T3sl4co1l and tautech are correct, it is an NTC. In series with the mains input, for inrush reduction. I knew it, just forgot when replying to blueskull.
The fuse definitely is pathetic, tiny and glass. It probably wouldn't actually quench a 240V arc. I suppose I should replace it, but don't much care. That's why we have circuit breakers.
Hrm, well, Idunno--
It might not quench a hard arc (i.e., the circuit fails shorted so hard that fault current (low ~kA) flows), but then the breaker should take care of that. There may be unusual scorching of the board, maybe a bit of a flameout too?
A softer failure (say the mains side electrolytic starts cooking off..??) might blow the fuse quietly, but wouldn't be enough to trip the breaker (since we're talking amps, not tens or hundreds of amps). If the fuse weren't there at all (i.e., depending on just the breaker), heat is able to build up, and you get an explosion, or fire, or something like that...
It might take a contrived situation to better illustrate this, but stranger things can happen in reality, so bear with me.
Example:
I was working on a prototype switching supply, working behind an isolation transformer. The trans was fairly undersized, so added significant series resistance to the supply. A shorted secondary wouldn't even trip the breaker! Now, the work was fused adequately, but the sequence of events that transpired was:
1. Supply working nicely like it should. Yay. Getting kind of warm though.
2. Main transistor (flyback topology) fails shorted. DC+, drain-source, and shunt resistor to DC-, draws mondo amps.
3. Transistor doesn't actually fail completely three-way-shorted, but blasts plasma for a while, dropping say 100V between pins. Erupts in flame.
4. A couple of seconds later, the equipment fuse (2A I think it was) burns out. But it doesn't really burn, it kind of splatters inside the glass tube, with much vapor residue. (The glass didn't crack or rupture.)
So the energy delivered was much greater than if the fuse had blown out promptly.
There might be a situation where this could happen with breakdown of insulation, and odd transistor failure modes (not quite the usual melt-into-short-circuit behavior), so I wouldn't discount it as purely a consequence of my crappy transformer.
Tim