A switch lets a person control a circuit. A fuse protects a circuit, automatically, when something goes wrong. Questions ask you to state these purposes, compare them, or choose a fuse value for a device.
This lesson follows on from calculating electrical energy from ratings, because the same current calculation decides the fuse.
What does each part do?
A switch opens or closes the circuit when the user chooses. On a mains appliance it should be in the live wire, so that with the switch off the appliance is cut off from the live supply.
A fuse contains a thin wire.
A current above its rating heats the wire until it melts, which breaks the circuit. It protects the wiring and the appliance from overheating. A circuit breaker has a similar purpose but is reset rather than replaced.
| Switch | Fuse | |
|---|---|---|
| Who decides? | The user | The current itself |
| Purpose | Control on and off | Protect against too large a current |
| Normal operation | Opened and closed often | Stays closed and carries the current |
| After use | Reusable | Melted fuse must be replaced by a qualified person |
How do you choose a fuse rating?
Find the normal current with I = P ÷ V. Then choose the smallest available fuse value that is above that current. A fuse much larger than needed would let a damaging current flow before it melts.
Worked example
An invented toaster is marked 230 V, 1000 W. The available fuses are 3 A, 5 A and 13 A. Which should be fitted?
Step 1, normal current: I = 1000 ÷ 230 = 4.35 A (to 3 significant figures).
Step 2, compare with the values: 3 A is too small, because the fuse would melt in normal use. 5 A is just above 4.35 A. 13 A is well above.
Step 3, choose: the 5 A fuse.
Step 4, reason: it carries the normal 4.35 A but melts if a fault raises the current above 5 A.
The mistake to watch for
A common slip is to fit the highest rating “to be safe”.
Mistaken answer: “Use the 13 A fuse, because it will never blow.”
A fuse that never blows does not protect anything. The flex could overheat well before 13 A melts the fuse.
The correction is to match the fuse to the device: just above the normal current. “Safe” means the fuse acts when it should.
A second common slip is to say a fuse is used to switch the device off. The user switches it off; the fuse acts only when the current is too large.
Check yourself
1. Give one difference between the purpose of a fuse and of a switch.
Show answer
A switch is used by a person to turn the device on or off. A fuse automatically breaks the circuit when the current becomes too large, to protect the wiring and appliance.
2. An invented lamp is marked 230 V, 60 W. Fuses available: 3 A, 5 A, 13 A. Which is suitable?
Show answer
I = 60 ÷ 230 = 0.26 A (to 2 significant figures). The smallest fuse above this is 3 A.
3. A heater is marked 230 V, 2800 W. Which fuse from 3 A, 5 A, 13 A?
Show answer
I = 2800 ÷ 230 = 12.2 A (to 3 significant figures). Only 13 A is above this, and 3 A and 5 A would melt in normal use.
Where this leads next
A fuse only works if the fault current can reach it. See how in interpreting earthing on a supplied diagram. Wider context is in electrical energy and safe interpretation.
If you understand the idea but write explanations that stop short, a teacher giving online one-to-one Physics tuition can practise full-mark wording with you on fresh questions.