"Gary Cavie" wrote in message
t...

The most likely cause is that you got a brief short between earth &
neutral (maybe one of the neutrals stroked the earthed strip on the back
of the socket plate?), which allowed a small amount of current (from the
other loads in the house, lights etc) to bypass the main RCD, which
unbalanced it, and caused just the effect that they are designed for. At
a guess you have an earth rod?

I don't know about an earth rod. Is that likely in a 15 year old house?

So this would happen even if the socket I was working on is unpowered?

Thankfully it didn't happen with the other 4 - that would have really
unnerved me! )

Daz