A fuse is a protective device. It is a weakest portion in the electrical circuit. It can thus be defined as, “the fuse is a safety device which protects the electrical circuit from taking an overload current”. The fuse essentially is a small piece of metal wire connected between two terminals mounted on an insulated base. It is always kept in series with the electrical circuit.
Necessity of fuse. The fuse is required in a circuit to avoid the flow of overload current through a circuit. If overload current flows, the apparatus, instruments, machines and even the wires may be damaged. So to protect these a fuse is essentially required to limit the current in the circuit.
Working of a fuse. A fuse wire is always connected in series with a circuit so that the Current of the circuit flows through the fuse. Obviously the heat is produced in a conductor. if it carries current and the heat produced is thus proportional to the square Of Current flowing in the circuit.
Generally, the fuse wire are Of low melting temperature. These are made either of lead tin alloy or tinned copper wire. In normal conditions the fuse carries the normal current safely without heating. But when the current exceeds the normal working current, the amount of heat produced with increase to the melting temperature and finally the circuit will be disconnected, because the fuse element is in series of the load. Thus an open circuit will save the circuit from being overloaded.
Define the following terms:
(a) Current rating of fuse wire (b) fusing current (c) fusing factor (d) fuse element.
(a) Current rating of fuse wire. It is the value of current which the fuse wire carry without any melting. It is also known as the safe current of fuse wire. All the fuses are rated at this current.
(b) Fusing current. It is the value of current at which the fuse melts. It is always more than the safe current rating of fuse.
(c) Fusing factor. It is the ratio of the fusing current and the safe current of the wire.
Fusing factor = Fusing current of the wire/Current rating of fuse wire.
It is always more than one.
(d) Fuse element. It is a fuse wire connected in series with the circuit. It is a protecting device which protects the circuit from taking an overload current.