Role and Protection of Neutral

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Role and protection ofNeutral in a LV installation

Technical article

Protection of goods and people

1

Contents

1. Introduction.............................................................. 2Function of the Neutral in LV distribution ......................................... 2Purpose of the paper ...................................................................... 2

2. The IEC 60364 standard and the Neutralconductor ..................................................................... 3

2.1 Protecting the Neutral against overloadsand short-circuits ......................................................................... 3Overloads and short-circuits ........................................................... 3Neutral conductor with a smaller cross-section than the phases .... 3Multipole breaking .......................................................................... 4Conclusion ...................................................................................... 5

2.2 Neutral and protection against insulation faults .................. 5Effect of Earthing systems .............................................................. 5TT System ...................................................................................... 6TN-S System (separate N and PE) ................................................. 7TN-C System (PEN) ....................................................................... 8IT System (ungrounded Neutral) ..................................................... 9

3. Development of the IEC 60364 standard:harmonics .................................................................... 10Influence of harmonic currents ....................................................... 10

3.1 Harmonics and protection of Neutral .................................... 11Switchgears to be installed ............................................................. 12Harmonic currents created by lighting ............................................ 13

3.2 Harmonics and Earthing System Arrangement ................... 15Avoid using the TN-C system if harmonics are present .................. 15Take care with source coupling in TN-S when harmonicsare present ..................................................................................... 15

4. Conclusions ............................................................. 17

2

1. Introduction

Function of the Neutral in LV distribution

When distributed, the Neutral in LV is mainly used to ensure a 230 V single-phase voltage to supply circuits such as lighting or control and monitoringauxiliaries, in addition to the 400 V three-phase voltage.

In a well-designed three-phase installation, with the exception of single-phase terminal distribution circuits, the Neutral conductor does not conveycurrent (or only very little: less than 15 % of phase current). Its potential withrespect to the ground is consequently often zero.

In practice this conductor is very rarely “neutral” and can be a source ofdisturbance for the operator if no precautions are taken.

Purpose of the paperThis paper aims at treating the Neutral conductor mainly in an upstreamthree-phase distribution, without going into the specific constraints of termi-nal single-phase distribution.The following points will be defined: the protection needs inherent in this conductor, i.e.:- overload and short-circuit protection of the Neutral conductor,- breaking of the Neutral conductor, if required, and breaking methods. the effect on these needs of the specific function of the Neutral in theEarthing Systems, i.e. the function of the Neutral conductor to ensure:- proper operation of the installation (safety with power on)- proper protection of persons in contact with de-energised parts of theinstallation (safety with power off).The installation rules laid down in the IEC 60364 standard consequentlyprovide a detailed definition of needs which result in the necessity (or not) toprotect, break or disconnect this conductor.Finally the increasing development of loads towards a multiplication of non-linear loads generates an harmonic current flow.Harmonic currents have not yet been fully taken into acount in the IEC 60364standard but are being studied in section 444.The effect of these phenomena on the TN-C and TN-S systems will thus bedescribed.In the conclusion, evaluation of these various constraints will show that afour-pole circuit-breaker with sudden multipole breaking guarantees properoperation of installations.

3

2. The IEC 60364 standard and the Neutral conductor

2.1 Protecting the Neutral against overloads and short-circuits

Overloads and short-circuitsIn the event of overloads on the Neutral or of a Phase to Neutral short-circuit,the same fault current, ld, flows through the conductors. Consequently 2possibilities must be examined:

Neutral conductor and phases with the same cross-sectionIf Sn= Sph, the Neutral conductor is protected in the event of a phase toNeutral fault by the phase overcurrent protection.The IEC 60364 standard then stipulates in § 431.2.1:"a) When the cross-section, Sn, of the Neutral conductor is at leastequivalent to the cross-section, Sph, of the phase conductors, there is noneed to provide an overcurrent detector or a breaking device on the Neutralconductor."

