11KV Cable Test

[Ahr35181]

Could anybody tell me what the minmium value of insulation resistance an existing 11KV cable should be when tested phase to earth at 5KV test voltage?

We have recently had an 11KV panel off for maintenance, when we came to restore power we tested the panel bus bars & the feeder cable together (as there is a joint in the feeder cable which is suspect).

Red & Blue phases were over 500Mohm to earth & Yellow phase was 130Mohm to earth (similar readings were recorded between phases).

The switchgear is ancient AEI oil filled, & the instruction manuals quote a minimum of 100Mohm to earth at 5KV test voltage.

Do current standards require the same value?

We succesfully restored power but will in due course replace the cable.

 

[ausphil]

We would accept 100MOhm for cables that have been in service and have been taken out for maintenance reasons or for IR tests after a fault. We also have a level of 5MOhm that we will put back into service, but will ensure we plan to replace the cable (or sections at least) within the near future.

Don't forget also that you are not just testing the cable, but also the insulation in the switchgear as well. This may well be the limiting factor of the IR in your measurement. However, if your switchgear manual says 100MOhm, and your total measurement including cable and switchgear is 130MOhm, then you can be sure that your switchgear and cable are both above 100MOhm.

 

[boomsystems]

A maintenace thumb rule is, highest system voltage plus 1 Mohms. If you are measuring IR of 11KV system phase with 5KV megger then you should have HSV+1= 12+1= 13Mohms to arrive at a firm decision to apply normal voltage to equipment ( charging).