AL_n wrote
Andy Burns wrote
AL_n wrote

I was sitting here at my desk and the whole upstairs lighting went
out. The circuit breaker had flipped. When I tried to un-flip it, I
saw a spark and it wouldn't flip back, so I conclude the short is
still present.

MCB or RCD?

No alterations have been made to the wiring for several weeks here,
so I can't imagine what might have happened. I don't have rats in the loft.

Try turning all lights off (or reversing them if they're 2-way or
3-way) and resetting the breaker, turn lights back one-by-one until
it trips again, might narrow it down, unless the problem is isn't in
a switched part of the circuit.

Thanks to you all, for the helpful suggestions. This is strange:
Is witched all the lighs off and then flipped the circuit breaker.
It flipped back to the "on" position.

I then turned all the lights on, one by one, expecting one of them to
cause the short again. None of them did! Now all the lights are working.

What does this mean? It makes no sense to me!

That you have a partial short somewhere in the system.

Most likely in what you changed in the changes you made several weeks ago.

Or its an intermittent fault somewhere, but thats not all
that likely unless you have got a water leak somewhere.