In point of fact, this type of protection is not always reliable or economic. Theuse of four-pole 4P 3D-N/2 circuit-breakers ("Half Neutral") is an optimumsolution which also guarantees breaking (often recommended) of the Neutralconductor.

Imax

Sn = SphN 1 2 3

Fig.3: Sn < Sph Fig. 4: Breaking of theNeutral is not compulsory

Fig. 5: However protec-tion by 4P 3D-N/2 circuit-breaker is an optimumsolution

In << Iph

4P3D-N/2

Neutral conductor with a smaller cross-section than the phasesThe Neutral conductor only conveys currents when there is a high unbalanceand even then these currents rarely exceed 10 % of phase current in well-designed installations. Consequently it is tempting and economicallyadvantageous to reduce its cross-section. The cross-section Sn = Sph / 2 isthe one normally chosen.Thus if Sn = Sph / 2, the Neutral conductor must be protected against Phaseto Neutral faults by a specific overcurrent protection. If this protection actsdirectly on the phases, protection of the Neutral is guaranteed without needto break.The IEC 60364 standard then stipulates in § 431.2.1:"b) When the cross-section, Sn, of the Neutral conductor, is smaller than thecross-section, Sph, of the phase conductors, it is necessary to provide anovercurrent detector on the Neutral conductor, adapted to the cross-sectionof this conductor. This detector must cause the Phase conductors, but notnecessarily the Neutral conductor, to break."

Sn = SphN 1 2 3

Ph

N

Id

Id

Fig.1: Sn = Sph Fig 2: Phase protection on the Phase to Neutral fault

4

Nevertheless the IEC 60364 standard accepts that: if the current normally flowing in the Neutral conductor is small (around10 % of Phase current), which means that there is no overload risk for thisconductor, then the phase overcurrent protection also guarantees protection of theNeutral conductor which does not need a specific protection.The existence of harmonic currents means that this condition may be difficultto satisfy with a TN type Earthing System.

Multipole breakingThe Neutral conductor is used to supply single-phase loads and, assuch, its upstream breaking must coincide with that of the phases.

Need for Neutral continuity in the case of single-phase loadsIf the Neutral is broken but the Phases are not, the Neutral can nolonger perform its original function, i.e. allow current to return to thesource and supply single-phase loads with 230 V. This accidentalbreaking may have serious consequences for these loads. Forsimplicity’s sake we shall consider the diagram in figure 6 whichcontains 2 single-phase loads of different impedances on 2 phases.

Fig. 6: Neutral breaking with single-phase loads

U3U1

U31 = 400 V

load 1 load 2impedance Z1 impedance 9 Z1

Fig. 7 : equivalent diagram to figure 6

For example, if we take the values of the impedances in figure 7, thiscorresponds to moving the potential of the neutral point.The potential of Phase 1 compared with the Neutral moves from 230 to 40 Vand the potential of Phase 3 from 230 to 360 V: consequently the potential ofPhase 2 moves to 347 V. Thus the Phase with the greatest load is inundervoltage and the phase with the lowest load in overvoltage. It is thusnecessary to break the Phases at the same time as the Neutral conductor.

123N

breaking

Y

load 1 : load 2 :impedance Z1 impedance 9 Z1

The current delivered by Phase 1 in load 1 and by Phase 3 in load 2 cannotreturn to supply via the Neutral, and thus moves from one phase to anotherthrough the two loads and the Neutral. This is equivalent to the voltagedivider in figure 7.

Take two loads connected in series. The voltage attheir terminals is a Phase to Phase voltage of400 V. We have a voltage divider, thus: U1 and U3in proportion to the impedances Z1 and Z2.

5

2. The IEC 60364 standard and the Neutral conductor (cont.)

Prevention of problems relating to single-phase loadsTo avoid such problems, the IEC 60364 standard stipulates in § 431.3 that:"When breaking of the Neutral conductor is specified, breaking and closing(making) of the Neutral conductor must be such that the Neutral conductor isnever broken before the phase conductors and that it is closed at the sametime or before the phase conductors."Thus for the Neutral conductor: breaking must take place at the same time or after phase breaking closing must take place at the same time or before phase breaking.

