18 EARTHING IEE Wiring IEE sltd(h oreo anITsystem iseither Isolated source (The of Combined = Separate I = C Neutral = S (from theFrench Earth word Terre) = N = T TN-S, TN-C-S,TT, TN-C,and IT. system: earthing BS 7671listsfive typesof prove helpful. intended toprovide that itishopedwill information article,This which isbasedonIEEGuidancenotes, is systems? earthing thevarious typesof of What are theadvantages anddisadvantages BS7671? What are therequirements of systems? What andunearthed are earthed By GeoffCronshaw ANSWERED QUESTIONS YOUR EARTHING: Matters | Autumn 2005 | www.iee.org systems are bothvery uncommonintheUK. source isnotdirectly Therefore earthed. TN-CandIT low voltage public supply intheUKbecause forRegulations, a andanITsystemisnotpermitted from the ElectricitySafety, QualityandContinuity isbecauseTN-Crequires2002. This anexemption Electricity Safety, Quality andContinuity Regulations for alow voltage supply given inaccordance withthe information. distributorsystem. The willbeable toprovide this earthing isthetypeof the firstthingstodetermine electrode.)connected toanearth aninstallation are exposed-conductive-parts of impedanceorisisolated All fromearthing Earth. through adeliberatelyconnected toearth introduced The systemwill eitherbeTN-S,TN-C-S(PME)orTT The When designinganelectricalinstallation, oneof 25 mm arrangements. Basedon equipotential bonding andmain earthing system). Schematicof (TN-S Fig 2: Fig 1: electricity distributor. always installedbythe Note: Anisolatorisnot from Table 54G. Cable sheathearth TN-S system 2 tails andselection Lhnk\^>Zkma ! E G G E I> G E TERMINAL EARTHING MAIN Lhnk\^h_lniier ISOLATOR COMPANY ELECTRICITY M connected with earth atonepointonly,connected withearth energy thesource of A TN-Ssystem,shown infig1,hasthe neutralof systemearthing 1.1 TN-S systems ofearthing 1. Overview into thepremises. thedistributor’s service cable of connected tothe metallicsheath orarmour istypicallypracticable tothesource, terminal andtheconsumer’searthing G&L MG&Llrlm^f BglmZeeZmbhg at orasnear IEE Wiring IEE as isr BglmZeeZmbhg >jnbif^gm >qihl^]&\hg]n\mbo^&iZkm Matters eason | Autumn 2005 ably | www.iee.org

19 EARTHING 20 EARTHING IEE Wiring IEE earthing (PME). Withearthing the thisarrangement isusually toas protectiveThis referred multiple at sourcewith earth andat intervals alongitsrun. adistribution mainconnected neutral conductorof A TN-C-Ssystem,shown infig3,has the supply systemearthing 1.2 TN-C-S distributor. installed bytheelectricity Note: Anisolatorisnotalways tails andselectionfromTable 54G. arrangements. Basedon25 mm and mainequipotential bonding system). Schematicofearthing Fig 4: Fig 3: TN-C-S system PME supply(TN-C-S PME Matters | Autumn 2005 | www.iee.org 2 *)): Lhnk\^>Zkma ikhm^\mbo^\hg]n\mhk!I>G" !\hf[bg^]g^nmkZeZg] bglhnk\^h_lniier% pbmaIF>Ziieb^]" > G I>G E M Lhnk\^>e^\mkh]^ G&< :]]bmbhgZe conductor. which islinkedterminal totheincomingneutral distributor willprovide aconsumer’searthing installation safely tothesource. To achieve this, the arisingin theconsumer’s currents fault earth distributor’s neutralconductorisalsousedtoreturn Ebgd !l^iZkZm^g^nmkZe!G"Zg] ikhm^\mbo^\hg]n\mhkl!I>" L pbmabgbglmZeeZmbhg" I> BglmZeeZmbhg >jnbif^gm installing asuitable electrode localtotheinstallation. earth consumer must provide theirown i.e. connectionto earth, by distributor With for theconsumer’searthing. TT, the connected asfor TN-S,but isprovided nofacility energy by the thesource of A TTsystem,shown above, has theneutralof systemearthing 1.3 TT electrodes. connections toearth be soughtwithregardsto recommendations should Manufacturers electricity distributor. always installedbythe Note: Anisolatorisnot from Table 54G. mm 25 system). Basedon (TT Figure 6: Figure 5: 2 tails andselection TT system TT No earth provided No earth *)): Lhnk\^>Zkma G E MM recognised whichrecognised rods, include plates earth and earth electrode. Regulation 542-02-01 liststhetypes earth typesof awidevarietyBS 7671recognises of electrodes Earth 2. RequirementsofBS7671 Zkma MMlrlm^f IEE Wiring IEE Matters | Autumn 2005 BglmZeeZmbhg >jnbif^gm | www.iee.org 21 EARTHING 22 EARTHING IEE Wiring IEE such anenclosure. bycable awiringenclosure orformed orcontainedin a protective conductor, i.e. onewhich of isnotpart underground structural metalwork. structural underground 2.5 mm conductor. Aminimumcross-sectional area of required thesize for acircuit protectivedetermine There are several which factors may influence or Sizing ofcircuitprotectiveconductors main bondingconductors. TN-C-S (PME)supply, itshouldbenosmallerthanthe