Actually you should do a Megger test BEFORE a hipot test per ANSI/NETA standards. Also, DC hipot tests can be destructive, and are not very effective. This has been discussed here at length many times, just search the forum for DC hipot test. For service aged MV cables a PD, TD, or VLF test should be performed, not DC.
We used to do a low voltage meggar test (500 V or 1 kV) after the DC high pot to verify we had cleared all grounds from the circuit before we left site.
It did not have anything to do with the cable. It was a quality control check of our procedures.
Are there any chances that the insulation level will improve after dc hipot test cable? Meggar reading before hipot is 10Gohms. Meggar reading after hipot test is 100Gohms. What is the reason behind this insulation improvement?
No, your difference is likely due to a difference in test conditions, tempature (Are those corrected values?), duration of megger test, humidity, corona supression, etc...
Depending on the time frame between the DC Hipot and the after Insulation Resistance test and depending on the test equipment being used, you may have gotten different results due to residual DC charge in the cable. If your concerned and have the opportunity, ground the cable for 6-8 hours and try the Insulation Resistance test again.
Up until recently I (like many others) resisted the shift towards "modern" cable testing techniques. After doing my homework I have come to the following conclusions...
1. VLF Hipot testing is very cool
2. Tan Delta testing is just a bit cooler...
3. Imcorp = assclowns
4. Partial discharge (when not monopolized by 60 Hz voodoo freaks)is the coolest...
With regards to the insulation improvement, Could it be there were moisture present in the air gap of the cable and the moisture was removed during hipot test and that led to an improvement of the insulation level?
Dear pmgohpelangi, yes, your explaination is valid. But it would only result in marginal increase in the resistance value, not 10 fold increase. I agree with others above, your test conditions matter the most in Megger.
