Given this diagram for a section of a mains circuit (seems like there's no varistor symbol) –
simulate this circuit – Schematic created using CircuitLab
I gather that it's important for this fuse to be present if the MOV short-fails. The fuse "time current characteristic curve", which I assume indicates when the fuse blows given a certain current and time:
It seems the "rated" current is not the melt current, but rather the current that the system is expected to typically draw; and that the melt current is much higher, dependent on time applied.
Are the circuit values appropriate? Is this compatible with residential mains?
Edit: I had been considering a PPTC fuse, but that idea was somewhat dead-on-arrival. To accommodate an AC motor with a startup surge potentially above 4A, the PPTC options are very limited, bulky, slow-acting and expensive. In comparison, filament fuses seem cheap, abundant and fast-acting.
Best Answer
Beware of the fault current ratings of pptc fuses .Say if you have 240VAC and the circuit wiring and the fuse has a resistance of say 1 ohm .The pptc fuse must interupt a prospective fault current of a whopping 240 amps .Double check your ratings ,most small pptc fuses will shatter despite having a say 265VAC rating.