Safety devices
4 marks · NormalA kitchen kettle is fitted with a 13 A fuse. The table below shows the fuses available and their ratings:
| Fuse | Rating (A) |
|---|---|
| A | 3 |
| B | 5 |
| C | 13 |
Explain why the fuse should be rated just above the normal working current of the kettle.
State and explain what would happen if someone used the 3 A fuse instead of the 13 A fuse in the kettle.
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) A fuse rated just above the working current will not blow in normal use but will break the circuit if the current gets too high. · must show fuse does not blow in normal use, fuse breaks circuit if current is too high
(b) The 3 A fuse would blow as soon as the kettle is switched on because the normal current is above 3 A, so the kettle cannot be used. · must show 3 A fuse blows immediately, kettle will not work
Common mistakes: Assuming higher rated fuses are always safer
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Fuse does not blow during normal operation
B2 (a) · 1 mark · Fuse blows and breaks circuit if current rises above safe value
B3 (b) · 1 mark · 3 A fuse blows immediately if working current is above 3 A
B4 (b) · 1 mark · Kettle cannot be used as fuse breaks circuit
Worked solution
a) If the fuse is rated just above the normal current, it allows normal use but blows if an unsafe current flows, protecting the kettle and user.
b) The 3 A fuse would blow as soon as the kettle is switched on because the current drawn is higher than 3 A, so the kettle will not work.