Thermistor in line with power wrapped to side of coil.

Thermistors detect the temperature of the surface of the fuser roller and let the system board know when to turn the heat lamp on and off.

I should have said "thermal fuse". They just pop open at a designated temp. I don't know if they are available in ranges applicable to pinball coils.

I would think if a coil got hot enough to pop a thermal fuse the sleeve is a melted glob anyway, so it's done. Protecting the rest of the game from a locked on coil is along the same line as unplugging them during a storm.