I have a new style consumer unit - all nice circuit breakers etc. The ring circuits for the 240V sockets have their own MCBs and they are all connected after a main RCD just for the ring circuits. Now, I am converting my loft, and the only reasonable way to install sockets in there was to put in a new ring circuit (there was no-where on the second floor where I could break into). So that's what I did. I bought a 32 MCB and wired it all up so I could plug lights into it while laying the flooring down. Fine! But when I wanted to send the cable somewhere else and hence remove the temporary socket, I switched off the MCB (for this new circuit) so I could work on it. Before I went into the socket I checked to see if it was live in the usual way with those electrical-testing screwdrivers. Nothing (as expected). So I started working away. After a few minutes, the RCD for the 240V ring circuits tripped(and hence all sockets in the house went off), but none of the individual MCBs had. This has happened a few times. Why should a non-live circuit cause the main RCD to trip?