Aubrey, polyswitches and the like are thermal devices and as such, are slow to respond. In a short circuit situation, you have a rapid rise of current limited only by the impedance of the source and the circuit. By the time a polyswitch has reacted, the triac is toast. Fuses react quickly, but you need a machanism to quench the arc, so high speed fuses usually have sand to quench this. Using a larger rated triac helps, but doesn't eliminate the problem. In light dimming circuits I used dual thyristors rated at 40A. Apart from being larger and much more expensive, they would withstand a short circuit multiple times whilst only being protected by a standard din-rail circuit breaker.