ConclusionThe Neutral conductor is protected by: short-circuit protection devices (SCPD) protecting the phase conductors ifthe Neutral and phases have the same cross-sections a “Half-Neutral” protection for smaller phase cross-sections.In practice:

Fuse protectionSwitchgear with fuses fitted with a compulsory striker must be used on theNeutral. These switchgear operate in such a manner that if the fuse on theNeutral conductor blows, the striker trips a multipole breaking system.However this solution is complex, space-consuming and costly and also callsfor a permanent standby supply of fuses with strikers of all ratings.

Circuit-breaker protectionArticle 530.3.1 of the IEC 60364 standard stipulates that:"All the moving contacts of all the poles of the multi-pole devices must bemechanically coupled so that their opening and closing is virtuallysimultaneous."In this case a four-pole circuit-breaker must be used which ensuressimultaneous opening and closing of phases and Neutral.We then have the multipole breaking and sudden closing required toguarantee proper operation of downstream single-phase loads.

2.2 Neutral and protection against insulation faults

Effect of Earthing systemsThe IEC 60364 standard has laid down installation rules to protect personsagainst electrical shocks. These rules stipulate the use of standardisedEarthing systems of the TT, TN or IT type.

The Earthing system defines the grounding mode: of the Neutral of the secondary of the HV/LV transformer which may begrounded (directly or via an impedance) or ungrounded of the frames of the installation which are always connected to the buildingground where they are installed, either directly or via the Neutral conductor.The functions and treatment of the Neutral conductor depend on theinstallation’s Earthing system. When the Neutral is distributed we thus needto check:

with power on, the effect of the Earthing system on: its protection its breakingif an insulation fault occurs.

with power off, that the installation or part of the installation which is de-energised is, and will continue to be, safe. Particular care should be taken tocheck that a medium voltage fault will not generate risks on the de-energisedLV part. If the Neutral is a live conductor, it needs to be disconnected.

6

TT SystemCharacteristics the Neutral (N) is directly grounded at transformer level (Neutral groundconnection) the installation frames are connected by a protective conductor (PE) to aground connection which may or may not be separate from the aboveconnection this system calls for the detection of insulation faults using a RCD(Residual Current Device). This device causes the overcurrent protections tobreak as these faults are normally too small to be directly tripped butnevertheless generate a dangerous contact voltage (Uc).

RA = 20 ΩRn = 10 Ω

Figure 8: example of a TT System

AdvantageSmall fault current (limited by the ground resistances) and thus limiteddestructive effectEffect on the Neutral conductor: energised installation protection: no effect as the insulation fault current (small) does not flowthrough this conductor Neutral breaking: no effect for the same reason de-energised installation disconnection compulsory as in the IEC 60364 § 536.2.Indeed in the event of overvoltage on the MV (transformer breakdown orfault) the Neutral potential rises causing a very dangerous potential toappear (a few hundred volts) between the Neutral and the application ground(fig. 9).

Figure 9: : effect of a fault on MV: dangerous contact risk

Consequently a person performing maintenance on the machine may in thiscase be in direct contact with the Neutral conductor at this high voltage andthe risk is at its greatest. Installation standards, in particular the IEC 60364§ 536.2 take account of this risk by stipulating disconnection of Neutralconductors. If disconnection is performed by a multi-pole breaking functionensuring both simultaneous breaking then disconnection of the phases andthe neutral, the result is increased safety of maintenance with power off.Disconnection is thus a necessity. A four-pole circuit-breaker enablingmultipole breaking and disconnection naturally meets all the requirements ofthe IEC 60364 standard.

