Earth Fault

At the end of a wash cycle when the door lock is waiting to disengage the machine trips the house circuit RCD.
If left in the same cyle position the RCD cannot be reset. If the position is moved the RCD can be reset and a new cycle begun. Checking the plug, there is a closed circuit between the earth and live (when in the cycle end position).
This fault occurs approx every 2nd cycle or every cycle. What could be causing this?