Electricity Hazards
Electric energy is distributed to various houses in a city from a power station by means of two wires. One wire is earthed at the power station so it is at zero potential. It is called neutral wire. The other wire is at a certain potential. It is called live wire. Our body is a good conductor of electricity through which current can easily pass.
Therefore, if a person holds live wire, then because of the presence of voltage in it, current will start flowing to ground through the human body which may prove fatal for the person. In order to avoid such a situation, the wire carrying electricity in the houses should never be naked. Rather it should be covered with an insulator. Such an insulation covered wire is called cable.
It is very essential that the layer of insulation in the cable is perfect and is not damaged. But sometimes a heavy current flow through the wire and it gets so hot that its insulation is burnt out and the wire becomes naked and it becomes dangerous (fig. 16. 22). Similarly, constant friction also removes the insulation from the wire whereas too much moisture also damages the insulation. In such a situation it is advisable to use a cable with two layers of insulation (fig. 16.23).
Take care
the overloading of the electric socket can cause severe damage.
Safe use of electricity in homes
Fig. 16.24 shows a typical circuit used in houses for the supply of electricity. The various household appliances such as electric bulb, fan electric iron and T.V. etc., are connected in parallel with the live and neutral wires through a switch. When any appliance is switched on, the current flows from the live wire to the neutral through the appliance and its switch. A specified amount of current can pass through the switch.
If the current exceeds this rated value, the switch is damaged after getting heated up. Before connecting a switch in the circuit, it should be ascertained that the maximum current that would pass through the switch lies within its rated value. It is very dangerous to connect an appliance directly with the live and the neutral wire. The connection is always made through a socket and a plug as shown in fig. 16. 25 when it is desired to connect any appliance in the circuit, then its plug is inserted into a socket. Thus terminals of the appliance are connected with live and neutral wires. When switch is turned on, an electric current flows through the appliance.
Fuse
Fig. 16. 24 shows a small wire connected in series with the live wire. It is known as fuse wire or fuse. A specified value of current can safely pass through it. When the current passing through it exceeds this limit, it gets so hot that it melts. The fuse wire is connected in series, in live wire. If we simultaneously switch on many appliances or due to fault in electrical wiring, live and neutral wires get short, a very heavy current passes which may damage the meter or burn the whole house.
Here the fuse wire plays its role. As the fuse wire is in series with the live wire, so the short circuit current also passes through it. This current being much higher than the rated value of the fuse, blows the fuse off. Thus the circuit gets open and the flow of current stops. After removing the fault of the circuit, the blown out fuse is replaced with a new one.
Take care
if you have to use extension wire for some appliance like heater, its resistance should be small so that its insulation does not melt due to overheating.
Using eq. 16. 15 we can determine the required fuse rating for a circuit. Suppose we want to insert a fuse for an air-conditioner or heater of power 3000w. If voltage supply is of 240V, then according to relation P= Vx I, we get I = 12.5A. The available fuses in the market are usually of rating 5A, 10A, 13A, 30A etc. Hence, suitable fuse for this circuit would be of 13A.
Do you know?
by passing more current than the specified value, the fuse wire melts and breaks the circuit.