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Home · IGCSE 0625 · Electricity and magnetism · The mains circuit · Practice
Core · Practice questions · The mains circuit

Live, neutral, earth.

Six original Cambridge-style questions on the mains circuit: the three wires and their colours, where the fuse and switch belong, how earthing and fusing make a faulty case safe, and how to choose a fuse rating.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
What earns the marks

Each wire, and how the protection works.

01
Recall
[3 marks]

Name the three wires in a mains cable and state the colour of each.

  • Live: brown. ✓
  • Neutral: blue. ✓
  • Earth: green and yellow. ✓
02
Application
[2 marks]

State in which wire the fuse and switch are connected, and explain why this is the safe choice.

  • Both go in the live wire. ✓
  • So that when they break the circuit, the appliance is cut off from the dangerous live side rather than left connected to it. ✓
03
Analysis
[4 marks]

A metal-cased appliance develops a fault in which the live wire touches the metal case. Explain how the earth wire and the fuse work together to make the appliance safe.

  • The earth wire gives the fault current a low-resistance path from the case to the ground. ✓
  • A large current flows through the live wire and the fuse. ✓
  • This current is above the fuse rating, so the fuse melts and breaks the circuit. ✓
  • The appliance is disconnected and the case is no longer live, so it is safe to touch. ✓
04
Calculation
[3 marks]

A kettle is rated at 2300 W and runs from the 230 V mains. Fuses of 3 A, 5 A and 13 A are available. Calculate the operating current and choose a suitable fuse.

I = P ÷ V = 2300 ÷ 230 = 10 A

choose the 13 A fuse

just above the 10 A operating current; 3 A and 5 A would blow at once

05
Application
[2 marks]

A plastic-cased hairdryer has only two wires, live and neutral, and no earth. Explain why it does not need an earth wire.

  • The plastic case is an insulator and cannot become live. ✓
  • So there is no metal body for a user to touch, and no earth is needed (the appliance is double insulated). ✓
06
Application
[2 marks]

State one advantage of a circuit breaker compared with a fuse for protecting a circuit.

  • A circuit breaker can be reset by switching it back on after a fault, rather than being replaced. ✓
  • It also trips faster and at a more precise current than a fuse. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.