Mains Electricity and Circuit Wiring Safety
The UK mains electricity supply is alternating current (a.c.) at approximately 230 V and 50 Hz, meaning the current repeatedly reverses direction, unlike the direct current (d.c.) supplied by a battery, which flows in one direction only. A three-pin plug and cable contain a live wire (brown, carries the alternating potential difference from the supply), a neutral wire (blue, completes the circuit, close to 0 V), and an earth wire (green and yellow, a safety wire connected to the case of an appliance and to the ground). Fuses, circuit breakers, earthing and double insulation protect people and appliances from electric shock and fire; calculating fuse ratings and the energy transferred by mains appliances extends the core GCSE course with quantitative fuse-current and energy calculations.
Before you start
Make sure you're comfortable with these topics first:
Method
- Identify the wire in question by its colour: live is brown, neutral is blue, and earth is green and yellow.
- Recall that the live wire alternates between a positive and negative potential difference relative to earth, while the neutral wire is at close to 0 V, so touching the live wire can cause a dangerous current to flow through a person to earth.
- Explain earthing: the earth wire connects the metal casing of an appliance to the ground, providing a low-resistance path for current if a fault causes the live wire to touch the casing, which causes a large current to flow and blows the fuse or trips the circuit breaker, disconnecting the appliance before it can cause an electric shock.
- For fuse-rating questions, calculate the normal operating current using current = power / potential difference, then choose a fuse rated just above this value from the standard ratings given in the question (commonly 3 A, 5 A or 13 A).
- Explain that a circuit breaker works like a fuse but uses an electromagnet to break the circuit when the current exceeds a set value, and can be reset and reused, unlike a fuse, which must be replaced.
- For energy-transferred questions, use energy transferred = power x time, or energy transferred = potential difference x current x time, keeping time in seconds unless the question specifies kWh.
Worked example
A 230 V kettle is rated at 2.3 kW. (a) Calculate the normal operating current of the kettle. (b) State which fuse, from 3 A, 5 A or 13 A, should be fitted in the plug, and explain your choice.
- Write down the equation linking power, potential difference and current: power = potential difference x current, so current = power / potential difference.
- Substitute the values: current = 2300 / 230 (2.3 kW converted to 2300 W).
- Calculate: 2300 / 230 = 10 A.
- Compare 10 A with the standard fuse ratings: a 3 A or 5 A fuse would blow immediately during normal use, since the operating current (10 A) is already higher than these ratings.
- State the final answer: a 13 A fuse should be fitted, since it is the smallest standard rating above the normal operating current of 10 A, so it protects the circuit without blowing during normal use.
Practice questions
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Q1State the colour of the live wire in a UK mains plug.Show answer
Answer: Brown.
Q2State the UK mains supply frequency.Show answer
Answer: 50 Hz.
Q3Explain the purpose of the earth wire in an appliance with a metal case.Show answer
Answer: It connects the case to the ground, so if the live wire touches the case a large current flows to earth, blowing the fuse and disconnecting the supply before the case becomes dangerous to touch.
Q4A 230 V toaster is rated at 920 W. Calculate its normal operating current.Show answer
Answer: 4.0 A (920/230).
Q5State one advantage of a circuit breaker over a fuse.Show answer
Answer: A circuit breaker can be reset and reused after it trips, whereas a blown fuse must be replaced.
Q6A 230 V appliance operates at a current of 8.0 A for 15 minutes. Calculate the energy transferred. Use energy = power x time, with power = potential difference x current.Show answer
Answer: 1656000 J (1.656 x 10^6 J); power = 230 x 8.0 = 1840 W, time = 15 x 60 = 900 s, energy = 1840 x 900 = 1656000 J.
Q7State why mains electricity is described as alternating current.Show answer
Answer: Because the current continually reverses direction, unlike direct current, which flows in one direction only.
Exam-style questions
Written in the style of a IGCSE Science exam paper, with a full mark scheme.
A 230 V electric heater is rated at 1610 W. (a) Calculate its normal operating current. (2 marks) (b) State, giving a reason, which fuse from 3 A, 5 A or 13 A should be fitted in the plug. (2 marks)
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Explain, with reference to the live wire, the neutral wire and the earth wire, how the wiring and fuse in a three-pin plug protect a person from electric shock if a fault causes the live wire inside an appliance to touch its metal casing.
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Free printable worksheet
Want more practice on paper? Download the mains electricity and circuit wiring safety worksheet pack - 6 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 25 of IGCSE Science Workbook, the whole course as one free printable PDF.
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