I was replacing a ceiling rose. Obviously I flipped the MCB off at the consumer unit first _and_ double checked that the power was off to the lights before I started.
Then, out of the blue, the main RCD popped out.
Scratched head. Checked again that the power to the lights was absolutely, definitely off and reset the RCD. It stayed in for a little while and then popped out again.
Long story short, I eventually realised that it was going out when I inadvertantly shorted the neutral to earth in the rose. Presumably, current drain on other circuits in the house was pulling the neutral a little bit away from the earth potential causing an unmatched current to flow when they were connected. Just wondered if this was normal or a possible fault. How far should the neutral get away from the earth?