IGCSE §2: Electricity · IGCSE
Mains Electricity and Safety
Revision notes on Mains Electricity and Safety for the OCR A-level Physics specification (H556). Free to read, with 5 practice questions in the app.
Alternating current (a.c.) — current that repeatedly reverses direction. The UK mains supply is a.c. at 230 V and a frequency of 50 Hz, meaning it reverses direction 100 times a second and completes 50 full cycles.
Direct current (d.c.) — current that flows in one direction only. Cells and batteries supply d.c.
The three wires in a mains cable:
| Wire | Colour | Job |
|---|---|---|
| Live | Brown | Carries the alternating supply at 230 V — the dangerous one |
| Neutral | Blue | Completes the circuit, at about 0 V |
| Earth | Green and yellow stripes | Safety wire, connected to the metal casing |
Why the live wire is dangerous. It alternates between large positive and negative voltages relative to earth. Because you are at earth potential, touching it gives the current a path through you to the ground. Touching the neutral wire is usually safer only because it sits near 0 V, not because it is harmless.
The fuse — a thin wire that melts and breaks the circuit if the current exceeds its rating.
The fuse must be in the live wire. Put it in the neutral and it will still blow, but the appliance stays connected to the live supply afterwards — so the casing can remain at 230 V and the fault is now hidden. This is the detail examiners ask for.
Choosing a fuse rating — just above the appliance's normal operating current, and at the next standard value up (3 A, 5 A, 13 A). Too high and it will not blow when it should; too low and it blows in normal use.
How earthing and the fuse work together. If a fault connects the live wire to a metal casing, the earth wire gives that current a very low-resistance path to the ground. The current becomes very large, the fuse melts, and the appliance is disconnected from the live supply. Two things have been achieved: the casing is not left live, and the circuit is cut.
Circuit breakers do the same job but switch off electromagnetically rather than melting. They act faster and can be reset instead of replaced.
Double insulation — an appliance with a plastic casing and no exposed metal cannot become live, so it needs no earth wire and is fitted with a two-core cable. Marked with a double-square symbol.
Example: a 2.3 kW heater on the 230 V mains draws I = P/V = 2300 ÷ 230 = 10 A, so it needs a 13 A fuse. A 3 A fuse would blow every time it was switched on.
5 questions on Mains Electricity and Safety
Reading is not revising. This topic has 5 questions in the app — multiple choice, calculations, and written answers marked against the mark scheme, point by point. Miss one and it comes back later with its options reshuffled, so you have to know it rather than remember where the answer sat.
- Every topic of Edexcel International GCSE Physics (4PH1)
- Flashcards and every equation, linked to the topic it comes from
- Full mock papers in the real paper structure
20 minutes a day is free. No card, nothing to cancel — and the paid version never renews itself either.
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