A three-pin mains plug has three wires. Each has a different job, and each has a standard colour for its insulation.
The live wire is brown. It carries the alternating potential difference from the supply to the appliance. The live wire is at about 230 V compared with the earth. The neutral wire is blue. It completes the circuit by carrying current back to the supply, and it stays at about 0 V.
The earth wire is green and yellow. It is a safety wire. It is connected to the metal case of an appliance and does not normally carry any current. If a fault makes the case live, the earth wire gives the current an easy path to the ground. The potential difference between the live wire and the neutral wire is about 230 V, and between the live wire and the earth wire it is also about 230 V. Between the neutral wire and the earth wire it is about 0 V.
A fuse is fitted in the live wire inside the plug. When wiring a plug, each coloured wire goes to the correct terminal and the cable grip holds the cable firmly.
