Overcurrent is the only thing that causes an MCB to trip. A difference in
current between live and neutral causes an RCD to trip. Thus a fault that
causes excess current to flow (slightly excess for a long period or gross
excess for a short period) trips an MCB (or other over-current device
such as a fuse). A few tens of milliamps leakage to earth trips an RCD -
even touching neutral and earth together on a circuit switched off at the
MCB or other single-pole switch in the circuit can cause enough earth-
loop current to flow to do it.



thats waht i was trying to say before i read this!
[g]