earthing: your questions answered

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Page 1: EARTHING: YOUR QUESTIONS ANSWERED

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This article, which is based on IEE Guidance notes, isintended to provide information that it is hoped willprove helpful.

BS 7671 lists five types of earthing system: TN-S, TN-C-S, TT, TN-C, and IT. T = Earth (from the French word Terre) N = Neutral S = Separate C = CombinedI = Isolated (The source of an IT system is either

connected to earth through a deliberately introducedearthing impedance or is isolated from Earth. Allexposed-conductive-parts of an installation areconnected to an earth electrode.)

When designing an electrical installation, one ofthe first things to determine is the type of earthingsystem. The distributor will be able to provide thisinformation.

The system will either be TN-S, TN-C-S (PME) or TTfor a low voltage supply given in accordance with theElectricity Safety, Quality and Continuity Regulations2002. This is because TN-C requires an exemptionfrom the Electricity Safety, Quality and ContinuityRegulations, and an IT system is not permitted for alow voltage public supply in the UK because thesource is not directly earthed. Therefore TN-C and ITsystems are both very uncommon in the UK.

EARTHING:YOUR QUESTIONS ANSWEREDBy Geoff Cronshaw

What are earthed and unearthed systems? What are the requirements of BS 7671? What are the advantages and disadvantages of the various types of earthing systems?

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1. Overview of earthing systems

1.1 TN-S system earthingA TN-S system, shown in fig 1, has the neutral of the source of energyconnected with earth at one point only, at or as near as is reasonablypracticable to the source, and the consumer’s earthing terminal is typicallyconnected to the metallic sheath or armour of the distributor’s service cableinto the premises.

Fig 2: Cable sheath earth(TN-S system). Schematic ofearthing and mainequipotential bondingarrangements. Based on 25 mm2 tails and selectionfrom Table 54G. Note: An isolator is notalways installed by theelectricity distributor.

Fig 1: TN-S system

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1.2 TN-C-S system earthingA TN-C-S system, shown in fig 3, has the supplyneutral conductor of a distribution main connectedwith earth at source and at intervals along its run.This is usually referred to as protective multipleearthing (PME). With this arrangement the

distributor’s neutral conductor is also used to returnearth fault currents arising in the consumer’sinstallation safely to the source. To achieve this, thedistributor will provide a consumer’s earthingterminal which is linked to the incoming neutralconductor.

Fig 3: TN-C-S system

Fig 4: PME supply (TN-C-Ssystem). Schematic of earthingand main equipotential bondingarrangements. Based on 25 mm2

tails and selection from Table 54G.Note: An isolator is not alwaysinstalled by the electricitydistributor.

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2. Requirements of BS 7671

Earth electrodesBS 7671 recognises a wide variety of types of earthelectrode. Regulation 542-02-01 lists the typesrecognised which include earth rods, earth plates and

Figure 5: TT system

Figure 6: No earth provided(TT system). Based on 25 mm2 tails and selectionfrom Table 54G. Note: An isolator is notalways installed by theelectricity distributor.Manufacturersrecommendations shouldbe sought with regards toconnections to earthelectrodes.

1.3 TT system earthingA TT system, shown above, has the neutral of the source ofenergy connected as for TN-S, but no facility is provided by thedistributor for the consumer’s earthing. With TT, theconsumer must provide their own connection to earth, i.e. byinstalling a suitable earth electrode local to the installation.

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underground structural metal work. The soil resistivity of the ground is probably the

single most important factor in the determination ofthe type of earth electrode. Rods can only be aseffective as the contact they make with thesurrounding material. Thus, they should be driveninto virgin ground, not disturbed (backfilled) ground.Where it is necessary to drive two or more rods andconnect them together to achieve a satisfactory result,the separation between rods should be at least equalto their combined driven depth to obtain maximumadvantage from each rod. In some locations low soilresistivity is found to be concentrated in the topsoillayer, beneath which there may be rock or otherimpervious strata which prevents the deep driving ofrods, or a deep layer of high resistivity. Only a test orknown information about the ground can reveal thiskind of information. In such circumstances, theinstallation of copper earth tapes, or pipes or plates,would be most likely to provide a satisfactory earthelectrode resistance value.

Whatever form an earth electrode takes, thepossibility of soil drying and freezing, and ofcorrosion, must be taken into account. Preferably,testing of an earth electrode should be carried outunder the least favourable conditions, i.e. afterprolonged dry weather. Further information onearthing principles and practice can be found in BS 7430 : 1998 ‘Code of Practice for Earthing’.

Earthing conductorsEarthing conductors which are defined in BS 7671 as aprotective conductor connecting the main earthingterminal of an installation to an earth electrode orother means of earthing must be adequately sizedparticularly where buried partly in the ground, andbe of suitable material and adequately protectedagainst corrosion and mechanical damage.

The size of an earthing conductor is arrived at inbasically the same way as for a circuit protectiveconductor, except that Table 54A of BS 7671 must beapplied to any buried earthing conductor. For a TN-C-S (PME) supply, it should be no smaller than themain bonding conductors.

Sizing of circuit protective conductorsThere are several factors which may influence ordetermine the size required for a circuit protectiveconductor. A minimum cross-sectional area of2.5 mm2 copper is required for any separate circuitprotective conductor, i.e. one which is not part of acable or formed by a wiring enclosure or contained insuch an enclosure.

