tieu chuan IEC chuong G

G

protection against electric shocks - G1

1. general

1.1 electric shock

when a current exceeding 30 mApasses through a part of a humanbody, the person concerned is inserious danger if the current is notinterrupted in a very short time.

the protection of persons againstelectric shock in LV installations mustbe provided in conformity withappropriate national standards andstatutory regulations, codes ofpractice, official guides and circulars,etc.Relevant IEC standards include:IEC 364, IEC 479-1, IEC 755,IEC 1008, IEC 1009and IEC 947-2 appendix B.

electric shockAn electric shock is the pathophysiologicaleffect of an electric current through thehuman body.Its passage affects essentially the circulatoryand respiratory functions and sometimesresults in serious burns. The degree ofdanger for the victim is a function of themagnitude of the current, the parts of thebody through which the current passes, andthe duration of current flow.IEC Publication 479-1 defines four zones ofcurrent-magnitude/time-duration, in each ofwhich the pathophysiological effects aredescribed (fig. G1). Any person coming intocontact with live metal risks an electric shock.

Curve C1 (of figure G1) shows that when acurrent greater than 30 mA passes through apart of a human being, the person concernedis likely to be killed, unless the current isinterrupted in a relatively short time.The point 500 ms/100 mA close to the curveC1 corresponds to a probability of heartfibrillation of the order of 0.14%.The protection of persons against electricshock in LV installations must be provided inconformity with appropriate nationalstandards and statutory regulations, codes ofpractice, official guides and circulars, etc.Relevant IEC standards include: IEC 364,IEC 479-1, IEC 755, IEC 1008, IEC 1009and IEC 947-2 appendix B.

0,1 0,2 0,5 1 2 5 10 20 50 100 200 500 1000 2000 5000 10000mA

current passing through the body Is

10

20

50

100

200

500

1000

5000

10000ms

2000

21 3 4

A B C1 C2 C3

duration of current flow t

∂ imperceptible∑ perceptible∏ reversible effects: muscular contractionπ possibility of irreversible effectsC1: no heart fibrillationC2: 5% probabilityof heart fibrillationC3: 50% probabilityof heart fibrillation

fig. G1: curve C1 (of IEC 479-1) defines the current-magnitude/time-duration limits whichmust not be exceeded.

1.2 direct and indirect contact

standards and regulations distinguishtwo kinds of dangerous contact:c direct contact,c indirect contact,and corresponding protectivemeasures.

direct contactA direct contact refers to a person cominginto contact with a conductor which is live innormal circumstances.

indirect contactAn indirect contact refers to a person cominginto contact with a conductive part which isnot normally alive, but has become aliveaccidentally (due to insulation failure or someother cause).

1 2 3 N

busbars

Is

insulationfailure

Is

insulationfailure1 2 3 PE conductor

Id

fig. G2: direct contact. fig. G3: indirect contact.

Is: touch current Id: insulation fault current

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G2 - protection against electric shocks

2. protection against direct contact

two measures of protection againstdirect-contact hazards are oftenimposed, since, in practice, the firstmeasure may not prove to beinfallible.

Two complementary measures are commonlyemployed as protection against the dangersof direct contact:c the physical prevention of contact with liveparts by barriers, insulation, inaccessibility,etc.c additional protection in the event that adirect contact occurs, despite the abovemeasures. This protection is based onresidual-current operated high-sensitivity fast-acting relays, which are highly effective in themajority of direct contact cases.

2.1 measures of protection against direct contact

IEC and national standardsfrequently distinguish betweendegrees of protectionc complete (insulation, enclosures)c partial or particular.

measures of complete protectionProtection by the insulation of live partsThis protection consists of an insulation whichconforms to the relevant standards. Paints,lacquers and varnishes do not provide anadequate protection.

Protection by means of barriers orenclosuresThis measure is in widespread use, sincemany components and materials are installedin cabinets, pillars, control panels anddistribution-board enclosures, etc. To beconsidered as providing effective protectionagainst direct-contact hazards, theseequipments must possess a degree ofprotection equal to at least IP2X or IPXXB(see Chapter F Sub-clause 7.2).Moreover, an opening in an enclosure (door,panel, drawer, etc.) must only be removable,opened or withdrawn:c by means of a key or tool provided for thepurpose, orc after complete isolation of the live parts inthe enclosure, orc with the automatic action of an interveningmetal shutter, removable only with a key orwith tools. The metal enclosure and all metalshutters must be bonded to the protectiveearthing conductor for the installation.

partial measures of protectionProtection by means of obstacles, or byplacing out of reachThis practice concerns locations to whichqualified, or otherwise authorized personnelonly, have access.

particular measuresof protectionProtection by the use of extra-low voltageSELV (Safety Extra Low Voltage) schemesThis measure is used only in low-powercircuits, and in particular circumstances, asdescribed in Sub-clause G3.5.

fig. G4: inherent direct-contact protectionby the insulation of a 3-phase cable withouter sheath.

fig. G5: example of direct-contactprevention by means of an earthed metalenclosure.

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2.2 additional measure of protection against direct contact

an additional measure of protectionagainst the hazards of direct contactis provided by the use of residual-current operated devices, whichoperate at 30 mA or less, and arereferred to as RCDs of highsensitivity.

All the preceding protective measures arepreventive, but experience has shown that forvarious reasons they cannot be regarded asbeing infallible. Among these reasons may becited:c lack of proper maintenance,c imprudence, carelessness,c normal (or abnormal) wear and tear ofinsulation; for example, flexure and abrasionof connecting leads,c accidental contact,c immersion in water, etc. - a situation inwhich insulation is no longer effective.In order to protect users in suchcircumstances, highly-sensitive fast-trippingdevices, based on the detection of residualcurrents to earth (which may or may not bethrough a human being or animal) are usedto disconnect the power supply automatically,and with sufficient rapidity to preventpermanent injury to, or death byelectrocution, of a normally healthy humanbeing.

fig. G6: high-sensitivity RCD.

These devices operate on the principle ofdifferential current measurement, in whichany difference between the current entering acircuit and that leaving it, must (on a systemsupplied from an earthed source) be flowingto earth, either through faulty insulation orthrough contact of an earthed object, such asa person, with a live conductor.Standard residual-current devices, referred toas RCDs, sufficiently sensitive for direct-contact protection are rated at 30 mA ofdifferential current. Other standard IECratings for high-sensitivity RCDs are 10 mAand 6 mA (generally used for individualappliance protection).This additional protection is imposed incertain countries for circuits supplying socketoutlets of ratings up to 32 A, and even higherif the location is wet and/or temporary (suchas work-sites for example).Chapter L section 3 itemizes various commonlocations in which RCDs of high sensitivityare obligatory (in some countries), but in anycase, are highly recommended as aneffective protection against both direct- andindirect-contact hazards.

IEC wiring regulations impose theuse of RCDs on circuits supplyingsocket outlets, installed in particularlocations considered to be potentiallydangerous, or used for specialpurposes. Some national wiringregulations impose their use on allcircuits supplying socket outlets.

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3. protection against indirect contact

national regulations covering LVinstallations impose, or stronglyrecommend, the provision of devicesfor indirect-contact protection.

the measures of protection are:c automatic disconnection of supply(at the first or second fault detection,depending on the system of earthing)c particular measures according tocircumstances.

Conductive material(1) used in themanufacture of an electrical appliance, butwhich is not part of the circuit for theappliance, is separated from live parts by the"basic insulation". Failure of the basicinsulation will result in the conductive partsbecoming live.Touching a normally-dead part of an electricalappliance which has become live due to thefailure of its insulation, is referred to as anindirect contact.Various measures are adopted to protectagainst this hazard, and include:c automatic disconnection of power supply tothe appliance concerned,

c special arrangements such as:v the use of class II insulation materials, or anequivalent degree of insulation,v non-conducting location(2) - out-of-reach orinterposition of barriers,v equipotential locality,v electrical separation by means of isolatingtransformers.(1) Conductive material (usually metal) which may betouched, without dismantling the appliance, is referred to as"exposed conductive parts".(2) The definition of resistances of the walls, floor and ceilingof a non-conducting location is given in Sub-clause G3.5

3.1 measure of protection by automatic disconnection of the supply

protection against indirect-contacthazards by automatic disconnectionof the supply can be achieved if theexposed conductive parts ofappliances are properly earthed.

principleThis protective measure depends on twofundamental requirements:c the earthing of all exposed conductive partsof equipment in the installation and theconstitution of an equipotential bondingnetwork (see Sub-clause F4-1),c automatic disconnection of the section ofthe installation concerned, in such a way thatthe touch-voltage/time safety requirementsare respected for any level of touch voltageUc(3).(3) touch voltage Uc:Touch voltage Uc is the voltage existing (as the result ofinsulation failure) between an exposed conductive part andany conductive element within reach which is at a different(generally earth) potential.The greater the value of Uc, the greater the rapidity ofsupply disconnection required to provide protection (seeTables G8 and G9). The highest value of Uc that can betolerated indefinitely without danger to human beings iscalled the "conventional touch-voltage limit" (U

L). installation

earth electrode Uc

fig. G7: in this illustration the dangeroustouch voltage Uc is from hand to hand.

reminder of the theoreticaldisconnecting-time limits*

in practice the disconnecting timesand the choice of protection schemesto use depend on the kind of earthingsystem concerned; TT, TN or IT.Precise indications are given in thecorresponding paragraphs.

assumed maximum disconnectingtouch time for the protectivevoltage device (seconds)(V) alternating direct

current current< 50 5 550 5 575 0.60 590 0.45 5120 0.34 5150 0.27 1220 0.17 0.40280 0.12 0.30350 0.08 0.20500 0.04 0.10

assumed maximum disconnectingtouch time for the protectivevoltage device (seconds)(V) alternating direct

current current25 5 550 0.48 575 0.30 290 0.25 0.80110 0.18 0.50150 0.12 0.25230 0.05 0.06280 0.02 0.02

table G8: maximum safe duration of theassumed values of touch voltage inconditions where U L = 50 V(1).

(1) The resistance of the floor and the wearing of shoes aretaken into account in these values.

* For most locations, the maximum permitted touch voltage(UL) is 50 V. For special locations, the limit is reduced to25 V. See G4-1 and Clause L3.

table G9: maximum safe duration of theassumed values of touch voltage inconditions where U L = 25 V.

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3.2 automatic disconnection for a TT-earthed installation

automatic disconnection for aTT-earthed installation is effected bya RCD having a sensitivity of

I∆n i UL = 50 V*

RA RA

where RA = resistance of theinstallation earth electrode* 25 V in some particular cases.

principleIn this scheme all the exposed andextraneous conductive parts of the installationmust be connected to a common earthelectrode. The supply system neutral isnormally earthed at a point outside the areaof influence of the electrode for theinstallation, but need not be so.The impedance of the earth-fault looptherefore consists mainly of the two earthelectrodes (i.e. the source and installationelectrodes) in series, so that the magnitude ofthe earth-fault current is generally too small tooperate overcurrent relays or fuses, and theuse of a differential-current form of protectionis essential.This principle of protection is also valid if onecommon earth electrode only is used, notablyin the case of a consumer-type substationwithin the installation area, where spacelimitations may impose the adoption of a TNearthing scheme, but where all otherconditions required by the TN system cannotbe fulfilled.

Automatic protection for a TT-earthedinstallation is assured by the use of a RCD ofsensitivity:

I∆n i UL = 50 V RA RA

whereRA = the resistance of the earth electrode forthe installation.I∆n = rated differential current operating level.For temporary supplies (to work-sites etc.)and agricultural and horticulturalestablishments, the value of UL in the above-mentioned formula must be replaced by 25 V.

exampleThe resistance of the substation neutral earthelectrode Rn is 10 ohms.The resistance of the installation earthelectrode RA is 20 ohms.The earth-fault current Id = 7.7 A.The touch-voltage Uc = IdRA = 154 V andtherefore dangerous, but,

I∆n = 50 = 2.5 A so that a standard 300 mA 20RCD will operate in 30 ms to clear a conditionin which 50 V touch voltage, or more,appears on an exposed conductive part.

