Safety devices
4 marks · NormalThe diagram below shows a lamp connected to the mains supply with a metal casing. The circuit includes a fuse and an earth wire.
Explain what happens if a fault causes the live wire to touch the metal casing of the lamp.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) earth wire
(b) If the live wire touches the casing, current flows to earth through the earth wire. This large current causes the fuse to melt (blow), disconnecting the circuit and turning the lamp off. This prevents the user from receiving an electric shock.
Common mistakes: Omitting the current path through the earth wire; Not stating that the fuse melts or blows
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies the earth wire as the safety feature protecting the user.
M1 (b) · 1 mark · States that a large current flows through the earth wire if the live wire touches the casing.
M2 (b) · 1 mark · States the fuse melts/blows due to the high current. (follow-through)
A1 (b) · 1 mark · Concludes that this disconnects the circuit and prevents electric shock to the user.
Worked solution
a) The earth wire protects the user if the live wire touches the metal casing.
b) If a fault causes the live wire to touch the casing, the earth wire provides a low-resistance path for the current. This causes a large current to flow through the earth wire and fuse. The fuse heats up and melts (blows), which breaks the circuit and switches the lamp off. This stops current flowing through the casing, so the user is protected from electric shock.