IGCSE Physics: Electrical Safety Practice Questions
Fuses and circuit breakers protect a circuit by breaking it when the current becomes too large. The earth wire protects the user by providing a low resistance path to earth if the live wire touches a metal casing.
Sub-topic 4.4 of Cambridge IGCSE Physics 0625 is short and heavily rewarded, because the answers are fixed. The one thing students routinely get wrong is what the fuse and the earth wire each protect. The questions below separate those roles clearly.
What you need to know for Electrical Safety
- Wire coloursLive is brown, neutral is blue, and earth is green and yellow striped. The live wire carries the high potential difference and is the dangerous one.
- FuseA thin wire that melts and breaks the circuit if the current exceeds its rating. It must be fitted in the live wire, so that the appliance is isolated from the high potential difference when it blows.
- Choosing a fuse ratingCalculate the normal operating current using I = P divided by V, then choose the next standard fuse rating above it, commonly 3 A, 5 A or 13 A.
- Earth wireConnects the metal casing of an appliance to earth. If the live wire touches the casing, a very large current flows to earth through the low resistance path, blowing the fuse and isolating the appliance.
- Circuit breakerA switch that trips automatically when the current is too large. It acts faster than a fuse and can be reset rather than replaced.
- Double insulationAn appliance with a plastic casing and no exposed metal parts needs no earth wire, because the casing cannot become live. Marked with a double square symbol.
IGCSE Physics Electrical Safety questions and answers
4 exam-style questions written to the 0625 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
An electric kettle is rated at 2200 W and operates from a 230 V mains supply. Calculate the normal operating current and state which fuse should be fitted from the range 3 A, 5 A and 13 A.
Show the worked answer
- Use P = IV, rearranged to I = P divided by V.
- I = 2200 divided by 230 = 9.565 A, which is 9.6 A to 2 significant figures.
- The fuse must be rated above the normal operating current, otherwise it would blow during ordinary use.
- The next standard rating above 9.6 A is 13 A, so a 13 A fuse should be fitted. A 3 A or 5 A fuse would blow as soon as the kettle was switched on.
Explain how the earth wire and the fuse work together to protect a user when a fault causes the live wire to touch the metal casing of an appliance.
Show the worked answer
- Without an earth wire, the metal casing would become live, and a person touching it would provide a path to earth and receive a serious shock.
- The earth wire connects the casing directly to earth and has a very low resistance.
- When the live wire touches the casing, a very large current flows through the earth wire to earth.
- This large current exceeds the fuse rating, so the fuse wire melts and the circuit is broken.
- The appliance is then disconnected from the live supply, so the casing is no longer at a dangerous potential and the user is protected.
Explain why the fuse must be fitted in the live wire rather than the neutral wire.
Show the worked answer
- The live wire is the one at a high potential difference relative to earth. The neutral wire is close to earth potential.
- If the fuse were in the neutral wire and it blew, current would stop flowing, but the appliance would remain connected to the live wire.
- Anyone touching the appliance could then still complete a path to earth and receive a serious shock.
- With the fuse in the live wire, blowing it disconnects the appliance from the high potential difference completely, so the appliance is safe to touch.
A plastic cased hairdryer has no earth wire. Explain why this is safe, and state two advantages of a circuit breaker over a fuse.
Show the worked answer
- The hairdryer is double insulated. Its outer casing is made of an insulating plastic and there are no exposed metal parts.
- Even if the live wire came loose inside, it could not make the casing live, so no current could pass to a person touching it. An earth wire is therefore unnecessary.
- Advantage 1 of a circuit breaker: it operates much more quickly than a fuse, so the circuit is broken sooner and the risk of injury or damage is lower.
- Advantage 2: it can be reset by flicking a switch rather than replaced, which is cheaper and more convenient.
Common mistakes in this topic
- Choosing a fuse rated below the normal operating current.
- Saying the fuse protects the user. The fuse protects the wiring and the appliance. The earth wire protects the user.
- Placing the fuse in the neutral wire.
- Confusing the wire colours. Live is brown, neutral is blue, earth is green and yellow.
- Suggesting a double insulated appliance needs an earth wire.
Exam tips
- Always calculate the operating current first with I = P divided by V, then pick the next standard rating up.
- Explain fuse and earth as a pair: the earth creates the large current, the fuse then breaks the circuit.
- Learn the three wire colours as a fixed list. It is a guaranteed easy mark.
- For any hazard question, name the specific danger: damaged insulation, overheating cables, damp conditions.
- Mention that a circuit breaker is faster and resettable. Those two points cover nearly every comparison question.
Practise 20 more questions like this, free
vStudyWise marks every answer instantly, tracks the topics you keep dropping marks on and turns them into a weekly study plan.
Electrical Safety FAQs
How do I choose the right fuse rating?
Calculate the appliance's normal operating current by dividing its power rating by the supply voltage, then choose the next standard fuse rating above that value. A fuse rated below the operating current would blow in normal use, and one rated far above it would not protect the appliance.
What does the earth wire actually do?
It connects the metal casing of an appliance to earth through a very low resistance path. If a fault makes the live wire touch the casing, a very large current flows to earth. That surge blows the fuse, which disconnects the appliance so the casing is no longer at a dangerous potential.
Why must the fuse be in the live wire?
The live wire is at a high potential difference relative to earth. If the fuse were in the neutral wire and blew, current would stop but the appliance would remain connected to the live supply, so touching it could still give a shock. A fuse in the live wire isolates the appliance completely.
What is double insulation?
A double insulated appliance has an outer casing made entirely of insulating material with no exposed metal parts, so the casing cannot become live even if a wire comes loose inside. Such appliances need no earth wire and are marked with a double square symbol.
Continue through the IGCSE Physics syllabus
Related IGCSE Physics topics
See all 24 IGCSE Physics practice topics ›
Written to the published Cambridge IGCSE Physics (0625) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.