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?
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).
You should stay within the ratings.
That particular fuse is rated at 12 max so it should be okay at 12V or less.
However, note that when you say "nominal" voltage that seems to contradict your statement that it will never exceed 12V- normally tolerances for voltage are +/- so a 12V nominal source with 5% or 10% tolerance could be as high as 12.6V or 13.2. Also, automotive "12V" is more like 14V in an ICE vehicle when the engine is running. In such case, choose something like the 15V or higher rated device. Derating is generally a good idea, especially if reliability is important to you.
Also note that the maximum break current is rated at only 10A so if your fault current can exceed that you should use a different fuse.
