Re: RCD trips while working on dead circuit

No, it means the RCD is working fine. Your MCB/fuse only isolated the live ("phase") wire. E and N are still connected back with all the Es and all the Ns on the load side of your RCD. The RCD's job is to make sure that (to within 30mA or whatever it's rating is) all of the current which goes out its L side is coming back up its N side. When you join the N to the E, you provide an alternative path back for some of the N current, and the RCD pops because you're out of balance.

The above explanation shows you one reason why relying on the MCB alone for isolation is less than best practice. You'd do better to switch off the whole supply, physically remove the L and N conductors from the MCB and the N busbar respectively - put the bare ends out of harm's way into a bit of connector block, or for best paranoia into spare ways in the earth block. Then you can restore power to the rest of the installation, and work at leisure on the properly-isolated (or even earthed) final circuit you just disconnected.

The cautious among us when first working on a circuit we haven't installed ourselves might even measure the voltage from the allegedly isolated N and L conductors to earth before grounding them, and/or do the temporary-connection-to-earth through a low-value glass fuse (so you could see it blow) or an incandescent bulb (so you could see it glow) Just In Case some eejut has cross-wired the final circuits; but that's taking caution a bit too far, IMHO. Me, I isolate the Ns and Ls, put them into free-waving terminal block, and then test AGAIN at the working position that there is no voltage between earth and the allegedly isolated Ls and Ns. 'Course, if you use a digital multimeter, you can be misled by very-low-current induced voltages - connecting a suitable resistance in parallel with the meter will soon sort out the difference between a tiny capactively-and-or-inductively coupled current, and a Nasty cross-connection.

HTH, Stefek

The N voltage to earth may only be a 100mV (hardly measurable - or less) but the path is usually less than 1 Ohm so you get the 30mA you need to trip the breaker.

Thanks to everyone for the explanation.

So... if I was to switch off just the circuit I want to work on, THEN confirm with a meter that there is no voltage across the N, L and E conductors... under what circumstances could that still put me at risk?

David

And, yes, it does happen - it happened to me!

The wazzock was the previous owner who fitted a two-way switch for the landing light. Clearly unaware of the existence of 3-core +E, he had a bit of a challenge when it came to wiring it up using 2-core+E. Then he had a bright idea. "I'll use the live from the upstairs lighting circuit for the upstairs switch and the one from the downstairs circuit (which was present in the downstaitrs switch box) for the downstairs switch and connect them together with the 2-core and earth". Hmmmmm. Muggins comes along and wants to work on a bedroom light. Pulls fuse, all OK, happy working away. SWMBO, in all innocence, turns on landing light. Disconnected neutral I am hanging onto rises to live potential through landing bulb which glows fitfully due to current through my sweaty fingers, while brain thinks WTF!? Guilty cable also buried in plaster with no conduit - hooray! Yes, it is wired correctly now. As they say in the X-files, "trust no-one".