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For safe operation of a PCB design, do I need to derate the voltage of a Fuse / PTC?

Can I for example assume, that a femtoASMDC008F can be used in a circuit with a nominal voltage of 12V (but gauranteed to be never above 12V)? If not, what would be the recommended derating values/safety margins I should keep?

Is there a difference between a fuse and PTC/polyfuse?

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Yes, and yes.

Regular fuses blow once and that's it.

Polyfuses heat up and go to an "open" state, where the voltage drop is high but some current continues to flow; the high power dissipation keeps the device hot, maintaining the "open" condition until reset. When it cools back down, the resistance returns mostly to nominal.

Check the manufacturer's application notes. They are safe to use up to rated voltage and current. If your source/load could draw more than the rated current, consider using a larger fuse, or increasing series resistance to guarantee staying within ratings.

Related: Polyfuse (resettable PTC) lifetime

As far as I know, PTCs generally fail open, so still act as regular fuses even if the PTC action eventually fails (or ratings are exceeded).

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