1. 1
   An electrician is rewiring a ring circuit socket outlet in a 1960s house. Before connecting the new socket, she must correctly identify the three wires inside the cable by their insulation colour.

   (a)

   Which colour is used for the live wire in a standard three-core mains cable?

   [1 mark]

   • ABrown
   • BBlue
   • CGreen and yellow
   • DBlack

   (b)

   Which colour is used for the earth wire in a standard three-core mains cable?

   [1 mark]

   • ABrown
   • BBlue
   • CGreen and yellow
   • DGrey

   (c)

   State the function of the neutral wire and explain why it is normally at, or close to, earth potential.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A washing machine develops a fault: worn insulation allows the live wire inside the appliance to touch its metal casing. A repair technician is asked to explain why this is dangerous and how the appliance's protective earthing prevents harm.

   (a)

   What is the potential difference of the live wire relative to earth in a UK mains supply?

   [1 mark]

   • A0 V
   • BApproximately 12 V
   • CApproximately 230 V, direct
   • DApproximately 230 V, alternating

   (b)

   If the metal casing is not earthed and becomes live, what is the main danger to a person who touches it while also touching an earthed object?

   [1 mark]

   • AThe appliance will automatically switch off
   • BA large current could flow through the person, causing an electric shock
   • CThe appliance will run more efficiently
   • DThe fuse will blow instantly with no risk to the person

   (c)

   Explain how connecting the earth wire to the metal casing protects a user if the live wire touches the casing.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A power station generates electricity at 25,000 V. This electricity must travel long distances along overhead cables to towns and cities before being used in homes, where step-down transformers reduce it back to 230 V.

   (a)

   A power station generates electricity at 25,000 V. Explain why a step-up transformer is used to increase this voltage to 400,000 V before the electricity is transmitted across the National Grid.

   [3 marks]

   (b)

   Explain why, after transmission, the voltage must be reduced again using step-down transformers before electricity reaches homes, and why it would be dangerous to supply 400,000 V directly to a house.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A national energy company is reviewing why its National Grid design, using step-up and step-down transformers around high-voltage transmission lines, has historically been chosen over transmitting electricity directly from power stations to homes at generation voltage.

   (a)

   A national energy company is comparing its ageing electricity grid, which loses a large fraction of generated power as heat, with a modernised grid design used in a neighbouring country. Explain, using ideas about voltage, current and resistance, how the National Grid system as a whole (step-up transformers, high-voltage transmission cables, step-down transformers) is designed to transfer electrical energy from power stations to consumers as efficiently as possible.

   [6 marks]

   (b)

   Evaluate why transmitting electricity at generation voltage (25,000 V) directly to every home, without any step-up transformers, would be a poor design choice for a national electricity grid, considering both energy efficiency and practicality.

   [6 marks]

   Total for question 4: 12 marks

End of questions