Units (Electricity)
5 marks · NormalA hairdryer plug shows the label: 220 V, 660 W. The hairdryer uses a two-core cable but does not have an earth wire.
Explain what is meant by 'double insulation' and why this hairdryer does not require an earth wire.
Calculate the current drawn by the hairdryer.
A fuse is chosen by dividing the power by the voltage, then rounding up to the next available size. State which value of fuse (from 3 A, 5 A, 13 A) is most suitable for this hairdryer.
Give two reasons why using a fuse is important for safety in 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.
Show answer
(a) Double insulation means the casing and inner parts are both insulated, so no part accessible to the user can become live. This removes the need for an earth wire.
(b) 3.0 A
(c) 5 A fuse
(d) If too much current flows, the fuse melts and breaks the circuit: this prevents the appliance overheating, reduces risk of fire, and protects the user from electric shock.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Explains double insulation concept relating to safety.
B2 (a) · 1 mark · States why an earth wire is unnecessary with double insulation.
A1 (b) · 1 mark · Calculates correct current (3.0 A). (follow-through)
B3 (c) · 1 mark · Chooses correct size fuse (5 A).
B4 (d) · 1 mark · Gives two valid safety reasons for using a fuse.
Worked solution
a) Double insulation means both the wires and the outer casing are made from insulating materials, so that the user cannot touch a live part even if a fault happens. Therefore, there is no need for an earth wire.
b) Current = Power / Voltage = 660 W / 220 V = 3.0 A
c) Choose the 5 A fuse, because 3.0 A is less than 5 A but above 3 A.
d) Fuses protect the hairdryer by breaking the circuit if there is too much current, preventing fire and electric shock.