A electric drill has a metal casing and is connected to the mains electricity supply using a three-core cable. The drill is switched on and begins to overheat, causing a large surge in current. Explain how the earth wire and fuse together 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 connects the metal casing to earth, giving a low‑resistance path for fault current.
This keeps the casing from becoming live, so the user cannot receive a shock.
When a large surge of current flows through the earth wire, the fuse heats and melts (or blows).
The fuse opening breaks the circuit, cutting power to the drill and making it safe.

Examiner tips

  • Use the word ‘earth’ and ‘low‑resistance path’ – examiners look for these terms. Explain the sequence: earth wire → prevents live casing → fuse blows → circuit opens. Show the cause‑effect chain clearly.
  • Include the key words ‘prevent the metal casing from becoming live’ and ‘fuse melts or blows’ to hit all points.

Common mistakes

  • Writing that the earth wire alone stops the shock without mentioning the fuse. Forgetting to state that the fuse blows because of the surge. Using vague phrases like ‘protects the user’ without explaining how the earth wire and fuse work together.

Mark scheme (4 marks)

  1. The earth wire connects the metal casing to earth, providing a low resistance path for current to flow
  2. This prevents the metal casing from becoming live / prevents the user receiving an electric shock
  3. The large surge of current through the earth wire causes the fuse to overheat and melt / blow
  4. The circuit is broken / the power supply to the drill is cut off, making the appliance safe

Key terms in this question

earth wire · fuse · metal casing · electric shock

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