what happen if measuring ct is used as protection ct

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Answer by Littlewats This can be done, but is usually not done because a metering CT is made to be very accurate at expected load current levels. A protection CT is made to be accurate at fault current levels (which is often significantly above load current). If the CT is too small, and has too small of a ratio, the CT will likely saturate, which will skew the results the protective device expects to get. An example: a metering CT may have a ratio of 100:5 (made for 100A load current, which will produce 5 amps secondary current). Fault current levels may be ~10,000A. You'd expect the secondary output of a perfectly linear CT to be 500A. One way resistance of the cable connecting the protective device to the CT may be on the order of .5 2 ohms. Ignoring the resistance of the CT and connected devices, secondary voltages would reach 250 1,000 volts for a phase fault, and 500 2,000 volts for a ground fault. The biggest CTs may be rated for 800 volts output below the knee point, so this 100:5 CT would clearly saturate. The real output current would likely be on the order of 150A at the most. ...And the above is ignoring any DC offset, which would cause the CT to saturate at unpredictable AC current levels. EDIT 5.2K CONFIDENCE VOTES What happen if measuring ct is used as protection ct?

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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

5.2KCONFIDENCEVOTES

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