AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
The earth wire is a safety wire to stop the appliance becoming live.
Full specification wording
Mains electricity: The earth wire is a safety wire to stop the appliance becoming live.
The earth wire joins the metal case of an appliance to the ground through a very low resistance path, so the case is held at 0 V. In normal use no current flows in it.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 5 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The earth wire joins the metal case of an appliance to the ground through a very low resistance path, so the case is held at 0 V. In normal use no current flows in it. Its job shows up in a fault: if the live wire works loose and touches the metal case, the case would otherwise rise to about 230 V and anyone touching it would complete a path to earth. Instead the earth wire gives the charge a much easier route, so a very large current passes through the live wire and the earth wire. That surge melts the fuse in the live line, or trips the circuit breaker, which disconnects the live supply and leaves the case safe to touch. The earth wire and the fuse work as a pair; neither protects the user alone.
What earns the marks
- The earth wire connecting the metal case to earth so that the case stays at 0 V.
- A fault in which the live wire touches the metal case.
- A large current in the earth wire because it is a low resistance path.
- The fuse melting or the circuit breaker tripping, disconnecting the live supply.
Common mistakes and how to put them right
Saying the earth wire carries a current all the time.
Claiming the earth wire absorbs or stores the extra charge rather than conducting it away.
Stopping at 'a large current flows' without saying the fuse then breaks the live circuit.
Connecting the earth wire to the neutral pin on a labelled plug diagram.
Exam tips
- Earth-wire answers are a chain of four steps, so write them as a numbered sequence and miss no link.
- Always finish with the fuse or breaker disconnecting the supply, because that is where the final mark sits.
- Say 'the case becomes live' rather than 'the appliance breaks' when describing the fault.
Try the lesson
The start of the Mains electricity: The earth wire is a safety wire to stop the appliance becoming live. lesson, made from checked teaching notes for this specification point.
Have a go first
In a three-core mains cable, which wire is connected to the metal case of an appliance to stop the case becoming live?
Listen
The earth wire joins the metal case of an appliance to the ground through a very low resistance path, so the case is held at 0 V. In normal use no current flows in it. Its job shows up in a fault: if the live wire works loose and touches the metal case, the case would otherwise rise to about 230 V and anyone touching it would complete a path to earth. Instead the earth wire gives the charge a much easier route, so a very large current passes through the live wire and the earth wire. That surge melts the fuse in the live line, or trips the circuit breaker, which disconnects the live supply and leaves the case safe to touch. The earth wire and the fuse work as a pair; neither protects the user alone. A strong answer earns marks for these points. the earth wire connecting the metal case to earth so that the case stays at 0 V. a fault in which the live wire touches the metal case. a large current in the earth wire because it is a low resistance path. the fuse melting or the circuit breaker tripping, disconnecting the live supply. Exam tip: Earth-wire answers are a chain of four steps, so write them as a numbered sequence and miss no link.
See it
Put it in order
Put these steps in the order they happen.
- 1. Case safe to touch
- 2. Live supply disconnected
- 3. Earth wire gives charge an easier route to earth
- 4. Fuse melts or circuit breaker trips
- 5. Live wire works loose and touches metal case
- 6. Case would become live
- 7. Very large current in live wire and earth wire
Key words
Say what each word means before you turn the card.
earth wire
Try answering aloud or in your head before turning the card.
fault current
Try answering aloud or in your head before turning the card.
fuse melts
Try answering aloud or in your head before turning the card.
metal case
Try answering aloud or in your head before turning the card.
See every definition
- earth wire
- A safety wire that joins the metal case of an appliance to the ground through a very low resistance path, so the case is held at 0 V.
- fault current
- A very large current that passes through the live wire and the earth wire when the live wire works loose and touches the metal case.
- fuse melts
- The surge melts the fuse in the live line, which disconnects the live supply and leaves the case safe to touch.
- metal case
- The part of an appliance joined to the ground by the earth wire, which would otherwise rise to about 230 V if the live wire touched it.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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