Why is voltage specification irrelevant in tripping a PPTC polyswitch?
The quick answer is that the power is not dissipated in the fuse but in the load. If, for sake of argument, the fuse has an internal resistance of 0.1 ohm then at it's full rated current it will be dissipating I^2R W = 1.6W and experience a voltage drop of IR = 0.4V. The rest of the voltage, be it 15.6V or 239.6V is across the load it is protecting.