Explain why the earth wire is an important safety feature in a mains appliance that has a metal casing.

AQA GCSE Physics (8463) — 4.2.3 Domestic uses and 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 live wire can become loose and touch the metal casing, making the casing live at 230 V. The earth wire is connected to the casing and gives a low‑resistance path to earth. If the live contacts the casing, the large fault current flows through the earth wire, causing the fuse to melt or blow. The fuse then breaks the circuit, preventing a shock to anyone touching the appliance.

Examiner tips

Use the word ‘earth’ not ‘ground’ unless the scheme allows it.
Show the fault‑current path: live → casing → earth wire → fuse → earth.
Explain that the fuse blows because the current is high, not because the earth wire itself is dangerous.
Mention that the earth wire protects the user, not the appliance itself.

Common mistakes

Confusing the earth wire with the neutral wire.
Failing to explain why the fuse blows (i.e. high current).
Using vague terms like ‘safety’ without describing the fault‑current path.

Mark scheme (4 marks)

The live wire may come loose and touch the metal casing, making the casing live / at 230 V
Without the earth wire, anyone touching the casing would receive an electric shock as current flows through them to the ground
The earth wire is connected to the metal casing and provides a low-resistance path to earth
The large current that flows through the earth wire causes the fuse to melt / blow, breaking the circuit and making the appliance safe

Key terms in this question

earth wire