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I had a bit of a phenomenon on the neighbour's electrical circuit.

What was observed:

  • The differential had blown

What was tried:

  • I tried to to isolate the faulty circuit by trying to get the differential stay closed.
  • Trial and error resulted in several circuit breakers set to "open" until this was managed, with no obvious pattern. For example, the circuit breaker for the fridge and the circuit breaker for the oven both had to be "open". Closing them would trip the differential. Even then, the differential randomly tripped after maybe 5 minutes of stability.

What fixed it:

  • The neighbor remembered a dodgy lamp plugged in. It was indeed doddy, as the power supply was broken open with the transformer spool clearly visible and the grounding cable had broken loose.
  • After unplugging said lamp, all the ciruit breakers could be closed and the differential could be satisfactorily closed and stayed this way.
  • I'm not sure whether I "isolated" that lamp through opening one of the ciruit breakers earlier, but probably yes (judging from the inscriptions on the breakers at least).

Question:

How is it possible that a single device on the "power plug" circuit causes the circuit breaker of the "stove" or "fridge" circuit to become differential-triggering-when-closed?

I actually suspected a major problem with the cabling at first, like water intrusion or a rat having a final meal. I reckon one should have a professional take a good look at this phenomenon?

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  • You'll need to tell us what country, and if not North America, whether you have 1-phase or 3-phase power in the building. Normally I can infer it from the text, but not this time. – Harper - Reinstate Monica Mar 16 at 19:21
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    This is an Eurozone setup: Luxembourg to be precise. 3-phase power: yes. But I will have to take a closer look at that panel and/or watch the screaming electrician when he drops by on Thursday. – David Tonhofer Mar 16 at 20:16
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Perhaps a neutral-to-ground short in the lamp, which is not disconnected by opening its breaker, is creating a current path impacting other circuits. I can't say exactly how as I don't know exactly how the multiple RCDs on a panel interact with one another through their common ground and neutral bus. But it's the only possible explanation. I'm guessing you are in a home with European-style RCD groups controlling everything in the house, is that right?

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  • Yes, that's right. The electrical panel is not top quality work but it should be "european standard" ca. 1998. I will definitely have a professional look at that. – David Tonhofer Mar 16 at 16:15
  • I don't think a neutral-ground fault on one RCD should cause a trip on another one. I don't really know that, but if the lamp is causing this while its breaker is off, then clearly, EITHER, it is normal for such a trip to occur in this circumstance, or you have BOTH something wrong with your lamp and something wrong with your wiring. Maybe for example the RCD neutral buses are not isolated correctly. – jay613 Mar 16 at 16:56
  • @jay613 If two RCDs are cross-connected to each other, I would expect a double trip. A typical reason for that would be "borrowing a neutral". – Harper - Reinstate Monica Mar 16 at 19:21
  • @Harper-ReinstateMonica sorry I'm learning this ... shouldn't borrowing a neutral from the wrong RCD cause a trip immediately even without any additional faults? – jay613 Mar 16 at 19:28
  • @jay only if there's current flow on the neutral, i.e. if a load is making use of the neutral. – Harper - Reinstate Monica Mar 16 at 21:11

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