Household Circuits

Three wires bring mains electricity into a house. The live (or line) wire is at a high potential of about 220 volts and carries current into the house. The neutral wire is at almost zero potential and carries the current back, completing the circuit. The earth (ground) wire is a safety wire connected to a metal plate buried in the ground; it is joined to the metal bodies of appliances. In India the colour code is red for live, black for neutral and green for earth. The supply is alternating current at 220 V and 50 Hz.

All the appliances in a house are connected in parallel between the live and neutral wires, not in series. This matters for three reasons. First, each appliance receives the full supply voltage of 220 V, which it needs to work. Second, each appliance can be switched on or off independently without affecting the others. Third, if one appliance fails or its fuse blows, the rest keep working. A switch is always placed in the live wire so that when it is off, the appliance is truly disconnected from the high potential.

A fuse is a short piece of thin wire with a low melting point, made of a tin-lead alloy. It is connected in the live wire. If the current grows too large, the fuse wire heats up and melts, breaking the circuit before the wiring catches fire. Two faults cause this large current. A short circuit happens when the live and neutral wires touch directly, so resistance drops almost to zero and current shoots up. Overloading happens when too many appliances draw current through one circuit. Modern homes use a Miniature Circuit Breaker (MCB), an automatic switch that trips on excess current and can simply be reset, unlike a fuse which must be replaced.

Earthing is the most important safety measure for appliances with a metal body, such as a refrigerator or iron. The earth wire connects the metal body to the ground through a very low-resistance path. If a fault lets the live wire touch the metal body, the body would otherwise become live and shock anyone who touches it. Instead, the fault current rushes through the earth wire to the ground, and this large current immediately blows the fuse, cutting off the supply. In a three-pin plug the earth pin is made longer and thicker than the other two, so earthing is established first and broken last.