Differential relay CT grounding 1

[rah]

Does anyone periodically check, via meggering, the insulation integrity of the ct secondary of a differential relay scheme. That is, carefully lift the one grounded point of the ct secondary and megger to ground the secondary wiring to ensure there is no insulation damage that could potentially result in a separate ground path and possible false relay operation.

If so, how often?

 

[NormGA]

Yes, that is an excellent way to determine the condition of your cables, relays, terminal blocks, internal panel wiring, and CT's. I have done this many, many, times and it has been scary every time, even though I open trip paths and trip cutout switches. The most common problem I encountered was defective direct burial current cables. We do this when we do our relay maintenance, which we do on a 10 year schedule, but most of us think that is too long. I think 5 years is about right. Doing this test is facilitated by having the one ground clearly marked and on a stud or terminal so that it can be removed without disturbing the rest of the circuit. Since I am now retired I can't remember what our standards for the meggar readings were, but they generally run pretty high for a good scheme, better than 50 megohms. For anything less than 1 megohm you need to give it prompt attention. I found one a couple years back that varied between 50 and 100 kilohms. I was afraid to put the ground back on so left it off until we could replace the cable.

 

[rah]

Thanks NORMGA,

I know it's the right thing to do. I also found that the BUREAU OF RECLAMATION test the insulation integrity of thier CT secondarys once every three years.

I will be adding this additional testing to be performed following calibration of our differential relays.

RAH