IGCSE Physics 4.5 Dangers of Electricity MCQs – 30 Practice Questions with Answers (Cambridge 0625)
Free original practice MCQs on Cambridge IGCSE Physics 0625 Section 4.5, covering fuses and circuit breakers, earthing, insulation and plug wiring, and common electrical hazards. A fuse rating question usually needs one quick calculation first: current = power ÷ voltage (I = P ÷ V) – then pick the fuse rating just above that current.
Formula recap: P = IV, so the normal working current is I = P ÷ V. A fuse is a thin wire in the live wire that melts and breaks the circuit when the current gets too large. Remind yourself of the key equations on the IGCSE Physics formula sheet.
1. Fuses & Circuit Breakers
Q1. How does a fuse protect an appliance and its cable?
- (A) It stores charge and releases it slowly
- (B) It diverts the current into the earth wire
- (C) It reduces the voltage supplied to the appliance
- (D) Its wire melts when the current becomes too large, breaking the circuit
Show Answer
Answer: D – A fuse contains a thin wire with a low melting point. If a fault makes the current too large, the wire gets hot, melts and breaks the circuit before the cable overheats.
Q2. In a correctly wired circuit, the fuse must be connected in which wire?
- (A) The neutral wire
- (B) The earth wire
- (C) The live wire
- (D) It does not matter which wire
Show Answer
Answer: C – The fuse goes in the live wire so that when it blows, the appliance is disconnected from the high-voltage live supply and becomes safe.
Q3. An electric heater is rated 2300 W and is used on a 230 V supply. The available fuse ratings are 3 A, 5 A and 13 A. Which fuse should be fitted?
- (A) 3 A
- (B) 5 A
- (C) 13 A
- (D) Any of these fuses is suitable
Show Answer
Answer: C – Working current I = P ÷ V = 2300 ÷ 230 = 10 A. The fuse rating must be just above 10 A, so a 13 A fuse is used. A 3 A or 5 A fuse would blow during normal use.
Q4. A lamp has a normal working current of 0.5 A. Which fuse, from 1 A, 5 A and 13 A, is the best choice?
- (A) 1 A
- (B) 5 A
- (C) 13 A
- (D) 13 A, because a bigger fuse lasts longer
Show Answer
Answer: A – Choose the smallest standard rating above the normal current: 1 A. This lets the lamp work normally but blows quickly if a fault raises the current.
Q5. A kettle rated at 1150 W on a 230 V supply has a faulty 13 A fuse fitted instead of the correct rating. Why is this dangerous?
- (A) The kettle will boil the water too quickly
- (B) The kettle will draw less current than normal
- (C) The fuse will blow as soon as the kettle is switched on
- (D) A fault current of, for example, 8 A could flow for a long time and overheat the cable without blowing the fuse
Show Answer
Answer: D – Correct working current I = P ÷ V = 1150 ÷ 230 = 5 A, so a 5 A fuse should be fitted. A 13 A fuse allows currents well above the safe level to keep flowing, overheating the flex and risking a fire.
Q6. The current in a circuit suddenly rises to a very large value because the live and neutral wires touch inside an appliance. What happens in a correctly fitted fuse?
- (A) The fuse wire cools down and switches the appliance off
- (B) The fuse wire gets hot, melts and cuts off the current
- (C) The fuse stores the extra current
- (D) The fuse passes the extra current to the earth wire
Show Answer
Answer: B – A short circuit causes a very large current. The fuse wire heats up almost instantly, melts and breaks the circuit, stopping the current.
Q7. How does a circuit breaker switch off a circuit when the current becomes too large?
- (A) A wire inside it melts and must be replaced
- (B) The water in it boils and opens a valve
- (C) An electromagnet pulls a switch open, and the breaker can be reset by hand
- (D) It changes the live wire into the neutral wire
Show Answer
Answer: C – In a circuit breaker, the fault current makes an electromagnet strong enough to trip a switch open. Once the fault is fixed, the breaker is simply reset – nothing needs replacing.
Q8. Which is an advantage of a circuit breaker compared with a fuse?
- (A) It can be reset and used again after it trips
- (B) It never needs a supply of electricity to work
- (C) It increases the current in the circuit
- (D) It removes the need for an earth wire in every appliance
Show Answer
Answer: A – A circuit breaker can be reset once the fault is corrected, works very quickly, and does not need a replacement wire like a blown fuse.
Q9. A residual current circuit breaker (RCCB) protects a person by switching off the supply when it detects that…
- (A) the voltage rises above 230 V
- (B) the fuse rating is too small
- (C) the appliance has been switched on for too long
- (D) the current in the live wire is different from the current in the neutral wire, showing current is leaking somewhere
Show Answer
Answer: D – In a healthy circuit the live and neutral currents are equal. If some current leaks away – for example through a person to earth – the RCCB detects the difference and switches off extremely quickly.
Q10. A tumble dryer draws a current of 9 A in normal use. Which fuse should be fitted in its plug?
