Think of it as a link, a wire, that in a dryer typically connects power source to heat source (heating element). A thermal fuse is there as a safety device to prevent dryer from overheating (or worse). When it senses that the temperature went over a certain limit, the wire will break, thus cutting power to the heating element.
Common causes for a thermal fuse to break:
1. Clogged dryer vent - will cause overheating and raise temperature inside the dryer, triggering the break in the fuse.
2. Faulty hi-limit thermostat
3. Nothing lasts forever. Sometimes a bad fuse is just a bad fuse.
So, if a fuse goes out, check the vent. You may want to replace the hi-limit t-stat just in case also.
Hope this helps.
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