HV/400V

substation earth electrode

Rn : 10 Ω

installationearth electrode

RA : 20 Ω

1

2

3

4

Uc

fig. G10: automatic disconnection for a TT-earthed installation.

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3. protection against indirect contact (continued)

3.2 automatic disconnection for a TT-earthed installation (continued)

the tripping times of RCDs aregenerally lower than thoseprescribed in the majority of nationalstandards; this feature facilitates theiruse and allows the adoption of aneffective scheme of discriminativeprotection.

specified disconnection timeRCD is a general term for all devicesoperating on the residual-current principle.RCCB* (residual current circuit breaker) asdefined in IEC 1008 is a specific class ofRCD.Type G (general) and type S (selective) havetripping time/current characteristics as shownin table G11. These characteristics allow acertain degree of selective tripping betweenthe several combinations of rating and type,as shown later in Sub-clause 4.3.

x I∆n 1 2 5 > 5instantaneous (ms) 300 150 40 40

domestic

type S (ms) 500 200 150 150

industrialsetting I** (ms) 150 150 150 150

* Merlin Gerin

table G11: maximum operating times ofRCCBs (IEC 1008).** Note : the use of the term "circuit breaker" does not meanthat a RCCB can break short-circuit currents. For suchduties RCDs known as RCBOs ("O" for overcurrent) asdefined in IEC 1009 must be employed.

3.3 automatic disconnection for a TN-earthed installation

the principle of the TN scheme ofearthing is to ensure that earth-faultcurrent will be sufficient to operateovercurrent protective devices(direct-acting tripping, overcurrentrelays and fuses) so that

Ia i Uo or 0.8 Uo* Zs Zc

principleIn this scheme all exposed and extraneousconductive parts of the installation areconnected directly to the earthed point of thepower supply by protective conductors.As noted in Chapter F Sub-clause 4.2, theway in which this direct connection is carriedout depends on whether the TN-C, TN-S, orTN-C-S method of implementing the TNprinciple is used. In figure G12 the methodTN-C is shown, in which the neutralconductor acts as both the Protective-Earthand Neutral (PEN) conductor. In all TNarrangements, any insulation fault to earthconstitutes a phase-neutral short-circuit.High fault current levels simplify protectionrequirements but can give rise to touchvoltages exceeding 50% of the phase-to-neutral voltage at the fault position during thebrief disconnection time.In practice, therefore, earth electrodes arenormally installed at intervals along theneutral of the supply network, while theconsumer is generally required to instal anearth electrode at the service position. Onlarge installations additional earth electrodesdispersed around the premises are often

provided, in order to reduce the touch voltageas much as possible.In high-rise apartment blocks, all extraneousconductive parts are connected to theprotective conductor at each level.In order to ensure adequate protection, theearth-fault current

Id = Uo or 0.8 Uo u Ia where Zs ZcUo = nominal phase-neutral voltage.Zs = earth-fault current loop impedance,equal to the sum of the impedances of: thesource, the live phase conductors to the faultposition, the protective conductors from thefault position back to the source.Zc = the faulty-circuit loop impedance (see"conventional method" Sub-clause 5.2).Note: the path through earth electrodes backto the source will have (generally) muchhigher impedance values than those listedabove, and need not be considered.Id = the fault current.Ia = a current equal to the value required tooperate the protective device in the timespecified.

example

RnA

N

321

PEN

A

F

NS160

50 m35 mm2

CD

35mm2

E

B

Uc

fig. G12: automatic disconnection for a TN-earthed installation.

In figure G12 the touch voltage

Uc = 230 = 115 V 2and is therefore dangerous.The impedance Zs of the loop = ZAB + ZBC +ZDE + ZEN + ZNA.If ZBC and ZDE are predominant, then:

ZS = 2ρ x L = 64.3 milli-ohm, so that SId = 230/64.3 = 3,576 A (≈ 22 In based on a160 A circuit breaker).The "instantaneous" magnetic tripping devicesetting of the circuit breaker is many times

less than this value, so that positive operationin the shortest possible time is assured.Note: some authorities base suchcalculations on the assumption that a voltagedrop of 20% occurs in the part of theimpedance loop BANE.This method, which is recommended, isexplained in chapter G Sub-clause 5.2"conventional method" and, in this example,will give a fault current of230 x 0.8 x 103 = 2,816 A (≈ 18 In) 64.3

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for TN earthing, the maximumallowable disconnection timedepends on the nominal voltage ofthe system.

specified maximumdisconnection timesThe times specified are a function of thenominal voltage phase/earth, which, for allpractical purposes on TN systems, is thephase/neutral voltage.

Uo (volts) disconnection timephase/neutral (seconds) U L=50 V

(see note 2)127 0.8230 0.4400 0.2> 400 0.1

table G13: maximum disconnection timesspecified for TN earthing schemes(IEC 364-4-41).

certain national regulations impose, theprovision of equipotential bonding of allextraneous and exposed conductive partsthat are simultaneously accessible, in anyarea where socket-outlets are installed, fromwhich portable or mobile equipment might besupplied. The common equipotential busbaris installed in the distribution-board cabinetfor the area concerned.

Note 2: when the conventional voltage limit is25 V, the specified disconnection times are:0.35 s. for 127 V0.2 s. for 230 V0.05 s. for 400 VIf the circuits concerned are final circuits, thenthese times can easily be achieved by theuse of RCDs.

Note 3: the use of RCDs may, as mentionedin note 2, be necessary on TN-earthedsystems. Use of RCDs on TN-C-S systemsmeans that the protective conductor and theneutral conductor must (evidently) beseparated upstream of the RCD. Thisseparation is commonly made at the serviceposition.

Note 1: a longer time interval than thosespecified in the table (but in any case lessthan 5 seconds) is allowed under certaincircumstances for distribution circuits, as wellas for final circuits supplying a fixedappliance, on condition that a dangeroustouch voltage is not thereby caused to appearon another appliance. IEC recommends, and

protection by means of a circuitbreakerThe instantaneous trip unit of a circuit breakerwill eliminate a short-circuit to earth fault inless than 0.1 second.In consequence, automatic disconnectionwithin the maximum allowable time willalways be assured, since all types of trip unit,magnetic or electronic, instantaneous orslightly retarded, are suitable: Ia = Im.The maximum tolerance authorized by therelevant standard, however, must always betaken into consideration.It is sufficient therefore that the fault currentUo / Zs or 0.8 Uo / Zc determined bycalculation (or established on site) be greaterthan the instantaneous trip-setting current, orthan the very short-time tripping thresholdlevel, to be sure of tripping whithin thepermitted time limit.

if the protection is to be provided by acircuit breaker, it is sufficient to verifythat the fault current will alwaysexceed the current-setting level ofthe instantaneous or short-time delaytripping unit (Im):

Im < Uo or 0.8*Zs Zc

* according to the "conventional" method ofcalculation (see sub-clause 5.2).

1

Im Uo/Zs

t

I

2

1 : instantaneous trip2 : short time-delayed trip

fig. G14: disconnection by circuit breakerfor a TN-earthed installation.

protection by means of fusesThe value of current which assures thecorrect operation of a fuse can beaccertained from a current/time performancegraph for the fuse concerned.The fault current Uo/Zs or 0.8 Uo/Zc asdetermined above, must largely exceed thatnecessary to ensure positive operation of thefuse.The condition to observe therefore is that:

Ia < Uo or 0.8 Uo as indicated in figure G15. Zs Zc

Example:The nominal phase-neutral voltage of thenetwork is 230 V and the maximumdisconnection time given by the graph infigure G15 is 0.4 seconds. The correspondingvalue of Ia can be read from the graph.Using the voltage (230 V) and the current Ia,the complete loop impedance or the circuitloop impedance can be calculated fromZs = 230/Ia or Zc = 0.8 x 230/Ia.Thisimpedance value must never be exceededand should preferably be substantially less toensure satisfactory fuse operation.

I

t

tc

Ia

= 0,4 s

Uo/Zs

fig. G15: disconnection by fusesfor a TN-earthed installation.

Ia can be determined from the fuseperformance curve. In any case,protection cannot be achieved if theloop impedance Zs or Zc exceeds acertain value.

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3.4 automatic disconnection on a second earth fault in an IT-earthed system


In this type of system:c the installation is isolated from earth, or theneutral point of its power-supply source isconnected to earth through a highimpedance,

c all exposed and extraneous conductiveparts are earthed via an installation earthelectrode.

in an IT scheme it is intended that afirst fault to earth will not cause anydisconnection.

first faultOn the occurrence of a short-circuit fault toearth, referred to as a "first fault", the faultcurrent is very small, such that the ruleId x RA i 50 V (see G3.2)is respected and no dangerous touchvoltages can occur.In practice the current Id is feeble, a conditionthat is neither dangerous to personnel, norharmful to the installation.However, in this scheme:c a permanent surveillance of the condition ofthe insulation to earth must be provided,together with an alarm signal (audio and/orflashing lights, etc.) in the event of a firstearth fault occurring,c the rapid location and repair of a first fault isimperative if the full benefits of the IT systemare to be realized. Continuity of service is thegreat advantage afforded by the scheme.

fig. G16: phases to earth insulationmonitoring relay (obligatory on IT-earthedinstallation).

RnA = 5 Ω

321

PE

HV/400 V

A

B

Zct1500 Ω

Id1

ZF

Id2

Uc

Id1Id2

Id2

Id2

Id1

Id2

Id1

fig. G17: fault-current paths for a first (earth) fault on an IT-earthed installation.

Example:For a network formed from 1 km ofconductors, the leakage (capacitive)impedance to earth ZF is of the order of3,500 ohms per phase. In normal (unfaulted)operation, the capacitive current* to earth isthereforeUo = 230 = 66 mA per phaseZF 3,500During a phase-to-earth fault, as shown infigure G17, the current passing through theelectrode resistance RnA is the vector sum ofthe capacitive currents in the two healthyphases. The voltages of the two healthyphases have (because of the fault) increasedto √3 the normal phase voltage, so that thecapacitive currents increase by the sameamount. These currents are displaced, onefrom the other by 60°, so that when addedvectorially, this amounts to 3 x 66 mA =198 mA i.e. Id2 in the present example.

The touch voltage Uc is therefore198 x 5 x 10-3 = 0.99 V, which is evidentlyharmless.The current through the short-circuit is givenby the vector sum of the neutral-resistorcurrent Id1 (= 153 mA) and the capacitivecurrent (Id2).Since the exposed conductive parts of theinstallation are connected directly to earth,the neutral impedance Zct plays practically nopart in the production of touch voltages toearth.* Resistive leakage current to earth through the insulation isassumed to be negligibly small in the example.

second fault situationOn the appearance of a second fault, on adifferent phase, or on a neutral conductor,a rapid disconnection becomes imperative.Fault clearance is carried out differently ineach of the following cases:1st case: concerns an installation in which allexposed conductive parts are bonded to acommon PE conductor, as shown in figureG19.In this case no earth electrodes are includedin the fault current path, so that a high level of

fault current is assured, and conventionalovercurrent protective devices are used, i.e.circuit breakers and fuses.The first fault could occur at the end of acircuit in a remote part of the installation,while the second fault could feasibly belocated at the opposite end of the installation.For this reason, it is conventional to doublethe loop impedance of a circuit, whencalculating the anticipated fault setting levelfor its overcurrent protective device(s).

the simultaneous existence of twoearth faults (if not both on the samephase) is dangerous, and rapidclearance by fuses or automaticcircuit breaker tripping depends onthe type of earth-bonding scheme,and whether separate earthingelectrodes are used or not, in theinstallation concerned.

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c if no neutral conductor is provided, then thevoltage to use for the fault-current calculationis the phase-to-phase value, i.e.

