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I have a main breaker + GFCI combo breaker in my panel. I decided to do some work on an outlet and accidentally bridged ground and neutral, with the room breaker off and the live wire dead. The GFCI popped. The house had electricity except for the room. Is this supposed to happen? If so, why?

Thank you.

EDIT: The breaker is formally an RCCB. The house is single phase, Europe.

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    Did you double check that the power was off to that outlet. Cables can use weird routes, just because an outlet is in a room, does not mean that room breaker powers it.
    – crip659
    May 24 at 20:45
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    I had a device connected to the outlet. When I flipped the room breaker, the device turned off. May 24 at 20:46
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    I also checked the wires with a wire tester(?) (a screwdriver with a lightbulb that you bridge with yourself) May 24 at 20:50
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    Where in Europe is this? The electrical codes across Europe differ quite a bit
    – Pelle
    May 25 at 10:54
  • I live in Greece. May 25 at 11:40

2 Answers 2

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You created a ground fault.

So it worked exactly as it should have.

Rather than the current in the live and the current in the neutral being equal (or within 30 mA for most European RCCBs, 5 mA for most USA/Canada GFCIs) some of the current (more than 30 mA, evidently) traveled on ground, and wasn't showing up as equal on the neutral, so the RCCB said "we have a leak, shut it down!"

That's exactly what it's supposed to do.

If that happened on your "Whole house" RCCB when the room breaker was off, you may have incorrectly shared neutral wires. The RCCB is still doing its job correctly, but your house may have some improper wiring. Since all neutrals are normally connected, and don't get shut off with the breaker, the "whole house" nature of the RCCB means that a neutral-ground connection anywhere in the house will make a fault. [this is not the way USA/Canada folks (or I, anyway) normally think since we have single-circuit GFCIs for the most part, and there, if the circuit is off, the GFCI on the circuit is also off; so a GFCI trip elsewhere would mean an improperly shared neutral.] With a whole house RCCB, that's perfectly normal, as I think it through..

The returning current from a device/appliance whose breaker was still on found the short to ground through the house's neutrals to be the path of least resistance.

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  • In Poland, and as far as I know in the whole European Union, a whole house GFCI is normal, and actually preferred, second only to having separate GFCI on all circuits, or "circuit blocks" (don't know the proper name, like all lights, all hardwired appliances, etc.). It is actually forbidden (exceptions apply) to make new installations with some circuits with GFCI and some without because this may cause false appearance of security. If any circuit in the panel requires it, all should have it so there is no chance someone will think circuit is protected against ground faults when it isn't.
    – Mołot
    May 26 at 10:32
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Yes, typically a European house has a "whole house RCD" aka GFPE that serves at least several circuits at a time, if not the whole house.

I won't call it a GFCI because GFCIs are human safety rated with 5mA sensitivity (which is too sensitive for a whole house)... and RCD/GFPE are not (with 30mA sensitivity).

Current flows in loops. Power comes out the live and back the neutral. One easy way to spot trouble is to compare the two currents. They should be equal at all times. If the currents are not equal, we know current is leaking out via a third path. It's not supposed to do that. This is how GFCIs and RCDs work.

Note this method does not need to care about the ground wire, and indeed, they are not connected to ground at all. (well GFCI receptacles are because they need ground for the socket, but the GFCI doesn't use it.)

Many normal branch circuits have circuit breakers that interrupt the live, but don't interrupt the neutral. Why bother, after all, since neutral current can't be more than live current unless it's crossed with another circuit.

When you shorted neutral to earth, you created a path around the RCD for ordinary neutral current in the house. Some of it came out your branch circuit (since the breaker does not disconnect it), over to earth, and back to the system equipotential bond which is on the utility side of the RCD. This path caused that neutral current to bypass the RCD. Third path! Trip!

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    This really depends on where in Europe you are. I have never seen a whole house GPFE, in NL there is a maximum of 4 16A circuits on one 30mA RCD. Also, a normal ciruit breaker here would disconnect BOTH live and neutral, so there should be no way for current to flow from neutral to ground.
    – Pelle
    May 25 at 11:00
  • @Pelle I see a lot of European and UK panels and yours is not the norm. "One RCD to rule them all" is common esp. on older or smaller homes, and neutral-switching is uncommon. Nonetheless, I couched the language to be less absolute. May 25 at 18:09
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    The (few) UK cases I've seen have RCDs for individual circuits and a ‘master’ one for the whole house. Is that common?
    – gidds
    May 25 at 19:55
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    In France, in 40 year old installations you get a 500mA whole house RCD, and unipolar breakers/fuses. In ~20 year old installations, 30mA whole house RCD plus bipolar breakers. More recent installations, several 30mA RCDs, usually one per line of breakers on the panel, and bipolar breakers. Having several RCDs is nice to save your freezer contents.
    – bobflux
    May 25 at 20:16
  • @gidds I suspect you are confusing MCBs with RCDs. May 25 at 20:32

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