Richard was bang on there.
Fuses, though a simple form of protection can in fact be quite complex in operation. It all depends on the fuse element heating up enough to melt and disconnect the circuit, before the wiring or protected component burns out! Ambient temperature will have an effect so fuses in a warm environment may not be able to carry the same current for as long as a fuse in a cooler environment, because they already have a head-start temperature wise.
The rating of a fuse is generally what it should be able to carry continuously, without melting. However, in the case of automobiles, it is advised in some quarters, that you should not exceed a maximum current loading of about 70% of the fuse rating for reliability.
Fuses are excellent under fault conditions, (short circuit/ very low resistance faults) creating disconnection in a fraction of a second.
However, under overload conditions, they are not quite so clever, or quick, as they take longer to heat up and melt. A typical auto fuse loaded up to say a third over its rating may take up to 10 minutes to finally blow and that can mean quite a cumulative temperature rise. ( just like Dave/ Montague has seen with his spot light circuit)
