2.3.2a Mains electricity and three-core cable

Three-core cable: three wires, three jobs

1. Most electrical appliances connect to the mains through a three-core cable.
2. Three-core means the cable holds three separate conducting wires inside one outer covering.
3. Each wire is usually made of copper so that charge flows through it easily when the appliance works.
4. The three wires are the live wire, the neutral wire and the earth wire, and each does a different job.
5. Every wire carries its own coloured insulation so you can tell the wires apart and connect them correctly.

Colour coding: brown, blue, green-and-yellow

1. The live wire insulation is brown.
2. The neutral wire insulation is blue.
3. The earth wire insulation has green and yellow stripes.
4. The colours matter because each wire must reach the correct part of the plug and appliance; a wrong connection can stop the appliance working and make it unsafe.

The live wire: it carries the alternating supply

1. The live wire brings the alternating potential difference from the supply, so its potential keeps changing direction as the mains alternates.
2. This makes the live wire the dangerous one: if a fault lets you touch a live part while you also provide a path to earth, charge can flow through your body and give you an electric shock.
3. The key point is simply that the live wire carries the alternating potential difference from the supply.

The neutral wire: it completes the circuit

1. An appliance only works if there is a complete circuit through it.
2. The live and neutral wires together connect the appliance across the mains supply, and the neutral wire completes that circuit.
3. Avoid saying the neutral wire does nothing: without it there is no complete circuit, so no charge flows and the appliance will not work.

The earth wire: a safety wire

1. Many appliances have a metal case.
2. If a fault let the live wire touch that case, the case could become live and give a shock to anyone who touched it.
3. The earth wire gives a safe path to earth, which stops the case staying live.
4. The essential point is that the earth wire is a safety wire whose job is to stop the appliance becoming live.

2.3.2b Mains electricity: potential differences and safety

Potentials in mains wiring: live at 230 V, neutral and earth near 0 V

1. The three wires in a mains cable sit at different potentials compared with the Earth.
2. The live wire is at about $230V$ relative to earth.
3. The neutral wire is at, or close to, earth potential, so it is roughly $0V$.
4. The earth wire is at $0V$.
5. So there is a potential difference of about $230V$ between the live wire and either the earth wire or the ground, and that potential difference can drive a dangerous current if a conducting path forms.

An open switch does not always make a wire safe

1. An open switch breaks the normal circuit, so current cannot flow around that circuit.
2. Breaking the circuit does not remove the potential difference between the live wire and earth.
3. The live wire on the supply side of the switch can stay connected to the mains, so it may remain at about $230V$ relative to earth even though no current flows through the appliance.
4. If you then touch that live wire while also connected to the ground, your body becomes a conducting path between live and earth, and the potential difference drives a current through you, giving an electric shock.

Why a live-to-earth connection is dangerous

1. A connection between the live wire and earth has about $230V$ across it, and that potential difference can drive a current through the connection.
2. The connection could be a person touching the live wire while standing on the ground.
3. It could be a fault that lets the live wire touch an earthed part of the appliance.
4. It could be any other conducting object joining live to earth.
5. A person who becomes part of this path can receive a serious or fatal electric shock.
6. A low-resistance fault path can also allow a very large current to flow, which produces dangerous heating and a fire risk.
7. During such a fault the earth wire carries current because it provides the path to earth; with no fault it stays at $0V$ and carries none.