A student uses a hairdryer at home. The hairdryer is connected to the mains supply using a three-pin plug fitted with a fuse and an earth wire. Explain how the earth wire and fuse together protect the user if the live wire inside the hairdryer accidentally touches the metal casing.

Cambridge International IGCSE Physics (0625) — 4.4 Electrical safety · 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 (0 V). If the live wire touches the casing, a large current flows through the low‑resistance earth wire. The surge current melts the fuse, which then blows. The fuse opening breaks the circuit, so no current can reach the user, protecting them from shock.

Examiner tips

  • Use the word ‘earth’ and ‘fuse’ explicitly; explain the sequence of events; mention low resistance of earth wire and that the fuse opens to break the circuit.
  • Show the cause (live touches casing) and effect (current, fuse blows, circuit opens).

Common mistakes

  • Forgetting to state that the earth wire connects to ground; confusing the fuse with a circuit breaker; not explaining that the fuse opens to stop current flow.

Mark scheme (4 marks)

  1. The earth wire connects the metal casing to earth (at 0 V / ground potential)
  2. A large/surge current flows through the earth wire (because the resistance of the earth wire is very low)
  3. The large current melts/breaks the fuse wire (fuse 'blows')
  4. The circuit is broken / disconnected so no current can flow, protecting the user from electric shock

Key terms in this question

earth wire · fuse · live wire · metal casing

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