Most homes have circuitry that's limited to a 3000w per set and you can have multiple sets such as when set runs the ceiling lights other sets run other rooms walls or sections of the house. However just 3,000 watt limit makes it so you can't run a frigerator the toaster and a heater or a microwave on the same circuit. If that was your case there would be some understanding of why you could be tripping the circuit when you open the door in microwave but I don't think that's your case. You should test the microwave on an unoccupied circuit (that is one that has little or nothing plugged into it most likely a garage). If the device were to trip the circuit all by itself that would mean that there is a short within the microwave and the door could be all or part of the trigger. A spot where our concurred should be noticeable you are in fact creating a surge which is what trips the breaker and this should be noticeable if you were to get inside it. Since you have a limited electrical knowledge I would recommend replacing the unit strictly on the grounds that is much safer than having a microwave burn down in your kitchen.