Trips?
If it trips a miniature circuit breaker (MCB) it either takes too much current or the MCB is faulty - or (most unlikely) other equipment and the uwave combine together to exceed the rated load for the MCB.
If it trips an earth leakage residual current device (RCD) then the oven may have a fault - it may be a design feature common to all similar ovens (doubtful) the RCD may be faulty or improperly specified or the wiring to the appliance may be defective.
Significant is the fact that two similar appliances fail in the same way.
You do not say if anything has changed recently - new fault - new oven - new wiring - was there a time when the fault didn't happen?
If it were mine I would consider calling in an electrician to check the integrity of the circuit that supplies it - insulation resistance.
Others on here may give you better answers and in fewer words - I am not qualified or experienced in these matters - not a certified electrician.
Good luck...
Peccavi
August 2013