From Norm Koon:
I have a question concerning cables that are approximately 45 years old.
The insulation resistance test read as follows:
Air Temp 82 deg F
One Minute Megohm readings Ph A 1220 Ph B 2540 Ph C 484
Ten Minute Megohm readings Ph A 1470 Ph B 4100 Ph C 510
PI Index Ph A 1.11 Ph B 1.61 Ph C 1.05
Given this test data, would it be advisable to reuse these cables?
These cables are partly running underground, inside a basement and outdoors, so
an accurate temperature reading was not used. The temperature is only the outside
air temperature. Also they are still hooked up to the primary bushings of the transformer.
So they are not true readings of the cables. I was wondering if the disparity of the readings could be due to leakage on the transformer bushings. The test voltage was 2500 vdc. The system voltage is 6900 volts ac.
Joe Tedesco, NEC Consultant