The current rating of a fuse specifies the current which the fuse will
carry without blowing. Once the current exceeds this value, the fuse
will eventually blow. The characteristic of a fuse is such that if the
current in a circuit is moderately higher than the fuse rating due to an
overload, the fuse will take some time to blow, possibly seconds,
minutes or tens of minutes, depending on the magnitude of the overload.
If the current is a lot higher e.g. in the case of a short circuit, the
fuse will blow in a few tens of milliseconds. While a low voltage or
high voltage fuse will blow when the current exceeds the current rating,
the current flow in a mains powered circuit can be extremely high
(possibly thousands of amps) in a short circuit scenario because the
current is coming from a large transformer outside the home. The
breaking capacity of a fuse is specified in joules or kiloamps and a
mains fuse could have a rating of 10000 amps or more. This is the
maximum current the fuse can carry without rupturing. Usually fuses for
mains equipment or distribution circuits have a ceramic body to
withstand the heat and shock when the fuse blows. The fuse is also
filled with quartz sand to absorb the heat and shock and quench the arc
when the fuse blows. A glass fuse could simply rupture and possibly
cause a fire if it carried such a heavy current. Glass/low voltage fuses
may be used on the secondary side of the transformer in the power
supply of equipment because the current output of the transformer in a
short circuit situation is limited.
