Quote:
Originally Posted by Ruffy
If you're only testing resistance with a regular multimeter then it's possible you're getting a false positive: Sometimes, when insulation is beginning to break down it takes the higher potential of the mains voltage to drive the short circuit.
This could explain why it seems OK on the bench but then trips when operated with mains.
Proper insulation test standards demand minimum 2x rated voltage for this reason. ("Megger it!" is the ubiquitous call from electricians - though other brands of insulation tester are available  )
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I was going to type out something similar but not many people have a mega lying around at home.
Only way to "prove" it's ok is to stick a known good one in and see if you get the same result.
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