(2) 0.8 e Uo* u Ia 2 Zc

Specified tripping/fuse-clearance timesDisconnecting times for 3-wire 3-phase ITschemes differ from those adopted for 4-wire3-phase IT schemes, and are given for bothcases in table G18.* based on the "conventional method" noted in the firstexample of Sub-clause 3.3.

1st case: where all exposedconductive parts are connected to acommon PE conductor conventionalovercurrent protection schemes(such as those used in TN systems)are applicable, with fault-levelcalculations and tripping/fuse-operating times suitably adapted.

c where the system includes a neutralconductor in addition to the 3 phaseconductors, the lowest short-circuit faultcurrents will occur if one of the (two) faults isfrom the neutral conductor to earth (all fourconductors are insulated from earth in an ITscheme). In four-wire IT installations,therefore, the phase-to-neutral voltage mustbe used to calculate short-circuit protectivelevels i.e.

(1) 0.8 Uo* u Ia where 2 ZcUo = phase/neutral voltageZc = impedance of the circuit fault-currentloop (see G3.3)Ia = current level for trip setting

Uo/U (volts) disconnection time (seconds) U L = 50 V (1)

Uo = phase-neutral volts 3-phase 3-wires 3-phase 4-wiresU = phase-phase volts127/220 0.8 5230/400 0.4 0.8400/690 0.2 0.4580/1000 0.1 0.2

table G18: maximum disconnection times specified for an IT-earthed installation(IEC 364-4-41).

(1) When the conventional voltage limit is25 V, the disconnecting times become:c in the case of a 3-phase 3-wire scheme,0.4 seconds at 127/220 V; 0.2 seconds at230/400 V and 0.06 seconds at 400/690 V,

c in the case of a 3-phase 4-wire scheme,1.0 second at 127/220 V; 0.5 seconds at230/400 V and 0.2 seconds at 400/690 V.

Example

Rn

321

PE

HV/400 V

HZct

IdA

K

RA

G D

E

C

50 m35 mm2

NS160160 A

F

B

J

busbars

50 m35 mm2

fig. G19: circuit breaker tripping on second (earth) fault when exposed conductive partsare connected to a common protective conductor.

The current levels and protective measuresdepend on the switchgear and fusegearconcerned:ccccc circuit breakersIn the case shown in figure G19, the levels ofinstantaneous and short time-delayovercurrent-trip settings must be decided.The times recommended in table G18 can bereadily complied with.Example: from the case shown in figure G19,determine that the short-circuit protectionprovided by the 160 A circuit breaker issuitable to clear a phase-to-phase short-circuit occurring at the load ends of thecircuits concerned.Reminder: In an IT system, the two circuitsinvolved in a phase-to-phase short circuit areassumed to be of equal length, with the samesized conductors; the PE conductors beingthe same size as the phase conductors.In such a case, the impedance of the circuitloop when using the "conventional method"(Sub-clause 5.2 of this chapter) will be twicethat calculated for one of the circuits in theTN case, shown in Sub-clause 3.3.

So that the resistance of circuit 1 loop FGHJ

= 2 RHJ = 2 ρ l mΩ awhere: ρ = the resistance in milli-ohm of acopper rod 1 metre long of c.s.a. 1 mm2

l = length of the circuit in metresa = c.s.a. of the conductor in mm2

= 2 x 22.5 x 50 = 64.3 mΩ 35and the loop resistance B, C, D, E, F, G, H, Jwill be 2 x 64.3 = 129 mΩThe fault current will therefore be:0.8 x ex 230 x 103 = 2.470 A 129c fusesThe current Ia for which fuse operation mustbe assured in a time specified according totable G18 can be found from fuse operatingcurves, as described in figure G15.The current indicated should be significantlylower than the fault currents calculated for thecircuit concerned,c RCCBsIn particular cases, RCCBs are necessary. Inthis case, protection against indirect contacthazards can be achieved by using one RCCBfor each circuit.

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3.4 automatic disconnection on a second earth fault in an IT-earthed system (continued)


2nd case: where exposed conductiveparts of appliances are earthedindividually or in separate groups,each appliance or each group must(in addition to overcurrent protection)be protected by a RCD.

2nd case: concerns exposed conductiveparts which are earthed either individually(each part having its own earth electrode) orin separate groups (one electrode for eachgroup).If all exposed conductive parts are notbonded to a common electrode system, thenit is possible for the second earth fault tooccur in a different group or in a separately-earthed individual apparatus. Additionalprotection to that described above for case 1,is required, and consists of a RCD placed atthe circuit breaker controlling each group andeach individually-earthed apparatus.The reason for this requirement is that theseparate-group electrodes are "bonded"through the earth so that the phase-to-phaseshort-circuit current will generally be limitedwhen passing through the earth bond, by the

electrode contact resistances with the earth,thereby making protection by overcurrentdevices unreliable. The more sensitive RCDsare therefore necessary, but the operatingcurrent of the RCDs must evidently exceedthat which occurs for a first fault.For a second fault occurring within a grouphaving a common earth-electrode system,the overcurrent protection operates, asdescribed above for case 1.Note 1: see also Chapter H1 Sub-clause 7.2,protection of the neutral conductor.Note 2: in 3-phase 4-wire installationsprotection against overcurrent in the neutralconductor is sometimes more convenientlyachieved by using a ring-type currenttransformer over the single-core neutralconductor, as shown in figure G20 (see alsoTable H1-65c).

Rn

HV/LV

PIM

RA

RCD

groupearth

case 1

N

Rn

HV/LV

PIM

RA 2

RCD RCD

RCD

RA 1

group 1earth

group 2earth

N

case 2

fig. G20: the application of RCDs when exposed conductive parts are earthed individuallyor by groups, on IT-earthed systems.

3.5 measures of protection against direct or indirect contact without circuit disconnection

extra-low voltage is used where therisks are great: swimming pools,wandering-lead hand lamps, andother portable appliances for outdooruse, etc.

the use of SELV (Safety by ExtraLow Voltage)Safety by extra low voltage SELV is used insituations where the operation of electricalequipment presents a serious hazard(swimming pools, amusement parks, etc.).This measure depends on supplying power atvery low voltage from the secondary windingsof isolating transformers especially designedaccording to national or to international(IEC 742) standards.The impulse withstand level of insulationbetween the primary and secondary windingsis very high, and/or an earthed metal screenis sometimes incorporated between thewindings. The secondary voltage neverexceeds 50 V rms.Three conditions of exploitation must berespected in order to provide satisfactoryprotection against indirect contact:c no live conductor at SELV must beconnected to earth,c exposed conductive parts of SELV-supplied

equipment must not be connected to earth, toother exposed conductive parts, or toextraneous conductive parts,c all live parts of SELV circuits and of othercircuits of higher voltage must be separatedby a distance at least equal to that betweenthe primary and secondary windings of asafety isolating transformer.These measures require that:c SELV circuits must use conduits exclusivelyprovided for them, unless cables which areinsulated for the highest voltage of the othercircuits are used for the SELV circuits,c socket outlets for the SELV system mustnot have an earth-pin contact. The SELVcircuit plugs and sockets must be special, sothat inadvertent connection to a differentvoltage level is not possible.Note: In normal conditions, when the SELVvoltage is less than 25 V, there is no need toprovide protection against direct-contacthazards. Particular requirements areindicated in Chapter L, Clause 3: "speciallocations".

the use of PELV (Protection byExtra Low Voltage)This system is for general use where lowvoltage is required, or preferred for safetyreasons, other than in the high-risk locationsnoted above. The conception is similar to thatof the SELV system, but the secondary circuitis earthed at one point.IEC 364-4-41 defines precisely thesignificance of the reference PELV. Protectionagainst direct-contact hazards is generallynecessary, except when the equipment is inthe zone of equipotential bonding, and thenominal voltage does not exceed 25 V rms,

and the equipment is used in normally drylocations only, and large-area contact with thehuman body is not expected.In all other cases, 6 V rms is the maximumpermitted voltage, where no direct-contactprotection is provided.

fig. G21: low-voltage supplies from a safetyisolating transformer, as defined in IEC 742.

230 V / 24 V

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FELV system (Functional ExtraLow Voltage)Where, for functional reasons, a voltage of50 V or less is used, but not all of therequirements relating to SELV or PELV arefulfilled, appropriate measures described inIEC 364-4-41 must be taken to ensureprotection against both direct and indirectcontact hazards, according to the locationand use of these circuits.

Note: Such conditions may, for example, beencountered when the circuit containsequipment (such as transformers, relays,remote-control switches, contactors)insufficiently insulated with respect to circuitsat higher voltages.

the separation of electric circuitsThe principle of separation of circuits(generally single-phase circuits) for safetypurposes is based on the followingreasoning.The two conductors from the unearthedsingle-phase secondary winding of aseparation transformer are insulated fromearth.If a direct contact is made with one conductor,a very small current only will flow into theperson making contact, through the earth andback to the other conductor, via the inherentcapacitance of that conductor with respect toearth. Since the conductor capacitance toearth is very small, the current is generallybelow the level of perception.As the length of circuit cable increases, thedirect contact current will progressivelyincrease to a point where a dangerouselectric shock will be experienced.Even if a short length of cable precludes anydanger from capacitive current, a low value ofinsulation resistance with respect to earth canresult in danger, since the current path is thenvia the person making contact, through theearth and back to the other conductorthrough the low conductor-to-earth insulationresistance.For these reasons, relatively short lengths ofwell-insulated cable are essential inseparation schemes.Transformers are specially designed for thisduty, with a high degree of insulation betweenprimary and secondary windings, or withequivalent protection, such as an earthedmetal screen between the windings.Construction of the transformer is to class IIinsulation standards.As indicated above, successful exploitation ofthe principle requires that:

the separation of electric circuits issuitable for relatively short cablelengths and high levels of insulationresistance. It is preferably used foran individual appliance.

c no conductor or exposed conductive part ofthe secondary circuit must be connected toearth,c the length of secondary cabling must belimited to avoid large capacitance values*,c a high insulation-resistance value must bemaintained for the cabling and appliances.These conditions generally limit theapplication of this safety measure to anindividual appliance.In the case where several appliances aresupplied from a separation transformer, it isnecessary to observe the followingrequirements:c the exposed conductive parts of allappliances must be connected together by aninsulated protective conductor, but notconnected to earth,c the socket outlets must be provided with anearth-pin connection. The earth-pinconnection is used in this case only to ensurethe interconnection (bonding) of all exposedconductive parts.In the case of a second fault, overcurrentprotection must provide automaticdisconnection in the same conditions asthose required for an IT scheme of powersystem earthing.* It is recommended in IEC 364-4-41 that the product of thenominal voltage of the circuit in volts and length in metres ofthe wiring system should not exceed 100 000, and that thelength of the wiring system should not exceed 500 m.

separationtransformer230 V / 230 Vclass II

fig. G22: safety supplies from a separationtransformer.

class II appliances

symbolThese appliances are also referred to ashaving "double insulation" since in class IIappliances a supplementary insulation isadded to the basic insulation. No conductiveparts of a class II appliance must beconnected to a protective conductor:c most portable or semi-fixed appliances,certain lamps, and some types of transformerare designed to have double insulation. It isimportant to take particular care in theexploitation of class II equipment and to verifyregularly and often that the class II standardis maintained (no broken outer envelope,etc.). Electronic devices, radio and televisionsets have safety levels equivalent to class II,but are not formally class II appliances,c supplementary insulation in an electricalinstallation (IEC 364-4-41: Sub-clause 413-2).Some national standards such asNF C 15-100 (France) (annex to 413.5

active part

basicinsulationsupplementaryinsulation

fig. G23: principle of class II insulationlevel.

Chapter 41) describe in more detail thenecessary measures to achieve thesupplementary insulation during installationwork.A simple example is that of drawing a cableinto a PVC conduit. Methods are alsodescribed for distribution boards.c for distribution boards and similarequipment, IEC 439-1 describes a set ofrequirements, for what is referred to as "totalinsulation", equivalent to class II,c some cables are recognized as beingequivalent to class II by many nationalstandards.

