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Megger tests: breakdown voltage puzzle
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taydin:
Here is a puzzle for you guys:

There are two pictures below. One is a pair of twisted 0.25 mm2 multi strand wire, the other is a single wire (same type) and I'm measuring insulation resistance on both.

1) Which one will have a higher insulation resistance?
2) Which one will have a higher breakdown voltage?
3) Is the insulation resistance the only factor determining the breakdown voltage (I don't know the answer to this one, but I did measure the above and know the answer)
Rerouter:
1. Probably the second provided the teeth of the clip are not digging into the insulation.
2. The second, unless you have extra insulation on the end, the spark only has to jump the gap at the end of the twist,
3. The breakdown voltage is determined by the dielectric strength, vs any air path, if you give enough voltage a certain thickness of dialectic will break down.
SG-1:
If a spark bridged the gap on the first test, you need to separate the ends, the test is invalid as an insulation test.

The second test is invalid, because the teeth on the test probe may puncture or weaken the insulation at a single point. You should tightly as possible wrap the single conductor in aluminum foil or tape, this will test the insulation over a much larger area. Do not get too close to the bare ends with the foil.
SG-1:
In both cases I am referencing insulation not air gap.

I would expect the insulation resistance test would max out each of the ranges on both single & twisted.

Your twisted pair will have the higher breakdown voltage.  I would expect it to be about twice that of the single conductor.




taydin:

--- Quote from: SG-1 on October 27, 2019, 08:45:58 pm ---If a spark bridged the gap on the first test, you need to separate the ends, the test is invalid as an insulation test.

--- End quote ---

There are no strands protruding off the end, they are well inside the insulation.


--- Quote from: SG-1 on October 27, 2019, 08:45:58 pm ---The second test is invalid, because the teeth on the test probe may puncture or weaken the insulation at a single point. You should tightly as possible wrap the single conductor in aluminum foil or tape, this will test the insulation over a much larger area. Do not get too close to the bare ends with the foil.

--- End quote ---

I know of no megger in the market that has teeth in their probes. Think about it, if the probe bites into the insulator, then IT damages the insulator instead of the high voltage :) Kinda defeats the purpose of using a megger. And besites, we are measuring Gigaohms, who is concerned about the nicer contact and a few ohms less resistance that teeth would provide?
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