1.4.4Electricity, Energy and Waves
Circuit Breakers — WJEC GCSE Science (Double Award) - Physics Revision Notes & Topical Questions
5 min read 7 parts Lesson 25 of 36
Section 1 of 7Mains supply to a home
Supply
- Supply Domestic electricity is supplied at 230 V with a maximum current of about 65 A.
Consumer unit
- Consumer unit The supply enters at the consumer unit, often called the fuse box.
- It contains circuit breakers that control and protect each circuit in the house.
Why protection is needed
- Why protection is needed A fault can cause a large surge of current that would overheat wires and risk fire or electric shock.
Section 2 of 7Types of protection
| Device | Where | What it protects | How it works |
|---|---|---|---|
| Miniature circuit breaker (MCB) | Consumer unit | One circuit, such as upstairs sockets, lights or a shower | Switch flips off if the current goes above its set limit |
| Residual current circuit breaker (RCCB) | Consumer unit | People from electrocution | Switch flips off if the current leaving and returning differ by more than its limit |
| Fuse | In the plug | An individual appliance | A thin wire melts if the current is too high |
Section 3 of 7Circuit breakers in action
1Fault occurs
- Fault occurs A fault such as a short circuit makes the current rise above the safe limit.
2Breaker trips
- Breaker trips The switch automatically moves to off and breaks the circuit.
3Fault found
- Fault found Because the circuit is dead, the cause can be found and repaired safely.
4Reset
- Reset The breaker is switched back on and nothing inside is damaged, unlike a fuse that must be replaced.
Section 4 of 7Fuses
Plug fuse
Construction
- Construction Usually a thin metal wire inside a glass cylinder, shown on circuit diagrams by a box with a wire through it.
Operation
- Operation If the current is too large the wire heats up and melts, breaking the circuit and disconnecting the appliance from the mains.
Rating
- Rating Its rating is the maximum current that can flow without melting; common values are 3 A, 5 A and 13 A.
- Choose the standard value just above the current the appliance needs.
Section 5 of 7Choosing a fuse (HT only)
1
Question: a 2 kW kettle runs from the 230 V mains. Which fuse should be fitted?
2
Convert the power: P = 2 kW = 2000 W, and V = 230 V.
3
Use P = V × I rearranged to I = P ÷ V.
4
Substitute: I = 2000 ÷ 230 = 8.7 A.
5
The fuse must be just above 8.7 A, and the choices are 3 A, 5 A and 13 A, so the correct fuse is 13 A.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Consumer unit | The box where the mains supply enters a home and is split between circuits. |
| Miniature circuit breaker | A resettable switch that cuts off one circuit when the current is too high. |
| Residual current circuit breaker | A device that cuts the supply if current leaving and returning differ, protecting against shock. |
| Fuse | A thin wire that melts when the current exceeds its rating, breaking the circuit. |
| Fuse rating | The maximum current a fuse will carry without melting. |
Section 7 of 7Exam technique
1
Choosing a fuse with a rating below the normal current, which would blow every time the appliance is used.
2
Choosing a fuse far above the current needed, which would not protect the appliance.
3
Forgetting to convert kW to W before using I = P ÷ V.
4
Saying that a circuit breaker must be replaced after tripping; it can be reset.
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