Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Explain why switches and fuses should be connected in the live wire of a…
Full specification wording
Topic 10 – Electricity and circuits: 10.39 Explain why switches and fuses should be connected in the live wire of a domestic circuit
The live wire carries the alternating potential that drives current through an appliance, while the neutral wire sits close to earth potential.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 37 of 39
Study notes
From the checked teaching notes for this specification point.
The idea
The live wire carries the alternating potential that drives current through an appliance, while the neutral wire sits close to earth potential. A switch placed in the live wire breaks the circuit on the high-potential side, so when it is open the appliance is isolated from the supply and its internal parts are not held at a dangerous potential. A fuse in the live wire melts when the current exceeds its rating, again cutting the supply at the live side. If either were fitted only in the neutral wire, opening the switch or blowing the fuse would leave the appliance still connected to the live supply, so a fault could make its casing live and give a shock. Choosing the live wire therefore protects the user during faults and normal switching.
What earns the marks
- States that the live wire carries the high alternating potential from the supply while the neutral wire is near earth potential.
- Explains that a switch in the live wire disconnects the appliance from the high-potential supply so its parts are not left live.
- Explains that a fuse in the live wire melts and cuts off the supply at the live side when current is too large.
- Identifies that fitting a switch or fuse only in the neutral wire would leave the appliance connected to the live supply and able to give a shock.
Common mistakes and how to put them right
Assumes the neutral wire is always safe to touch.
Put it right: Recognise that a fault can raise the potential of parts connected to neutral.
Places the fuse in the neutral wire and expects it to protect the user.
Put it right: Connect it in the live wire so the supply is cut at the high-potential side.
Believes a switch in the live wire stops current flowing in the earth wire.
Put it right: Explain that it isolates the appliance from the supply instead.
Treats the fuse as protecting the appliance from too little current.
Put it right: State that it melts when current exceeds its rating.
Exam tips
- Link each device to the idea of cutting the supply at the live side, not just breaking the circuit anywhere.
- Use the phrase 'isolates the appliance from the live supply' to show why the position matters.
- Contrast live-wire and neutral-wire placement explicitly to earn the explanation mark.
Try the lesson
The start of the Topic 10 – Electricity and circuits: 10.39 Explain why switches and fuses should be connected in the live wire of a domestic circuit lesson, made from checked teaching notes for this specification point.
Have a go first
In a domestic circuit, why is the switch placed in the live wire rather than in the neutral wire?
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