Electrical safety — Know

What are the three wires in a mains circuit, and why must a switch be connected to the live wire?

A mains circuit consists of a live wire (line wire), a neutral wire, and an earth wire. A switch must be connected to the live wire so that when it is open (off), the live wire supply to the appliance is cut off; if the switch were on the neutral wire instead, the appliance could remain live and dangerous even when 'switched off'.