The only realy effective way to sort this problem of tripping the RCD is to use a multi meter of some description and check all the live and negative wireing connections against the earth or cabinet. Set the meter on Megohms this will register the slightest earth leek. The most common earth leak is on the motor (if it has carbon brushes)the carbon builds up and forms a bridge across the caseing of the motor. The next is the heater/s. If as you say the fault is intermitent it could be a chaffed wire, so check where any wiring could be rubbing against the cabinet.
workman1, May 2008
