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| Megger tests: breakdown voltage puzzle |
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| taydin:
@SG-1, I had repeated the insulation resistance test after wrapping a piece of that blue wire with aluminum foil, like you suggested. First I casually wrapped the piece with foil, not caring much for the foil to be tight. The isolation resistance dropped from 885 GΩ to 95 GΩ Then I tightly wrapped the piece of wire with foil, making sure that it is snug. The isolation resistance remained at 95 GΩ |
| SG-1:
Taydin, I was also amazed at the breakdown voltage. At one point wondering if the test set is working :-BROKE, then flames appeared ! :clap: On the first test, I started on the 10KV scale, figuring it would be over fast, then had to switch to the 25KV scale, then on to the 50KV scale, WOW, what a show ! :popcorn: A normal dielectric test voltage for 600V rated wire is either 1500VAC RMS for one minute or 2200VAC RMS for one second. Thanks, for peaking my interest, it was fun. I think the biggest two factors in breakdown is probably time & humidity. As for Mr. Murphy, his legal address is the same as where I work. |
| taydin:
Everything indicates that AC breaks down an insulator more easily. I tried to confirm this by googling around, but wasn't able to find a good source for this. Here is a "theory" of mine to explain why AC breaks down insulators more easily. Would love to hear what others think or what others know about this. For DC, as a result of polarization effect, free electrons in an insulator are drawn towards the poles of the high voltage power source, so the DC voltage need to reach a level where this polarization effect is being countered by the force of the electric field, and this level is quite high. I can confirm that insulation resistance increases with voltage (up to a certain voltage level at least). But for AC, the polarization effect cannot occur, because the poles of the voltage source keep changing rapidly. So the free electrons exist in a more or less homogenous distribution. This reduces insulation resistance quite a bit and makes break down easier. |
| SG-1:
I have always been told that AC Dielectric Tests are destructive & DC tests do not harm the insulation unless you actually cause an insulation failure. This would seem to be correct because Meggers are used for preventive maintenance programs. I'm on holiday this week, but will try to look for the IEEE test standard for this when I get back, it may shed some light on the subject. |
| SG-1:
What are you seeing that makes you think the resistance is changing with voltage ? |
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