Just as most homes are equipped with fuses or circuit breakers, microwave ovens also have fuses that short out or blow in the event of an overload. If your microwave oven blows a fuse, there are a couple possible causes. And if it blows the fuse for your kitchen, there also is a likely cause for that.
Fuses are simple devices containing a wire made of a metal that melts at high temperature. When a short circuit occurs, the current through the fuse soars; the higher current means more heat, and the wire in the fuse melts, breaking the circuit. This process destroys the fuse, but it protects other more valuable components in the circuit that might otherwise be damaged by the high current. Basically, then, a blown fuse in a microwave is the result of a surge in the current. This surge could result from several causes.
Your microwave has what engineers call interlock switches that disable the oven when the door is open. Malfunctioning interlock switches or a badly misaligned door can cause the fuses to blow. It's also possible the microwave's cooling fan may have failed, leading to overheating and melting the element in the fuse. Another possible problem is a short in the controller or near the line cord that feeds power into the microwave unit.
If you have a short in the controller or near the line cord, the new fuse usually blows again immediately. The fuse also blows again if there is a short in the microwave magnetron or generator, the part of the microwave that actually produces the microwave radiation to cook the food. The location of the generator depends on the model you have; typically it's located behind a perforated screen overlooking the chamber that houses your food. You can disconnect the generator by unplugging the wiring connecting it to the cavity thermal fuse and the other circuitry; the location of these elements depends on the model, so consult the manufacturer's instructions for details. The microwave will not heat food with the generator disconnected, but if you are able to select a cycle and initiate it as you normally would without blowing the fuse again, then the short may be within the generator itself.
Sometimes there is no short, and the fuse has gone bad without any particular cause. In this case, replacing the fuse solves the problem. If the fuse blows again, however, then it is likely you have a larger problem such as a short in the controller. And if using your microwave blows the fuse for the whole kitchen, the problem is usually fairly straightforward: You have too many appliances on the same circuit. Disconnecting one or more of them should solve the problem.