Mains Electricity
5 marks · NormalA metal-cased toaster and a plastic-cased kettle are shown below.
| Appliance | Casing material | Earth wire? | Double insulation? |
|---|---|---|---|
| Toaster | Metal | Yes | No |
| Kettle | Plastic | No | Yes |
Explain why the toaster requires an earth wire, but the kettle does not.
State what would happen if the live wire in the toaster became loose and touched the metal case. Explain how the user is protected.
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 toaster needs an earth wire because its metal case could become live if a fault occurs. The kettle does not need an earth wire because its plastic case provides double insulation, so the user cannot touch any live parts. · must show metal conducts electricity, danger of touch, plastic insulates
(b) If the live wire touches the metal case, a current flows through the earth wire causing the fuse to blow. This disconnects the toaster and prevents electric shock. · must show current to earth, fuse blows, user protected
Common mistakes: Assuming plastic appliances need earthing; Not mentioning the fuse blows
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States the toaster needs an earth wire because it has a metal case.
M2 (a) · 1 mark · Explains that metal can become live if a fault occurs.
A1 (a) · 1 mark · States the plastic kettle does not need an earth wire due to double insulation.
M3 (b) · 1 mark · States that current flows to earth wire if the live touches metal case.
A2 (b) · 1 mark · States that the fuse will blow and user is protected.
Worked solution
a) The toaster requires an earth wire because it has a metal casing which could become live if a fault occurs, such as a wire coming loose. Metal is a conductor, so the user could receive an electric shock. The plastic kettle is double insulated: all live parts are covered by plastic, so the user cannot touch anything live, and so no earth wire is needed.
b) If the live wire in the toaster touches the case, a large current will flow from the live wire to earth. This large current will cause the fuse to blow, switching the appliance off and protecting the user from electric shock.