You know that each electrical plug has three pins. These three pins are connected to three wires which in turn are connected to an electrical appliance. Two of these three pins are of the same size while the third pin is thicker than the other two. It is usually connected to a green wire. This is known as the earth wire. Do you know what the function of the third pin is?
The third pin and the third wire (green) in an electrical lead carry no current when everything is normal. One end of this lead is connected to the body of the electrical appliance and the other end is connected to a metal plate buried in the ground. In fact the third pin connects the body of the electrical appliance to the earth. This is known as electrical grounding. It is a precaution in case a live wire accidentally comes into contact with the metal frame of the appliance.
Sometimes the electrical appliance may develop a fault which makes the electricity flow to the metal casing. Then a person touching it receives an electric shock. The electricity flows from the casing, through the person, into the ground. The shock felt by the person depends on the intensity of current. If, for example, the person has wet hands, the shock experienced would be of a greater intensity, because wet skin conducts electricity better than dry skin.
Third pin or earthing is a method used to prevent faulty appliances from giving shocks. So it is very important to ensure that the equipment running off the mains is properly grounded. It is done with the help of the third pin of a plug.
In case a properly grounded appliance develops a fault, the electricity would flow into the ground from the metal work through the earth wire. A person touching the appliance would not receive a shock. Since the resistance of the wire is lower than that of a person, it is easier for the current to flow through the wire. Therefore, very little current flows through a person if he or she touches the faulty appliance. Since the wire’s resistance is very low, a very large amount of current flows through it. This causes the fuse wire to melt and cut off the electrical supply. Hence the third pin of the plug serves as a safety device against electrical shocks.