A student discovers that the plastic casing of an electric kettle has become cracked, exposing the metal heating element inside. Explain why touching the exposed metal could be dangerous, and describe two safety features built into the mains electrical system that are designed to protect the user in such a situation.

Cambridge International IGCSE Physics (0625) — 4.4 Electrical safety · Explain and Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The metal heating element may be connected to the live conductor, so touching it could allow a large current to flow through the body to earth, producing an electric shock.

1. The earth (green‑yellow) wire is bonded to the kettle’s metal casing. If the casing becomes live, the current is directed safely to earth rather than through a person.
2. The fuse (or RCCB) is designed to interrupt the circuit when a fault current flows through the earth wire. A fuse melts or an RCCB trips, breaking the circuit and stopping the current.

Examiner tips

  • Use the term ‘live conductor’ and explain the risk of a shock; mention the potential difference to earth.
  • Show the two safety features in separate points and state how each protects the user.
  • Include the word ‘earth’ and ‘fuse’ or ‘RCCB’ as key terminology.

Common mistakes

  • Confusing the earth wire with the neutral wire; not recognising that the fuse or RCCB is the second safety feature.
  • Writing that the metal is ‘hot’ without explaining the risk of a shock or the role of the earth connection.

Mark scheme (4 marks)

  1. The exposed metal could become live (connected to the live wire) / at a high potential difference relative to earth, so a current could pass through the person's body to earth, causing an electric shock.
  2. The earth wire (green and yellow) connects the metal casing to earth so that if the casing becomes live, current flows safely to earth rather than through the user.
  3. The large fault current that flows through the earth wire causes the fuse to melt / blow, breaking the circuit and cutting off the current.
  4. A residual current circuit breaker (RCCB) detects a difference in current between the live and neutral wires and switches off / trips the circuit very quickly, protecting the user.

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