Edexcel IGCSE Physics · Spec 2.2-2.4
Electrical Heating, Fuses and Circuit Protection
Covers explain heating in a resistor, Distinguish insulation, earthing, fuses and circuit breakers and Select a suitable fuse using P = IV.
Physics revision video
Electrical Heating, Fuses and Circuit Protection
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Explained
Why wires get hot, and the four things that keep you safe
When current flows through a resistor, the moving electrons collide with the ions in the metal lattice and transfer energy to them. The ions vibrate more, so the component's thermal store increases and it gets hot.
Sometimes that is the point, as in a kettle element, a toaster or a filament lamp. Sometimes it is a waste, as in the cables of the national grid, and sometimes it is a hazard, which is what this topic is really about.
Insulation and double insulation
Basic insulation is the plastic covering around the live conductors. Plastic does not conduct electricity, so it stops the current reaching you and prevents an electric shock.
A double insulated appliance goes further and has an insulating outer case as well. Because no part you can touch is metal, no fault can make the case live, so it needs no earth wire. Those appliances carry a double square symbol.
Earthing
An appliance with a metal case needs a different solution. The earth wire connects the case to the ground through a path of very low resistance.
If a live wire works loose and touches the case, the case would otherwise become live and anyone touching it would receive a shock. Instead, the low resistance path means a very large current flows, and that surge is what blows the fuse or trips the circuit breaker, cutting off the supply.
Notice that the earth wire does not make the appliance safe on its own. It works by causing a large enough current to operate the fuse, and answers that leave that step out have described only half the mechanism.
Fuses and circuit breakers
A fuse is a thin wire in the live side of the circuit. Too much current heats it until it melts, breaking the circuit.
A circuit breaker does the same job with a switch that trips. It works faster and more sensitively than a fuse, and it can be reset rather than replaced, which is cheaper over time.
To choose a fuse, work out the normal current with power equals current times voltage, then pick the next fuse rating above it. A 690 W appliance on 230 V draws 3 A, so a 5 A fuse is right. A 3 A fuse would blow in normal use and a 13 A fuse would let a dangerous current flow before acting.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme asks why a plastic covering is used and awards two marks: that plastic is an electrical insulator, allowing the idea that it does not conduct or a reference to double insulation, and that it prevents an electric shock. It then instructs the examiner to ignore references to current transferring to the surroundings.
The examiner report on that paper shows where the second mark went. Some candidates gave responses that were too vague, referring to risks of harming or to safety in general rather than the specific risk of shock or electrocution. Others discussed thermal insulation instead of electrical insulation.
The report also covers the fuse comparison. The idea that a circuit breaker is resettable was frequently seen, but candidates used all four lines elaborating on it. Very few knew that a circuit breaker is more sensitive or faster, and those who did were generally the ones who scored both marks. Candidates often wrote that the circuit breaker was safer rather than faster.
Safer is a conclusion, not a reason. Faster and more sensitive are the reasons it is safer, and those are what earn the mark.
The three wires
Live is brown and carries the alternating supply at 230 V. Neutral is blue and completes the circuit, staying close to zero volts. Earth is green and yellow striped and carries no current at all in normal use.
The fuse and the switch both go in the live wire, and for the same reason: breaking the live side isolates the appliance from the dangerous voltage. Putting either in the neutral wire would stop the appliance working while leaving it live to touch.
Spec 2.2-2.4
What you need to know
- Explain heating in a resistor
- Distinguish insulation, earthing, fuses and circuit breakers
- Select a suitable fuse using P = IV
Active recall
Quick check
Answer each question before opening the answer.
How does a resistor heat up?
Electrons transfer energy in collisions with lattice ions
What is the difference between a fuse and circuit breaker?
A fuse melts once; a circuit breaker trips and can be reset
Which fuse suits an 8.7 A appliance?
13 A
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