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I tracked down an AC appliance (a dishwasher) which started tripping my home current circuit breaker (RCD). I happen to have a professional insulation resistance meter and I was able to measure about 10× difference in insulation resistance when switching its polarity.

Before doing anything, I try to understand whether this can be a sign of failure of power input circuits.

What I see when attaching meter terminals (+/-) to the appliance terminal (L/N/PE):

L (+) <-------> PE (-) : 113 MΩ
N (+) <-------> PE (-) : 113 MΩ

L (-) <-------> PE (+) : 1.3 GΩ
N (-) <-------> PE (+) : 1.3 GΩ

Measurements carried at 250 V.
The values can be consistently reproduced.

For non-European readers, according to European standards (images via Google):

  • L = live (brown)
  • N = zero (blue)
  • PE = protected/earth (connected to the chassis of the appliance) (yellow/green)
miroxlav
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  • I don't think so because all you need are diodes in there somewhere. – DKNguyen Oct 18 '21 at 22:27
  • @DKNguyen – I would understand that if measuring between L and N. But I did measurements against PEN which is connected to chassis of the appliance . – miroxlav Oct 18 '21 at 22:34
  • What is PEN? is that some other way of saying ground/earth? – Hearth Oct 18 '21 at 22:39
  • What's the N stand for? I've seen PE for "protective earth", but the N threw me off. – Hearth Oct 18 '21 at 22:43
  • @Hearth – question edited. I thought the letters are international enough, but obviously not. – miroxlav Oct 18 '21 at 22:46
  • Might be an indication of galvanic action taking place (ie. the electrical wiring got wet). Is it an RCD breaker? – Spehro Pefhany Oct 19 '21 at 00:20
  • That is quite large impedance to worry. Unbalance may have something to do with the dielectric and circuitry, but I would not conclude anything by that alone. Current measurement, during operation, probably give better explanation. Does the appliance use water? – jay Oct 19 '21 at 00:24
  • Assuming this is an RCD breaker tripping, try measuring again immediately after a trip. If its leaking, the water might have evaporated by the time you measured. – user1850479 Oct 19 '21 at 02:44
  • Probably the most critical device - heater wasn't measured at all, because disconnected by relay. You should track when it does trip the RCD, mostly when warming water due to heater failure. Next: PEN is PE +N it's an incoming wire to the house, then it is split into PE and N. So you don't have a PEN on the machine, rather PE. – Marko Buršič Oct 19 '21 at 06:35
  • @Hearth 4-wire system with combined neutral and protective earth. A TN-C system. – winny Oct 19 '21 at 06:52
  • @user1850479 - thank you, I agree and I added an answer how I currently understand this. – miroxlav Oct 20 '21 at 14:01
  • @MarkoBuršič – absolutely correct Marko, I typed "PEN" without thinking. Question fixed, thank you. – miroxlav Oct 20 '21 at 14:02
  • @SpehroPefhany – yes, it is and RCD, question edited. – miroxlav Oct 20 '21 at 14:08

1 Answers1

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I attempt to answer my question with what I learned in the meantime:

Any contact of two different metals creates de-facto semiconductor and resistance of such is dependent on polarity.

In case of this appliance, even the lower of two measured resistance values is in normal range so it alone does not look like evidence of malfunction. These appliances have several points where components are grounded to the chassis and they mix metals there (for example copper wire → aluminium clamp → steel frame) so this can contribute to the effect on resistances depending upon polarity.

Another possible hypothesis (credits) is that a galvanization started if circuits got wet from water. I will be able to verify this after fixing potential leaks problem, ensuring that everything is dry and then by measuring again.


For addressing your kind hints in discussion (on where the actual fault can be) I make an off-topic note that most likely the humidity is leaking via old seal to surface of 2-in-1 heater+pump assembly so it is time to fit new seals and possibly also replace the assembly if it is permanently damaged by water leaks.

miroxlav
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