Effect of multiple neutral earthing systems on consumers installations Originally published in Vector August 1983 Anhistoricalexaminationintoelectricalfaults hasrevealedthatthemajorcausesofearth leakagetrip-outsarefaultsinelectrical appliancesandflexiblecords.Thesecondmost prevalentfaulthasbeenfoundtobeinadvertent connectionsbetweentheconsumersneutral conductor and earth. Thearticledescribesaninvestigationintothe effects of multiple neutral earthing systems upon theefficacyofsensitiveearthleakageprotection devices installed in the consumers premises. Inthecaseofinadvertentconnectionbetween neutralandearth,thisconditionwouldonlycause the earth leakage relay or circuit breaker to trip if the neutralconductorwasatapotentialhighenoughto drivethenecessary20to30 mA required to trip the device. Dependingupontheneutralconductorimpedance betweenthesystemearthpointandtheconsumer andthesystemloadcurrentbeingdrawn,itisquite commonfortheneutraltoriseseveralvoltsabove earth,particularlyinthecaseofsinglepointneutral earthing,orwherethelastneutralearthingpointis relatively distant from the consumer. Therelativelyhighincidenceoftheseneutral-earth faultshasbeenlargelyduetoPVCinsulation damageinconjunctionwiththehightemperature andtemperatureexcursionsencounteredinlighting fittings, roof areas, etc. ItisinterestingtonotethatseveralMunicipalities have reported cases of earth leakage devices having beenfoundtobeinperfectworkingorderwhen testedintheirworkshops,afterfailingtooperate whentestedinsitu,duetothelimited(approx40 mA)testcurrentavailablefromportabletest equipment. Thismal-operationcanbeshowntobeadirect resultofthedesensitisingeffectofthesocalled shorted neutral condition. Specifications Thisshortcominghasbeenconsideredserious enough to justify the inclusion in several national and internationalspecificationsofsomereference, requirement,ortestprocedure.Forexample, UnderwritersLaboratoriesoftheUnitedStates,and CSA of Canada include a self-protecting requirement intheeventoftheneutral-earthimpedancefalling below some predetermined value. ThedraftrevisedspecificationSABS767(Core balanceearthleakagecircuitbreakers)imposesa limitonthedegreeofdesensitisationataparticular loop impedance. IEC and BSS have a clause titled Verification of the behaviour of residual circuit breaker in case of earth fault of the neutral conductor on the line side and on theloadsideofthedevice(Clause8.3.6.)under consideration. Investigation into the problem TheElectricityDepartmentofJohannesburgCity Council, some time ago, conducted a series of tests to investigate the seriousness of this condition. It was found that the desensitising effect on the earth leakage devices was dependent, in the main, on: (a) the impedance of the neutral conductor between the systemearthandtheearthfault;and(b)the impedanceoftheresidualdetectorcircuitwhen referred to the primary. Neutral impedance Iftheneutralconductoristakentobeastranded copper conductor of 16 mm2 cross-section, it can be shownthattheneutralimpedance(mainlyresistive) wouldbeapproximately0,006 foraconductor length of 5 m up to, say, 1,2 for a conductor length of 1 km. Earth fault impedance Intheeventofaneutral-earthfaultonthe consumers installation, the impedance (again mainly resistive) of the earth path between the system earth andtheearthfault,isunlikelytobelessthanthe impedance of the neutral path and could conceivably be some multiple of the neutral impedance. Residual detector impedance Figure1representstheequivalentcircuitofacore balanceearthleakagedetectorwithaneutral-earth loop impedance shown. IfthevalueIr,isconsideredtobethecurrentinthe detector circuit at the point of tripping, then it can be shown that the current in the residual detector circuit andintheneutral-earthloopcircuitdivideasina branch with parallel impedances. Itcanbefurthershownthataneffectivedetector impedancecanbecalculatedandassignedtothe detectorcircuit,dependingupontheparticular designofthedetector.Thisisdependentuponthe turns ratio of the device, and the tripping circuit. Figure1.Equivalentcircuitofacorebalanceearthleakage detector. Tests Aseriesofindependenttestscarriedoutbythe ElectricityDepartmentofJohannesburgestablished thedegreeofthisdesensitisingeffectofaneutral-earth fault on various types of earth leakage devices, byplottingtheactualno-loadsensitivityagainstthe neutral-earth loop impedance. This is shown in figure 2. Byconsideringtheactualcircuitimpedances involvedandtheparallelbranchdivisionofthese impedances,theactualloadcurrentsrequiredto overridethisdesensitisingeffectcanbecalculated and confirmed by test. Figure2.E.lc.b.operatingcurrentagainstresistanceofshorted turn. Magnitude of the problem Thedesensitisingeffectoftheneutral-earthloopis seentobeminimalondetectorswhichhavealow