AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
The earth wire is at 0 V, it only carries a current if there is a fault.
Full specification wording
Mains electricity: The earth wire is at 0 V, it only carries a current if there is a fault.
In normal operation both ends of the earth wire sit at 0 V, so there is no potential difference along it and no current in it: the working current goes out along live and back along neutral.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 7 of 8
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In normal operation both ends of the earth wire sit at 0 V, so there is no potential difference along it and no current in it: the working current goes out along live and back along neutral. The earth wire only acts when something goes wrong. If the live core touches the metal case, one end of the earth wire is suddenly at the live potential while the other is still at 0 V, and that potential difference drives a large current through it because the path has very low resistance. The size of that current is the point, because it is far larger than the appliance's normal current and the fuse melts quickly. A residual current device spots the same fault differently, by detecting that the live and neutral currents no longer match.
What earns the marks
- No current in the earth wire under normal conditions.
- There being no potential difference along the earth wire when there is no fault.
- A fault putting the case, and so the earth wire, at the live potential.
- The resulting current being large because the earth path has very low resistance.
Common mistakes and how to put them right
Saying the earth wire carries a small current continuously to keep the case safe.
Explaining the fault current from the low resistance alone, never naming the potential difference that drives it.
Confusing the earth wire with the neutral wire, which does carry current in normal use.
Claiming a fuse melts because of a high potential difference rather than a high current.
Exam tips
- Answer 'when does the earth wire carry a current?' with 'only during a fault', then name the fault.
- Link low resistance to a large current using I = V ÷ R whenever the command word is explain.
- Do not describe a residual current device unless the question mentions one; the fuse answer is what is expected.
Try the lesson
The start of the Mains electricity: The earth wire is at 0 V, it only carries a current if there is a fault. lesson, made from checked teaching notes for this specification point.
Have a go first
In a mains circuit, what is the potential difference of the earth wire under normal working conditions?
Listen
In normal operation both ends of the earth wire sit at 0 V, so there is no potential difference along it and no current in it: the working current goes out along live and back along neutral. The earth wire only acts when something goes wrong. If the live core touches the metal case, one end of the earth wire is suddenly at the live potential while the other is still at 0 V, and that potential difference drives a large current through it because the path has very low resistance. The size of that current is the point, because it is far larger than the appliance's normal current and the fuse melts quickly. A residual current device spots the same fault differently, by detecting that the live and neutral currents no longer match. A strong answer earns marks for these points. no current in the earth wire under normal conditions. there being no potential difference along the earth wire when there is no fault. a fault putting the case, and so the earth wire, at the live potential. the resulting current being large because the earth path has very low resistance. Exam tip: Answer 'when does the earth wire carry a current?' with 'only during a fault', then name the fault.
Key words
Say what each word means before you turn the card.
earth wire at 0 V
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no current in normal use
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low resistance path
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See every definition
- earth wire at 0 V
- The earth wire sits at 0 V, so in normal operation both ends are at 0 V and there is no potential difference along it.
- no current in normal use
- In normal operation there is no potential difference along the earth wire, so no current flows in it and the working current goes out along live and back along neutral.
- low resistance path
- The earth path has very low resistance, so the resulting current is large, far larger than the appliance's normal current, and the fuse melts quickly.
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