Edexcel IGCSE Physics · Spec 2.17-2.19
Mains Electricity & Safety
Mains electricity, the three-pin plug and electrical safety.
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Mains Electricity & Safety
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Explained
Mains electricity, and which safety device does which job
Mains electricity in the United Kingdom is supplied at about 230 volts as alternating current, meaning the direction of the current reverses many times each second. The mains frequency is 50 Hz, so it reverses 100 times a second.
A battery supplies direct current, which flows one way only. On an oscilloscope, direct current is a straight horizontal line and alternating current is a wave crossing above and below zero.
Alternating current is used for the supply because transformers only work with it, and transformers are what make efficient long distance transmission possible.
The three wires
The live wire is brown. It carries the alternating voltage from the supply, alternating between a large positive and a large negative value, and it is the dangerous one.
The neutral wire is blue. It completes the circuit and stays at close to zero volts.
The earth wire is green and yellow striped. It carries no current at all in normal use and exists purely for safety.
Touching the live wire is dangerous because your body completes a path to earth, and a current passes through you. Touching the neutral wire is far less dangerous, because it is already at nearly the same potential as the ground, so there is little to drive a current.
The two safety systems, and what each protects
This is the part questions test hardest, because the fuse and the earth wire protect against different things.
The fuse is a thin wire in the live side of the circuit that melts and breaks the circuit if the current exceeds its rating. It protects the appliance and the wiring from overheating and catching fire. A circuit breaker does the same job by switching off automatically, and it can be reset rather than replaced.
The earth wire protects the user. If a fault lets the live wire touch the metal case of an appliance, that case would otherwise become live and anyone touching it would receive a shock. The earth wire provides a low resistance path from the case to the ground, so a very large current flows through it instead, and that surge is what blows the fuse and disconnects the appliance.
The two therefore work together, but they answer different questions. If a question asks about fire or overheating, that is the fuse. If it asks about shock, that is the earth wire. Double insulated appliances, marked with a double square symbol, have plastic casings that cannot become live, so they need no earth wire at all.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme sets a six mark question covering both devices, and its two rejections are mirror images of each other.
Asked what a circuit breaker does, it credits any two of: that it acts if the current is too high, allowing current surge; the idea of an isolating device, allowing stops the current, breaks the circuit or cuts the power; and the idea that there is no danger of overheating, allowing that it prevents fires. Beside those it says: ignore comments relating to electric shock.
Asked how an earth wire works, it credits any two of: in the event of a fault, allowing a description such as the live wire touching the metal case or drilling through a cable; that current flows in the earth wire, allowing that it goes to earth or provides a low resistance path; and that it trips the circuit breaker, allowing the idea that it prevents an electric shock. Beside those it says: ignore comments relating to too much current.
Read the two instructions together. Shock earns nothing on the overcurrent question, and too much current earns nothing on the earthing question. Each device has its own vocabulary and the other device's vocabulary is discarded, so a general answer about safety will collect marks on neither.
Between them the mark scheme asks for two differences between a fuse and a circuit breaker, and credits that a fuse cannot be reset and has to be replaced, and that a fuse is slower to break the circuit or less sensitive. It then instructs the examiner to ignore fuse melts or blows.
That last one is worth pausing on. Melting is what a fuse does, not how it differs from a circuit breaker, and the question asked for a difference. It is true, relevant, and worth nothing.
Choosing a fuse rating
Work out the normal operating current from power equals current times voltage, then choose the next standard rating above it.
A rating too far above the working current lets a dangerous current flow before the fuse acts. A rating below it blows every time the appliance is switched on. The common ratings are 3 A, 5 A and 13 A.
Wiring a plug
Live to the right hand pin as you look at the open plug with the cable at the bottom, neutral to the left, earth to the top. The fuse sits in the live side, so that breaking it disconnects the appliance from the dangerous wire rather than the safe one.
The cable grip clamps the outer insulation rather than the individual wires. If the cable is pulled, the grip takes the strain and the connections stay in place.
Spec 2.17-2.19
What you need to know
- Tell a.c. and d.c. apart
- Describe the wiring of a three-pin plug
- Explain how fuses and earthing keep us safe
Active recall
Quick check
Answer each question before opening the answer.
State the colours and roles of the live, neutral and earth wires (UK).
Live — brown (carries the alternating voltage); neutral — blue (completes the circuit); earth — green-and-yellow (safety).
How do a fuse and earth wire protect an appliance?
A large fault current flows to earth and melts the fuse, disconnecting the live supply.
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