On 3 Dec 2003 23:10:42 GMT, wrote:

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?

Thanks for explaining. I did wonder!

(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?"...)

And there was me thinking I must have been charged up with static.....

PoP