RCD tripped when cutting cable with breaker off

Switching the breaker off for the circuit you're working on only isolates the live, so when you cut the cable you briefly short the neutral and earth.

The RCD works by making sure that (within limits) the amount of current supplied via its live returns via its neutral, your short allows some of the current from circuits *other* than the one you're working on to return via earth instead of live, so it trips.

^^^^^ neutral

This can be quite common, it's due to earth and neutral being shorted together.

Switching off the circuit breaker (or removing the circuit fuse) only interrupts the line conductor. The neutrals for all the circuits in your house are connected together in your consumer unit so when you short earth and neutral together on your circuit you are providing an additional return path for the other circuits via your earth conductor. This results in the RCD seeing less return current in the neutral than supply in the live if anything is in use on the other circuits, causing it to trip.

Thanks for the quick and reassuring replies. I guess I've just been lucky and it's never happened before.

Thanks, BraileTrail

You can reduce the chance of this happening if you snip in one conductor at a time and open up the "wound" as you go by bending the cable so that the jaws of your cutter don't touch the exposed ends.

an earth neutral short will trip an RCD.

Guaranteed when cutting a 'dead ' cable that's still connected at the CU.

Hence the reason I check to see if anyone is using a computer before I do anything.