2 possibles
water heater or the main motor damp/carbon dust tracking
check the heater element is not s/c
open machine find the heater element and remove the 2 heavy wires , turn on power if rcd stays in replace heater element if rcd still trips check the motor in the same way by removing the jack plug .(connect the heater back first)
will be one or the other
wirecutterman
March 2010