7

TN-S System (separate N and PE)Characteristics the Neutral (N) and the protective conductor (PE) are directly grounded attransformer level (Neutral ground connection) the Neutral (N) and the protective conductor (PE) are separate in the event of an insulation fault, this system requires breaking of theovercurrent protections, which in turn assumes control of fault loopimpedances (ABCDE) to be certain of trip release effect if an insulation fault occurs, the fault current is very high and thusdestructive.


In this case the maintenance operator will be in direct contact with theovervoltage. We strongly recommend disconnection and thus breaking of theNeutral in the TN-S system.Example 2 - High rise buildingsIt is harder to guarantee the quality of grounding connections in the variousstoreys of high rise buildings. The potential of the frames moves away fromthe potential of the ground at the bottom of the foundations due to theexceptionally long cables used. To avoid generating dangerous situations, werecommend breaking the Neutral.

Figure 11: consequence of a MV lightning stroke

AdvantageNo additional switchgear required. Protection is directly provided by theSCPD (provided that the condition governing maximum cable length toguarantee loop impedance allowing tripping is respected).Effect on the Neutral conductor: energised installation protection: no effect as the fault current does not flow through thisconductor. breaking: no effect for the same reason de-energised installation disconnection: the IEC 60364 standard recommends disconnection in § 536.2.Some countries make disconnection a requirement on the basis that, just asin TT, the potential of this conductor cannot be guaranteed. The 2 examplesbelow highlight these problems.Example 1 : If a lightning wave (frequency around MHz) reaches the MV itwill not be stopped by grounding the transformer (as the inductivecomponent - Lω - of the grounding Neutral connection is dominant at thesefrequencies since ω = 2π f) and the potential (dangerous) will beautomatically transmitted to the Neutral conductor.

Figure 10: TN-S System

123NPE

Uc

Uc

8

TN-C System (PEN)Characteristics the Neutral is directly grounded at transformer level (Neutral groundconnection) the Neutral (N) and the protective conductor (PE) are combined in onesingle conductor (PEN). in the event of an insulation fault, this system requires (just like the TN-S)breaking of the overcurrent protections, which in turn assumes control of faultloop impedances (ABCDE) to be certain of trip release effect if an insulation fault occurs, the fault current is very high and thusdestructive.

Figure 12: TN-C System

Neutralgroundconnection

AdvantageNo additional switchgear required, and 4-conductor instead of 5-conductordistribution. Protection is directly provided by the SCPDs (provided that thecondition governing maximum cable length to guarantee loop impedanceallowing tripping is respected).Effect on the Neutral conductor: energised installationThe Neutral and the PE are combined:=> the fault currents flow through the Neutral=> the normal Neutral currents flow through the PE.This will give rise to a certain number of problems (see following chapter). protection: forbidden as the PEN, in its capacity as PE protectiveconductor, has to withstand all normal and abnormal currents (IEC 60364 §543.1.), breaking (1): forbidden as the PEN, in its capacity as protective conductor,must never be broken. de-energised installation disconnection: forbidden just like breaking. This calls for a systematic,multiple grounding of the PEN conductor in order to guaranteeequipotentiality.(1) the PEN must have a "mechanical" resistance (to prevent its rupture) witha cross-section of at least 10 mm² in Cu and 16 mm² in alu (IEC 60364§ 543.1.).

9


IT System (ungrounded Neutral)Characteristics the Neutral (N) is ungrounded (in fact it is connected by a capacitiveleakage impedance due to cables of around 3500 Ω/km) the installation frames are connected by a protective conductor (PE) to aground connection unlike the last two systems, this system only stipulates breaking the powersupply if two insulation faults occur. This is because the first fault, limited bythe ground resistances, presents no risk for persons but must be detectedand eliminated.

Rn RA

Figure 13: IT system in a first fault situation

The Neutral of B is designed for 100 A. The phase of A, designed for 1000 A,will not protect it, hence: protection: compulsory breaking: compulsory (multipole) de-energised installation disconnection.As the Neutral is not grounded, the effects of MV overvoltages are greaterthan with a TN/TT grounding system, and disconnection is compulsory.(IEC 60364 § 431.2.2).