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3.5 measures of protection against direct or indirect contact without circuit disconnection(continued)

in principle, safety by placingsimultaneously-accessibleconductive parts out-of-reach, or byinterposing obstacles, requires also anon-conducting floor, and so is not aneasily applied principle

out-of-reach or interpositionof obstacles.By these means, the probability of touching alive exposed conductive part, while at thesame time touching an extraneousconductive part at earth potential, isextremely low. In practice, this measure canonly be applied in a dry location, and isimplemented according to the followingconditions:c the floor and the walls of the chamber mustbe non-conducting, i.e. the resistance to earthat any point must be:> 50 kΩ (installation voltages i 500 V),> 100 kΩ (500 V < installation voltagesi 1000 V).Resistance is measured by means of"MEGGER" type instruments (hand-operatedgenerator or battery-operated electronicmodel) between an electrode placed on thefloor or against the wall, and earth (i.e. thenearest protective earth-conductor).The electrode contact area and pressure

must evidently be the same for all tests.Different instrument suppliers provideelectrodes specific to their own product, sothat care should be taken to ensure that theelectrodes used are those supplied with theinstrument. There are no universallyrecognized standards established for thesetests at the time of writing.c the placing of equipment and obstaclesmust be such that simultaneous contact withtwo exposed conductive parts or with anexposed conductive part and an extraneousconductive part by an individual person is notpossible.c no exposed protective conductor must beintroduced into the chamber concerned.c entrances to the chamber must bearranged so that persons entering are not atrisk, e.g. a person standing on a conductingfloor outside the chamber must not be able toreach through the doorway to touch anexposed conductive part, such as a lightingswitch mounted in an industrial-type cast-ironconduit box, for example.

electricalapparatus

electricalapparatus

< 2 m

electricalapparatus

insulatedwallsinsulated

obstacles

> 2 m

insulated floor

2.5 m

fig. G24: protection by out-of-reach arrangements and the interposition of non-conductingobstacles.

between a live conductor and the metalenvelope of an appliance will result in thewhole "cage" being raised to phase-to-earthvoltage, but no fault current will flow. In suchconditions, a person entering the chamberwould be at risk (since he/she would bestepping on to a live floor). Suitableprecautions must be taken to protectpersonnel from this danger (e.g. non-conducting floor at entrances, etc.).Special protective devices are also necessaryto detect insulation failure, in the absence ofsignificant fault current.*Note: extraneous conductive parts entering (or leaving) theequipotential space (such as water pipes, etc.) must beencased in suitable insulating material and excluded fromthe equipotential network, since such parts are likely to bebonded to protective (earthed) conductors elsewhere in theinstallation.

earth-free equipotentialchambersIn this scheme, all exposed conductive parts,including the floor (see *Note) are bonded bysuitably large conductors, such that nosignificant difference of potential can existbetween any two points. A failure of insulation

earth-free equipotential chambersare associated with particularinstallations (laboratories, etc.) andgive rise to a number of practicalinstallation difficulties.

insulatingmaterial

conductive floor

M

fig. G25: equipotential bonding of all exposed conductive partssimultaneously accessible.

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4. implementation of the TT system

4.1 protective measures

the application to living quarters iscovered in Chapter L Clause 1.

protection against indirectcontactGeneral caseProtection against indirect contact is assuredby RCDs, the sensitivity I∆n of whichcomplies with the condition:

I∆n i 50 V (1)

RA

(1) 25 V for work-site installations, agriculturalestablishments, etc.

The choice of sensitivity of the differentialdevice is a function of the resistance RA ofthe earth electrode for the installation, and isgiven in table G26.

I∆n maximum resistanceof the earth electrode(50 V) (25 V)

3 A 16 Ω 8 Ω1 A 50 Ω 25 Ω500 mA 100 Ω 50 Ω300 mA 166 Ω 83 Ω30 mA 1666 Ω 833 Ω

table G26: the upper limit of resistance foran installation earthing electrode whichmust not be exceeded, for givensensitivity levels of RCDs at U L voltagelimits of 50 V and 25 V.

Case of distribution circuitsIEC 364-4-41 and a number of nationalstandards recognize a maximum tripping timeof 1 second in installation distribution circuits(as opposed to final circuits). This allows adegree of selective discrimination to beachieved:c at level A: RCD time-delayed, e.g. "S" type,c at level B: RCD instantaneous.

A

B

fig. G27: distribution circuits.

Case where the exposed conductive partsof an appliance, or group of appliances,are connected to a separate earthelectrodeProtection against indirect contact by a RCDat the circuit breaker controlling each groupor separately-earthed individual appliance.In each case, the sensitivity must becompatible with the resistance of the earthelectrode concerned. RA 2RA 1

a distant location

fig. G28: separate earth electrode.

high-sensitivity RCDsIEC 364-4-471 strongly recommends the useof a RCD of high sensitivity (i 30 mA) in thefollowing cases:c socket-outlet circuits for rated currentsof i 32 A at any location(1),c socket-outlet circuits in wet locations at allcurrent ratings(1),c socket-outlet circuits in temporaryinstallations(1),c circuits supplying laundry rooms andswimming pools(1),c supply circuits to work-sites, caravans,pleasure boats, and travelling fairs(1).This protection may be for individual circuitsor for groups of circuits,c strongly recommended for circuits of socketoutlets u 20 A (mandatory if they areexpected to supply portable equipment foroutdoor use),c in some countries, this requirement ismandatory for all socket-outlet circuitsrated i 32 A.(1) these cases are treated in delail in Chapter L Clause 3.

fig. G29: circuit supplying socket-outlets.

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4. implementation of the TT system (continued)

4.1 protective measures (continued)

in areas of high fire riskRCD protection at the circuit breakercontrolling all supplies to the area at risk isnecessary in some locations, and mandatoryin many countries.The sensitivity of the RCD must be i 500 mA.

fire-risk area

fig. G30: fire-risk location.

protection when exposedconductive parts are notconnected to earth(in the case of an existing installation wherethe location is dry and provision of anearthing connection is not possible, or in theevent that a protective earth wire becomesbroken)RCDs of high sensitivity (i 30 mA) will affordboth protection against indirect-contacthazards, and the additional protection againstthe dangers of direct-contact .

fig. G31: unearthed exposed conductiveparts (A).

4.2 types of RCDRCDs are commonly incorporated in thefollowing components:c industrial-type moulded-case differentialcircuit breakers conforming to IEC 947-2 andits appendix B,c domestic-type differential circuit breakers(RCCBs)* conforming to IEC 755, 1008, and1009 (RCBOs),*see NOTE concerning RCCBs at the end ofSub-clause 3.2.

c differential switches conforming to particularnational standards,c relays with separate toroidal (ring-type)current transformers, conforming to IEC 755.RCDs are mandatorily used at the origin ofTT-earthed installations, where their ability todiscriminate with other RCDs allows selectivetripping, thereby ensuring the level of servicecontinuity required.

fig. G32: industrial-type CB with RCD module.DIN-rail circuit breaker with RCD module

Adaptable differential circuit breakers,including DIN-rail mounted units, areavailable, to which may be associated anauxiliary module. The ensemble provides a

comprehensive range of protective functions(isolation, short-circuit, overload, andsensitive earth-fault protection).

the international standard forindustrial differential circuit breakersis IEC 947-2 and its appendix B.

G


the international standards fordomestic circuit breakers (RCBOs) isIEC 1009.

The incoming-supply circuit breaker can also have time-delayed characteristics (type S).

"Monobloc" type of earth-fault differential circuit breakersdesigned for the protection of socket-outlet circuits and finalcircuit protection.

fig. G33: domestic earth-fault differential circuit breakers.

In addition to the adaptable industrial circuitbreakers which comply to industrial anddomestic standards, there are ranges of

"monobloc" differential circuit breakersintended for domestic and tertiary sectorapplications.

Differential switches (RCCBs) are used for the protection ofdistribution or sub-distribution boards.

RCDs with separate toroidal CTs can be used in associationwith circuit breakers or contactors.

fig. G34: differential switches (RCCBs). fig. G35: RCDs with separate toroidalcurrent transformers.

differential switches are covered byparticular national standards(NF C 61-140 for France).RCDs with separate toroidal currenttransformers are standardizedin IEC 755.

RCCBs, RCBOs and CBRsRCCBs (Residual Current Circuit Breakers)These devices are more-accurately describedin the French version of IEC 1008 as"interrupteurs" which is generally translatedinto English by "load-break switches"."Residual-current load-break switches" wouldbe a more accurate description of a RCCB,which, although assigned a rated making andbreaking capacity, is not designed to breakshort-circuit currents (the unique feature of acircuit breaker) so that the term RCCB can bemisleading.As noted in sub-clause 7.3 a SCPD (Short-Circuit Protective Device) must always beseries-connected with a RCCB.

RCBOsThe "O" stands for "Overcurrent" which refersto the fact that, in addition to sensitivedifferential earth-fault protection, overcurrentprotection is provided. The RCBO has a ratedshort-circuit breaking capability and isproperly referred to as a circuit breaker.IEC 1009 is the international referencestandard.

Note: Both RCCBs and RCBOs asstandardized in IEC 1008 and 1009respectively, provide complete isolation whenopened. These units are designed fordomestic and similar installations.

CBRsAmendment 1 (1992) of the product standardIEC 947-Part 2: "Circuit Breakers" includesAppendix B, which covers the incorporationof residual-current protection into industrial-type LV circuit breakers. The Appendix isbased on the relevant requirements of IEC755,IEC 1008 and IEC 1009. Circuit breakers soequipped are referred to as CBRs.

4.3 coordination of differential protective devicesDiscriminative-tripping coordination isachieved either by time-delay or bysubdivision of circuits, which are thenprotected individually or by groups, or by acombination of both methods.Such discrimination avoids the tripping of anycircuit breaker, other than that immediatelyupstream of a fault positionc with equipment currently available,discrimination is possible at three or fourdifferent levels of distribution, viz:v at the main general distribution board,

v at local general distribution boards,v at sub-distribution boards,v at socket outlets for individual applianceprotectionc in general, at distribution boards (and sub-distribution boards, if existing) and onindividual-appliance protection, devices forautomatic disconnection in the event of anindirect-contact hazard occurring are installedtogether with additional protection againstdirect-contact hazards.

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A

B

RCD 300 mAtype S

RCD30 mA

4. implementation of the TT system (continued)

4.3 coordination of differential protective devices (continued)

discrimination between RCDsDiscrimination is achieved by exploiting theseveral levels of standardized sensitivity:30 mA, 100 mA, 300 mA and 1 A and thecorresponding tripping times, as shown belowin figure G36.

time(ms)

40

10

60

100130150200250

500

1000

300

10000

15 current(mA)

30

100

15060

300

500

600

1000

1 1,5 10 100 500 1000 (A)

300 mA selective RCDs(i.e. time-delayed)

domestic Stime delayed

RCD 30 mA general domestic

industrial(settings I and II )

I

II

and industrial setting 0

fig. G36: discrimination between RCDs.

discrimination at 2 levelsProtection:c level A: RCD time-delayed setting 1 (forindustrial device) type S (for domestic device)for protection against indirect contacts,c level B: RCD instantaneous, with highsensitivity on circuits supplying socket-outletsor appliances at high risk (washing machines,etc. See also Chapter L Clause 3).

fig. G37.

discrimination at 3 or 4 levelsProtection:c level A: RCD time-delayed (setting III),c level B: RCD time-delayed (setting II),c level C: RCD time-delayed (setting I) ortype S,c level D: RCD instantaneous.

A relay with separatetoroidal CT 3 Adelay time 500 ms

RCCB 1 Adelay time 250 ms

B

C

D

RCCB 300 mAdelay time 50 ms or type S

RCCB 30 mA

fig. G38: discrimination at 3 or 4 levels.

