Popcorn is a low-moisture food which can cause a microwave to overheat, as can a failure of the cooling fan or blocked vents.
You should always turn a paper plate upside-down on the turntable (slightly off-center for better heating) and place the popcorn container on top of that.
The paper plate elevates the popcorn for better exposure to the energy and protects the turntable from excessive heat.
Placing food off-center instead of in the center increases exposure to the energy and prevents the same spot on the turntable from being overheated repeatedly, which leads to material fatigue and cracking.
It's also a good idea to put about 1/2 cup of water in a 1-cup microwave-safe container in the corner of the oven when cooking low-moisture foods like popcorn, bacon, and potatoes.
The microwave's magnetron has a thermal protector (thermostat, TCO, or thermal fuse) attached to it.
If the magnetron gets too hot, it can be damaged or destroyed, so the thermal protector opens and disconnects the power to the whole oven.
If it is a resettable device, the power will be restored when the device cools. If it is not resettable, the device must be replaced.
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- William E. Miller, AS-EET
Microwave Oven Control Panel Repair Nationwide
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