ConclusionThe effect of Earthing Systems on a Neutral conductor is two-fold: generally disconnection is required as soon as the Neutral is broken more specifically, in the IT system, the Neutral conductor must beprotected separately from the Phases.

AdvantageContinuity of service on the first fault. However to preserve this advantageContinuous Insulation Monitors (CIM) must be used (recommended by theIEC 60364 and a requirement of certain national standards) together withFault Tracking Devices (FTD).Effect on the Neutral conductor: energised installationIt is recommended not to distribute the Neutral.If a double fault occurs, one of which concerns the Neutral conductor, theconductor may be subjected to overload independently from the currentflowing in the Phases (see fig. 14).

Figure 14: IT system in a double fault situation

10

3. Development of the IEC 60364 standard: harmonics

Influence of harmonic currentsEffects of order 3 and multiple of 3 harmonicsHarmonics are generated by the non-linear loads of the installation(computers, ballast lighting, rectifiers, power electronic choppers) and canproduce high currents in the Neutral.In particular order 3 or multiple of 3 harmonics of the three Phases have atendency to cumulate in the Neutral as: fundamental currents are out-of-phase by 2π/3 so that their sum is zero on the other hand, order 3 harmonics of the three Phases are alwayspositioned in the same manner with respect to their own fundamental, andare in phase with each other (fig 15).

I1 H3+

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I2 H3

I3H3

I1 H1

I2 H1

I3 H1

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3

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Figure 15: order 3 harmonics are in phase and cumulate in the Neutral

These order 3 harmonic generators are increasingly numerous, and the IECis looking into how this problem can be handled. In the absence of standards,the following recommendations are made:

Management of harmonic current in the Neutral conductorThe installation rules – particularly IEC 60364 - take into account valorisationof harmonic current H3 in the Neutral conductor for correct sizing of liveconductors; this depends on THDiH3 in the phases:THDiH3 < 15 %: we can consider that there is no significant H3 current value.There is no specific protection or sizing of the Neutral conductor.15 % < THDiH3 < 33 % the H3 current is significant, which implies 2constraints: constant currents in the 4 conductors: consideration in the thermal sizing ofcables (normally a derating of around 0.85), sizing of the Neutral conductor at least equal to that of the Phase, andsetting of the Neutral conductor protection is equivalent to that of the Phase.33 % > THDiH3 the H3 current is very high: the 2 above-mentionedconstraints become: thermal sizing of Phase conductors remains the same (same derating ofaround 0.85), sizing of the conductor considering the maximum neutral current (1.7 timesthat of phases), but without allowing for thermal derating and specific Neutralconductor protection (greater than that of the Phase).

Positioning problems in electrical distributionThe influence of non-linear loads generating H3 currents in the neutral isparticularly great at the single-phase load grouping point. - Downstream, in final distribution, the conductors of these single-phaseloads require no specific oversizing arrangements - Upstream, in power distribution, as a rule three-phase linear loads suppliedin parallel lead to a small to very small valorisation of the H3 currentcompared to phase current and thus require no specific precautions (currentin neutral around 50 % of phase current).As a result, these problems are more particularly significant in the currentvalue range of 100 A and 630 A, i.e. Medium Power Distribution.

11

3.1 Harmonics and protection of Neutral

UpstreamMain LV board and PowerDistribution, the averageTHDI is normally less than15 %

DB

1038

16

At intermediate levelMedium PowerDistribution, on feedersgrouping non-linear single-phase loads, the THDI canbe very high (> 50 %)

DownstreamFinal DistributionSizing of single-phasefeeders is not specificallyaffected by the THDi

Typical diagram of LV installation with the sum up of single phase non-linear loads in the sub-distributionswitchboard.

