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.
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.
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!
