A student plugs a kettle into a mains socket at home. The kettle has a metal casing and is connected to the live wire, the neutral wire, and the earth wire. Explain why the kettle has three pins on its plug rather than two.

Eduqas GCSE Physics — 7.4 Domestic uses and safety · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Mains electricity in the UK is supplied at 230 V. The plug of the kettle has a live pin, a neutral pin, and an earth pin.

Model answer (4 marks)

The live pin supplies the 230 V AC to the kettle. The neutral pin provides the return path for the current, completing the circuit. The earth pin is a safety wire that is connected to the metal casing of the kettle. If a fault makes the casing live, the earth wire provides a low‑resistance path for the fault current, causing the fuse to blow or the circuit breaker to trip, thereby protecting the user.

Examiner tips

  • Use the exact terms ‘live’, ‘neutral’ and ‘earth’ as the mark scheme does.
  • Explain the function of each pin in the order given in the scheme.
  • Show the safety function of the earth pin clearly.

Common mistakes

  • Confusing the earth pin with the neutral pin.
  • Failing to mention that the earth wire protects against a live casing.
  • Using vague phrases like ‘safety wire’ without linking it to the fuse or breaker.

Mark scheme (4 marks)

  1. The live wire carries the alternating current / voltage to the appliance
  2. The neutral wire completes the circuit / provides the return path for the current
  3. The earth wire is a safety wire connected to the metal casing
  4. If a fault occurs and the casing becomes live, current flows through the earth wire, which blows the fuse / trips the circuit breaker, making the appliance safe

Key terms in this question

live wire · neutral wire · earth wire · metal casing

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