OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Explain that a live wire may be dangerous even when a switch in a mains…
Full specification wording
Powering Earth: explain that a live wire may be dangerous even when a switch in a mains circuit is open, and explain the dangers of providing any connection between the live wire and earth (to include: the protection offered by insulation of devices)
A switch placed in the live wire stops current flowing when open, but the live wire on the supply side of the switch stays at a high potential relative to earth.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 9 of 9
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A switch placed in the live wire stops current flowing when open, but the live wire on the supply side of the switch stays at a high potential relative to earth. Touching that section, or a fault that connects it to an earthed metal case, completes a path to earth and a large current can flow through a person, causing a shock. A direct live-to-earth connection also risks a large current, overheating and fire. Insulation around wires and the casing of devices prevents contact between the live conductor and a person or earthed metal, so intact insulation is a key protection; damaged insulation removes that protection and makes the danger real.
What earns the marks
- Explains that the live wire on the supply side of an open switch remains at a high potential relative to earth.
- Explains that touching the live wire, or a fault linking it to an earthed case, completes a path to earth so current flows through a person.
- Explains that a connection between live and earth can cause a large current, overheating or fire.
- Explains that insulation of wires and devices prevents contact between the live conductor and a person or earthed metal.
Common mistakes and how to put them right
Assumes an open switch makes the whole circuit safe.
Put it right by noting the live wire before the switch is still at high potential.
Claims insulation increases the current through a person.
Put it right by saying insulation prevents contact and so protects the user.
Ignores the fire risk of a live-to-earth fault.
Put it right by including overheating and fire as consequences.
Exam tips
- Name which side of the switch is still live before explaining the danger.
- Link a live-to-earth path to current through a person and to overheating.
- State that insulation protects by preventing contact, not by reducing voltage.
Try the lesson
The start of the Powering Earth: explain that a live wire may be dangerous even when a switch in a mains circuit is open, and explain the dangers of providing any connection between the live wire and earth (to include: the protection offered by insulation of devices) lesson, made from checked teaching notes for this specification point.
Have a go first
A lamp is switched off at its switch, but the switch has been fitted into the neutral wire rather than the live wire. Why can the lamp still be dangerous to touch?
Listen
A switch placed in the live wire stops current flowing when open, but the live wire on the supply side of the switch stays at a high potential relative to earth. Touching that section, or a fault that connects it to an earthed metal case, completes a path to earth and a large current can flow through a person, causing a shock. A direct live-to-earth connection also risks a large current, overheating and fire. Insulation around wires and the casing of devices prevents contact between the live conductor and a person or earthed metal, so intact insulation is a key protection; damaged insulation removes that protection and makes the danger real. A strong answer earns marks for these points. Explains that the live wire on the supply side of an open switch remains at a high potential relative to earth. Explains that touching the live wire, or a fault linking it to an earthed case, completes a path to earth so current flows through a person. Explains that a connection between live and earth can cause a large current, overheating or fire. Explains that insulation of wires and devices prevents contact between the live conductor and a person or earthed metal. A common slip: Assumes an open switch makes the whole circuit safe. Instead, noting the live wire before the switch is still at high potential. Exam tip: Name which side of the switch is still live before explaining the danger.
Key words
Say what each word means before you turn the card.
Danger of the live wire with an open switch
Try answering aloud or in your head before turning the card.
Live-to-earth faults and shock
Try answering aloud or in your head before turning the card.
Overheating and fire risk
Try answering aloud or in your head before turning the card.
Protective role of insulation
Try answering aloud or in your head before turning the card.
See every definition
- Danger of the live wire with an open switch
- A switch in the live wire stops current when open, but the live wire on the supply side stays at a high potential relative to earth.
- Live-to-earth faults and shock
- Touching the live wire, or a fault linking it to an earthed case, completes a path to earth so a large current can flow through a person, causing a shock.
- Overheating and fire risk
- A direct connection between the live wire and earth also risks a large current, overheating and fire.
- Protective role of insulation
- Insulation around wires and the casing of devices prevents contact between the live conductor and a person or earthed metal, so intact insulation is a key protection.
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