Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Explain the function of an earth wire and of fuses or circuit breakers in…
Full specification wording
Topic 10 – Electricity and circuits: 10.38 Explain the function of an earth wire and of fuses or circuit breakers in ensuring safety
The earth wire connects the metal case of an appliance to the ground at zero potential.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 36 of 39
Study notes
From the checked teaching notes for this specification point.
The idea
The earth wire connects the metal case of an appliance to the ground at zero potential. If a fault lets the live wire touch the case, current flows through the low-resistance earth path to earth, and the large current melts the fuse or trips the circuit breaker, disconnecting the live supply. A fuse contains a thin wire that melts when the current exceeds its rating; a circuit breaker uses an electromagnet or bimetallic strip to open a switch when current is too large, and can be reset. Both break the live wire, so the case cannot stay live. The earth wire only protects Class I appliances with metal cases; double-insulated appliances need no earth.
What earns the marks
- States that the earth wire connects the metal case of an appliance to the ground at zero potential.
- Explains that a fault sending the live wire to the case causes a large current to flow through the earth wire.
- Explains that the fuse melts or the circuit breaker trips when the current exceeds its rating, disconnecting the live supply.
- Describes the fuse as a thin wire that melts to break the circuit, and the circuit breaker as a resettable switch opened by excess current.
- Recognises that breaking the live wire prevents the case from remaining at a dangerous potential.
Common mistakes and how to put them right
Says the earth wire carries current in normal use.
Put it right by explaining that it carries current only when a fault occurs.
Claims a fuse protects by reducing the current to a safe value.
Put it right by explaining that the fuse melts and breaks the circuit entirely.
Assumes every appliance needs an earth wire.
Put it right by noting that double-insulated appliances with plastic cases do not require one.
Exam tips
- Describe the fault path: live wire touches case, current flows to earth, fuse melts or breaker trips.
- Contrast fuse and circuit breaker: the fuse must be replaced, the breaker can be reset.
- State clearly that the safety device is fitted in the live wire so the appliance is isolated from the supply.
Try the lesson
The start of the Topic 10 – Electricity and circuits: 10.38 Explain the function of an earth wire and of fuses or circuit breakers in ensuring safety lesson, made from checked teaching notes for this specification point.
Have a go first
What is the main safety function of the earth wire in a mains appliance with a metal case?
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