Mains electricity — AQA GCSE Combined Science

Mains cables contain copper conductors surrounded by flexible plastic insulation. That insulation is coloured so that anyone working on a plug, socket or appliance can identify each wire quickly and safely. In a three-core cable, the live wire is brown, the neutral wire is blue, and the earth wire is green and yellow stripes. The colour is on the insulation, not on the copper itself, and it is a safety feature: correct identification prevents the live wire being confused with the neutral or earth. For example, when wiring a three-pin plug you match each coloured core to the correct terminal, and you check the colours again before closing the plug. The same principle applies to flexes, fixed wiring and appliance leads.

The live wire in a modern UK mains cable is covered with brown insulation. It carries the alternating potential difference from the supply and is the wire that is switched and fused for safety. In a three-pin plug the brown core connects to the terminal marked L, which links to the fuse and then to the live pin. The live wire is dangerous because it is at a high potential difference relative to earth, so it must never be touched when the circuit is live. For example, in a table lamp the brown core goes to the live pin, the blue core to the neutral pin and the green-and-yellow core to the earth pin. Older cables may use red for the live wire, so the colour must be checked rather than assumed.

In a three-core mains cable the neutral conductor is identified by blue insulation. It completes the circuit: current flows from the live wire, through the appliance, and back along the neutral to the supply. In normal operation the neutral stays close to earth potential, so it is not the wire that delivers the dangerous driving voltage; the live wire does that. The blue colour is a safety code, letting anyone wiring a plug or inspecting a cable connect the correct conductor to the correct terminal. A common example is a table lamp: the blue core is clamped into the neutral pin, the brown core into the live pin, and the green-and-yellow core to earth. If the blue core were swapped with the brown in the plug, the appliance could still run, but the appliance's switch would sit in the neutral conductor, leaving parts live when apparently off.

The earth wire in a three-core mains cable is covered in green and yellow striped insulation. It is a safety conductor, not part of the normal current path. It connects the metal case of an appliance to earth, so its potential stays at zero volts. If a fault lets the live wire touch the metal case, a large current flows through the earth wire to earth. That surge melts the fuse or trips the circuit breaker, disconnecting the live supply before anyone can receive a shock. A metal-cased kettle is the classic example: its earth wire protects the user if a live conductor works loose inside. Appliances with plastic cases are often double insulated and need no earth connection.

A three-core mains cable contains live, neutral and earth wires with distinct jobs. The live wire carries the alternating potential difference from the supply, so its potential difference repeatedly changes direction and magnitude, typically peaking near 325 V for a stated 230 V rms supply. The neutral wire completes the circuit, providing the return path so current can flow through the appliance. The earth wire is a safety wire connected to the metal case of an appliance; it stops the case becoming live because a fault current flows to earth and the low-resistance earth path makes a large current that melts the fuse or trips the circuit breaker, disconnecting the live wire. In a correctly wired plug, the live wire connects to the fuse and the brown insulation, neutral to blue, and earth to green-and-yellow.

In a domestic mains supply, the live wire alternates in potential difference relative to earth, and the stated value of about 230 V is the root-mean-square value, so the peak is higher, about 325 V. Earth is the reference at 0 V. The neutral wire is at, or close to, earth potential, about 0 V, because it is connected to earth at the supply substation; it still completes the circuit and carries the normal operating current. The earth wire is at 0 V and normally carries no current; it only carries a current if there is a fault, when a live conductor touches the metal case and a large current flows to earth, melting the fuse or tripping the circuit breaker. A multimeter set to a.c. voltage would show about 230 V between live and earth, near 0 V between neutral and earth, and 0 V between earth and earth.

This statement is the lead-in to the mains electricity explanations that follow. It signals that students must do more than recall labels: they must give a causal account of how the mains supply and its safety features behave. In practice, an explanation links a named part or quantity to a consequence. For example, the live wire alternates between a positive and negative potential relative to earth, so a person touching it can provide a path to earth and receive a current. A fuse melts when the current exceeds its rating because the heating effect of the current raises the temperature of the fuse wire. Earthing provides a low-resistance path so a fault current is large enough to melt the fuse. Answers should name the component or quantity, state what happens to it, and say why that matters for safety.

An open switch breaks the circuit, so the appliance stops working, but it does not make the live wire safe. The live wire remains at the alternating mains potential relative to earth because it is still connected to the supply. A person who touches the exposed live wire can provide a path to earth, so a current flows through them. This is why switches and fuses are placed in the live wire: opening the switch or melting the fuse isolates the appliance from the high potential. The neutral wire, by contrast, is near earth potential in normal operation. A useful check is to ask whether the wire is still connected to the supply; if it is, treat it as live.

In the UK, the live wire alternates between about +325 V and −325 V relative to earth, while the neutral wire stays near 0 V and the earth wire is a safety route at 0 V. If a person or a conductor creates a low-resistance path between live and earth, a large current flows. Through a person this can cause electric shock, burns, or death by disrupting heart rhythm; through a metal case it can overheat cables and start a fire. The earth wire and fuse or circuit breaker reduce this danger by giving fault current a low-resistance path so the fuse melts or the breaker trips and disconnects the live supply. Double insulation avoids the need for an earth connection. The danger is greatest when the path bypasses the normal circuit resistance, because current is then limited only by the resistance of the path.