detectorimpedancereflectedtotheprimaryside, according to the relationship: 1T12 However,thisisnotthecasefordetectorswhich havearelativelyhighimpedancereflectedtothe primary side, as can be seen from figure 2. Suchdevices can be desensitised from the nominal 20 mA leveltomanyhundredsofmilliamps,evenwithloop impedances as high as 0,25 or 0,50 Neutral earthing Single point earthing For the case of single point earthing, particularly with the earth point at the transformer neutral, the neutral conductorimpedancebetweenthepointofthe consumers neutral-earth fault and the system earth, could conceivably be several ohms. Multiple earthed neutral (MEN) According to the AMEU draft code of practice for the applicationofmultipleearthingtolow-voltage distributionsystems,theneutralandearth conductorsaresolidlyconnectedatthetee-off point on the lv. distributor. The probability is then that the distance between this earth connector on the neutral and any neutral-earth fault on the consumers premises would probably not belessthansay50mandcouldbeuptoafew hundred metres. For16mm2 copperconductor,theimpedance betweentheearthpointandthefaultcouldthenbe expectedtobebetweenabout0,05andsay 0,25 As can be seen from figure 2 the desensitising effect withdeviceslabelled1and2couldstillberather seriouscertainlywaybeyondthelimitsimposed by SABS 767. PME systems Theobligatoryearthingoftheneutralatthe consumerssupplypointobviouslydecreasesthe neutralconductorimpedancesbetweenthatpoint andthepointofanyneutral-earthfault,sincethe additional length of conductor to the tee-off point can no longer make a contribution. Thiscanonlymakethedesensitisingeffectthat much worse. Black box analogy Itisinterestingtoconsidertheblackboxanalogyof Figure4whereAistheactualfaultcurrent,Bthe residualdetectorcurrentandCistheneutral-earth loop current. Asoneexample,letusexaminethecaseofequal divisionofthefaultcurrentbetweenthedetector circuitandtheneutral-earthcircuit,i.e.whenthe effectivedetectorimpedanceisequaltotheloop impedance. For the detector to operate at 20 mA (B), theneutral-earthcurrentwouldalsobe20mA(C) andthetotalfaultcurrentwouldthenbe40mA (A = B + C). The intersection of the total fault current calculated in thisway,withthetrippingcurvesofFigure2, approximatesthecalculatedreflecteddetector impedanceonthehorizontalscaleofFigure2.The rangeofneutral-earthloopimpedanceswhich reducesthesensitivitybygreaterthan50%isalso shown in Figure 2 for three different types of devices havingdetectorimpedancesof0,0130,35and 5,13 Load current As load current is drawn on the protected circuit, the desensitisingeffectoftheneutral-earthloopwill eventuallybeovercomeandthedevicewilltrip purelybyvirtueofthedivisionofloadcurrent through the parallel impedances. However,withcertaintypesofdetectors,theload current required to cause spontaneous tripping could bemanyamperes,oreventensofamperes, particularly when the impedance of the neutral circuit is low. Thiscaneasilybeseenfromtherelationship(see Figure 3): Zn+ Zf+ Zr Where: lp =Loadcurrentrequiredtooperatetheearthleakage detector. If= Nominal tripping sensitivity of the detector Zr=Effectivedetectorimpedance(reflectedtothe primary). Zf=Impedanceoftheearthpathbetween thesystem earth and the earth fault. Zn= Impedanceoftheneutralconductorbetween the system earth and the earth fault. Figure 3. Equivalent circuit showing load current dependence. Integrity of the neutral connection IthasbeenclearlydemonstratedthattheMEN systemremovesthehazardousconditionsimposed ontheconsumerbyabrokenneutralinthePME system. Themultipleearthconnectionsoftheneutralinan MENsystemprovideintegritytotheneutralreturn path.Thisinitselfprovidesnotonlycontinuityof supplytotheconsumers,butensuresoperationof earth leakage devices which require this supply. Ingeneral,suchdevicesareofthelow-impedance type. Conclusions Theneutral-earthloopconditionresultsin desensitisationofearthleakagedevices.Thisis aggravatedbylowerloopimpedancesencountered with MEN or PME systems, but this effect is minimal with the low-impedance type of devices. Thepotentialeconomicadvantagesofferedbya multiple earthed neutral system should therefore not bediscounted,sinceadequateprotectioncanbe affordedbythecorrectapplicationof sensitive earth leakage protection devices. Acknowledgements TheauthorisindebtedtotheElectricityDepartment of the City of Johannesburg for permission to include andreproducereferencestoacommunicationfrom thatDepartmenttotheSouthAfricanBureauof Standardsandcalculations,testsandcurves contained therein. Acknowledgementisalsoaccordedtothe Engineering Staff of Fuchs Electrical Industries, who assisted with tests and advice. ReferenceismadetothedraftAMEUcodeof practicefortheapplicationofmultipleearthingto low-voltage distribution systems.