ECN Forum Forums Code Related Discussion NEC & other Code issues Insulation Resistance

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.

"...still hooked up to the primary" should make individual readings meaningless, unless I misinterpreted something.

Norm
The 2500 volt DC test does not provide an adequate check of the insulation integrity.
Is the cable in use now? I would want to Hi Pot test the cable. New 15 kv cable is tested at about 60 kv but that is not what you want on cable this old. Check with the mfg annd see if they can give you a value to test the cable.