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Fuses & Circuit Breakers

Key idea: O Level practical electricity: how fuses and circuit breakers protect circuits, why they are connected in the live wire, and how to choose a fuse rating.

  • SEC G3 Physics 2027
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Learning objectives

  • Explain electrical heating in common appliances
  • Apply P = VI
  • Apply E = VIt
  • Calculate electrical energy and cost in kWh
  • Identify the hazard from damaged insulation
  • Identify the hazard from overheating cables
  • Identify the hazard from damp conditions
  • Explain how fuses and circuit breakers protect circuits
  • Choose and justify an appropriate fuse rating
  • Explain why metal casings are earthed
  • Explain why double-insulated appliances do not need an earth wire
  • State the meanings and roles of live, neutral and earth
  • Describe the wiring of a mains plug
  • Explain live-wire placement of switches, fuses and circuit breakers

1. Definition

A. Fuse

A fuse is a safety device that melts and breaks the circuit when the current is too large.

B. Circuit breaker

A circuit breaker is an automatic switch that trips and breaks the circuit when the current is too large (and it can be reset).

What you need for this course

You should be able to explain how fuses and circuit breakers protect a circuit, choose a suitable fuse rating and explain why these devices are fitted to the live wire.

2. Key Ideas

  • Fuses and circuit breakers protect against large current (overload / short circuit) which can cause cable overheating and fires.
  • They are connected in the live wire, so when they open the circuit the appliance is disconnected from the live supply.
  • Fuse: melts (“blows”) → must be replaced. Circuit breaker: trips → can be reset.
  • Fuse rating selection:
    • find normal current: I = P/V
    • choose a rating just above the normal current (next available rating)

3. Detailed Explanations

A. What hazard are they preventing?

Large currents heat up wires and cables. If insulation melts, wires can touch and sparks can occur, causing an electric fire.

Fuses and circuit breakers are designed to break the circuit when the current becomes dangerously large.

B. Why must they be in the live wire?

If a fuse/circuit breaker is in the neutral wire, the appliance may still be connected to the live wire even after the protection device opens the circuit. Parts of the appliance can still be at live potential, which is a shock hazard.

Why mains switches and protective devices belong in the live wireTwo simplified mains circuits compare a switch in the live wire with a switch in the neutral wire. In the safe circuit, opening the live switch isolates the appliance. In the unsafe circuit, opening the neutral switch stops current but leaves the appliance connected to the live supply.Device in LIVE — isolated when openLIVEopen switch / fuseapplianceisolatedNEUTRAL✓ live connection openedNo connection to live beyond the open pointnormal current = 0 and internal parts are isolatedDevice in NEUTRAL — still connected to liveLIVEappliancestill liveNEUTRAL⚠ live connection remainsNo normal current does not mean no dangera fault or touch can complete a path to Earth
Scroll diagram horizontally to read all labels.
Opening a switch or protective device in the live conductor isolates the appliance from high potential. Opening only the neutral conductor stops normal current but can leave the appliance connected to live.

C. Choosing a fuse rating (O Level method)

  1. Calculate normal current:

I = P/V

  1. Choose a fuse rating that is just above the normal current.

Examples of common ratings (country-dependent): 3 A, 5 A, 13 A.

Safety

Never replace a blown fuse with thicker wire or foil. A higher-than-needed rating can allow dangerous currents to flow before the fuse blows.

4. Common Mistakes

  • Choosing a fuse rating that is too high (“it won’t blow”) → unsafe.
  • Saying a fuse is mainly to prevent electric shock (its main role is to prevent overheating/fire from large currents).
  • Putting the fuse/circuit breaker in the neutral wire in explanations/diagrams.
  • Forgetting units (A, V, W).

5. Exam Tips

  1. Use the chain: fault → large current → heating → fuse melts / breaker trips → circuit opens (in live wire).
  2. For fuse ratings: compute I = P/V, then choose a rating slightly higher.
  3. If asked “why live wire?”, write: “so the appliance is disconnected from the live supply when the circuit opens”.

