CrossRoads:
PTC won't protect against reverse voltage tho. By the time they blow, the damage is done.
The idea is that the circuit is actually protected by a fat rectifier that creates a short across the power lines if the polarity is wrong. Then the short circuit current blows the fuse or the PTC or what have you. The circuit itself sees only the -0.7V voltage drop of the rectifier.
When power is hooked up the right way around, there's no voltage drop at all from the protection circuit -- if it uses a fuse.