New ring circuit

[notb665]

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?

 

[breezer]

you said you turned off the mcb to work on the circuit (as you should) yes?

trouble is the mcb will only turn off the live (as it should) that leaves the neutral and earth still connected, if you short the neutral to earth the rcd will trip, which is what i guess you have done

 

[notb665]

Ah ha! Yes, I did turn the MCB off before commencing. I see. Thank you very very much. I must have touched the neutral to the earth at some point then.

I don't understand why it would do this (as neutral has no current (?)) but I guess I don't need to. But I wouldn't mind anyway. I am vaguely aware that the current does alternate between the two but I am not sure. Any links/books to explain stuff like this? I left school in the 1990s, went to do a degree in Chemistry - we just never did stuff like this at school - it still galls me! At some point I want to do a nightclass or something in electrics and/or carpentry.

Thanks again for your help.

 

[breezer]

in theory you should't but its quite common to get an "imbalance" between neutral and earth, when the two touch current then flows and becaus its an imbalance the rcd trips.

to find out how an rcd works you may want to read this