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discriminative protection at three levels

Rp

Rn

RA

321NPE

N123PE

NPhPE

M

M

MERLIN GERIN

differential relaywith separate toroidalCT setting level ≤ 50/RAtime-delay setting level II

main circuit breaker

HV/LV

MERLIN GERIN

MERLIN GERIN

SM20

IN OUT

earth leakagecurrent monitor

differentialrelay withseparate CT

discontactor

T

VigicompactNS100setting level300 mA

T

TT E S T

30mA

XC40diff.30 mARCD

DPN Vigi30 mA

NS400 NS80H-MA

instantaneous300 mA

distributionbox

discontactor

RCDMCBMCB

MCB + RCD

300 mA type Stime-delayedRCD

remotely-controlledactuator

fig. G39: typical 3-level installation, showing the protection of distribution circuits in a TT-earthed system.One motor is provided with specific protection.

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5. implementation of the TN system

5.1 preliminary conditionsAt the design stage, the maximum permittedlengths of cable downstream of a controllingcircuit breaker (or set of fuses) must becalculated, while during the installation workcertain rules must be fully respected.

imposed conditionsCertain conditions must be observed, aslisted below and illustrated in figure G40.1. earth electrodes should be provided atevenly-spaced points (as far as practicalconditions allow) along the PE conductor.Note : This is not normally done for a singledomestic installation; one earth electrode onlyis usually required at the service position.2. the PE conductor must not pass throughferro-magnetic conduit, ducts, etc. or bemounted on steel work, since inductive and/or proximity effects can increase the effectiveimpedance of the conductor.3. in the case of a PEN conductor (a neutralconductor which is also used as a protectiveconductor), connection must be made directlyto the earth terminal of an appliance (see 3 infigure G40) before being looped to the neutralterminal of the appliance.4. where the conductor i 6 mm2 for copper or10 mm2 for aluminium, or where a cable ismovable, the neutral and protectiveconductors should be separated (i.e. aTN-S scheme should be adopted within theinstallation).5. earth faults should be cleared byovercurrent-protection devices, i.e. by fusesand circuit breakers.The foregoing list indicates the conditions tobe respected in the implementation of aTN scheme for the protection against indirectcontacts.

RpnA

PEN

TN-C

PE

TN-C-S

N

2 2

34

5

5 5

1

fig. G40: implementation of the TN systemof earthing.

note(1) the TN scheme requires that the LV neutral of the HV/LVtransformer, the exposed conductive parts of the substationand of the installation, and the extraneous conductive partsin the sub-station and installation, all be earthed to acommon earthing system.(2) for a substation in which the metering is at low-voltage, ameans of isolation is required at the origin of the LVinstallation, and the isolation must be clearly visible.(3) a PEN conductor must never be interrupted under anycircumstances. Control and protective switchgear for theseveral TN arrangements will be:c 3-pole when the circuit includes a PEN conductor,c preferably 4-pole (3 phases + neutral) when the circuitincludes a neutral with a separate PE conductor.

5.2 protection against indirect contact

Three methods of calculation arecommonly used:c the method of impedances, basedon the trigonometric addition of thesystem resistances and inductivereactances.c the method of composition.c the conventional method, based onan assumed voltage drop and theuse of prepared tables.

methods of determining levels ofshort-circuit currentIn TN-earthed systems, a short-circuit toearth will, in principle, always providesufficient current to operate an overcurrentdevice. The source and supply mainsimpedances are much lower that those of theinstallation circuits, so that any restriction inthe magnitude of earth-fault currents will bemainly caused by the installation conductors(long flexible leads to appliances greatlyincrease the "fault-loop" impedance, with acorresponding reduction of short-circuitcurrent).The most recent IEC recommendations forindirect-contact protection on TN earthingschemes only relates maximum allowabletripping times to the nominal system voltage(see table G13 in Sub-clause 3.3).The reasoning behind theserecommendations is that, for TN systems, thecurrent which must pass in order to raise thepotential of an exposed conductive part to50 V or more is so high that one of twopossibilities will occur:c either the fault path will blow itself clear,practically instantaneously, orc the conductor will weld itself into a solidfault and provide adequate current to operateovercurrent devices.To ensure correct operation of overcurrentdevices in the latter case, a reasonablyaccurate assessment of short-circuit earth-

fault current levels must be determined at thedesign stage of a project.A rigorous analysis requires the use of phase-sequence-component techniques applied toevery circuit in turn. The principle isstraightforward, but the amount ofcomputation is not considered justifiable,especially since the zero-phase-sequenceimpedances are extremely difficult todetermine with any resonable degree ofaccuracy in an average LV installation.Other simpler methods of adequate accuracyare preferred. Three practical methods are:c the "method of impedances", based on thesummation of all the impedances (positive-phase-sequence only) around the fault loop,for each circuit,c the "method of composition", which is anestimation of short-circuit current at theremote end of a loop, when the short-circuitcurrent level at the near end of the loop isknown,c the "conventional method" of calculating theminimum levels of earth-fault currents,together with the use of tables of values forobtaining rapid results.These methods are only reliable for the casein which the cables that make up the earth-fault-current loop are in close proximity (toeach other) and not separated by ferro-magnetic materials.

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for calculations, modern practice is touse software agreed by NationalAuthorities, and based on themethod of impedances, such asECODIAL 2 (Merlin Gerin).National Authorities generally alsopublish Guides, which include typicalvalues, conductor lengths, etc.

method of impedancesThis method summates the positive-sequence impedances of each item (cable,PE conductor, transformer, etc.) included inthe earth-fault loop circuit from which theshort-circuit earth-fault current is calculated,using the formula:

where (∑R)2 = (the sum of all resistances inthe loop)2

I =U/ (∑R)2+(∑X)

2

and (∑X)2 = (the sum of all inductivereactances in the loop)2

and U = nominal system phase-to-neutralvoltageThe application of the method is not alwayseasy, because it supposes a knowledge of allparameter values and characteristics of theelements in the loop. In many cases, anational guide can supply typical values forestimation purposes.

method of compositionThis method permits the determination of theshort-circuit current at the end of a loop fromthe known value of S.C. at the sending end,by means of the approximate formula:

I = U Isc where U + Zsc IscIsc = upstream short-circuit currentI = end-of-loop short-circuit currentU = nominal system phase voltageZsc = impedance of loop

Note: in this method the individualimpedances are added arithmetically* asopposed to the previous "method ofimpedances" procedure.

* This results in a calculated current value which is less thanthat which would actually flow. If the overcurrent settings arebased on this calculated value, then operation of the relay,or fuse, is assured.

conventional methodThis method is generally considered to besufficiently accurate to fix the upper limit ofcable lengths.Principle:The principle bases the short-circuit currentcalculation on the assumption that thevoltage at the origin of the circuit concerned(i.e. at the point at which the circuit protectivedevice is located) remains at 80% or more ofthe nominal phase to neutral voltage. The80% value is used, together with the circuitloop impedance, to compute the short-circuitcurrent.This coefficient takes account of all voltagedrops upstream of the point considered. In LVcables, when all conductors of a 3-phase4-wire circuit are in close proximity (which isthe normal case), the inductive reactanceinternal to* and between conductors isnegligibly small compared to the cableresistance.

This approximation is considered to be validfor cable sizes up to 120 mm2.Above that size, the resistance value R isincreased as follows:core size (mm2) value of resistanceS = 150 mm2 R+15%S = 185 mm2 R+20%S = 240 mm2 R+25%

* causes proximity and skin effects, i.e. an apparentincrease in resistance.

Example:

Id

L

C

PE

SPE Sph

AB

the maximum length of any circuit ofa TN-earthed installation is:

Lmax = 0.8 Uo Sph ρ (1+m) Ia

fig. G41: calculation of L max. for aTN-earthed system, using theconventional method.

The maximum length of a circuit in a TN-earthed installation is given by the formula:

Lmax = 0.8 Uo Sph metres, where: ρ (1+m) IaLmax = maximum length in metresUo = phase volts = 230 V for a 230/400 Vsystemρ = resistivity at normal working temperaturein ohm-mm2/metre= 22.5 10-3 for copper= 36 10-3 for aluminiumIa = trip current setting for the instantaneousoperation of a circuit breaker, orIa = the current which assures operation ofthe protective fuse concerned, in thespecified time.m = Sph / SPESph = cross-sectional area of the phaseconductors of the circuit concerned in mm2

SPE = cross-sectional area of the protectiveconductor concerned in mm2

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nominal cross- instantaneous or short-time-delayed tripping current Im (amperes)sectional areaof conductorsmm 2 50 63 80 100 125 160 200 250 320 400 500 560 630 700 800 875 1000 1120 1250 1600 2000 2500 3200 4000 5000 6300 8000 10000 125001.5 103 81 64 51 41 32 25 20 16 13 10 9 8 7 6 6 52.5 171 136 107 85 66 53 42 34 26 21 17 15 13 12 10 10 8 8 7 54 274 217 171 137 109 85 68 54 43 34 27 24 21 19 17 16 14 12 11 8 7 56 410 326 256 205 164 126 102 82 64 51 41 36 32 29 25 23 20 18 16 13 10 8 6 510 427 342 273 214 171 137 107 85 68 61 54 49 42 39 34 30 27 21 17 14 10 8 7 516 436 342 274 219 171 137 109 97 87 78 68 62 55 49 44 34 27 21 17 13 11 8 7 525 428 342 267 213 171 152 135 122 107 98 85 76 66 53 43 34 27 21 17 13 10 8 735 479 374 299 239 214 190 171 150 136 120 107 96 75 80 48 37 30 24 19 15 12 950 406 325 290 258 232 203 185 162 145 130 101 81 65 50 40 32 26 20 16 1270 479 427 380 342 299 274 239 214 191 150 120 96 75 60 48 38 30 24 1995 464 406 371 325 290 260 203 162 130 101 81 65 51 40 32 26120 469 410 366 328 256 205 165 128 102 82 65 51 41 33150 446 398 357 279 223 178 139 111 89 71 56 44 36185 471 422 329 264 211 165 132 105 84 66 53 42240 410 328 263 205 164 131 104 82 66 52

5. implementation of the TN system (continued)

5.2 protection against indirect contact (continued)

the following tables* give the lengthof circuit which must not beexceeded, in order that persons beprotected against indirect contacthazards by protective devices.

* Based on tables given in the guide UTE C15-105.

tablesThe following tables, applicable to TNsystems, have been established according tothe "conventional method" described above.The tables give maximum circuit lengths,beyond which the ohmic resistance of theconductors will limit the magnitude of theshort-circuit current to a level below thatrequired to trip the circuit breaker (or to blowthe fuse) protecting the circuit, with sufficientrapidity to ensure safety against indirectcontact.

The tables take into account:c the type of protection: circuit breakers orfuses,c operating-current settings,c cross-sectional area of phase conductorsand protective conductors,c type of earthing scheme (see fig. G47),c type of circuit breaker (i.e. B, C or D).The tables may be used for 230/400 Vsystems.Equivalent tables for protection by Compactand Multi 9 circuit breakers (Merlin Gerin) areincluded in the relevant catalogues.

Correction factor mTable G42 indicates the correction factor toapply to the values given in tables G43 toG46 according to the ratio SPH/SPE, the typeof circuit, and the conductor materials.

circuit conductor m = S PH/SPE (or PEN)material m = 1 m = 2 m = 3 m = 4

3P + N or P + N copper 1 0.67 0.50 0.40aluminium 0.62 0.42 0.31 0.25

table G42: correction factor to apply to the lengths given in tables G43 to G46 forTN systems.

Circuits protected by general-purposecircuit-breakers

table G43: maximum circuit lengths for different sizes of conductor and instantaneous-tripping-current settings for general-pur posecircuit breakers.

