neff inset oven keeps tripping the rcd?

can anyone tell me what may be wrong with my neff b1421 electric fan assisted oven with with seperate top grill. when i turn the knobs to start the fan oven the rcd keep tripping the whole house.no individual trips go off thou. i have gone as far as disconnecting the live to the 2 ring element around the raer fan and the other elements and fan and stat seem to operate fine.i know it doesn't have to be the element as i have broken the circuit that controls that so wherever the fault is may not be powering up with the element disconnected but it is a start.no visible sign of any cracks in the element either. any ideas guys.thanks.