My outside PIR Lighting trips my RCD ?

My outside PIR lighting trips out my RCD just as it is about to turn off on timer. It switches on, and works fine prior to turning itself off. I have just fitted a brand new light in it's place, and this one is doing the same thing. This has only just started to happen after the recent snow storms. The old unit was not water damaged, and my replacement one is brand new. Any ideas please.

Steve Nash
February 2009