supplementary equipotentialbonding need not bephysically carried out by theinstallation of single-coregreen-and-yellow conductorsin every instance. There maybe a situation where, forexample, two simultaneouslyaccessible metallic parts arein reliable contact and theresistance between the twoparts is sufficiently low.

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Where doubt existsregarding the effectivenessof supplementaryequipotential bonding,Regulation 415.2.2 requiresthat the resistance, R,between simultaneouslyaccessible exposed-conductive-parts andextraneous-conductive-partsfulfils the followingcondition:

In a.c. systems R ≤ 50 VIa

Where:Ia is the operating currentin amperes of either: the protective device forRCDs, IΔn.

overcurrent devices, thecurrent causingautomatic operation in 5 s

Example 1Let's take the scenario thatthe protective device is anRCD rated at 30 mA:

R ≤ 50 VIa

R ≤ 50 V0.03

R ≤ 1666 Ω

Therefore, a maximumresistance of 1666 Ω willensure there is sufficientcurrent to operate the RCD.Note that the touch voltagemay rise above 50 v; thevalue of 50 v is used as aconstant in the formula toensure sufficient current isflowing to operate the RCD.

Example 2Let's take the scenario thatthe protective device is aBS EN 60898 Type B circuit-

breaker, rated at 32 A. Firstwe must establish thecurrent causing operation ofthe circuit-breaker byreferring to the correcttime/currentcharacteristics graph inAppendix 3 of BS 7671.Therefore, looking at fig. 1

(extract from fig.3.4 ofBS 7671:2008), we canestablish that the currentcausing operation of thecircuit breaker is 160 A.

R ≤ 50 VIa

R ≤ 50 V160

R ≤ 0.31 Ω

Therefore, a maximumresistance of 0.31 Ω willensure there is sufficientcurrent to operate thecircuit breaker within fiveseconds.Note that in fig. 1 the

current causing operation ofthe device between 0.1 and5 s is the same; this will notbe the case for fuses. If manycircuits are present in aparticular area and areprotected by differentprotective devices/types, etc.,the worst case characteristicshould be used.The IET publication,

Guidance Note 3, with thelatest edition due forimminent publication,advises that the resistanceof supplementaryequipotential bondingconductors should be nomore than 0.05 Ω. Whenverifying supplementaryequipotential bondingconductors, Regulation612.2.1 requires that a

continuity test be madewith a recommendation thatthe test instrument has ano-load voltage of between4 V and 24 V, d.c. or a.c. anda short-circuit current ofnot less than 200 mA.

WORKING STANDARDSThe installation ofsupplementaryequipotential bonding doesnot mean that a lowerstandard of work ispermitted, nor therequirements for faultprotection or the need todisconnect the supply forother reasons, such asprotection against fire,thermal stresses inequipment, etc., can beomitted.Regulation 522.6.1

requires that the correctwiring system is selected forthe application to minimizethe risk of damage arisingfrom mechanical stress, e.g.by impact, abrasion,penetration, tension orcompression duringinstallation, use ormaintenance.Fundamentally, this

means that if there is adanger of a metallic part

not forming part of theelectrical installationbecoming live due todamage to a cable, then theprotection of that cable isparamount and must formpart of the design. Simplyinstalling supplementaryequipotential bonding as ameans of omittingmechanical protection willjust result in more metallicparts becoming live in theevent of damage to a cable.

SIZING OF SUPPLEMENTARYEQUIPOTENTIALCONDUCTORSBS 7671:2008 hasrequirements for the sizingof supplementaryequipotential bondingconductors in Regulation544.2, the table shownoverleaf, fig. 2, will aid withthe choice of conductor.

WHERE SUPPLEMENTARYEQUIPOTENTIAL BONDINGIS REQUIREDUltimately, responsibility iswith the designer of theinstallation, who is acompetent person, fullyaware of the installationconditions and will usetheir skill and engineering

Figure 1: Time/current characteristics of circuit-breakers toBS EN 60898 Type B and BS EN 61009-1

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judgement to design theinstallation accordingly.

Where supplementaryequipotential bonding isrequired, it may involve theentire installation, a part ofthe installation, an item ofequipment or a location, etc.Where supplementaryequipotential bonding isinstalled, it should includeall simultaneouslyaccessible exposed-conductive-parts of fixed

equipment and allextraneous-conductive-parts.

Exposed-conductive-partConductive part ofequipment which can betouched and which is notnormally live, but which canbecome live when basicinsulation fails.

An example of an exposed-conductive-part is themetallic outer case of anelectrical class I

appliance, designed to beconnected to the means ofearthing at all times ofoperation

Extraneous-conductive-partA conductive part liable tointroduce a potential,generally Earth potential,and not forming part of theelectrical installation.

An example of anextraneous-conductive-part isa metallic water pipe whichis buried in the groundand subsequently entersa building

To generalise, as statedearlier, supplementaryequipotential bonding isrequired where adisconnection time can notbe met or where a SpecialInstallation or Location, i.e.those in Part 7 ofBS 7671:2008, has anincreased risk of electricshock. The followingSections of Part 7 directlyreference supplementaryequipotential bonding - notethat other measures will benecessary to meet therequirements ofBS 7671:2008.

