Recall the differences in function between the live, neutral and earth…

The idea

In UK domestic mains supply the three wires have distinct jobs. The live wire carries the alternating potential difference from the supply and alternates between a positive and a negative potential difference relative to the neutral wire; its root-mean-square value is about 230 V, with a peak of roughly 325 V. The neutral wire completes the circuit and is held at about zero potential difference, close to earth potential. The earth wire is a safety wire connected to the metal case of an appliance; it carries no current in normal use but provides a low-resistance path to earth if a fault connects the live wire to the case, so the case cannot stay at a dangerous potential difference. The potential difference between live and neutral is the full mains value, while the potential difference between live and earth is also about the mains value; between neutral and earth it is close to zero.

What earns the marks

• States that the live wire carries the alternating potential difference from the supply and is at about 230 V root-mean-square relative to neutral.
• States that the neutral wire completes the circuit and is at about zero potential difference, close to earth potential.
• States that the earth wire is a safety wire that carries current only when a fault occurs and provides a path to earth.
• Recognises that the potential difference between live and neutral is about the full mains value, while that between neutral and earth is close to zero.

Common mistakes and how to put them right

• Describes the 230 V mains value as the peak potential difference.

  Put it right: State instead that 230 V is the root-mean-square value and the peak is about 325 V.

• Claims the earth wire carries current during normal operation.

  Put it right: Explain that it carries current only when a fault occurs.

• Treats the neutral wire as the wire that supplies the potential difference.

  Put it right: State instead that the live wire carries the alternating potential difference and the neutral completes the circuit.

Exam tips

• Match each wire to its function first, then to its typical potential difference, so the two parts of the question are both answered.
• Remember that 230 V is the root-mean-square mains value and the peak is about 325 V when a question probes the alternating nature of the supply.
• For safety questions, explain that the earth wire gives a low-resistance path to earth so a faulty case cannot remain at a high potential difference.