How do I fit a light which requires an earth?

Matt's house of course has earths but possibly doesn't have earths run with the lighting circuits, as this is a requirement only introduced in recent years. If its an older house, earths weren't required at lights.
There could be a fair bit of work involved in running in earths to existing light fittings and switches but it can be done and they can be run back to the distribution board.
If you want avoid doing this, the best answer is fit a double insulated light fitting (i.e. not a metal one) and this will avoid the earth issue. Double insulated fittings are marked with a symbol consisting of 2 squares, one inside the other.

J
March 2009

Your house will have an earth somewhere unless it has a very rare type of wiring system.

What colour are the wires in your switches?

Adam
March 2009