Mains electricity: The live wire carries the alternating potential difference from the supply. The neutral wire completes the circuit. The earth wire is a safety wire to stop the appliance becoming live.

The idea

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.

What earns the marks

• The live wire carries the alternating potential difference from the supply, so its potential difference changes direction and magnitude with time.
• The neutral wire completes the circuit by providing the return path for current through the appliance.
• The earth wire is a safety wire connected to the metal case of an appliance.
• The earth wire stops the appliance becoming live because a fault current flows to earth and the large current melts the fuse or trips the circuit breaker.
• The live wire is brown, the neutral wire is blue and the earth wire is green-and-yellow in a standard three-core cable.
• The earth wire normally carries no current; it carries current only when a fault occurs.

Common mistakes and how to put them right

• Thinking the neutral wire is always at 0 V and therefore safe to touch.

  Put it right: The neutral wire can be at a potential difference above 0 V when current flows, so it is not assumed safe.

• Believing the earth wire carries current all the time.

  Put it right: It normally carries no current and only carries current during a fault.

• Confusing the live and neutral functions.

  Put it right: The live wire carries the alternating potential difference from the supply, while the neutral wire completes the circuit.

• Assuming the earth wire protects by insulating the case.

  Put it right: It protects by providing a low-resistance path to earth so a fault current is large enough to melt the fuse or trip the circuit breaker.

Exam tips

• Link each wire to its function in one sentence: live carries the alternating potential difference, neutral completes the circuit, earth is the safety wire.
• When explaining safety, state the sequence: fault makes case live, large current flows through earth wire, fuse melts or circuit breaker trips, live wire disconnected.
• Use the standard colours brown, blue and green-and-yellow accurately when identifying wires in a plug or cable.
• Avoid saying the earth wire 'absorbs' or 'blocks' current; describe it as a low-resistance path that causes a large current to operate a safety device.