Home › Learning Hub › IGCSE Physics › 4d Electrical safety
Topic 4 · 4.4

Electrical safety

Core · Papers 1–6

🎯What you need to be able to do

  • State the hazards of damaged insulation, overheating cables, damp conditions and overloading plugs, extension leads and sockets.
  • Know that a mains circuit has a live, a neutral and an earth wire, and explain why a switch must be in the live wire.
  • Explain the use and operation of fuses and trip switches, and choose suitable ratings.
  • Explain why an appliance’s outer casing must be earthed or non-conducting (double-insulated).

📚The physics

Hazards

damaged insulation: exposed live wires can give an electric shock or cause a short circuit and fire
overheating cables: too much current heats the wires, melting insulation and starting fires
damp conditions: water conducts, so current can flow through a person; never touch switches with wet hands
overloading: plugging many appliances into one socket or extension lead draws too much current, overheating the cable

The mains circuit

A mains circuit has a live (line) wire, a neutral wire and an earth wire. The live wire is at a high voltage; the neutral wire is close to 0 V. The switch must be in the live wire, so that when it is off the appliance is disconnected from the high voltage; with the switch in the neutral wire, the appliance would still be live and dangerous even when switched off.

The inside of a three-pin plug. The green and yellow earth wire goes to the top pin, which connects to the metal case of the appliance. The brown live wire goes through the fuse to the right-hand pin. The blue neutral wire goes to the left-hand pin. A cable grip holds the outer sheath of the cable.
The fuse is always in the live wire.

Fuses and trip switches

A fuse is a thin wire that heats up and melts if the current is larger than its rating, breaking the circuit. A trip switch (circuit breaker) does the same job with an electromagnet that opens a switch; it can be reset. Both protect the wiring from overheating. Choose a rating a little above the normal current: small enough to break the circuit in a fault, large enough not to break it in normal use.

An appliance with a metal case connected to live, neutral and earth wires, with a fuse in the live wire. If a fault makes the live wire touch the case, a large current flows through the earth wire to earth, the fuse melts and the case is safe to touch.
Earthing and a fuse work together.

An appliance’s outer casing must be either earthed (if it is metal) or non-conducting (double-insulated), so a person touching it cannot become part of a circuit to the live wire. A fuse without an earth wire still protects the circuit and cabling of a double-insulated appliance.

✏️Worked example

A kettle is rated 2.3 kW, 230 V. (Modelled on 0625/32 June 2026 Q8.) (a) Calculate the normal current. [2] (b) Choose a fuse: 3 A, 5 A or 13 A. Explain your choice. [2] (c) Explain how the fuse protects the circuit. [2] (d) Give one reason why large currents are dangerous. [1]

(a) I = P/V = 2300 / 230 = 10 A.

(b) 13 A: the smallest rating above the normal 10 A. A 3 A or 5 A fuse would melt every time the kettle was used.

(c) If the current becomes larger than 13 A, the fuse wire overheats and melts, breaking the circuit before the cables overheat.

(d) They heat the cables, which can melt the insulation and cause a fire (or a shock).

Check it. The fuse rating must always be larger than the normal current but as close to it as possible.
“The fuse protects the person.” A fuse protects the wiring and appliance; it is the earth wire (with the fuse) that makes a faulty metal case safe.

📝Practise

In the style of the multiple-choice and theory papers.

1. (Multiple choice.) An electric oven takes a current of 26 A. Which trip switch rating is most suitable? A: 6 A. B: 20 A. C: 32 A. D: 100 A. (Modelled on 0625/12 June 2026 Q33.)
C. Just above the normal current.
2. (Multiple choice.) Which situation is most likely to make an extension lead overheat? A: fuses of low rating in the plugs. B: the lead is kept dry. C: several high-power appliances plugged into it at once. D: the earth wire is connected.
C. Overloading draws a large total current.
3. (Theory.) Explain why a light switch must be connected in the live wire. [2]
When the switch is off, the lamp is disconnected from the high voltage of the live wire, so it is safe to change the bulb. In the neutral wire, the lamp would still be connected to the live wire when switched off.
4. (Theory.) A hair dryer has a plastic case and no earth wire. Explain why it is still safe to use. [2]
It is double-insulated: the outer case does not conduct, so even if a live wire came loose inside, the case could not become live.
5. (Theory.) Give two advantages of a trip switch over a fuse. [2]
It can be reset (no need to replace a fuse); it acts faster (and can be more sensitive).
6. (Theory.) A 60 W lamp works from 230 V. Which fuse is suitable: 1 A, 3 A or 13 A? [2]
I = 60 / 230 = 0.26 A, so a 1 A fuse.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • BBC Bitesize — mains electricity and safety
  • Electrical Safety First — home electrical safety guidance