A lawnmower has a metal casing and is connected to the mains electricity supply using a three-core cable. The three-core cable contains a live wire, a neutral wire and an earth wire. Explain the role of the fuse and the earth wire in keeping a user safe if the live wire inside the lawnmower accidentally touches the metal casing.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The live wire touching the metal casing creates a short circuit, so a large current flows through the earth wire because it has very low resistance. The earth wire therefore gives a safe, low‑resistance path for the fault current to earth, preventing the casing from becoming live. The large current also passes through the fuse. The fuse wire overheats and melts (blows), breaking the circuit. Once the fuse blows the power supply to the lawnmower is disconnected, stopping any electric shock or fire risk.
Examiner tips
- Use the word "short circuit" to show understanding of the fault. Mention that the earth wire is low resistance and provides a safe path. Explain that the fuse melts because of the high current. Show the sequence: fault → earth wire → fuse → circuit break.
- common_mistakes
- :
- Failing to state that the earth wire is low resistance. Not explaining that the fuse melts due to overheating. Omitting that the fuse breaking stops the supply and prevents shock.
Mark scheme (4 marks)
- The live wire touching the metal casing causes a large/surge of current to flow through the earth wire (because the earth wire has a very low resistance).
- The earth wire provides a safe/low resistance path for the current to flow to earth, preventing the metal casing from becoming live.
- The large current through the fuse causes the fuse wire to overheat and melt/blow.
- The fuse breaking the circuit disconnects/cuts off the power supply to the lawnmower, preventing electric shock or fire.
Key terms in this question
earth wire · fuse · live wire
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Mains electricity questions
- A student notices that the plastic insulation on a mains cable has been damaged,…
- A washing machine has a metal casing and is connected to the mains electricity s…
- A hairdryer has a plastic casing and is sold as a double-insulated appliance. Ex…
- A kettle has a metal casing and is connected to the mains electricity supply usi…
- A fuse and a circuit breaker can both be used as safety devices in mains electri…
- A electric drill has a metal casing and is connected to the mains electricity su…