How does mains electricity reach our homes safely, and how is electrical power calculated?
Alternating and direct current, the live, neutral and earth wires, fuses and earthing, and the electrical power equation P = V I.
A CCEA GCSE Physics answer on alternating and direct current, the live, neutral and earth wires in a plug, how fuses and earthing keep us safe, and using the electrical power equation P = V I.
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What this dot point is asking
CCEA wants you to know the difference between alternating and direct current, the roles of the live, neutral and earth wires, how fuses and earthing keep us safe, and how to use the electrical power equation P = V I. The fuse and earth explanation is a common extended-answer question.
The answer
Alternating and direct current
The three wires
Fuses and earthing
Double-insulated appliances have a plastic case and no earth wire, because the case cannot become live.
Electrical power
Worked example: choosing a fuse
Examples in context
- Example 1. A metal-cased toaster
- If a loose live wire touches the metal body, the earth wire carries a surge of current to earth, blowing the fuse before anyone touching the toaster receives a shock.
- Example 2. A phone charger
- A small charger is often double insulated, with a plastic case and only live and neutral wires, because the plastic case can never become live, so no earth wire is needed.
- Example 3. A circuit breaker
- Many modern homes use circuit breakers instead of fuses. A circuit breaker is a switch that trips and breaks the circuit if the current gets too high. Unlike a fuse, it can simply be reset rather than replaced, but it does the same protective job.
A clear way to remember the safety system is: the fuse and circuit breaker protect against too large a current, and the earth wire keeps the case at a safe voltage and provides a low-resistance path for fault current. Together they ensure that a fault is detected and the live supply is cut off before anyone is harmed.
Try this
Q1. State the UK mains voltage. [1 mark]
- Cue. About 230 V.
Q2. What is the colour of the earth wire? [1 mark]
- Cue. Green and yellow.
Q3. A appliance runs at . Find the current. [2 marks]
- Cue. .
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style3 marksAn electric kettle is rated at 2300 W and runs from the 230 V mains. Calculate the current it draws and suggest a suitable fuse from 3 A, 5 A and 13 A.Show worked answer →
Rearrange the power equation for current:
A fuse must be rated just above the normal current, so a 13 A fuse is suitable (3 A and 5 A would blow under the normal 10 A current).
Markers reward , the value 10 A, and choosing the 13 A fuse with a reason.
CCEA style4 marksExplain the role of the earth wire and the fuse in protecting a person using an appliance with a metal case.Show worked answer →
The earth wire connects the metal case to earth. If a fault makes the live wire touch the case, a large current flows to earth through the low-resistance earth wire rather than through a person.
This large current melts (blows) the fuse, breaking the circuit and disconnecting the live supply, so the case is no longer live and the user is protected.
Markers reward: earth wire carries fault current safely to earth; large current blows the fuse; circuit broken so the appliance is isolated and safe.
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Sources & how we know this
- CCEA GCSE Physics specification — CCEA (2017)