A kettle has a metal casing and is connected to the mains electricity supply using a three-core cable containing a live wire, a neutral wire, and an earth wire. A fault develops inside the kettle so that the live wire comes into contact with the metal casing. Explain how the earth wire and fuse work together to protect the user from electric shock.

Pearson Edexcel International GCSE Physics (4PH1) — 2.1 Mains electricity · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The earth wire gives a low‑resistance path for the fault current to flow from the metal casing to the earth. When the live conductor touches the casing, a large surge of current travels through the earth wire. This high current heats the fuse wire until it melts or blows, breaking the circuit. Once the fuse is blown the kettle is disconnected, so the casing is no longer live and the user cannot receive an electric shock.

Examiner tips

  • Use the word ‘earth’ and ‘fuse’ explicitly; mention ‘low resistance path’ and ‘high current’.
  • Show the sequence: fault → earth → fuse → circuit broken.
  • Use the term ‘blows’ or ‘melts’ for the fuse action.

Common mistakes

  • Confusing the earth wire with the neutral wire.
  • Failing to mention that the fuse blows because of the high current.
  • Using vague terms like ‘protects’ without explaining the mechanism.

Mark scheme (4 marks)

  1. The earth wire provides a low resistance path for current to flow from the metal casing to earth
  2. A large / surge of current flows through the earth wire
  3. The large current causes the fuse wire to overheat and melt / blow
  4. The circuit is broken / disconnected, so the casing is no longer live and the user cannot receive an electric shock

Key terms in this question

earth wire · fuse · live wire · electric shock

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