what happen if measuring ct is used as protection ct
Post on 08-Nov-2015
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AnswerbyLittlewats
Thiscanbedone,butisusuallynotdonebecauseameteringCTismadetobeveryaccurateatexpectedloadcurrentlevels.AprotectionCTismadetobeaccurateatfaultcurrentlevels(whichisoftensignificantlyaboveloadcurrent).IftheCTistoosmall,andhastoosmallofaratio,theCTwilllikelysaturate,whichwillskewtheresultstheprotectivedeviceexpectstoget.
Anexample:ameteringCTmayhavearatioof100:5(madefor100Aloadcurrent,whichwillproduce5ampssecondarycurrent).Faultcurrentlevelsmaybe~10,000A.You'dexpectthesecondaryoutputofaperfectlylinearCTtobe500A.OnewayresistanceofthecableconnectingtheprotectivedevicetotheCTmaybeontheorderof.52ohms.IgnoringtheresistanceoftheCTandconnecteddevices,secondaryvoltageswouldreach2501,000voltsforaphasefault,and5002,000voltsforagroundfault.ThebiggestCTsmayberatedfor800voltsoutputbelowthekneepoint,sothis100:5CTwouldclearlysaturate.Therealoutputcurrentwouldlikelybeontheorderof150Aatthemost.
...AndtheaboveisignoringanyDCoffset,whichwouldcausetheCTtosaturateatunpredictableACcurrentlevels.EDIT
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Whathappenifmeasuringctisusedasprotectionct?
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