Thanks, Joe - I forget that terms vary. These things are amazingly sensitive. Some years ago, I was installing some new ceiling roses in the house. I had switched off the lighting circuit breaker at the board and was working with a lead light from the power circuit. The circuit breakers isolate the live feed only.

I had stripped the new wires and was using both hands to feed them tidily into the connection strip, when BANG!. No light! I went to the board and found the main RCD had tripped - I had no idea why. I closed it and went back to work, only to have the same thing happen 5 minutes later. I closed the RCD again and used my meter to check all the wires I was working on - zero volts.

This time, I summoned assistance and, with someone at the board, systematically tested the various wires with my hands, tripping out now and again. Turned out that holding the neutral and earth wires between my finger and thumb would trip the RCD every time. On our supply, there is theoretically no voltage difference between neutral and earth.

It took two weeks of investigation to find an old lighting switch cable buried direct in the plaster in the oldest part of the house. At some time n the past, a nail had been driven through the cable and the wires were in contact with the plaster. This hadn't tripped the new RCD because we never used the switch to which it led.

Phil