FUSE
If a circuit is closed but no resistor is in the circuit, a heavy or high current will flow and produce large amounts of heat in the wire and draw large quantities of power from the battery. The result is that the wire becomes very hot and in many cases burns. This is called a short circuit and is the cause of many fires. It may result in an electrical appliance becoming inoperative. To prevent this happening, each normal circuit is protected by a fuse which is device that holds a piece of thin wire and is placed in the circuit. If the heat in the wire should rise above an acceptable level, the wire in the fuse will heat and melt and open the circuit. No damage will be done to any device or wire and when the cause of the heating is fixed and the fuse replaced, the circuit is ready for use again. Fuse wires are bare wires made of easily melting materials having high specific resistance. Usually standard alloy (63% tin and 37% lead) fuse wires are used for small currents, say up to 5 A and tinned copper for large value of currents.
CLASSIFICATION OF FUSES :
Fuses may be classified into two groups
(i) Those designed to protect the circuit both from overload and short circuits.
(ii) Those designed to protect the circuit from short circuits only.
In the first condition fuse element breaks the circuit above 25% overload, and in the second case the fuse wire melts at a current which is several times greater than the normal current.
TYPES OF FUSES :
1.REWIRABLE FUSE
These fuses are also known as semi-enclosed fuses or kit-kat fuses and are used in domestic installation. In these types of fuses, the fuse wire is neither totally enclosed nor is it kept in free air. Figure shows semienclosed fuse. In these types of fuses the fusing current of a copper conductor is nearly double the rated current and has a low rupturing capacity.
2.CARTRIDGE FUSE :
The cartridge fuse is an enclosed type of fuse. It is of two types,
(i) D-type cartridge fuse.
(ii) Link-type cartridge fuse