Section 701 - Locationscontaining a Bath orShowerSection 701 now has arelaxed requirement forsupplementaryequipotential bonding whenthe following threeconditions of Regulation701.415.2 are met;(i) All final circuits of thelocation comply with therequirements for automaticdisconnection(ii) All final circuits of thelocation have additional

protection by means of a30 mA RCD(iii) All extraneous-conductive-parts of thelocation are effectivelyconnected to the protectiveequipotential bonding

Section 702 - SwimmingPools and Other BasinsSupplementaryequipotential bonding willconnect all extraneous-conductive-parts in zones 0,1 and 2 to the protectiveconductors of exposed-conductive-parts ofequipment situated in thesezones, in Regulation702.411.3.3.Regulation 702.522.21

requires that in zones 0, 1and 2, any metallic sheathor metallic covering of awiring system shall beconnected to thesupplementaryequipotential bonding. Thenote at the end of thisRegulation states that cablesshould preferably beinstalled in conduits madeof insulating material. Thisis a relaxation from the16th Edition as Regulation602-06-01 states that in zonesA and B, a surface wiringsystem shall not employmetallic conduit or metallictrunking or an exposedmetallic cable sheath or anexposed earthing orbonding conductor.Regulation 702.55.1

permits the installation ofan electric heating unitembedded in the floor,provided that itincorporates an earthedmetallic sheath, is coveredby an embedded earthedmetallic grid and connectedto the supplementary

Figure 3: Application of supplementary equipotential bonding

Figure 2: Minimum cross-sectional area of supplementaryequipotential bonding

Size ofCPC

mm²

Minimum cross-sectional area of supplementary bonding conductors

Exposed-conductive-part to extraneous-conductive-part

Exposed-conductive-part to exposed-conductive-part

Extraneous-conductive-part to extraneous-conductive-part (1)

mechanicallyprotectedmm²

mechanicallyprotectedmm²

mechanicallyprotectedmm²

mechanicallyprotectedmm²

mechanicallyprotectedmm²

mechanicallyprotectedmm²

1.0 1.0 4.0 1.0 4.0 2.5 4.0

1.5 1.0 4.0 1.5 4.0 2.5 4.0

2.5 1.5 4.0 2.5 4.0 2.5 4.0

4.0 2.5 4.0 4.0 4.0 2.5 4.0

6.0 4.0 4.0 6.0 6.0 2.5 4.0

10.0 6.0 6.0 10.0 10.0 2.5 4.0

16.0 10.0 10.0 16.0 16.0 2.5 4.0

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equipotential bonding ofthe location (otherrequirements are alsonecessary).

Section 705 - Agriculturaland Horticultural PremisesSupplementaryequipotential bonding isrequired to connect allexposed-conductive-partsand extraneous-conductive-parts that can be touched bylivestock, the metal grid laidin the floor, concretereinforcement in general orreinforcement of cellars forliquid manure (otherrequirements are alsonecessary), in Regulation705.415.2.1.Regulation 705.544.2

requires thatsupplementaryequipotential bondingconductors are protectedagainst mechanical damageand corrosion and chosen toavoid electrolytic effects,with examples given as:(i) Hot-dip galvanized steelstrip with dimensions of atleast 30 mm × 3 mm(ii) Hot-dip galvanizedround steel of at least 8 mmdiameter(iii) Copper conductorhaving a minimum cross-sectional area of 4 mm2.Other suitable materialsmay be used.

Section 706 - ConductingLocations with RestrictedMovementIn a conducting locationwith restricted movement,Regulation 706.410.3.10requires that a supply tofixed equipment shallincorporate supplementaryequipotential bonding isused to connect exposed-

conductive-parts of fixedequipment and theconductive parts of thelocation where automaticdisconnection of the supplyis the protective measure.Part e) of this Regulationrequires that where theprotective measure is PELV,equipotential bonding isprovided between allexposed-conductive-parts,all extraneous-conductive-parts inside the conductinglocation with restrictedmovement, and theconnection of the PELVsystem to Earth.

Section 740 - TemporaryElectrical Installations forStructures, AmusementDevices and Booths atFairgrounds, AmusementParks and CircusesRegulation 740.415.2.1reiterates the requirementsof 705.415.2.1, shown aboveunder Agricultural andHorticultural Premises.

CONCLUSIONTo summarise,supplementaryequipotential bonding isrequired where adisconnection time can notbe met or where a SpecialInstallation or Location, i.e.those in Part 7 ofBS 7671:2008, has anincreased risk of electricshock. Confusion created byRegulation 413-7 of the 15thEdition of the IEE WiringRegulations, whicheffectively requiredsupplementaryequipotential bonding toconnect all accessiblemetallic parts within theequipotential zone, has beenclarified by Regulations

411.3.2.6 and 415.2. Otherinstallations, such as thoseof hazardous locations, willhave further requirementsfor supplementaryequipotential bonding toreduce the risks of sparkingdue to the build up of staticelectricity, for example. Insuch installations, therequirements of BS 7671will be supplemented by therequirements orrecommendations of otherBritish Standards or by therequirements of the personordering the work.

FURTHER INFORMATION BS 7671:2008Requirements forElectrical Installations,IEE Wiring Regulations,

Seventeenth EditionGuidance Note 3 -Inspection and Testing

PD CLC/TR 50404:2003Electrostatics — Code ofpractice for the avoidanceof hazards due to staticelectricity

BS EN 60079 Electricalapparatus for explosivegas atmospheres (suite ofstandards)

BS EN 60079−14:2003Electrical apparatus forexplosive gasatmospheres - Part 14:Electrical installations inhazardous areas (otherthan mines)

Thanks to Richard Rennieof Yorkshire Water for theimage used.

Figure 4: Application of supplementary equipotential bonding ina Special Location