Recall the potential differences between the live, neutral and earth mains…

The idea

UK mains supply is an alternating supply of about 230 V. The live wire alternates between a positive and negative potential relative to earth, with a peak value well above the stated root-mean-square value, so its potential is described as about 230 V alternating. The neutral wire is held at approximately 0 V, close to earth potential, and completes the circuit. The earth wire is also at approximately 0 V and provides a low-resistance path to earth for safety, carrying current only when a fault occurs. Learners should recall these approximate potential differences: live about 230 V alternating, neutral about 0 V and earth about 0 V. The exact peak value is not required, but recognising that the live potential varies with time prevents the error of treating it as a steady value.

What earns the marks

• Recalls that the live wire has a potential of about 230 V alternating relative to earth.
• Recalls that the neutral wire is at approximately 0 V.
• Recalls that the earth wire is at approximately 0 V.
• Recognises that the live potential alternates rather than staying at a fixed value.

Common mistakes and how to put them right

• States that the neutral wire carries a large potential.

  Put it right: Recall that it is approximately 0 V.

• Gives the earth wire a high potential.

  Put it right: State that it is approximately 0 V and provides a safety path.

• Describes the live wire as a steady direct potential.

  Put it right: Recognise that it alternates about 230 V.

• Confuses the peak value with the stated supply value.

  Put it right: Recall the approximate 230 V figure without inventing a precise peak.

Exam tips

• Learn the three approximate values together: live about 230 V alternating, neutral about 0 V, earth about 0 V.
• Add the word 'alternating' whenever you quote the live potential.
• Keep the values approximate; do not invent a precise peak voltage.