applied toany conductor. buried earthing For a BS7671mustbe conductor, that except Table 54Aof basically thesameway asfor acircuit protective andmechanical damage. corrosion against suitable material andadequately protected be of particularly where and buried intheground, partly mustbeadequately earthing sized other meansof electrode aninstallation or toanearth of terminal protective conductor connectingthemainearthing conductorswhichEarthing are definedinBS7671asa conductors Earthing BS 7430:1998 principlesand practice canbefound in earthing prolonged dryweather. on information Further under theleastfavourable conditions, i.e. after out electrode shouldbecarried anearth testing of corrosion, mustbetaken corrosion, andof soildryingandfreezing, possibility of electrode resistance value. would bemostlikely toprovide asatisfactory earth tapes, copperearth orpipesplates, installation of Insuch circumstances, information. the kind of canreveal aboutknown theground information this highresistivity. Only atestor rods, layer oradeep of impervious strata which prevents driving thedeep of layer, beneath which there may berock orother resistivity isfound tobeconcentrated inthetopsoil advantage from each rod. Insomelocations low soil to theircombineddriven toobtainmaximum depth betweenthe separation rods shouldbeat leastequal toachieveconnect themtogether asatisfactory result, Where itisnecessarytodrive two ormore rods and notdisturbed(backfilled) ground. into virginground, material. Thus,surrounding theyshouldbedriven effective asthecontacttheymake withthe electrode. earth Rods canonly beas the typeof of inthedetermination factor single mostimportant h olrssiiyo the soilresistivityThe of The size of an earthing conductor isarrived anearthing at in of size The Whatever electrode takes, anearth the form 2 Matters copper isrequired for any circuit separate | Cd fPractice for Earthing’. ‘Code of Autumn 2005 | www.iee.org into account.Pr ground ispr ground obably eferably, the provided is4mm theminimumsize mechanical protection isnot through it.If current produceddynamic by fault effects passingearth chemical by deterioration orbedamaged electro- or mechanical itisliable damage tosuffer protected if t overcurrent -seeRegulation 413-02-05. result of theirtemperature riseasa circuit conductors, of of circuit protective awiringenclosure. Such a fixed totheoutsideof conductor clipped toasurface, onacable run tray or (I the circuit impedancesandthelimitingcapability of limitingeffect thecurrent account beingtaken of flow through theassociated protective device, due impedance, negligible which can for of afault current I in mm S where: orshallbeobtainedby referenceformula toBS7454. by thefollowingnot lessthanthevalue determined cross-sectionalThe area, where calculated, shallbe Regulation 543-01-03states: quite acceptable. theassociated circuit. is This the live conductorsof protective conductor having asmallercsathanthat of willinmany instancesresult ina the formula therefore onthesafeside. errs Even so, application of protective and fault conductor duringanearth It assumesthat noheat isdissipated from the calculationscircuit (see Regulation current 434-03-03). equation’ andisthesameasthat usedfor short- iscommonly toasthe‘adiabatic referred formula The thephaseconductor(s). area of it employs asimplerelationship tothecross-sectional thanisstrictlyproduce size necessary, alarger since protective from conductorsize Table 54Gbut thismay the Table easiermethod istodetermine 54A).The conductors(seealso conductors including earthing equivalent. I S = 2 Account shall be taken of the effect, on the resistance theeffect, Account shallbetaken of An examplewould beabare orinsulated copper The secondmethodinvolvesThe calculation. aformula BS 7671provides two methodsfor sizingprotective steoeaigtm fthedisconnectingdevice in is theoperating time of stevlei mee rs o .. ffault foris thevalue a.c.) inamperes (rms. of stenmnlcosscinlae fthe conductor is thenominalcross-sectional area of )o that protective device. t) of 2 . k 2 t conductor mustalso 2 copper or be suit ably Ze specifiedby adistributor is0.35 connection.Forwith anearth examplethemaximum providing customers effective andreliable methodof thissystemisthat it provides an advantageThe of Such a supply systemisdescribedinBS 7671asTN-C-S. (PME). 