ELECTRICAL POWER
3 marks · NormalSome electrical appliances have double insulation and do not need an earth wire.
Describe what is meant by double insulation.
Explain why double insulated appliances do not need an earth wire.
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) All electrical parts are surrounded by insulating material so they cannot be touched. · must show electrical parts covered by insulator
(b) No live part can touch the casing, so even if a fault happens, the user cannot get an electric shock, removing the need for an earth wire. · must show no contact between live part and casing, no risk of shock so no earth needed
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Define double insulation as electrical parts fully covered by insulator.
B2 (b) · 1 mark · No risk of live part contacting casing or user (prevents shock).
A1 (b) · 1 mark · Correctly link double insulation to no need for earth wire because user cannot touch live parts.
Worked solution
a) Double insulation means all the electrical parts are surrounded by insulating (non-conducting) material so the user cannot touch them.
b) Since there is no path for current to pass to the user, no earth wire is needed for safety. If there is a fault, the double insulation prevents contact between live parts and the user.