What is the function of an earth wire? Why is it necessary to earth metallic casings of electric appliances?
Earth wire acts as a safety measure. When the live wire touches the metallic casing of an electric appliance, the electric current flows from the casing of the appliance to the earth through the copper wire. As the Earth offers very low or almost no resistance to the flow of current, so large current passes through the copper wire instead of human body. This large current heats the circuit and hence, the fuse in the circuit melts. As a result of this, the circuit gets switched off automatically and hence, the electric appliance is saved from burning and the human body suffers no electric shock.