...what kind of overvoltage is to be expected at all. A tiny polyfuse with low voltage rating might not withstand rough surges.

Polyfuses have several disadvantages: They show huge tolerances, the threshold heavily depends on ambient temperature and the voltage rating and fusing capability is such limited, that you wish to have a fuse for the polyfuse. Also, they can take very long times (up to an hour or so) to recover.

I would only use them (and I do so in some projects!) when the replacing of a standard fuse is absolutely no option.

Instead, I would use a fat Transzorb (protected by standard slow blow melting fuse), rated such high, that only the haviest overvoltages will blow the fuse.

The Transzorb offers a big advantage: The failing condition of a Transzorb (or Transil, -> ST.com) is the short circuit, so that your circuit is even protected, when the transil becomes damaged somehow.

Kai Klaas