Get this problem a lot, plenty of phonecalls about this one.
Possibly an earth fault. When you say it trips the switch is it tripping the mcb or the RCD ?
The mcb is a small unit the RCD is a larger unit.The RCD wil disconnect all the circuits that it protects the mcb will only disconnect the one circuit.
If it trips the RCD then its an earth leakage fom the unit. The appliance needs looking at . Can be just a loose cable sometimes.
( Qualified Electrician)
Keith
June 2007