2.9 - The Three-Wire Mains Cable
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.
TermDefinition
Live wireThe brown wire in a mains cable that carries the alternating potential difference from the supply.
Neutral wireThe blue wire in a mains cable that completes the circuit and sits at about 0 V.
Earth wireThe green and yellow safety wire that connects the metal case of an appliance to the ground, so a current can flow safely to earth if there is a fault.
Three-core cableA cable connecting an appliance to the mains that contains separate live, neutral and earth wires.