You will usually see fuses blow for these different reasons.
- - Thermal shock. Hot to cold, back to hot repeatedly.
- - Vibrations - constant vibrations - its metal.
- - Heat - like way too hot, such as under the hood of a car, near the exhaust.
- - A dead short.
Usually the wire just opens up for reasons one, two and three. If the visual
area of the fuse is black, like a flash bulb, then there usually is a more serious problem; a dead short.
When the fuse has a current load of perhaps 150-200%, it will usually cause
the fuse wire to melt in the middle, with a ball of metal on the ends of
the fuse wire. Thermal reasons usually cause the fuse wire just to break
(crack).