A toaster has a metal casing and is connected to the mains electricity supply using a three-core cable. The live wire inside the toaster accidentally touches the metal casing, creating a short circuit. 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 the metal casing a very low‑resistance path to earth.
When the live conductor touches the casing a large surge of current flows through the earth wire.
This surge raises the temperature of the fuse element until it melts or blows.
The fuse then opens the circuit, breaking the supply so no current can reach the casing and the user is protected from shock.

Examiner tips

  • Use the word "earth" for the safety conductor, not "ground". Show the sequence: low resistance → surge → fuse melts → circuit opens. Mention that the fuse protects by breaking the circuit, not by stopping the current directly.
  • Use clear, exam‑style wording: "provides a low‑resistance path", "large surge of current", "fuse melts", "circuit is broken".

Common mistakes

  • Confusing the earth wire with the live or neutral conductor. Failing to explain that the fuse blows because of the surge, not because it is a safety device itself. Using vague terms like "protects the user" without linking to the fuse action.

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 to melt/blow
  4. The circuit is broken/disconnected, preventing current from reaching the metal casing and protecting the user from electric shock

Key terms in this question

earth wire · fuse · live wire · short circuit · mains electricity

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