Circuits protected by Compact* or Multi 9*circuit breakers for industrial or domesticuse

SPH rated current (A)mm2 1 2 3 4 6 8 10 13 16 20 25 32 40 45 50 63 80 1001.5 1227 613 409 307 204 153 123 94 77 61 49 38 31 27 25 19 15 122.5 681 511 341 256 204 157 128 102 82 64 51 45 41 32 28 204 1090 818 545 409 327 252 204 164 131 102 82 73 65 52 41 336 818 613 491 377 307 245 196 153 123 109 98 78 61 4910 1022 818 629 511 409 327 256 204 182 164 130 102 8216 1006 818 654 523 409 327 291 262 208 164 13125 1022 818 639 511 454 409 325 258 20435 894 716 636 572 454 358 28850 777 617 485 388

table G44: maximum circuit lengths for different sizes of conductor and rated currents fortype B (1) circuit breakers.* Merlin Gerin products.(1) For the definition of type B circuit breaker refer to chapter H2 Sub-clause 4.2.

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SPH rated current (A)mm2 1 2 3 4 6 8 10 13 16 20 25 32 40 45 50 63 80 1001.5 613 307 204 153 102 77 61 47 38 31 25 19 15 14 12 10 8 62.5 1022 511 341 256 170 128 102 79 64 51 41 32 26 23 20 16 13 104 818 545 409 273 204 164 126 102 82 65 51 41 36 33 26 20 166 818 613 409 307 245 189 153 123 98 77 61 55 49 39 31 2510 1022 681 511 409 315 256 204 164 128 102 91 82 65 51 4116 818 654 503 409 327 262 204 164 145 131 104 82 6525 1022 786 639 511 409 319 256 227 204 162 128 10235 894 716 572 447 358 318 286 227 179 14350 777 607 485 431 389 309 243 194

table G45: maximum circuit lengths for different conductor sizes and for rated currentsof circuit breakers of type C (1).(1) For the definition of type C circuit breakers refer to chapter H2 Sub-clause 4.2.

SPH rated current (A)

mm 2 1 1.6 2 2.5 3 4 6 6.3 8 10 12.5 13 16 20 25 32 40 45 50 63 80 100

1.5 438 274 219 175 146 110 73 70 53 44 35 34 27 22 18 14 11 10 9 7 5 4

2.5 730 456 365 292 243 183 122 116 88 73 58 56 46 37 29 23 18 16 15 12 9 7

4 730 584 467 389 292 195 186 141 117 93 90 73 58 47 37 29 26 23 19 14 12

6 876 701 584 438 292 279 211 175 140 135 110 88 70 55 44 39 35 28 21 18

10 974 730 487 465 352 292 234 225 183 146 117 91 73 65 58 46 35 29

16 779 743 564 467 374 359 292 234 187 146 117 104 93 74 58 47

25 881 730 584 562 456 365 292 228 183 162 146 116 88 73

35 1022 818 786 639 511 409 319 258 227 204 162 123 102

50 867 692 558 432 347 308 277 220 174 139

table G46: maximum circuit lengths for different conductor sizes and for rated currentsof circuit breakers of type D or MA Merlin Gerin (1).

(1) For the definition of type D circuit breakers refer to chapter H2 Sub-clause 4.2.For typical use of an MA circuit breaker, refer to Chapter J figure J5-3.

Example:A 3-phase 4-wire (230/400 V) installation isTN-C earthed. A circuit is protected by acircuit breaker rated at 63 A, and consists ofan aluminium cored cable with 50 mm2 phaseconductors and a neutral conductor (PEN) of25 mm2.What is the maximum length of circuit, belowwhich protection of persons against indirect-contact hazards is assured by theinstantaneous magnetic tripping relay of thecircuit breaker?Table G44 gives 617 metres, to which mustbe applied a factor of 0.42 (table G42 form = SPH/SPE = 2).The maximum length of circuit is therefore:617 x 0.42 = 259 metres.

particular case where oneor more exposed conductivepart(s) is (are) earthed to aseparate earth electrodeProtection must be provided against indirectcontact by a RCD at the origin of any circuitsupplying an appliance or group ofappliances, the exposed conductive parts ofwhich are connected to an independent earthelectrode.The sensitivity of the RCD must be adaptedto the earth electrode resistance (RA2 infigure G47).Downstream of the RCD, the earthingscheme must be TN-S.

RA 2RA 1

a distant location

fig. G47: separate earth electrode.

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IEC 364-4-471 strongly recommends the useof a RCD of high sensitivity (i 30 mA) in thefollowing cases:c socket-outlet circuits for rated currentsof i 32 A at any location(1),c socket-outlet circuits in wet locations at allcurrent ratings(1),c socket-outlet circuits in temporaryinstallations(1),c circuits supplying laundry rooms andswimming pools(1),c supply circuits to work-sites, caravans,pleasure boats, and travelling fairs(1).This protection may be for individual circuitsor for groups of circuits,c strongly recommended for circuits of socketoutlets u 20 A (mandatory if they areexpected to supply portable equipment foroutdoor use),c in some countries, this requirement ismandatory for all socket-outlet circuitsrated i 32 A.(1) these cases are treated in delail in Chapter L Clause 3.

5. implementation of the TN system (continued)

5.3 high-sensitivity RCDs


5.4 protection in high fire-risk locationsIn locations where the risk of fire is high, theTN-C scheme of earthing is often prohibited,and the TN-S arrangement must be adopted.Protection by a RCD of sensitivity 500 mA atthe origin of the circuit supplying the fire-risklocation is mandatory in some countries.

fire-risk area


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5.5 when the fault-current-loop impedance is particularly highWhen the earth-fault current is restricted dueto an inevitably high fault-loop impedance, sothat the overcurrent protection cannot berelied upon to trip the circuit within theprescribed time, the following possibilitiesshould be considered:

2 i Irm i 4In

PE or PEN

unusuallylong cable

fig. G50: a circuit breaker with low-setinstantaneous magnetic trip.

Suggestion 1:install a circuit breaker which has aninstantaneous magnetic tripping element withan operation level which is lower than theusual setting, for example:2In i Irm i 4InThis affords protection for persons on circuitswhich are abnormally long. It must bechecked, however, that high transientcurrents such as the starting currents ofmotors will not cause nuisance trip-outs.

Suggestion 2:install a RCD on the circuit. The device neednot be highly-sensitive (HS) (several amps toa few tens of amps). Where socket-outletsare involved, the particular circuits must, inany case, be protected by HS (i 30 mA)RCDs; generally one RCD for a number ofsocket outlets on a common circuit.

phasesneutralPE

TN-S

phasesPEN

TN-C

fig. G51: RCD protection on TN systemswith high earth-fault-loop impedance.

Suggestion 3:increase the size of the PE or PENconductors and/or the phase conductors, toreduce the loop impedance.

Suggestion 4:add supplementary equipotential conductors.This will have a similar effect to that ofsuggestion 3, i.e. a reduction in the earth-fault-loop resistance, while at the same timeimproving the existing touch-voltageprotection measures. The effectiveness ofthis improvement may be checked by aresistance test between each exposedconductive part and the local main protectiveconductor.For TN-C installations, bonding as shown infigure G52 is not allowed, and Suggestion 3should be adopted.

fig. G52: improved equipotential bonding.

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6. implementation of the IT system

The basic feature of the IT scheme ofearthing is that, in the event of a short-circuitto earth fault, the system can continue tofunction without interruption.Such a fault is referred to as a "first fault". Inthis scheme, all exposed conductive parts ofan installation are connected via PEconductors to an earth electrode at theinstallation, while the neutral point of thesupply transformer is isolated from earth orconnected to earth through a high resistance(commonly 1,000 ohms or more).This means that the current through an earthfault will be measured in milli-amps, which willnot cause serious damage at the faultposition, or give rise to dangerous touchvoltages, or present a fire hazard. Thesystem may therefore be allowed to functionnormally until it is convenient to isolate thefaulty section for repair work.In practice, the scheme requires certainspecific measures for its satisfactoryexploitation:

c permanent monitoring of the insulation withrespect to earth, which must signal (audiblyor visually) the occurrence of the first fault,c a device for limiting the voltage which theneutral point of the supply transformer canattain with respect to earth,c a "first-fault" location routine by an efficientmaintenance staff. Fault location is greatlyfacilitated by automatic devices which arecurrently available,c automatic high-speed tripping ofappropriate circuit breakers must take placein the event of a "second fault" occurringbefore the first fault is repaired. The secondfault (by definition) is an earth fault affecting adifferent phase than that of the first fault or aneutral conductor*.The second fault results in a short-circuitthrough the earth and/or through PE bondingconductors.* on systems where the neutral is distributed, as shown infigure G58.

6.1 preliminary conditionsPreliminary conditions are summarized intable G53 and fig. G54.

minimum functions components examplesrequired and devices (MG)protection against overvoltages (1) voltage limiter Cardew Cat system frequencyneutral earthing resistor (2) resistor impedance Zx(for impedance earthing variation)overall earth-fault monitor (3) permanent insulation Vigilohm TR22Awith alarm for first fault condition monitor PIM with alarm feature or XM 200automatic fault clearance (4) four-pole circuit breakers Compact circuit breakeron second fault and (if the neutral is distributed) or RCD-MSprotection of the neutral all 4 poles + tripconductor against overcurrentlocation of first fault (5) with device for fault-location Vigilohm system

on live system, or by successiveopening of circuits

table G53: essential functions in IT schemes.

L1L2L3N

HV/LV

4

4

2 1 3

5

4

fig. G54: 3-phase 3-wire IT-earthedsystem.

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6.2 protection against indirect contactfirst-fault conditionThe earth-fault current which flows under afirst-fault condition is measured in milli-amps.The touch voltage with respect to earth is theproduct of this current and the resistance ofthe installation earth electrode and PEconductor (from the faulted component to theelectrode). This value of voltage is clearlyharmless and could amount to several voltsonly in the worst case (1,000 Ω earthingresistor will pass 230 mA* and a poorinstallation earth-electrode of 50 ohms, wouldgive 12.5 V, for example).An alarm is given by the permanent earth-fault monitoring.

Principle of earth-fault monitoringA generator of very low frequency a.c.current, or of d.c. current, (to reduce theeffects of cable capacitance to negligiblelevels) applies a voltage between the neutralpoint of the supply transformer and earth.This voltage causes a small current to flowaccording to the insulation resistance to earthof the whole installation, plus that of anyconnected appliance.

Low-frequency instruments can be used ona.c. systems which generate transient d.c.components under fault conditions. Certainversions can distinguish between resistiveand capacitive components of the leakagecurrent.Modern developments permit themeasurement of leakage-current evolution,so that prevention of a first fault can beachieved.

Examples of equipment and devices**c manual fault-location (fig. G55).The generator may be fixed (example:XM200) or portable (example: XGRpermitting the checking of dead circuits) andthe receiver, together with the magnetic tong-type pick-up sensor, are portable.* On a 230/400 V 3-phase system.** The equipment and devices described to illustrate theprinciples of fault location, are manufactured by M.G.

P12 P50P100 ON/OFF

XGRXRM

XM200

MERLIN GERIN

XM100

fig. G55: non-automatic (manual) fault location.

c fixed automatic fault location (fig. G56)The monitoring relay XM200, together withthe fixed detectors XD301 (each suppliedfrom a toroidal CT embracing the conductorsof the circuit concerned) provide a system ofautomatic fault location on a live installation.Moreover, the level of insulation is indicatedfor each monitored circuit, and two levels arechecked: the first level warns of unusually lowinsulation resistance so that preventivemeasures may be taken, while the secondlevel indicates a fault condition and gives analarm.

XD301

XD312

1 to 12 circuits

toroidal CTs

XM200

MERLIN GERIN

XM100

XD301 XD301

fig. G56: fixed automatic fault location.

modern monitoring systems greatlyfacilitate first-fault location and repair.

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6. implementation of the IT system (continued)


c automatic monitoring, logging, and faultlocation.The Vigilohm System also allows access to aprinter and/or a PC which provides a globalreview of the insulation level of an entireinstallation, and records the chronologicalevolution of the insulation level of eachcircuit.The central monitor XM300C, together withthe localization detectors XL308 and XL316,associated with toroidal CTs from severalcircuits, as shown below in figure G57,provide the means for this automaticexploitation.

