A fuse consists of a replaceable part (the fuselink) and a fuse holder. The simplest fuselink is a length of wire. It is mounted by screw connections in a holder which partly encloses it. When an overcurrent or short-circuit current flows, the wire starts to melt and arcing commences at various positions along it. The arc voltage causes the current to fall and once it has fallen to zero, the arcs are extinguished.
The larger the wire cross section, the larger is the current that the fuselink will carry without operating. In the UK, fuses of this type are specified for use at voltages up to 250 V and currents up to 100 A. They are known as semi-enclosed or rewireable fuses.
The most common fuselink is the cartridge type. This consists of a barrel (usually of ceramic) containing one or more elements which are connected at each end to caps fitted over the ends of the barrel. If a high current breaking capacity is required, the cartridge is filled with sand of high chemical purity and controlled grain size. The entire fuselink is replaced after the fuse has operated and a fault has been disconnected. Cartridge fuses are used for a much wider range of voltages and currents than semi-enclosed fuses.
Fuselinks can be divided into current-limiting and non-current-limiting types. A sand-filled cartridge fuselink is of the current-limiting type; when it operates, it limits the peak current to a value which is substantially lower than the prospective current. A non-current-limiting fuse, such as a semi-enclosed fuse, does not limit the current significantly.
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