Then what causes my bulk eraser to shut off when it overheats
-- then come back when it cools off?

That's a resettable *thermal* fuse, NOT a fuse designed for
overcurrent protection.

I had always thought that at least some polyfuses /were/ resettable.

Polyfuses (in the sense of the term I've always heard used) are
self-resetting, unless you hit 'em so hard that you damage them.

They aren't designed for use at mains voltages/currents, though... at
least, I haven't seen any that are.

The self-resetting thermal "fuses" that you'd find in a bulk eraser or
similar "not intended for a 100% duty cycle - let it cool down!"
device are (I think) of the bimetallic type, somewhat akin to what you
would find in a traditional wall thermostat.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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