Safety devices
4 marks · NormalA student notices that some hairdryers have a plastic casing, while others have a metal casing and an earth wire.
Explain why appliances with a metal casing must be earthed.
Describe how the earth wire and fuse work together to protect the user if a fault occurs inside a metal-cased 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) Metal casing can become live if a fault occurs, so it must be connected to earth wire to ensure safety. · must show metal casing can become live, earth provides a safe path
(b) If a live wire touches the metal casing, the earth wire carries the current safely to earth, causing a large current that blows the fuse and breaks the circuit. · must show earth wire carries current away, fuse blows due to large current, circuit breaks, user protected
Common mistakes: Thinking the earth wire is needed for plastic casing; Missing the link between fuse and earth wire
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States that metal casing can become live if fault occurs
B2 (a) · 1 mark · States earth wire provides a safe path for current instead of passing through user
B3 (b) · 1 mark · States earth wire carries current away from casing if live wire contacts it
B4 (b) · 1 mark · States that the large current then blows the fuse, breaking the circuit and protecting the user
Worked solution
a) Appliances with a metal casing must be earthed because if the live wire comes loose and touches the casing, the casing could become live. Earth wire ensures current flows safely to ground instead of through a person.
b) The earth wire provides a low-resistance path for current if the live wire touches the casing. This makes a large current flow, which causes the fuse to blow, breaking the circuit and stopping electricity — preventing electric shock.