IR Test Result interpretation

[NLocus]

Hi Everybody

I would be glad if someone can shed some light on IR Test.

(I) What is the minimum acceptable value for a power cable for an IR Test with respect to installation for VSD, soft starter & Star Delta. Does the starting mode has an effect on the IR. Once, for a soft starter cable, the minimum IR for a cable should be > 80 Mohm to be able to start the motor.

(II) when performing an IR Test on a motor, based on what IR result can we conclude that the motor has humidity inside or the motor is grounded

(III) If a power cable during wet condition, the IR of a power cable drop to 10 Mohm and after hours the IR increase to 80 Mohm, how can we determine where the cable is damaged since it is in a duct.

Any other points concerning IR test/results and real experience sharing.

Thanks in advance.

 

[electricpete]

(II) when performing an IR Test on a motor, based on what IR result can we conclude that the motor has humidity inside or the motor is grounded

IEEE 43-2000 specifies minimum 5 megaohms for "for most machines with random-wound stator coils and formwound coils rated below 1 kV". This is temperature-corrected to 40C. Also must pass the PI (>2). There is some situational judgement involved in interpreting results imo. If history is available, trends help recognize what is abnormal for a machine/family.

Cable limits - typically depends on length of cable to my understanding. Perhaps better in the power forum.

=====================================
(2B)+(2B)' ?

 

[ScottyUK]

The operating voltage makes a difference.

An LV cable I would (reluctantly) put back in to service at only 1M[Ω] if I had no real alternative, but I'd be very reluctant to energise an 11kV cable with that IR value, regardless of alternatives, because it would probably fail upon energisation leaving me with an even bigger problem.

 

[oldfieldguy]

The OLD rule used to be "A thousand ohms per volt". That's not in a code or specification, just information handed down from one old grizzled electrician to another.

It seems that doubling that value lines up with more modern ideas. Couple the absolute value with a polarization index of two or better, and you'd be safe but I'd certainly want to see better.

Cables don't fit the PI model. I tested literal miles of 15 kV PILC and the average PI was around 1.15.

old field guy