The fuse is a device used in an electrical circuit for protecting electrical devices against overloads and short circuit. It is the simplest and cheapest devices used for interrupting an electrical circuit under short circuit or excessive overload current magnitudes.
It is used for overload or short circuit protection in high voltage up to 66 KV and low voltage up to 400 KV. In some places, their use is restricted to those applications where their performance characteristics are especially suitable for the current interruption.
The action of fuse depends on the heating effect of the electrical current. In normal operating conditions, when the current flowing through the circuit is within the safe limits, the heat developed in the fuse element carrying this current is readily dissipated into the surrounding air, and therefore, a fuse element remains at a temperature below its melting point.
When the fault (short circuit) occurs or when the load connected in the circuit exceeds its capacity, the current exceed the limiting value, the heat generated due to this excessive current cannot be scattered fast enough and the fusible element gets heated, melts and break the circuit. It thus protects a machine or apparatus or an installation from damage due to excessive current.
A fuse consists of a fusible element in the form of a metal conductor of a specially selected small cross-sectional area, a case or cartridge to hold the fusible element, and in some cases, provided with a means to aid arc extinction.
The functions of the fuse wire are to carry the normal working current safely without heating and to break the circuit when the current exceeds the limiting current.
Advantages of an Electrical Fuse
- It is the cheapest form of protection, and it does need any maintenance.
- Its operation is completely automatic and requires less time as compared to circuit breakers.
- The smaller sizes of fuse element impose a current limiting effect under short-circuit conditions.
- Its inverse time-current characteristic enables its use for overload protection.
Disadvantages of an Electrical Fuse
- Considerable time is required in replacing a fuse after the operation.
- The current-time characteristic of a fuse cannot always be correlated with that of the protective device.
Fuses are used for the protection of the cables in low voltages light, and power circuits and for transformers of rating not exceeding 200 KVA, in the primary distribution system. Fuses are used in low and moderate voltage applications where the frequent operation is not expected or where the use of a circuit breaker is uneconomical.