State the purpose of a fuse in a household electrical circuit and explain what happens inside the fuse when a fault causes the current to exceed the fuse's rated value.

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).

Household appliances are connected to the mains supply using plugs that contain a fuse. The fuse is a short length of thin wire with a known melting point.

Model answer (4 marks)

A fuse protects the appliance (and the wiring) from damage by excessive current and acts as a safety device to prevent overheating or fire.
When the current exceeds the fuse’s rated value the thin fuse wire heats up.
The heat causes the wire to melt and break the circuit.
Once the circuit is broken, current stops flowing and the appliance is switched off.

Examiner tips

  • Use the word "protects" and mention overheating/fire. Explain the sequence: current > rating → heating → melting → circuit open. Show the final effect: no current reaches the appliance.

Common mistakes

  • Saying the fuse simply "turns off" without explaining heating and melting. Forgetting to mention that the fuse protects the wiring/appliance. Using vague terms like "breaks" without describing the melting process.

Mark scheme (4 marks)

  1. A fuse protects the appliance (and/or wiring/circuit) from damage due to excessive current / acts as a safety device to prevent overheating or fire
  2. When the current exceeds the rated value, the fuse wire heats up
  3. The fuse wire melts / breaks the circuit
  4. Current stops flowing / the appliance is switched off / no current reaches the appliance

Key terms in this question

fuse · current · circuit · rated value

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