In uk.d-i-y, PoP wrote:



..................................... I know the armoured cable comes
thru the house consumer unit 100A RCD because (with the circuit RCD
switched off) the 100A RCD tripped the house out when I chopped thru
the mains cable in the garage - presumably due to some small current
flowing between live and earth even with the circuit off.

No, the live was isolated. But the neutral wasn't (single-pole breaker,
neutrals commoned on the RCD side). When you chopped through the cable,
you gave the current flowing back to the neutral block and back to
the balance-detection circuitry of the RCD another path back to earth
(through the N conductor of the SWA and back down the earth conductor).
Doesn't take much of an imbalance to make the RCD pop, right?

(This comes up often enough that Someone ought to write it up for the
FAQ under "why does my RCD pop when I've disconnected a circuit with
the MCB?"...)

Stefek