Three wires in a mains cable

1. Most mains appliances connect to a socket through a plug and a flexible three-core cable.
2. Each core is a bundle of fine copper wires surrounded by its own layer of coloured plastic insulation, and an outer plastic sheath holds the three cores together.
3. The live wire has brown insulation.
4. The neutral wire has blue insulation.
5. The earth wire has green and yellow stripes.
6. The colours let an electrician identify each wire instantly, so the wires are never connected to the wrong pins.
7. Each of the three wires does a different job in the circuit.

The live wire

1. The live wire brings the alternating potential difference from the mains supply into the appliance.
2. Its potential alternates continuously, but it stays at about $230V$ relative to the earth.
3. The switch that turns an appliance on and off is connected in the live wire.
4. The live wire is the most dangerous wire in the circuit, because touching it can let a current flow through your body.

The neutral wire

1. The neutral wire completes the circuit by carrying the current from the appliance back to the supply.
2. In normal use the current flows into the appliance through the live wire and out through the neutral wire, so the two wires carry the same current.
3. The neutral wire is joined to the earth at the local substation, which keeps it at or very close to $0V$.

The earth wire

1. The earth wire is a safety wire that joins the metal case of an appliance, through the earth pin of the plug, to a connection in the ground outside the building.
2. It holds the metal case at $0V$, the same potential as the ground you are standing on.
3. In normal use the earth wire carries no current.
4. If a fault makes the live wire touch the metal case, a large current flows through the low-resistance earth wire into the ground instead of the case staying at $230V$.
5. The case therefore cannot reach a dangerous potential, so a person who touches it does not receive a shock.
6. Appliances with metal cases, such as washing machines, fridges and electric ovens, must have an earth wire.

Potential differences between the wires

1. The potential difference between two wires is the difference between their potentials.
2. The potential difference between the live and neutral wires is about $230V$, and this is what drives the current through an appliance.
3. The potential difference between the live and earth wires is also about $230V$, because the earth wire is at $0V$ like the neutral wire.
4. The potential difference between the neutral and earth wires is about $0V$, because both are held at the potential of the ground.
5. A current flows only along a conducting path that joins two points with a potential difference between them.
6. A path between live and neutral is the normal working circuit, while a path between live and earth is a fault or a shock.
7. No current would flow along a path between neutral and earth, because there is no potential difference to drive it.