A washing machine is connected to the mains electricity supply using a three-core cable. The cable contains a live wire, a neutral wire, and an earth wire. Explain the role of the earth wire and the fuse in protecting a user if the live wire accidentally touches the metal casing of the washing machine.

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 the earth, giving a low‑resistance path for fault current. If the live wire touches the casing, a large current flows through the earth wire. This current heats the fuse wire until it melts (blows), breaking the circuit. The appliance is then disconnected from the live supply, protecting the user from electrocution.

Examiner tips

  • Use the word "earth" and explain it is a low‑resistance path
  • Show the sequence: live touches casing → fault current → fuse blows → circuit opens
  • Mention that the fuse protects the user by disconnecting the supply

Common mistakes

  • Confusing the earth wire with the neutral wire
  • Failing to explain that the fuse blows because of the fault current
  • Using vague terms like "protects" without describing the mechanism

Mark scheme (4 marks)

  1. The earth wire connects the metal casing to the ground (earth), providing a low-resistance path for current to flow.
  2. A large current flows through the earth wire when the live wire touches the casing.
  3. The large current causes the fuse wire to heat up and melt (blow).
  4. The circuit is broken, disconnecting the appliance from the live supply and protecting the user from electrocution.

Key terms in this question

earth wire · live wire · fuse

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