AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Mains electricity: that a live wire may be dangerous even when a switch in the mains circuit is open
An open switch breaks the circuit, so the appliance stops working, but it does not make the live wire safe.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An open switch breaks the circuit, so the appliance stops working, but it does not make the live wire safe. The live wire remains at the alternating mains potential relative to earth because it is still connected to the supply. A person who touches the exposed live wire can provide a path to earth, so a current flows through them. This is why switches and fuses are placed in the live wire: opening the switch or melting the fuse isolates the appliance from the high potential. The neutral wire, by contrast, is near earth potential in normal operation. A useful check is to ask whether the wire is still connected to the supply; if it is, treat it as live.
What earns the marks
- An open switch stops the current through the appliance but does not disconnect the live wire from the supply.
- The live wire remains at the alternating mains potential relative to earth even when the switch is open.
- Touching the live wire can drive a current through the person to earth, which is the danger.
- Switches and fuses are placed in the live wire so that opening or breaking the circuit isolates the appliance from the high potential.
- The neutral wire is near earth potential in normal operation, so it is not the wire that supplies the dangerous driving potential.
- A safe isolation check is to confirm that the wire is no longer connected to the supply, not merely that the appliance has stopped working.
Common mistakes and how to put them right
Assuming that because the appliance is switched off it is safe to touch.
Put it right: The live wire may still be at mains potential, so the supply must be isolated before touching it.
Thinking the switch removes the potential difference from the live wire.
Put it right: The switch breaks the circuit, but the live wire remains connected to the supply and stays at mains potential.
Believing the neutral wire is always dangerous.
Put it right: The neutral wire is near earth potential in normal operation; the live wire is the one that can drive a current through a person.
Exam tips
- State clearly that the appliance stops working but the live wire is still at mains potential.
- Use the phrase path to earth when explaining why contact with the live wire is dangerous.
- Link the position of the switch in the live wire to the purpose of isolating the appliance from the supply.
Try the lesson
The start of the Mains electricity: that a live wire may be dangerous even when a switch in the mains circuit is open lesson, made from checked teaching notes for this specification point.
Have a go first
A mains circuit is incorrectly wired so that the switch is placed in the neutral wire. The switch is open and the appliance is off. Why is it still dangerous to touch the exposed live wire inside the appliance?
Listen
An open switch breaks the circuit, so the appliance stops working, but it does not make the live wire safe. The live wire remains at the alternating mains potential relative to earth because it is still connected to the supply. A person who touches the exposed live wire can provide a path to earth, so a current flows through them. This is why switches and fuses are placed in the live wire: opening the switch or melting the fuse isolates the appliance from the high potential. The neutral wire, by contrast, is near earth potential in normal operation. A useful check is to ask whether the wire is still connected to the supply; if it is, treat it as live. A strong answer earns marks for these points. An open switch stops the current through the appliance but does not disconnect the live wire from the supply. The live wire remains at the alternating mains potential relative to earth even when the switch is open. Touching the live wire can drive a current through the person to earth, which is the danger. Switches and fuses are placed in the live wire so that opening or breaking the circuit isolates the appliance from the high potential. A common slip: Assuming that because the appliance is switched off it is safe to touch. Instead, the live wire may still be at mains potential, so the supply must be isolated before touching it. Exam tip: State clearly that the appliance stops working but the live wire is still at mains potential.
Key words
Say what each word means before you turn the card.
Live wire potential relative to earth
Try answering aloud or in your head before turning the card.
Function and placement of switches
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Isolation from the supply
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Electrical safety in the home
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See every definition
- Live wire potential relative to earth
- The live wire remains at the alternating mains potential relative to earth even when the switch is open, because it is still connected to the supply.
- Function and placement of switches
- Switches and fuses are placed in the live wire so that opening the switch or melting the fuse isolates the appliance from the high potential.
- Isolation from the supply
- A safe isolation check is to confirm that the wire is no longer connected to the supply, not merely that the appliance has stopped working.
- Electrical safety in the home
- Touching the live wire can drive a current through the person to earth, which is the danger.
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