M

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NS4004P 3D400 A

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NT10 NT12

1000 kVA GE GE

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Switchgears to be installedProtection in case of a high H3 harmonic level (15 % to > 33 %)Switchgear to be implemented.THDi > 33 %The current flowing in the Neutral conductor is greater than that of the phaseconductors (1.7 maximum)For protection of feeders with high H3 current, an offer of the Compact NSOSN (OverSized Neutral) type must be implementedThis consists of installing a Compact NS circuit-breaker sized in rating for the Neutral current (i.e. 1.6 times phase current) whose Phase protection is set for the phase current, and (if necessary), the Neutral protection is set specifically at 1.6 timesphase protection.The following table gives the various possibilities

Compact NS OSN Feeder I phase25 A 63 A 100 A 160 A 200 Ato to to to to63 A 100 A 160 A 250 A 400 A

THDi of around 33 % but unknownInstallation rules allow 2 solutions sizing all the conductors for the maximum constraint: safe but costly andtotally improbable. sizing the installation normally and guarding against Neutral conductoroverload risks, i.e. providing Neutral protection Compact NS 4P, 4D monitoring THDi (particularly for the third-order harmonic) of the networkFor feeder protection, implement a Compact NS 4P 4D sized for thecharacteristics of the feeder to be considered equipped with acommunication module. This module is used to directly retrieve THDiinformation on a PM measurement module or centrally on a dataconcentrator. A dedicated software for use of these data on a PC is available.

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Lamp type Typical Setting mode Typical THDiH3

power

Incandescentlamp 100 W Light dimmer 5 to 85 %with dimmer

ELV halogen lamp 25 W Electronic ELV 5 %transformer

Fluorescent tube 100 W Magnetic ballast 10 %

< 25 W Electronic ballast 85 %

> 25 W + PFC 30 %

Discharge lamp 100 W Magnetic ballast 10 %

Electrical ballast 30 %

Harmonic currents created by lightingThe principle of an electronic ballast is to supply the fluorescent tube with ahigh frequency AC voltage. It consists of an AC-DC converter (rectifier)associated with a DC converter generating the high frequency voltage (20 to60 kHz).For low power lamps (particularly fluo-compact lamps), the rectifier draws avery deformed network current, the standard form of which is shown below:

The third-order harmonic can reach 85 %.

For devices of higher power (> 25 W), the rectifier is equipped with a filteringor power factor correction device (Power Factor Correction, PFC), used toreduce the third-order harmonic to less than 30 %.

H3 harmonics created by lighting

0.03 0.035 0.04 0.045 0.05 0.055 0.06 0.065

0

T(s)

3. Development of the IEC 60364 standard: harmonics (cont.)

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Example of distribution of H3 currents and loads in office premises

Load type Number Unit current drawn Total current THDIH3

Total third-order(A) (A) (%) harmonic current

(A)

PC 10 0.5 5 85 4.25

PC + printer 5 1.45 7.3 35 2.55

Photocopier 2 0.32 0.64 65 0.42

Fluo tubes 20 0.2 4 25 1

Heating 10 10 0 0

Total 27 8.2

Calculation of THDiH3 gib=ves:

H3 level is thus considerably reduced: the over-sizing of Neutral conductor isvery rare and it is often due to specific applications (computer room,greenhouse...).

i h3

(%) = 100. = 0.308.227

15

Avoid using the TN-C system if harmonics are presentThe Neutral and PE are combined in a PEN which is connected to as manypoints as possible on the building’s metal structure in order to guaranteeequipotentiality. This gives rise to two types of problems: electromagnetic radiation of stray currents.The currents flowing in the PEN and in particular the 3rd order harmonics,may take uncontrolled paths (frames of communicating equipment cables - avariety of conductive components). Consequently the vectorial sum of thecurrents ceases to be zero in the cableways (3Ph + PEN) andelectromagnetic radiation occurs. The same phenomenon is observed inconductive structures.To give an example, a cathode tube (TV, microcomputer) is disturbed by a0.7 A/m magnetic field which is the field generated by a 10 A current at adistance of 2 metres! This value can be easily reached.The TN-C system is thus unsuited to modern ElectroMagnetic Compatibility(EMC) requirements. voltage drop in the PENThe flow of currents in the frames and structures and the unbalance currentsresulting from the single-phase loads cause potential drops in the PEN whichaffect its equipotentiality.

