Thanks folks for the information on this thread; it has proven most helpful. Our 5-year-old over-the-range GE microwave has been blowing fuses, but I think that your input has helped resolve the problem.
I live in a northern Canadian city. Our microwave is mounted on an exterior wall and vents straight to the outside. Yes it gets cold. A couple years ago, when it was really cold outside (below -30C) it blew a fuse when the door opened. Last year, this started to happen more often, when we were only at -20C. Earlier this winter it started happening at -10C.
I found this thread and the next time the fuse blew, I took a look at the switches and found that they had some play in where they were positioned. My guess is that the cold weather caused them to shift such that when the door opened they did not open/close in the right order. I shimmed them so that the switches were better fixed in place and would trigger in the required order. So far (and I’m knocking on wood here) even though we are going through a -20C cold patch, the microwave is continuing to operate without blowing a fuse.
So thank you for your insight and advice!