RCD tripping

[cdbe]

My brother in law called me as the real electrician in the family was nursing a hangover.

He'd been replacing a faulty outside light (with the MCB off) and thinks he gave the cable a bit of a tug where it came through the wall and one of the RCD's tripped (wylex dual RCD board). It wasn't the RCD that the lighting circuit was on. It wouldn't reset - even with the live feed to the circuit disconnected (I suppose I should have removed N and E as well?).

I swapped the RCD's to rule it out and they both reset, he fixed his light and then it tripped again when he used the shower. He managed to reset it about 10 minutes later.

The cable to the light comes through the wall - he said there used to be a switch - presumably an FCU or something, at the top of the kitchen wall but it was in the way of the coving they wanted so it was replaced with a junction box and stuffed behind the coving, as you do!

I've told him he needs to investigate that JB in the first instance, but I'm a bit confused about what's going on with it being the RCD on the other side that's tripping?

So the lighting circuit is at the "bottom" of the board (yes - its installed on its end!) and the RCD that's tripping is whichever one is mounted at the "top"

 

[trojanhawrs]

Old fcu probably taken off the downstairs sockets.

 

[ericmark]

I would guess a neutral to earth fault. Since the neutral is often not switched, it is a get the meter out time.

Without an insulation tester, you're going to have a hard time finding it.

 

[Harry Bloomfield]

Despite turning the MCB off, that only isolates the L - the N remains connected. Shorting the N, to the E, can also trip the RCD.

The FCU, was likely there for a good reason - Probably that supply, came from a supply with a higher rating than needed for lighting, so it was there to limit the current to a more suitable value. Fitting a joint box instead, was a bad move, it need the FCU refitted.

 

[Taylortwocities]

Hopefully the hangover has abated now and the miscreant electrician can concentrate on the problem!