Why a British Standard 13A rated fuse does not blow at 13A?

Michael Osborn from connected-fidelity™ gives more insight into fuses.

You would not think it, but a 13A British Standard (BS) rated fuse is not designed to fail at 13A. In fact, a 13A fuse designed to BS 1362 allows a current of 20A to pass indefinitely without blowing.

The wire inside a fuse heats up when a current passes through it. The more current that passes, the hotter the wire will get and when the current is high enough, the wire melts and the fuse is ‘blown’.

The rated current of the fuse (in this case 13A) is the current that it can safely pass whilst maintaining a low enough temperature not to overheat the plug it is in. BS 1362 specifies a maximum power dissipation (heat) of one watt when carrying the rated current to ensure this, it is a safety standard to prevent fire hazard. The actual current where a BS 1362 fuse will blow is about 1.66 times the rated value, so for a 13A fuse this is about 21.6A.

All metals heat up when an electric current is carried, the flow of electron charge generates heat along with the energy. Electrical resistance changes slightly in heated conducting materials and it may be partly this effect that can alter the sound quality of an audio system powered through one or more fuses, and in part why some fuse designs and treatments do make a difference to audio and A/V output quality.