- (A) 3 A
- (B) 5 A
- (C) 10 A
- (D) 30 A
Show Answer
Answer: C – The rating must be just above the normal current of 9 A, so a 10 A fuse is correct. The 3 A and 5 A fuses would blow in normal use, and 30 A would not protect the cable in a fault.
2. Earthing, Insulation & Plug Wiring
Q11. The metal case of an electric oven becomes live because the live wire touches it. The oven is correctly earthed. What happens?
- (A) Nothing – the case stays live but harmless
- (B) The neutral wire carries the current away slowly
- (C) The oven works normally but uses more power
- (D) A very large current flows through the low-resistance earth wire and blows the fuse, disconnecting the supply
Show Answer
Answer: D – The earth wire gives the current a very low-resistance path to earth. The resulting large current blows the fuse (or trips the breaker), so the case is no longer live.
Q12. Why is the earth wire able to carry a large current safely away from a faulty appliance?
- (A) It has a very low resistance
- (B) It is made of plastic
- (C) It has a very high resistance
- (D) It is connected to the neutral wire
Show Answer
Answer: A – A low-resistance earth path means a fault produces a large current, and it is this large current that blows the fuse and makes the appliance safe.
Q13. Which appliance most needs an earth wire?
- (A) A metal-bodied washing machine
- (B) A battery-powered torch
- (C) A plastic-cased hair dryer with double insulation
- (D) A wooden lamp stand
Show Answer
Answer: A – Any appliance with an exposed metal case needs earthing, because if the case becomes live a user touching it could receive a shock. The earth wire and fuse together remove this danger.
Q14. An appliance is described as double insulated. What does this mean for its wiring?
- (A) It needs two earth wires
- (B) Its wires are twice as thick as normal
- (C) It must use two fuses in its plug
- (D) It has two layers of insulation and a non-conducting case, so it does not need an earth wire
Show Answer
Answer: D – A double-insulated appliance has extra insulation and usually a plastic case that cannot become live, so it is supplied with only a live and a neutral wire – no earth wire is needed.
Q15. In a three-pin plug, which colour is the insulation of the live wire?
- (A) Blue
- (B) Brown
- (C) Green and yellow
- (D) Black
Show Answer
Answer: B – Live is brown, neutral is blue and earth is green and yellow. Knowing the colours matters so the fuse and switch end up in the live wire.
Q16. The green-and-yellow wire in a plug is connected to…
- (A) the earth pin and the metal case of the appliance
- (B) the fuse
- (C) the neutral pin
- (D) the on/off switch of the appliance
Show Answer
Answer: A – The green-and-yellow earth wire joins the top (earth) pin of the plug to the metal case of the appliance, ready to carry a fault current safely to earth.
Q17. Why must a switch be connected in the live wire rather than in the neutral wire?
- (A) So that when the switch is off, the appliance is disconnected from the live supply and is safe to touch inside
- (B) So the fuse can be a smaller rating
- (C) So the appliance uses less energy when switched off
- (D) So the current flows faster when the switch is on
Show Answer
Answer: A – If the switch were in the neutral wire, the appliance would stop working when switched off but its insides would still be at live potential – a shock hazard for anyone opening it.
Q18. A fuse is wrongly fitted in the neutral wire of an appliance with a metal case. The fuse blows in a fault. Why is the appliance still dangerous?
- (A) The case may still be connected to the live wire at high potential, so touching it can give a shock
- (B) The appliance will now draw twice the normal current
- (C) The fuse will keep blowing every time it is replaced
- (D) The earth wire will melt
Show Answer
Answer: A – Blowing a neutral-wire fuse stops the current but leaves the appliance connected to the live supply. Fuses and switches must always be in the live wire so a blown fuse makes the appliance safe.
Q19. When wiring a plug, which safety point is correct?
- (A) Bare wires may touch each other as long as the plug case is closed
- (B) The cable grip should hold the outer insulation, and each wire should be firmly connected with no stray strands
- (C) The fuse should be removed to make the plug safer
- (D) The earth pin may be cut off appliances that are used indoors
Show Answer
Answer: B – A correctly wired plug has the cord grip clamping the cable sheath, tight connections and no loose strands that could touch another terminal and cause a short circuit.
Q20. A metal-cased kettle is used with its earth wire broken. The live wire then touches the case. Why is this a serious hazard?
- (A) The kettle will take longer to boil
- (B) The fuse will blow immediately
- (C) No fault current can flow to blow the fuse, so the case stays live and a person touching it may receive a shock through their body to earth
- (D) The water in the kettle will become charged and unusable
Show Answer
Answer: C – With no earth connection, the fault current can only flow when someone touches the case – through that person. The earth wire exists so the fault current flows at once and blows the fuse instead.
3. Hazards & Safe Practice
Q21. The insulation on an appliance flex is cracked and frayed, so the wires inside are exposed. What is the main danger?