XM300 C

MERLIN GERIN

XM100

XL308 XL316

MERLIN GERIN

XL08

897

MERLIN GERIN

XL16

678

fig. G57: automatic fault location and insulation-resistance data logging.

Implementation of permanent insulation-monitoring (PIM) devicesc connectionThe PIM device is normally connectedbetween the neutral (or articificial neutral)point of the power-supply transformer and itsearth electrode,c supplyPower supply to the PIM device should betaken from a highly reliable source. Inpractice, this is generally directly from theinstallation being monitored, throughovercurrent protective devices of suitableshort-circuit current rating,c impedance of the PIM deviceIn order to maintain the level of earth-faultwithin safe limits, the current passing througha PIM device during a short-circuit to earth isnormally limited to a value < 30 mA.Where the neutral point is earthed through animpedance, the total current passing throughthe PIM device and the impedance (inparallel with it) must be < 500 mA.This means that a touch voltage of less than50 V will occur in the installation as long asthe installation earth-electrode resistance isless than 100 ohms, and that fire risk ofelectrical origin is avoided,c level settingsCertain national standards recommend a firstsetting at 20% below the insulation level ofthe new installation. This value allows thedetection of a reduction of the insulationquality, necessitating preventive maintenancemeasures in a situation of incipient failure.The detection level for earth-fault alarm willbe set at a much lower level.

By way of an example, the two levels mightbe:v new installation insulation level: 100 kΩv leakage current without danger: 500 mA(fire risk at > 500 mA)v indication levels set by the consumer:- threshold for preventive maintenance:0.8 x 100 = 80 kΩ- threshold for short-circuit alarm: 300 mA.

Notes:v following a long period of shutdown, duringwhich the whole, or part of, the installationremains de-energized, humidity can reducethe general level of insulation resistance. Thissituation, which is mainly due to leakagecurrent over the damp surface of healthyinsulation, does not constitute a faultcondition, and will improve rapidly as thenormal temperature rise of current-carryingconductors reduces the surface humidity.v the PIM device (XM) can measure theresistive and the capacitive currentcomponents of the leakage current to earth,separately, thereby deriving the trueinsulation resistance from the total permanentcurrent leakage.

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the case of a second faultA second earth fault on an IT system (unlessoccurring on the same conductor as the firstfault) constitutes a phase-phase or phase-to-neutral fault, and whether occurring on thesame circuit as the first fault, or on a differentcircuit, overcurrent protective devices (fusesor circuit breakers) would normally operate toeffect an automatic fault clearance.The settings of overcurrent tripping relaysand the ratings of fuses are the basicparameters that decide the maximumpractical length of circuit that can besatisfactorily protected, as discussed inSub-clause 5.2.Note: in normal circumstances, the faultcurrent path is through common PEconductors, bonding all exposed conductive

parts of an installation, and so the fault loopimpedance is sufficiently low to ensure anadequate level of fault current.Where circuit lengths are unavoidably long, andespecially if the appliances of a circuit areearthed separately (so that the fault currentpasses through two earth electrodes), reliabletripping on overcurrent may not be possible.In this case, an RCD is recommended oneach circuit of the installation.Where an IT system is resistance earthed,however, care must be taken to ensure thatthe RCD is not too sensitive, or a first faultmay cause an unwanted trip-out. Tripping ofresidual current devices which satisfy IECstandards may occur at values of 0.5 I∆n toI∆n, where I∆n is the nominal residual-currentsetting level.

at the remote end of a loop, when the level ofshort-circuit current at the near end of theloop is known. Complex impedances arecombined arithmetically in this method,c the conventional method, in which theminimum value of voltage at the origin of afaulty circuit is assumed to be 80% of thenominal circuit voltage, and tables are usedbased on this assumption, to give directreadings of circuit lengths.These methods are reliable only for the casesin which wiring and cables which make up thefault-current loop are in close proximity (toeach other) and are not separated by ferro-magnetic materials.

A reasonably accurate assessment of short-circuit current levels must be carried out atthe design stage of a project.A rigorous analysis is not necessary, sincecurrent magnitudes only are important for theprotective devices concerned (i.e. phaseangles need not be determined) so thatsimplified conservatively approximatemethods are normally used. Three practicalmethods are:c the method of impedances, based on thevectorial summation of all the (positive-phase-sequence) impedances around a fault-current loop,c the method of composition, which is anapproximate estimation of short-circuit current

three methods of calculating short-circuit current levels are commonlyemployed:c method of impedances, whichtakes account of complexrepresentation of impedances,c method of composition, is aconservatively approximate method,which combines impedancesarithmetically,c conventional method, is asimplified method based on anassumed minimum voltage duringfault, and the use of tables.

Method of impedancesThis method as described in Sub-clause 5.2,is identical for both the IT and TN systems ofearthing.

the software Ecodial 2 (Merlin Gerin)is based on the "method ofimpedances".

Method of compositionThis method as described in Sub-clause 5.2,is identical for both the IT and TN systems ofearthing.

Conventional methodThe principle is the same for an IT system asthat described in Sub-clause 5.2 for a TNsystem, viz: the calculation of maximumcircuit lengths which should not be exceededdownstream of a circuit breaker or fuses, toensure protection by overcurrent devices.It is clearly impossible to check circuit lengthsfor every feasible combination of twoconcurrent faults.All cases are covered, however, if theovercurrent trip setting is based on theassumption that a first fault occurs at theremote end of the circuit concerned, while thesecond fault occurs at the remote end of anidentical circuit, as already mentioned in Sub-clause 3.4. This may result, in general, in onetrip-out only occurring (on the circuit with thelower trip-setting level), thereby leaving thesystem in a first-fault situation, but with onefaulty circuit switched out of service.c for the case of a 3-phase 3-wire installationthe second fault can only cause a phase/phase short-circuit, so that the voltage to usein the formula for maximum circuit length iseUo.The maximum circuit length is given by:

Lm = 0.8 eUo Sph metres 2 ρ (1+m) Ia

the maximum length of an IT earthedcircuit is:c for a 3-phase 3-wire scheme

L max = 0.8 Uo ex Sph 2 ρ Ia (1+m)c for a 3-phase 4-wire scheme

L max = 0.8 Uo S1 2 ρ Ia (1+m)

For the case of a 3-phase 4-wire installationthe lowest value of fault current will occur ifone of the faults is on a neutral conductor. Inthis case, Uo is the value to use forcomputing the maximum cable length, and,

Lm = 0.8 Uo S1 metres 2 ρ (1+m) Ia(i.e. 50% only of the length permitted for a TNscheme). Reminder: there is no length limitfor earth-fault protection on a TT scheme,since protection is provided by RCDs of highsensitivity.In the preceding formulae:Lmax = longest circuit in metresUo = phase-to-neutral voltage (230 V on a230/400 V system)ρ = resistivity at normal operatingtemperature= 22.5 x 10-3 ohms-mm2/m for copper= 36 x 10-3 ohms-mm2/m for aluminiumIa = overcurrent trip-setting level in ampsor Ia = current in amps required to clear thefuse in the specified timem = Sph/SPESPE = cross-sectional area of PE conductorin mm2

S1 = S neutral if the circuit includes a neutralconductor.

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Id

PE CD

Id

AB

N

Id

PE

Id

N

fig. G58: calculation of Lmax. for an IT-earthed system, showing fault-current path for adouble-fault condition.

TablesThe following tables have been establishedaccording to the "conventional method"described above.The tables give maximum circuit lengths,beyond which the ohmic resistance of theconductors will limit the magnitude of theshort-circuit current to a level below thatrequired to trip the circuit breaker (or to blowthe fuse) protecting the circuit, with sufficientrapidity to ensure safety against indirectcontact. The tables take into account:c the type of protection: circuit breakers orfuses,c operating-current settings,c cross-sectional area of phase conductorsand protective conductors,c type of earthing scheme,c correction factor: table G59 indicates thecorrection factor to apply to the lengths givenin tables G43 to G46, when considering an ITsystem.

the following tables* give the lengthof circuit which must not beexceeded, in order that persons beprotected against indirect contacthazards by protective devices.

* The tables are those shown in Sub-clause 5.2 (tables G43to G46).However, the table of correction factors (table G59) whichtakes into account the ratio Sph/SPE, and of the type ofcircuit (3-ph 3-wire; 3-ph 4-wire; 1-ph 2-wire) as well asconductor material, is specific to the IT system, and differsfrom that for TN.

circuit conductor material m = S ph/S PE (or PEN)m = 1 m = 2 m = 3 m = 4

3 phases copper 0.86 0.57 0.43 0.34aluminium 0.54 0.36 0.27 0.21

3ph + N or 1ph + N copper 0.50 0.33 0.25 0.20aluminium 0.31 0.21 0.16 0.12

table G59: correction factors, for IT-earthed systems, to apply to the circuit lengths givenin tables G43 to G46.

ExampleA 3-phase 3-wire 230/400 V installation isIT-earthed.One of its circuits is protected by a circuitbreaker rated at 63 A, and consists of analuminium-cored cable with 50 mm2 phaseconductors. The 25 mm2 PE conductor is alsoaluminum. What is the maximum length ofcircuit, below which protection of personsagainst indirect-contact hazards is assured bythe instantaneous magnetic tripping relay ofthe circuit breaker?Table G44 indicates 617 metres, to whichmust be applied a correction factor of 0.36(m = 2 for aluminium cable).The maximum length is therefore 222 metres.

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6.3 high-sensitivity RCDsIEC 364-4-471 strongly recommends the useof a RCD of high sensitivity (i 30 mA) in thefollowing cases:c socket-outlet circuits for rated currentsof i 32 A at any location(1),c socket-outlet circuits in wet locations at allcurrent ratings(1),c socket-outlet circuits in temporaryinstallations(1),c circuits supplying laundry rooms andswimming pools(1),c supply circuits to work-sites, caravans,pleasure boats, and travelling fairs(1).This protection may be for individual circuitsor for groups of circuits,c strongly recommended for circuits of socketoutlets u 20 A (mandatory if they areexpected to supply portable equipment foroutdoor use),c in some countries, this requirement ismandatory for all socket-outlet circuitsrated i 32 A.(1) these cases are treated in delail in Chapter L Clause 3.


6.4 in areas of high fire-riskRCD protection at the circuit breakercontrolling all supplies to the area at risk, ismandatory in many countries. The sensitivityof the RCD must be i 500 mA.

fire-risk area


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6.5 when the fault-current-loop impedance is particularly highWhen, during the design stage of theinstallation, it is found that the fault-currentloop impedance of a circuit will be inevitablyhigh, so that the overcurrent protectioncannot be relied upon to operate within theprescribed time, the following possibilitiesshould be considered:

Note: this is also the case when one (of two)earth faults occurs at the end of a longflexible lead, for example.

Suggestion 1:instal a circuit breaker which has aninstantaneous magnetic tripping element withan operation level which is lower than theusual setting, for example: 2In i Irm i 4 In.This affords protection on circuits which areabnormally long. It must be checked,however, that high transient currents such asthe starting currents of motors will not causenuisance trip-outs.

2In i Irm i 4In

PE or PEN

unusuallylong cable

fig. G62: a circuit breaker with low-setinstantaneous magnetic trip.

Suggestion 2:instal a RCD on the circuit of low sensitivity(several amps to a few tens of amps, since itmust not operate for a first fault).If the circuit is supplying socket outlets, it will,in any case, be protected by a high-sensitivityRCD (i 30 mA).

phasesneutralPE

TN-S

fig. G63: RCD protection.

Suggestion 3:increase the size of the PE conductors and/orthe phase conductors, to reduce the loopimpedance.

fig. G64: improved equipotential bonding.

Suggestion 4:add supplementary equipotential conductors.This will have a similar effect to that ofsuggestion 3, i.e. a reduction in the earth-fault-loop resistance, while at the same timeimproving the existing touch-voltageprotection measures. The effectiveness ofthis improvement may be checked by aresistance test between each exposedconductive part and the local main protectiveconductor.For TN-C installations, bonding as shown infigure G52 is not allowed, and Suggestion 3should be adopted.

