A hairdryer has a plastic casing and is sold as a double-insulated appliance. Explain why a double-insulated appliance does not need an earth wire, and state one advantage of using a circuit breaker rather than a fuse to protect the hairdryer's mains circuit.

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).

A hairdryer is connected to the mains electricity supply. It is labelled with the double-insulation symbol and its mains cable contains only a live wire and a neutral wire.

Model answer (4 marks)

A double‑insulated appliance has two layers of insulation around the live parts, so the plastic casing is non‑conductive and there is no conductive path from the live wire to the outer shell. Therefore the casing cannot become live and a user cannot receive a shock, so an earth wire is unnecessary.

A circuit breaker can be reset after it has operated, so it does not need to be replaced, and it operates more quickly than a fuse, breaking the circuit more rapidly.

Examiner tips

  • Use the term "double‑insulated" and explain the two layers of insulation. Mention that the plastic casing is non‑conductive. State that the lack of a conductive path prevents the casing from becoming live. For the second part, give one advantage: either the resettable nature or the faster response time.

Common mistakes

  • Confusing the role of the earth wire with a fuse. Saying the casing is insulated but still conductive. Forgetting to mention that the double insulation eliminates the need for an earth wire.

Mark scheme (4 marks)

  1. The plastic casing does not conduct electricity / electrical charge cannot flow through a plastic casing
  2. There is no conducting path from the live wire to the outer casing, so the casing cannot become live / a user cannot receive an electric shock from the casing
  3. A circuit breaker can be reset after it has operated / does not need to be replaced
  4. A circuit breaker operates/responds faster than a fuse / breaks the circuit more quickly

Key terms in this question

earth wire · circuit breaker · fuse

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