I have one of those three-pin plugs that serve as an RCD. It plugs into the socket and then you plug in at the back. I use it for the lawnmower/hedge-trimmer outside. When I press the test button before use, it trips on one of the sockets I use, but not the other. The earth wire in the socket is connected OK, so what else can I try short of replacing it with a dedicated RCD socket? SOD's law, of course; the dodgy socket is the only one conveniently near the garden door. Are these devices just as good as the ones built into some sockets?