Mains Electricity
4 marks · EasyA student has a lamp that is fitted with a 3 A fuse and is connected to a 230 V mains supply. The lamp casing is made from metal.
Explain why the lamp must be fitted with an earth wire.
The lamp uses a current of 0.4 A. State whether the 3 A fuse is suitable for this lamp, giving a reason for your answer.
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) The earth wire provides a path for current to flow safely to the ground if a fault occurs and the live wire touches the metal casing. This prevents the user from receiving an electric shock. · must show Current flows to earth in fault, Prevents electric shock
(b) The 3 A fuse is not suitable because it is much higher than the required current (0.4 A). The fuse might not blow if a fault increases the current just above the normal value, so it would not protect the lamp effectively. · must show Fuse rating too high, May not blow at correct current
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Marking points
B1 (a) · 1 mark · States that the earth wire allows current to flow to earth if there is a fault.
B2 (a) · 1 mark · Explains that this protects the user from electric shock.
B3 (b) · 1 mark · Correctly identifies that the 3 A fuse is not suitable because it is too high for a 0.4 A device.
B4 (b) · 1 mark · Explains that the fuse may not blow when needed, so it does not protect properly.
Worked solution
(a) The lamp has a metal casing. If a fault causes the live wire to touch the casing, the earth wire provides a path for the current to flow to earth. This prevents the user from receiving an electric shock.
(b) The lamp uses only 0.4 A, but the fuse is 3 A. This means the fuse will only blow if the current is much higher than normal. It may not protect the lamp if the current increases only slightly above 0.4 A. Therefore, the 3 A fuse is not suitable.