AEMC 6503 - Test Voltage; Spot Testing

MegohmmeterModels6501/6503 13
4.3.1 Test Voltage
Nopublishedstandardtellswhichvoltagetochooseforanygiven
winding. However, published recommendations could be summa-
rizedasfollows:
Rated Voltage of Motor Test Voltage
Below 115 250V
115 250 or 500V
230 500V
460 500 or 1000V
4.3.2 Spot Testing
Asageneralruleinspottesting,testvoltageshouldbeapplieduntil
novariationinreadingisnotedfor15secondsorappliedforaxed
60seconds.Oncapacitancecircuits,crankingforuptoaminuteor
moremaybenecessarytofullychargethesample.Whatminimum
valueshouldbemeasured?TheIEEEstandardN°43-2000states
thatitisimpossibletospecifythevalueofinsulationresistanceat
whichawindingwillfailelectrically,butonmotors,minimumread-
ingsgenerallystatedare:
Rated Voltage R minimum
250 or less 2 MΩ
460 2 MΩ
There is no xed gure for determining what is good and bad in
resistancereadings,butagoodguidewouldbe1MΩforeveryone
hundredappliedoperatingvolts,asaminimumgure.Thisapplies
to motors and transformers. When the insulation resistance gets
thislow,anelectricalbreak-downcanbeexpectedatanytimeand
rewindingorreplacingshouldbeconsidered.
It is not unusual for a winding to be 10 to 100 times the recom-
mended minimum value (IEEE standard 43-2000: Recommended
Practice for Testing Insulation Resistance of Rotating Machinery),
butthisvarieswithtemperatureandhumidity.
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