Insulation Test Problem

It is actually quite common to see this with digital multimeters. I would suggest checking the circuits with an analog meter.

Switchman,

Thank you for the advice, I will try with analog meter and compare the results.

Induced voltages on de-energized ungrounded equipment is normal in the presence of other energized equipment. It is best to ground one end of whatever you are testing to minimize potential stored.

Please correct me if I mis-interpret your terms,

You are trying to test a generator, a surge tester can do a DC winding insulation to ground resistance (hipot or megger) and a ringing comparision of the windings which would tell if there are shorted turns in the coils. A portable 12kV surge tester can test a Generator size roughly to 2500kW if 1200 rpm, larger kW for higher rpm.

The surge tester can also be used to do a DC hipot or megger of a conductor.

Always ground items under test while connecting and maintain one ground when testing.

Hi Ron,

You don't need an analogue meter you need one with a low impedance.

If you are indeed using a digital meter it will probably be 10MΩ input impedance if of high spec or 1MΩ if lower spec. You can always shunt down this voltage with a resistor of say 10kΩ. If this is just capacitive or inductive coupling then it will drop it to millivolts.

However why not use your Megger? You will be using 500V or 1kV so 7.5V will not do it any harm.

If I understand your question correctly you are testing the cable between your main switch room and the machine's isolator. If so, where have you been measuring the currents recorded?

Regards

Chas

Hi Chas,

Thank you for the advice;

1. Yes, I used a digital multimeter with an input impedance of 10MΩ, 30pF nominal.

2. The cable tested is from the Main Switch Room Circuit Breaker [Email-Westinghouse, Series C, 400Amp max., 600Vac, cat. KD3400F]to the Machine Isolator.

3. The voltage and currents recorded were similar when measured from:

(a) Isolator Line Side.

(b) Main Circuit Breaker Load Side.

4. Thank you, I used the Megger at 500Vdc and 1000Vdc, and all went well as you advised, "However why not use your Megger? You will be using 500V or 1kV so 7.5V will not do it any harm",.

Cheers!