Just trippin'

[Jimbo]

I wired a spur into an existing socket at the weekend. I switched off the relevant MCB and checked that everything on the socket ring had gone off, then got to work.

However, I found that once I was playing with the bare wires, I kept tripping the main power on the consumer unit, despite the MCB being switched off. The same thing happened a few evenings ago when I was wiring in a new ceiling light (different MCB of course).

Any ideas what's causing this problem?

???

 

[Hamiltonandy]

Residual current circuit breaker will trip if earth leakage exceedes rated value perhaps 30mA. Now you switched MCB off disconnecting Live but this does not disconnect Neutral.
Neutral can have a voltage difference to earth from a volt or so in town near supply transformer to large value in rural area. The leakage caused when you earthed Neutral wire tripped RCD.
Safe practice would mean switching off RCD/isolating switch at consumer unit before connecting new wiring to existing.

 

[Jimbo]

Thanks for the information. I would normally switch off the main power, but I was doing the work in the evening so this wasn't possible. (Although ultimately, that's what I had to do -- I finished the job by torchlight!)

From now on I'll do this sort of thing on a Saturday morning when there's plenty of natural light around!

Jim