3.1 Protectivemultipleearthing advantages anddisadvantages 3.0 Type systems, ofearthing final temperatures. conductor material, andtheappropriate initialand the andheattemperature capacity of coefficient k Iamperes. current tothefault seconds corresponding Figure 8: Figure 7: earth atpitchsupplypoint earth supply, separationfromPME PME atmaindistributionboard earth supply, separationfromPME PME safco aigacuto theresistivity, taking accountof is afactor Typical sitedistributionfora Typical sitedistributionfora I>G E MG<Lniier!IF>" BlheZmhk I>G E MG<Lniier!IF>" BlheZmhk I> G E I> G E bglmZeeZmbhgZg]lniier^Zkmabg` Ω K^lblmZg\^Zk^Zlh_ for TN-C-S mh[^l^iZkZm^] ^ZkmaZg]\ZkZoZgikhm^\mbo^\hg]n\mhkl !Zkfhnk(la^Zma"l^i^kZm^]_khfK<= ^Zkmabg` K<=!Lmri^"bg =blmkb[nmbhg provide aneffective andreliable connection earth guidance onPME systems. Whilst PMEsystems ElectricityAssociationThe publications provides Special Locations is considered tobesmall. within thebuilding complying withBS 7671,therisk points onthesupply network andbondingisprovided However,earth. sincethere are multipleearthing massof andthegeneral terminal to thePMEearth can develop between theconductive connected parts totheinstallation) apotential open circuit external thesupply becoming conditions (PENconductorof supplies compared to0.8 However, supply system fault undercertain Ibm\alniierihbgm @^g^kZeinkihl^ ,)f:K<= ,)f:K<= Ibm\alniier IEE Wiring IEE Ω for TN-Ssupplies. Matters | Autumn 2005 Ibm\ahnme^ml Ibm\ahnme^ml | www.iee.org 23 EARTHING 24 EARTHING IEE Wiring IEE consumer’s earth terminal, unless inappropriate for terminal, consumer’s earth neutral orprotective conductor for connectiontothe distributor isrequired tomake available hissupply for which theyare fully responsible. electricity The the consumertomake suitable andsafearrangements sitetemporaryinstallation,construction leaving itto outdoorinstallations, for certain earthing e.g. a becoming somehow disconnectedoreven stolen. wire either theearth where there isthelikelihood of back tothesource, e.g. alow voltage overhead supply, connection distributor cannotguaranteetheearth for theconsumerareearthing usually where the in which adistributor willnotprovide ameansof electrode localtotheinstallation. circumstances The i.e.connection toearth, by installingasuitable earth With TT, theconsumermustprovide theirown system 3.2 TT installations sites. andconstruction andhorticultural toagricultural PME earth electricity distribution companiesmightnotprovide a bonding allaccessible extraneous-conductive-parts to thePMEterminal. from any systembonded system which issegregated aquarry, musthave anearthing production sideof taken shaft,orfor useinthe toanunderground shops andrestaurants. may buildings suchearth as beusedfor permanent thedistribution network. APME of the PMEearth petrol andthat fillingarea of earthing of segregation isrequiredalternative toensure the arrangement supply for hazardous areas. Petroleum, which recommends aTT Institute of and Explosives Administration (APEA)andthe stations”, published by theAssociation for Petroleum petrol filling modification andmaintenanceof publication is“Guidancefor thedesign,construction, precautions needtobetaken. reference The premises andany barsorshops, etc. premises onthesites, such asthesiteowner’s living supplies, althoughtheymay beusedfor fixed beingusedfor caravansterminals orboat mooring metalwork inacaravan orboat. prevents This PME conductor tobeconnectedelectrically toany does notallow thecombinedneutralandprotective Electricity Safety, QualityandContinuity Regulations the special locations. For exampleRegulation 9(4)of precautions needtobetaken when dealingwith A distributor alsomightnotprovide meansof in thepracticaldifficulties Finally, becauseof areAlso, anotherarea. Asupply minesandquarries electrode andRCD earth A separate orother Petrol fillingstations are anotherarea where Matters | Autumn 2005 | www.iee.org Guidance Note 7. on speciallocations isavailableinformation inIEE available in IEEGuidanceNote5.Alsomore isTN-S. systemearthing of situations theusualform installed adjacenttoorwithin theirpremises. Insuch dedicated totheirinstallation and transformers instance, anoverhead supply. the premises protective ortoaseparate conductorof, for thedistributor’s servicecable into of sheath orarmour istypically connectedtothemetallic terminal earthing reason at onepointonly,connected withearth at orasnearis energy thesource of A TN-Ssystemhastheneutralof system 3.3 TN-S taken, electrode. such asanadditionalearth inappropriate unlessadditionalprecautions are orswimmingsite, poolinstallations mightbe farm ESQCR).Construction safety(Reg 24of reasons of More information on earthing andbondingis onearthing More information consumers mayLarge have oneormore HV/LV ably practicable tothe source andtheconsumer’s