6. Worked Examples

Modelled example 1

Choosing a fuse rating

Core

Problem

An iron is rated 600 W on a 240 V supply. Calculate normal current and choose a suitable fuse from 3 A, 5 A and 13 A.
Study the worked solution

  1. Calculate normal current

    Method

    Use I = P/V.

    Reason

    The appliance rating gives power at the supply voltage.

    Working

    I = 600/240 = 2.5 A

  2. Choose the protective rating

    Method

    Select the 3 A fuse.

    Reason

    It is the smallest available rating above the 2.5 A operating current.

    Working

    2.5 A < 3 A

Guided practice 2

Maximum power before a breaker trips

About 5 min

Problem

A circuit breaker trips at 20 A on a 240 V supply. What total power corresponds to its trip current?

Map the current limit to a power limit

Unit: kW

Hints

Hint 1: relationship
Use P = VI.
Hint 2: conversion
4800 W = 4.8 kW.
View solution step by step

  1. Calculate at the trip current

    Method

    Multiply voltage by current.

    Reason

    The stated current is the breaker’s limiting current.

    Working

    P = (240)(20) = 4800 W = 4.8 kW

Common misconception 3

Live vs neutral placement

Find and correct the mistake

Learner response

A learner says a fuse works equally safely in neutral because opening either conductor stops normal current. Diagnose the claim.

Distinguish stopping current from isolating live potential

View solution step by step

  1. Identify the incomplete idea

    Method

    Acknowledge that opening neutral can stop normal current.

    Reason

    The circuit is broken, but that alone does not make the appliance isolated from live.

    Working

    Neutral open: normal current = 0, but the live connection can remain.

  2. State the safe placement

    Method

    Place the fuse in live.

    Reason

    When it melts, the appliance is disconnected from the live supply and exposed internal parts are not left live through that connection.

    Working

    Live open → appliance isolated from live supply.

Examiner practice 4

Fuse rating for a heater

4 marks

Examination question

A heater is rated 1100 W at 240 V. Calculate normal current and choose from 3 A, 5 A and 13 A. Justify the choice. [4 marks]

Calculate before choosing

View solution step by step

  1. Find normal current

    2 marks

    Method

    Use I = P/V.

    Reason

    The rating describes normal power at the supply p.d.

    Working

    I = 1100/240 = 4.58 A

  2. Select and justify

    2 marks

    Method

    Choose the 5 A fuse.

    Reason

    It is the smallest available rating above 4.58 A.

    Working

    4.58 A < 5 A≪13 A

Challenge 5

Is the fuse too low?

Minimal support

Rating-evaluation transfer

An appliance rated 1000 W at 240 V is fitted with a 3 A fuse. Evaluate whether it can operate normally and recommend the suitable listed rating.

Compare the existing rating with normal current

Hints

Hint 1: normal current
Calculate 1000/240.
Hint 2: comparison
A normal current above the fuse rating can melt the fuse in normal use.
View solution step by step

  1. Calculate normal current

    Method

    Use I = P/V.

    Reason

    The existing fuse must be compared with operating current.

    Working

    I = 1000/240 ≈ 4.17 A

  2. Evaluate and replace

    Method

    Reject 3 A and choose 5 A.

    Reason

    3 A is below normal current; 5 A is the next suitable standard rating above it.

    Working

    3 A < 4.17 A < 5 A

7. Mind Stretchers

Mind stretcher 1: Too-high fuse ratingExtension

A 60 W lamp is protected by a 13 A fuse. Is this safer or less safe than using a 3 A fuse? Explain.

Show Answer

Less safe. A 13 A fuse allows much larger currents to flow before it blows, so the wire/cable could overheat in a fault before the fuse breaks the circuit. A 3 A fuse is closer to the normal current and provides better protection for the lamp circuit.

Mind stretcher 2: Short circuit vs overloadExtension

Which situation is more likely to produce the larger current: a short circuit or an overload? Explain briefly.

Show Answer

A short circuit. The resistance becomes very small, so current becomes very large (I = V/R). Overload increases current too, but usually not as dramatically as a short circuit.

8. Practice and next step

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027