If the RCD is correctly wired then current going from active to earth will trip it. The earth-nuetral link *should* (don't assume) be on the opposite side of the RCD to your house wiring. Disclaimer: I don't live in the UK nor am I an electrician, but it would be stupid to wire the earth-nuetral link on the same side of the RCD as your house wiring. It would not be able to trip if you were touching something earthed and something active simultaneously. I do not recommend using your earth wire as a return path for current.