8/3/2019 NWfall08 Quiz85 Grounding

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www.netaworld.org Fall 2008 NETA WORLD

by Jim WhiteShermco Industries

Grounding

1.AccordingtoANSI/NEAMS-07,howotenshouldgroundsystemsbeelectricallytested?a.Onceayear c.Onceeverythreeyearsb.Onceeverytwoyears d.Onlyeveryouryears

2.Temaximumgroundresistanceoanindustrialacilityshouldbea.1ohm c.4ohmsb.2ohms d.5ohms

3.Temaximumgroundresistanceoageneratingstationshouldbea.1ohm c.4ohmsb.2ohms d.5ohms

4.WhatIEEEstandardisusedasareerenceorall-o-potentialtesting?

a.Standard71 c.Standard99b.Standard81 d.Stanardd101

5. Foragroundsystemwithairlyuniormsoiland testelectrodesdrivenatsucientspacingrelativetothesizeothatgroundsystem,whatdistanceromthetestedelectrodeshouldthepotentialelectrodebeplacedtomeasurethetheoreticallycorrectgroundresistancewhenperormingaall-o-potentialtest?a.20eetb.100eetc.62percentothedistancebetweenthecurrentelec -trodeandthetestedelectrode

d.57.7percentothedistancebetweenthecurrentelec-trodeandthetestedelectrode

Answers

1.Withaveragereliabilityandaveragecondition,thetableinAppendixBindicatesatestintervalo24months.Tistimerecommendationchangesaccordingtocriticalityandcondition.

2.Section7.13givesamaximumo5ohmsoranindustrialacility.Ingrounding,though,thelowerthebetter.

3.Section7.13shows1ohmmaximumorgeneratingandtransmissionacilities.4.TeANSI/IEEEStandard81,IEEEGuideorMeasuring

EarthResistivity,GroundImpedance,andEarthSuracePotentialsoaGroundSystemisthereerence.

5.Ithesoilisairlyhomogeneousincontent,thedistancethevoltageelectrode,P2,isdrivenisusually62percent.Otenthatmaynotprovidethetrueresistance,though.TebestprocedureistodriveP2inseverallocations,inlinewiththetestedelectrode,C1,andthetestcurrentelectrode,C2.ObtaintestdataorP2positionsbeginningabout50percentothedistanceromC1toC2untilabout70percentothatdistance.PlottheresultingearthresistancesrelativetotheP2distanceromC1.Wheretheplottedvalueslevelofisthetrueearth

resistance.Itheplottedvaluesdonotlevelof,thatindicatesmoredistanceisneededbetweenC1andC2andthetestsneedtoberunagain.

JimWhiteistherainingDirectororShermcoIndustriesandtheprincipalShermcorepresentativeontheNFPA70BCommittee.Jim

isthealternateNEArepresentativeontheNFPA70ECommitteeandservesastheNEArepresentativeontheIEEE/NFPAArc-FlashHazardWorkGroup(RPC)AdHocCommittee.HeservedastheChairmanothe 2008 IEEEElectricalSaetyWorkshop. Jimis aNEACertifedLevelIVElectricalestingechnicianandamemberotheNEASaetyCommittee.