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7. residual current differential devices (RCDs)

7.1 descriptionprincipleThe essential features are showndiagrammatically in figure G65 below. Amagnetic core encompasses all the current-carrying conductors of an electric circuit andthe magnetic flux generated in the core willdepend at every instant on the arithmeticalsum of the currents; the currents passing inone direction being considered as positive,while those passing in the opposite directionwill be negative.In a normally healthy circuit (figure G65)i1 + i2 = 0 and there will be no flux in themagnetic core, and zero e.m.f. in its coil. Anearth-fault current id will pass through thecore to the fault, but will return to the source

via the earth, or via protective conductors in aTN-earthed system. The current balance inthe conductors passing through the magneticcore therefore no longer exists, and thedifference gives rise to a magnetic flux in thecore.The difference current is known as the"residual" current and the principle is referredto as the "differential current" principle.The resultant alternating flux in the coreinduces an e.m.f. in its coil, so that a currenti3 flows in the tripping-device operating coil. Ifthe residual current exceeds the valuerequired to operate the tripping device, thenthe associated circuit breaker will trip.

P

Ni1 i2

i3S

N

Ø

id

fig. G65: the principle of RCD operation.

7.2 application of RCDs

earth-leakage currents exist whichare not due to a fault, as well astransient overvoltages, either or bothof which can lead to unwantedtripping by RCDs.Certain techniques have beendeveloped to overcome theseoperational problems.

permanent earth leakagecurrentsEvery LV installation has a permanentleakage current to earth, which is mainly dueto imperfect insulation, and to the intrinsiccapacitance between live conductors andearth.The larger the installation the lower itsinsulation resistance and the greater itscapacitance with consequently increasedleakage current.On 3-phase systems the capacitive leakagecurrent to earth would be zero if theconductors of all three phases had equalcapacitance to earth, a condition that cannotbe realized in practical installations. Thecapacitive current to earth is sometimesincreased significantly by filtering capacitorsassociated with electronic equipment(automation, informatics and computer-basedsystems, etc.). In the absence of more-precise data, permanent leakage current in agiven installation can be estimated from thefollowing values, measured at 230 V 50 Hz,and abstracted from "Bulletin de l'UTE"April 1992.Fax terminal 0.5 to 1.0 mAIT* workstation 1 to 2 mAPrinter (IT*) < 1 mAIT* terminal 1 to 2 mAPhotocopier 0.5 to 1.5 mA* Information Technology.

transient leakage currentsThe initial energization of the capacitancesmentioned above gives rise to high-frequencytransient currents of very short duration,similar to that shown in figure G66. Thesudden occurrence of a first-fault on an IT-earthed system also causes transient earth-leakage currents at high frequency, due to thesudden rise of the two healthy phases tophase/phase voltage above earth.

10 µs (f = 100 kHz)

t

100%90%

10%

ca.0.5 µs

60%

fig. G66: standardized 0.5 µs/100 kHzcurrent transient wave.

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7. residual current differential devices (RCDs) (continued)

7.2 application of RCDs (continued)

influence of overvoltagesElectrical power networks are subjected toovervoltages of various origins; atmospheric,or due to abrupt changes of system operatingconditions (faults, fuse operation, switching,etc.). These sudden changes often causelarge transient voltages and currents insystem inductive and capacitive circuits,before a new stable state is reached.Records have established that, on LVsystems, overvoltages remain generallybelow 6 kV, and that they can be adequatelyrepresented by the conventional 1.2/50 µsimpulse wave (figure G67).These overvoltages give rise to transientcurrents represented by a current impulsewave of the conventional 8/20 µs form,having a peak value of several tens ofamperes (figure G68). The transient currentsflow to earth via the capacitances of theinstallation surge arresters or through aninsulation failure.

electromagnetic compatibilityThe high-frequency (or unidirectionalimpulse) transient overvoltages and currentsmentioned above, together with otherelectromagnetic disturbance sources(contactor coils, relays, dry contacts),electrostatic discharges, and radiatedelectromagnetic waves (radio, ignitionsystems, etc.) are part of the increasinglyimportant field of EMC (electromagneticcompatibility). For further details, theTechnical publications nos. 120 and 149, byMerlin Gerin, may be consulted.It is essential that RCDs be immune topossible malfunction from the effects ofelectromagnetic-surge disturbances. Inpractice, the levels shown in table G70 arecomplied with in design and manufacturingspecifications*.* Merlin Gerin products.

implementationc every RCD installed must have a minimumlevel of immunity to unwanted tripping inconformity with the requirements of tableG70. RCDs type "S" or time-delay settinglevels I or II (see figure G36) cover alltransient leakage currents, including those oflightning arresters (see installation layouts inChapter L, Sub-clause 1.3) of a duration lessthan 40 ms,c permanent leakage currents downstream ofa RCD must be studied, particularly in thecase of large installations and/or where filtercircuits are present, or again, in the case ofan IT-earthed installation. If the capacitancevalues are known, the equivalent leakage

fig. G67: standardized voltage-impulsewave 1.2/50 µs.

fig. G68: standardized current-impulsewave 8/20 µs.

fig. G69: standardized symbol used insome countries, to indicate proof againstincorrect operation due to transients.

current for the choice of the sensitivity of aRCD is: i mA* = 0.072 C at 50 Hzi mA = 0.086 C at 60 Hzwhere C = capacity (in n F) of one phase toearth.Since RCDs complying with IEC and manynational standards may operate within therange 0.5 I∆n - I∆n for a nominal rating of I∆n,the leakage current downstream of a RCDmust not exceed 0.5 I∆n.The limitation of permanent leakage currentto 0.25 I∆n, by sub-division of circuits, will, inpractice, eliminate the influence of allcorresponding current transients.For very particular cases, such as theextension, or partial renovation of extendedIT-earthed installations, the manufacturersmust be consulted.

i mA* = 230 V x 100 π x 103 C (n F) 109

i mA* = 0.072 C (n F) at 50 Hz

t

U

1.2 µs 50 µs

0.5 U

U max

t

0.5

0.9

0.1

8 µs

20 µs

I

table G70: electromagnetic compatibility withstand-level tests for RCDs.

disturbance type of test required withstand quantityovervoltage 1.2/50 µs impulse 6 kV peaktransient current 0.5 µs/100 kHz impulse 200 A peak*

8/20 µs impulse 200 A peak60 A peak for 10 mA RCDs5 kA peak for types "S"or time-delayed models (see note)

switching repetitive transient bursts IEC 801-4 4 kVstatic electricity electrostatic discharges IEC 801-2 8 kVradiated waves radiated electromagnetic fields IEC 801-3 3 V/m

* for RCDs having I∆n < 10 mA this test is not required (IEC 1008-1).Note: Time-delayed RCDs are normally installed near the service position of installations, where current surges of external originare the most severe. The 5 kA peak test reflects this high-performance duty requirement.

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direct current componentsAuxiliary d.c. supplies for control andindication of electrical and mechanicalequipment are common, and certainappliances include rectifiers (diodes, triacs,thyristors).In the event of an earth fault downstream of arectifier, the fault current can include a d.c.component.The risk depends on the level of insulation ofthe d.c. circuits in an appliance, and eachcase must be considered individually.Problems of this kind generally concernindustrial applications.The IEC classifies RCDs according to theirability to function correctly in the presence ofd.c. components in the residual current.

3 classes are distinguished:Class AC: operates due to a.c. current only.Class A: operates if residual current consistsof uni-directional pulses.Class B: operates on pure d.c.Note:For general use Class AC RCDs are normally installed.Class A are available for specific requirements as a specialvariation of Class AC devices.

recommendations concerningthe installation of RCDs withseparate toroidal currenttransformersThe detector of residual current is a closedmagnetic circuit (usually circular) of very highmagnetic permeability, on which is wound acoil of wire, the ensemble constituting atoroidal (or ring-type) current transformer.Because of its high permeability, any smalldeviation from perfect symmetry of theconductors encompassed by the core, andthe proximity of ferrous material (steelenclosure, chassis members, etc.) can affectthe balance of magnetic forces sufficiently, attimes of large load currents (motor-startingcurrent, transformer energizing current surge,etc.) to cause unwanted tripping of the RCD.Unless particular measures are taken, theratio of operating current I∆n to maximumphase current Iph (max.) is generally lessthan 1/1,000.This limit can be increased substantially(i.e. the response can be desensitized) byadopting the measures shown in fig. G71,and summarized in table G72.

Centralize the cables in the ring core

Use an oversized magnetic ring core

Insert a tubular magnetic screen.

L

L = twice the diameter of the magnetic ring core

fig. G71: means of reducing the ratioI∆n/Iph (max.).

measures diameter (mm) sensitivitydiminutionfactor

careful centralizing of cables through the ring core 3oversizing of the ring core ø 50 > ø 100 2

ø 80 > ø 200 2ø 120 > ø 200 6ø 50 4ø 80 3ø 120 3ø 200 2

These measures can be combined. By carefully centralizing the cables in a ring core of 200 mmdiameter, where a 50 mm core would be large enough, and using a sleeve, the ratio 1/1,000could become 1/30,000.

use of a steel or soft-iron shielding sleevec of wall thickness 0.5 mmc of length 2 x inside diameter of ring corec completely surrounding the conductors and overlappingthe circular core equally at both ends

table G72: means of reducing the ratio I ∆n/Iph (max.).

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7. residual current differential devices (RCDs) (continued)

7.3 choice of characteristics of a residual-current circuit breaker (RCCB - IEC 1008)rated currentThe rated current of a RCCB is chosenaccording to the maximum sustained loadcurrent it will carry, estimated in accordancewith the methods described in Chapter BSub-clause 4.3.c if the RCCB is connected in series with,and downstream of a circuit breaker, therated current of both items will be the same,i.e. In u In1* (fig. G73 (a)),c if the RCCB is located upstream of a groupof circuits, protected by circuit breakers, asshown in fig. G73 (b), then the RCCB ratedcurrent will be given byIn u ku x ks (In1 + In2 + In3 + In4).* Some national standards include a thermal withstand testat a current greater than In in order to ensure correctcoordination of protection.

In1

In

(a)

In1

In

In2 In3 In4

(b)

fig. G73: residual current circuit breakers(RCCBs).

electrodynamic withstandrequirementsProtection against short-circuits must beprovided by an upstream SCPD (Short-CircuitProtective Device) but it is considered thatwhere the RCCB is located in the samedistribution box (complying with theappropriate standards) as the downstreamcircuit breakers (or fuses), the short-circuitprotection afforded by these (outgoing-circuit)SCPDs is an adequate alternative.Coordination between the RCCB and theSCPDs is necessary, and manufacturersgenerally provide tables associating RCCBsand circuit breakers or fuses (see table G74).

Coordination of circuit breakers and RCCBs- max. short-circuit current in kA (r.m.s.)upstream circuit breaker type C60a C60N C60H C60L NC100H NC100Ldownstream 2 p 25 A 10 16 20 45 45RCCB 40 A 10 16 20 40 45

63 A 16 20 30 5 4580 A 5

4 p 25 A 5 8 10 25 2240 A 5 8 10 25 2263 A 8 10 15 5 22

Coordination of fuses and RCCBs- max. short-circuit (not applicable to aM fuses)upstream fuses gl(not applicable to aM fuses) 16 A 25 A 32 A 40 A 50 A 63 A 80 A 100 Adownstream 2 p 25 A 100 100 100RCCB 40 A 100 100 80 10 (1)

63 A 80 50 30 20 10 (1)

80 A 30 204 p 25 A 100 100 100 10 (1)

40 A 100 100 80 10 (1)

63 A 80 50 30 20 10 (1)

80 A 30 20 10 (1)

table G74: typical manufacturers coordination table for RCCBs, circuit breakers,and fuses.(1) A 100 A fuse with several RCCBs downstream: the thermal withstand of the RCCBs is not certain.

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