Do thermal cutoff fuses wear out if used below rated temperature?

Question

I tend to keep my electrical applicances alive for decades and have had several thermal fuses (the non-resettable axial kind) repeatedly wear out for no apparent reason, some of them in expensive devices from reputable brands. I know not to solder these things so I don't think I'm pre-stressing them when I replace them.

This is the kind of component I'm talking about: https://www.cantherm.com/wp-content/uploads/2017/05/SDF_JUNE_2016.pdf

Question: does the waxy stuff inside the fuse wear out at temperatures well below the rated temperature?

Answer 1

Yes they do, as you have observed. When heated significantly, but still below melting point, metal crystal structures tend to rearrange into larger crystals and ultimately the electrical path will degrade- have a look at an old car headlamp bulb. Premature ageing can result from soldering but even in normal use these devices do degrade.

Perhaps consider a self-resetting PTC fuse if that’s an option.