Skip to main content
WalesPhysicsSyllabus dot point

How is mains electricity supplied to homes, and how are people and appliances protected?

Domestic mains electricity, the live, neutral and earth wires, fuses, circuit breakers, earthing and double insulation, electrical power and the cost of electrical energy.

A focused answer to WJEC GCSE Physics topic 1.4 on domestic electricity, covering the mains supply, live, neutral and earth wires, fuses, circuit breakers and earthing, the power equations, and calculating the cost of electrical energy in kilowatt-hours.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section
  1. What this topic is asking
  2. The mains supply
  3. Fuses and circuit breakers
  4. Earthing and double insulation
  5. Electrical power and energy cost
  6. Try this

What this topic is asking

WJEC wants you to describe the mains supply and the three wires in a cable, explain how fuses, circuit breakers and earthing keep people safe, use the power equations, and calculate the cost of domestic electrical energy. This is topic 1.4 Domestic electricity in Unit 1 of WJEC GCSE Physics (3420).

The mains supply

Domestic appliances are connected in parallel so each receives the full mains voltage and can be switched independently.

Fuses and circuit breakers

You choose a fuse rated just above the appliance's normal working current. A residual current circuit breaker (RCCB) acts faster and trips on a small difference between the live and neutral currents, which indicates a leak.

Earthing and double insulation

Electrical power and energy cost

Try this

Q1. State the colour and job of the earth wire. [2 marks]

  • Cue. Green and yellow; it keeps the metal case at 0 V0\,\text{V} for safety.

Q2. A 2 kW2\,\text{kW} kettle runs for half an hour. Calculate the energy used in kWh. [2 marks]

  • Cue. E=P×t=2×0.5=1 kWhE = P \times t = 2 \times 0.5 = 1\,\text{kWh}.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC 20193 marksCalculate the energy used by a 2.5 kW2.5\,\text{kW} electric heater running for 33 hours, and find the cost if electricity costs 20 p20\,\text{p} per kilowatt-hour.
Show worked answer →

A topic 1.4 cost calculation. Energy in kilowatt-hours is power in kilowatts times time in hours: E=P×t=2.5×3=7.5 kWhE = P \times t = 2.5 \times 3 = 7.5\,\text{kWh} (2 marks). The cost is units times price: 7.5×20=150 p=£1.507.5 \times 20 = 150\,\text{p} = \pounds 1.50 (1 mark). Markers reward keeping the power in kilowatts and the time in hours, and converting pence to pounds. A common error is to convert to joules unnecessarily or to mix units.

WJEC 20214 marksExplain the purpose of the earth wire and the fuse in a metal-cased appliance, and what happens if a fault makes the case live.
Show worked answer →

A topic 1.4 Explain question. The earth wire connects the metal case to earth so the case stays at 0 V0\,\text{V} and is safe to touch (1 mark). If a fault connects the live wire to the case, a large current flows from live to earth through the low-resistance earth wire (1 mark). This large current melts the fuse (or trips the circuit breaker), breaking the live connection (1 mark), so the case is no longer live and the user is protected (1 mark). Markers reward the earthing, the surge of current, the fuse melting and the resulting safety.

Related dot points

Sources & how we know this