Why does my microwave all of a sudden doesn’t heat and blows a circuit

I have tried all the troubleshooting suggestions relevant to the problem. I have also tried using the "Add 30 seconds" button and also tried manually entering a cook time (thinking maybe it was button related), but the problem continues to occur either way. The oven will go on as if it is operating normally for a split second, then shut off and blow the fuse (the switch is thrown at the circuit breaker upstairs).