- (A) The appliance will work more slowly
- (B) A user may touch a live wire and receive a shock, and exposed wires that touch can cause a short circuit
- (C) The flex will become too long
- (D) The fuse rating will change
Show Answer
Answer: B – Damaged insulation removes the protective barrier around live conductors. It risks both electric shock and a short circuit, which can start a fire.
Q22. Several high-power appliances are plugged into one socket through adaptors. Why is this dangerous?
- (A) The total current may exceed the safe rating of the socket wiring, overheating it and possibly causing a fire
- (B) Each appliance receives a higher voltage
- (C) The fuses in all the plugs will blow at once
- (D) The appliances will interfere with each other’s switches
Show Answer
Answer: A – Currents add up in parallel circuits. Too many appliances on one socket overload the cable, which heats up – a common cause of electrical fires, even before any fuse blows.
Q23. An extension lead rated at 5 A is used, still coiled up on its drum, to power a 2300 W heater on a 230 V supply. Why is this dangerous?
- (A) The coiled lead gives the heater a lower voltage
- (B) The heater draws current I = P ÷ V = 2300 ÷ 230 = 10 A, which is above the lead’s rating, and the coil cannot lose heat easily, so the insulation may melt
- (C) The fuse in the heater plug will be bypassed
- (D) The lead will act as an earth wire
Show Answer
Answer: B – A current of 10 A in a lead rated at 5 A overheats it. Because the cable is coiled, the heat cannot escape, the insulation melts, and a short circuit or fire can follow. Extension leads should be fully uncoiled for high-power use.
Q24. Why is it more dangerous to touch a live wire when your hands and the floor are wet?
- (A) Water increases the supply voltage
- (B) Water makes the fuse work more slowly
- (C) Water changes the live wire into an earth wire
- (D) Wet skin and a damp floor give a lower-resistance path, so a larger current flows through the body
Show Answer
Answer: D – Water conducts electricity and wet skin has a much lower resistance than dry skin. For the same voltage, a larger and possibly fatal current can pass through the body.
Q25. A bird sits on a single overhead power line and receives no shock. Why not?
- (A) Birds have insulated feathers that block all current
- (B) The wire carries no current
- (C) Both of the bird’s feet are on the same wire, so there is no potential difference across its body and no current flows through it
- (D) The bird is too light to complete the circuit
Show Answer
Answer: C – Current only flows through the body if there is a potential difference across it. Both feet are at the same potential on one wire, so no current passes through the bird. It would be shocked if it touched a second wire or the ground at the same time.
Q26. Why should you never operate an electrical switch with wet hands?
- (A) The switch will rust
- (B) The water may drip inside the switch and provide a conducting path, and wet skin has low resistance, so a shock is more likely
- (C) Wet hands switch the circuit on too quickly
- (D) The fuse will blow every time
Show Answer
Answer: B – Water can bridge the insulation inside a switch and wet skin lowers the body’s resistance, making any shock much more severe. Always dry your hands first.
Q27. A person receives an electric shock from an appliance even though its 13 A fuse does not blow. How is this possible?
- (A) Fuses only work with direct current
- (B) The fuse was fitted in the earth wire
- (C) The current through the person can be dangerous at well under 1 A – far below the fuse rating – so the fuse does not melt
- (D) The person was wearing rubber shoes
Show Answer
Answer: C – A fuse protects cables and appliances against large currents, but a current of a fraction of an ampere through the chest can be fatal. A fuse therefore cannot protect a person against every shock – earthing, insulation and RCCBs are also needed.
Q28. Which action is safe when using electrical appliances?
- (A) Pulling a plug out by its flex
- (B) Using an appliance with a cracked case near a sink of water
- (C) Switching off and unplugging an appliance before cleaning it or opening it up
- (D) Replacing a blown fuse with a thicker wire so it does not blow again
Show Answer
Answer: C – Isolating the appliance first removes the shock risk. Pulling plugs by the flex damages insulation, water near electricity is hazardous, and replacing a fuse with thick wire removes the protection entirely.
Q29. A hair dryer is accidentally dropped into a bath of water while switched on. Why is this situation so dangerous?
- (A) The water becomes hot very quickly
- (B) The person in the bath provides a path to earth through the water, and the wet body’s low resistance allows a large shock current
- (C) The hair dryer will explode
- (D) The fuse in the plug will make the water live permanently
Show Answer
Answer: B – Water connects the live parts of the dryer to the bather, whose wet skin has low resistance, so a large current can flow through the body to earth. Electrical appliances must be kept away from baths and sinks.
Q30. Long cables carrying large currents to an appliance can overheat even when no fault is present. What reduces this overheating?
- (A) Using thinner cables
- (B) Coiling the spare cable tightly
- (C) Fitting a larger fuse
- (D) Using thicker cables, which have lower resistance and so less heating for the same current
Show Answer
Answer: D – Electrical heating in a wire increases with resistance. Thicker cables have lower resistance, run cooler at high currents, and are the correct choice for power-hungry appliances.
Keep practising: Browse all boards and topics on the Physics MCQs hub, and keep the key facts for charges, currents and circuits at hand with the IGCSE Physics formula sheet.
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