Figure 16: insulation fault with Neutral not broken

Take care with source coupling in TN-S when harmonics arepresentFigures 17 and 18 show the diagram of an electrical distribution supplied by2 normally separate sources (coupling normally upstream) where eachsource is grounded using a TN-S system. This distribution can supply 2separate private networks. Fire protection is provided by GFP typeswitchgear (GFP: Ground Fault Protection). general problem relating to source coupling in the event of aninsulation fault and Neutral not brokenWhen two sources, S1 and S2, are coupled using the TN-S system withoutbreaking of the Neutral (fig. 17), if an insulation fault occurs, part of the faultcurrent (ld1) returns normally to the source via the PE, but another part (ld2)may return to the source via the metal structures.This ld2 current is detected by the GFP protection of source S2.According to current distribution, the protection of source S1 where the faultoccurred may not trip as the sensitivity threshold is no longer reached.However the ld2 current may, on the contrary, disturb the GFP and causeuntimely tripping if its threshold is lower than that of the coupling switch.‘

3. Development of the IEC 60364 standard: harmonics (cont.)

3.2 Harmonics and Earthing System Arrangement

16

NeutralId2

GFP

S1

PE Ground busLoads

GFP

S2

Serviceground

Serviceground

NeutralId2

GFP

S1

PE Ground busLoads

GFP

S2

Serviceground

Id2

Id1

Serviceground

Figure 17: insulation fault with Neutral not broken

Figure 18 : harmonic current without insulation fault with Neutral not broken

problem relating to source coupling when harmonics are presentLet us now reconsider the two sources, S1 and S2, still coupled in TN-Swithout breaking of the Neutral but this time without an insulation fault andwith the presence of 3rd order harmonics (fig. 18).These harmonics cumulate in the Neutral which conveys a non-negligiblecurrent. As a result of the Neutral connections with the metal frames, thiscurrent may return to the source S1 via the Neutral conductor and the PE ofthe installation, and a non-negligible current may thus flow in the structureseven if the 2 networks are independent.This natural “pollution” of S2 can be considerable and serious if theequipment supplied is sensitive.For example, if in figure 18 a distribution of 3000 A is considered, 5 % of 3rdorder harmonics cumulating in the Neutral generate a flow of 3 x 3000 x 5 %= 450 A. With 10 % of current returned via the coupling, this gives a currentof 45 A in the PE and structures, a zero sequence current, which thusgenerates electromagnetic radiation.Multipole breaking on the coupling switch will eliminate this harmonicpollution. Moreover if breaking is performed on both the source circuit-breakers, proper operation of the installation is guaranteed in allcircumstances.A transfer switch of 4-pole is recommended by IEC 60364 § 444.4.9.

DB

1038

11D

B10

3805

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4. Conclusions

This concise study shows that in all three-phase distributions:The TN-C system requires Neutral continuity, as the Neutral is also the PEprotective conductor. This Earthing System is not recommended if thedevices supplied are harmonic current generators as is more and morefrequently the case.For the other Grounding Systems, specific protection, breaking anddisconnection of the Neutral conductor may not be necessary. However forthe sake of: safety with power off, disconnection is recommended and in some cases isa requirement proper operation, multipole breaking is recommended.As a result, protection of circuits using a four-pole circuit-breaker/disconnector guarantees the quality of the electrical power supply duringoperation, and safety with power off.

As standards, specifications and designs develop from time to time, always ask for confirmation ofthe information given in this publication.

Published by: Schneider Electric Industries SASPrinted by:

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Role and Protection of Neutral

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