8.11 - Mains Wiring & Safety

A mains cable has three wires

Most appliances connect to the mains by a three-core cable - three separate wires, each in coloured plastic insulation.

The job of each wire

• Live wire (brown) - Carries the alternating potential difference from the mains, at about 230 V.
• Neutral wire (blue) - Completes the circuit, carrying current back out. It stays at about 0 V.
• Earth wire (green and yellow stripes) - A safety wire connected to the metal casing to stop it becoming live. It carries current only when there's a fault, and sits at 0 V.

Potential differences between the wires

• Live to neutral = about 230 V (the supply p.d.).
• Live to earth = about 230 V.
• Neutral to earth = 0 V, because both are at 0 V.

The switch in a socket is fitted in the live wire so that switching off isolates the appliance from the high voltage.

Why the live wire is dangerous

• Your body, like the earth, is at about 0 V. The live wire is at about 230 V.
• Touching the live wire makes your body a link between the supply and the earth, creating a large potential difference across you.
• This drives a current through your body, causing an electric shock that can injure or kill.

The danger stays when switched off

• Even with an appliance switched off, the live wire before the switch is still at 230 V.
• Touching it would still complete a circuit to earth, causing a shock.

Live-to-earth connections

Any connection between the live wire and earth is dangerous. If that connection is a low-resistance path, a very large current flows, which can start a fire.