Insulation Resistance Testing

[mas2006]

A fellow engineer and I were discussing insulation resistance testing and he mentioned that several times he obtains an infinite reading on his instrument without the leads connected, but as soon as he connects the leads, he gets less than infinite (the leads are connected to the instruments, but not to a specimen), although still a very high reading. Does anybody knows the reason for such strange behavior?
Thanks,
MAS2006.

 

[electricpete]

Leakage current.

From probe, along the lead to instrument, along the other lead to other probe.

From probe along your friends arm to his other arm to the other lead.

If you're talking gigaohm range, it doesn't take much leakage current to change the measurement.

As you add more and more things in parallel the resistance goes down. Total resistance will be lower than the lowest of all the parallel things.

If you're measuring 5 gigaohms now with only leads connected and you connect to your motor and read (after one minute) 500 megaohms, then the motor is darned close to 500 megohams and this extra resistance isn't affecting you.

If you obtain any satisfactory measurment, you needn't worry about the effects of the leads (unless you are trending), they only make the reading lower.

If you have 100 megaohms with the lead and 90 megaohms with the motor also connected and you're expecting 200 megaohms, time to troubleshoot that leakage.

I'm not sure if there might be some kind of meter zeroing device to remove the effects of that leakage current... or maybe not.

What type of instrument are we talking about?

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[mas2006]

Dear ElectricPete, thanks for your reply. I will discuss it with my friend. He was using an AEMC, but I don't remember the model.

MAS2006.