when you open the door the house earth trip switches off
Do you mean the RCD (residual current device, interuptor diferential) or the fuse?
In case of the RCD, a small imbalance in current between neutral and live wires (typically 30 mA for homes) trips the switch as it is assumed that the difference in current flows to earth somewhere creating danger of electric shock to persons. Imho there are two possibilities:
- When opening the microwave door a current path is created between live (or neutral) ans earth. Possibly turning on the light within the microwave chamber may have that effect.
- The RCD is out of specification and trips at too small a current.
Check #2 first by swapping the RCD with another one. If that still shows the same effect, the problem is within the microwave. As microwaves operate internally with dangerously high voltages and he tripping of the RCD shows that something potentially lethal is going on, I recommend you do not try to repair the unit by yourself. Get some expert help.