An example would be a bare or insulated copperconductor clipped to a surface, run on a cable tray orfixed to the outside of a wiring enclosure. Such acircuit protective conductor must also be suitablyprotected if it is liable to suffer mechanical damage orchemical deterioration or be damaged by electro-dynamic effects produced by passing earth faultcurrent through it. If mechanical protection is notprovided the minimum size is 4 mm2 copper orequivalent.

BS 7671 provides two methods for sizing protectiveconductors including earthing conductors (see alsoTable 54A). The easier method is to determine theprotective conductor size from Table 54G but this mayproduce a larger size than is strictly necessary, sinceit employs a simple relationship to the cross-sectionalarea of the phase conductor(s).

The second method involves a formula calculation.The formula is commonly referred to as the ‘adiabaticequation’ and is the same as that used for short-circuit current calculations (see Regulation 434-03-03).It assumes that no heat is dissipated from theprotective conductor during an earth fault andtherefore errs on the safe side. Even so, application ofthe formula will in many instances result in aprotective conductor having a smaller csa than that ofthe live conductors of the associated circuit. This isquite acceptable.

Regulation 543-01-03 states:The cross-sectional area, where calculated, shall benot less than the value determined by the followingformula or shall be obtained by reference to BS 7454.

where:

S is the nominal cross-sectional area of the conductorin mm2.

I is the value in amperes (rms. for a.c.) of faultcurrent for a fault of negligible impedance, which canflow through the associated protective device, dueaccount being taken of the current limiting effect ofthe circuit impedances and the limiting capability (I2 t) of that protective device.

Account shall be taken of the effect, on the resistanceof circuit conductors, of their temperature rise as aresult of overcurrent - see Regulation 413-02-05.t is the operating time of the disconnecting device in

S = I2tk

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seconds corresponding to the fault current I amperes.k is a factor taking account of the resistivity,temperature coefficient and heat capacity of theconductor material, and the appropriate initial andfinal temperatures.

3.0 Type of earthing systems, advantages and disadvantages

3.1 Protective multiple earthing (PME). Such a supply system is described in BS 7671 as TN-C-S.The advantage of this system is that it provides aneffective and reliable method of providing customerswith an earth connection. For example the maximumZe specified by a distributor is 0.35 Ω for TN-C-S

supplies compared to 0.8 Ω for TN-S supplies. However, under certain supply system fault

conditions (PEN conductor of the supply becomingopen circuit external to the installation) a potentialcan develop between the conductive parts connectedto the PME earth terminal and the general mass ofearth. However, since there are multiple earthingpoints on the supply network and bonding is providedwithin the building complying with BS 7671, the riskis considered to be small.

Special LocationsThe Electricity Association publications providesguidance on PME systems. Whilst PME systemsprovide an effective and reliable earth connection

Figure 7: Typical site distribution for aPME supply, separation from PMEearth at main distribution board

Figure 8: Typical site distribution for aPME supply, separation from PMEearth at pitch supply point

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precautions need to be taken when dealing withspecial locations. For example Regulation 9(4) of theElectricity Safety, Quality and Continuity Regulationsdoes not allow the combined neutral and protectiveconductor to be connected electrically to anymetalwork in a caravan or boat. This prevents PMEterminals being used for caravans or boat mooringsupplies, although they may be used for fixedpremises on the sites, such as the site owner’s livingpremises and any bars or shops, etc.

Petrol filling stations are another area whereprecautions need to be taken. The referencepublication is “Guidance for the design, construction,modification and maintenance of petrol fillingstations”, published by the Association for Petroleumand Explosives Administration (APEA) and theInstitute of Petroleum, which recommends a TTsupply for hazardous areas.

A separate earth electrode and RCD or otheralternative arrangement is required to ensure thesegregation of petrol filling area earthing and that ofthe PME earth of the distribution network. A PMEearth may be used for permanent buildings such asshops and restaurants.

Also, mines and quarries are another area. A supplytaken to an underground shaft, or for use in theproduction side of a quarry, must have an earthingsystem which is segregated from any system bondedto the PME terminal.

Finally, because of the practical difficulties inbonding all accessible extraneous-conductive-partselectricity distribution companies might not provide aPME earth to agricultural and horticulturalinstallations and construction sites.

3.2 TT systemWith TT, the consumer must provide their ownconnection to earth, i.e. by installing a suitable earthelectrode local to the installation. The circumstancesin which a distributor will not provide a means ofearthing for the consumer are usually where thedistributor cannot guarantee the earth connectionback to the source, e.g. a low voltage overhead supply,where there is the likelihood of the earth wire eitherbecoming somehow disconnected or even stolen.

A distributor also might not provide means ofearthing for certain outdoor installations, e.g. aconstruction site temporary installation, leaving it tothe consumer to make suitable and safe arrangementsfor which they are fully responsible. The electricitydistributor is required to make available his supplyneutral or protective conductor for connection to theconsumer’s earth terminal, unless inappropriate for

reasons of safety (Reg 24 of ESQCR). Constructionsite, farm or swimming pool installations might beinappropriate unless additional precautions aretaken, such as an additional earth electrode.

3.3 TN-S systemA TN-S system has the neutral of the source of energyconnected with earth at one point only, at or as near as isreasonably practicable to the source and the consumer’searthing terminal is typically connected to the metallicsheath or armour of the distributor’s service cable intothe premises or to a separate protective conductor of, forinstance, an overhead supply.

Large consumers may have one or more HV/LVtransformers dedicated to their installation andinstalled adjacent to or within their premises. In suchsituations the usual form of system earthing is TN-S.

More information on earthing and bonding isavailable in IEE Guidance Note 5. Also moreinformation on special locations is available in IEEGuidance Note 7.