1.11 - Wires & the Ring Main
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.
The ring main
- House sockets are wired on a ring main - a looped circuit running from the consumer unit, out to all the sockets, and back again.
- The live, neutral, and earth wires all follow this loop.
Advantages of the ring main
- Sockets can be placed anywhere on the loop.
- Sockets are connected in parallel, so each receives the full 230 V and can be operated independently.
- Current can reach a socket along two paths, so less current flows through each section of cable - meaning thinner wires can be used.
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.
Ring mainA looped parallel circuit running from the consumer unit that connects the sockets in a house.