The electric wires used in domestic wiring are made of copper metal because copper is a good conductor of electricity having very low resistance. Now, the copper wires chosen for household wiring are of such thickness so as to allow a certain maximum current to passe through them. If the current passing through wires exceeds this maximum value, the copper wires get over- heated and may even cause a fire. An extremely large current can flow in domestic wiring under two circumstances: short circuting and overloading.
1. Short Circuiting:- If the plastic insulation of the live wire and neutral wire gets torn, then the two wires touch each other. This touching of the live wire and neutral wire directly is known as short circuit. When the two wires touch each other, the current flowing through the wires becomes very large and heats the wires to a dangerously high temperature, and a fire may be started.
2. Overloading:- The current flowing in domestic wiring at a particular time depends on the power ratings of the appliances being used. If too many electrical appliances of high power rating (like electric iron, water heater, air conditioner, etc.,) are switched on at the same time, they draw an extremely large current from the circuit.
This is known as overloading the circuit. Overloading can also occur if too many appliances are connected to a single socket. Now, due to an extremely large current flowing through them, the copper wires of household wiring get heated to a very high temperature and a fire may be started.
It is obvious that we should have some device which may disconnect the electricity supply when a short circuit or overloading occurs so that the electric fires are prevented in our homes. To avoid this danger of electric 💡 fires we use an electric fuse in the wiring.
So, when a building is wired, the wiring protected by fuses. We will now describe what a fuse is and how it works.
A fuse is a safety device having a short length of a thin, tin- plated copper wire having low melting point, which melts and breaks the circuit if the current exceeds a safe value. The thickness and length of the fuse wire depends on the maximum current allowed through the circuit. An electric fuse works on the heating effect of current. The fuse for protecting our domestic wiring is fitted just above our main switch on the switch board. A fuse wire is connected in series in the electric circuits.
The main fuse in domestic wiring consists of a porcelain fuse holder H having two brass terminals T1 and T2 in it. This is connected in the live wire. The other part of the fuse is a removable fuse grip G which is also made of porcelain. The fuse grip has a fuse wire fixed in it. When fuse grip is inserted in the holder, then the circuit of our domestic wiring is completed. So, under normal circumstances when the current is within limit, the fuse wire is intact and electric current is available in our wiring.
When a short circuit takes place, or overloading takes place, the current becomes large and heats the fuse wire too much. Since the melting point of wire is much lower than copper wires, the fuse wire melts and breaks the circuit as shown picture.
When the fuse wire breaks, electricity supply is automatically switched off before any damages can be done to the rest of the wiring (or the electrical appliances being used).
We will now give some important points about the fuse wire to be used in electrical circuits. First of all we should know why we use a thin wire as a fuse wire and not a thick wire. We use a thin wire in a fuse because it has a much greater resistance than the rest of connecting wires. Due to its high resistance, the heating effect of current will be much more in the fuse wire than anywhere else in the circuit.
This will melt it will have a low resistance and hence it will not get heated to its melting point easily. The fuse wire is usually made from tin- plated copper wire having low melting point so that it may melt easily. A pure copper wire cannot be used as fuse wire because it has a high melting point due to which it will not melt easily when a short circuit takes place.
