Meggering and HV DC Tests

The HV test can damage insulation, resulting in degraded isolation.

emc_c

Meggering is done once with HV. Megger testing is to catch cable that has bad insulation due to rocks in conduits, bends, or a contractor is using old old wire to deplete his inventory.

Once meggering test has passed and complete then the system is ready to go hot. I don't see a reason to megger with HV again. If that were the case there would be alot of power outtage because P&GE is meggering cable every xx months.

Meggerring resistance should always be in the giga or infinity range for good wire insulation.

It takes years for wire insulation to go bad so it does not make sense with new wire to megger after with HV.

Also there is difference between the word "Megger" and "Hi-Pot" testing. Meggering is done 480VAC or less and Hi-Pot is done on MV (12000Volts and up) cable.

Sorry rabat, I usually don't jump in or respond like this but too much of your response is incorrect for it to go without saying.

Insulation resistance testing and insulation integrity testing are both part of proper acceptance testing, troubleshooting, and long term performance documentation to aid in preventive and predictive maintenance and cost budgeting.

Once meggering test has passed and complete then the system is ready to go hot. I don't see a reason to megger with HV again. If that were the case there would be alot of power outtage because P&GE is meggering cable every xx months.

This statement just does not make good sense and at minimum it certainly implies that you need a better understanding of maintenance standards.

Meggerring resistance should always be in the giga or infinity range for good wire insulation.

Minimum insulation resistance should be approximately one Mega-ohm for each 1000 volts of operating voltage, with a minimum value of one Mega-ohm. While a higher insulation resistance is obviously better there is nothing that says only good insulation is up in the "giga range", unless it's called out in spec.

It takes years for wire insulation to go bad so it does not make sense with new wire to megger after with HV.

Please pardon my tone but this statement is just nonsense and bordering on irresponsible. I've seen cable runs that had good resistance values with a Megger and flat out fail Hi-Pot leakage tests before we get to 25%. I've seen cables that have had perfect values at startup, both high resistance and low leakage current, and 3 months later the owner is searching for the hole that popped for no apparent reason. I could go on about this one but....

Also there is difference between the word "Megger" and "Hi-Pot" testing. Meggering is done 480VAC or less and Hi-Pot is done on MV (12000Volts and up) cable.

Megger is a trademark that Biddle started and, unless they sold, AVO now owns the name as a subsidiary. A "Megger" is a layman's term for an insulation resistance tester.

Your statement of insulation resistance tests being done on 480volt or less and Hipot being done on cables 12kV and up is incorrect and is actually what prompted me to respond. That's just so wrong that at this point I'm not going to go off down that road... you should know better and if not, do some searching and find out. Go browse Megger.com and Hipotronics for a start.

My apology if I'm tired tonight and my tone is too harsh. I've just simply cleaned up too many messes and seen too many injuries because people got bad information.

A rule of thumb that I use is - 1 Megohm per 1000 volts plus 1 Megohm.

Because during HV Test, the leakage current thro the insulation increases which might cause additional heating and subsequent de-grading of the insulation. To ensure that this has not happened, you megger after the HV Test and compare these IR values with those got before the HV Test and ensuring that the IR has not gone down drastically after the HV Test.

Thank you very much for reply