Safety devices
4 marks · NormalA manufacturer decides to use a plastic casing for a new electric hand mixer. The mixer is labeled 'double insulated'.
Describe what is meant by double insulation in this context.
Explain why an earth wire is not needed for this appliance.
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) Double insulation means that both the inside electrical parts and the outer casing are insulated, using non-conducting material like plastic. There are two separate layers of insulation to prevent any live wires from touching the outside of the appliance.
(b) An earth wire is not required because if a fault occurs, the double insulation stops any live wire from touching the casing, so there is no risk of electric shock to the user.
Common mistakes: Saying the earth wire is needed despite double insulation; Confusing 'plastic' with 'double insulation'
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States both internal and external insulation.
B2 (a) · 1 mark · States that this prevents live wire contact with casing or outside.
M1 (b) · 1 mark · Reason why an earth wire is not required (no contact possible with casing).
A1 (b) · 1 mark · Explains that there is no electric shock risk because of the insulation.
Worked solution
a) Double insulation means there are two layers of insulating material: one around the electrical wires/components inside, and a second insulating layer for the outer plastic casing. This prevents any electrical part from touching the outside if a fault occurs.
b) The earth wire is not needed because with double insulation, no electrical part can contact the outside casing, so there is no way for current to reach the user, even if a fault develops.