Welcome! The sockets in your home can deliver enough electricity to kill – yet we plug things in safely every day. In this lesson you’ll learn how mains electricity and plugs keep us safe, then become a plug detective and track down dangerous faults. It takes about one hour. Work through the steps in order.
Before you start
- You need: your exercise book or paper, a pen and a calculator.
- Write the title and date: Mains electricity – Plug Detective.
- The golden rule: write your answer first, then tap Show answer. Correct mistakes in a different colour.
- Look out for the blue 📝 boxes. Copy each one into your book neatly – these are your revision notes.
📝 Copy into your book
By the end of this lesson I will be able to:
1. describe UK mains electricity
2. name the three wires in a cable and what each one does
3. explain why touching the live wire is dangerous
4. spot unsafe wiring and explain why it’s dangerous.
Step 1: Starter (5 minutes)
A battery in a torch is about 1.5 V. A plug socket in the UK is about 230 V. Write down three differences you think there are between the electricity from a battery and the electricity from the mains.
Show answer
The mains has a much higher potential difference (230 V, not 1.5 V), so it can be deadly. A battery gives direct current (dc), which flows one way. The mains gives alternating current (ac), which keeps changing direction. The mains also never runs out, whereas a battery goes flat.
📝 Copy into your book
UK mains electricity is an ALTERNATING CURRENT (ac) supply with a frequency of 50 Hz and a potential difference of about 230 V.
Batteries give DIRECT CURRENT (dc), which flows in one direction only.
Step 2: Read and write (8 minutes)
First, copy these three questions into your book. Then read the text below carefully and answer them in full sentences.
- What colour is each of the three wires in a mains cable?
- What is the job of the earth wire?
- Why is touching the live wire dangerous?
📖 Read: Inside a mains cable
Most appliances are connected to the mains with a three-core cable. The insulation around each wire is colour-coded. The live wire is brown. It carries the alternating potential difference from the supply, at about 230 V. The neutral wire is blue. It completes the circuit and is at, or close to, 0 V. The earth wire is green and yellow. It is a safety wire that stops an appliance becoming live, and it is at 0 V. It only carries a current if there is a fault.
Your body is at 0 V. If you touch the live wire, there is a large potential difference between your hand and the ground, so a current flows through your body. This electric shock can kill. The live wire can be dangerous even when a switch is turned off. Any connection between the live wire and the earth – for example through a person – can also cause a fire.
Plugs also contain a fuse, connected to the live wire. If too much current flows because of a fault, the thin wire inside the fuse melts and breaks the circuit.
Show answers
(a) Live – brown; neutral – blue; earth – green and yellow.
(b) It is a safety wire that stops the appliance (e.g. a metal case) becoming live if there is a fault.
(c) Your body is at 0 V and the live wire is at 230 V, so there is a large potential difference across you and a current flows through your body – an electric shock.
📝 Copy into your book
| Wire | Colour | Potential difference | Job |
|---|---|---|---|
| Live | Brown | About 230 V | Carries the alternating p.d. from the supply |
| Neutral | Blue | At or close to 0 V | Completes the circuit |
| Earth | Green and yellow | 0 V | Safety wire – stops the appliance becoming live |
Step 3: Choosing a fuse (8 minutes)
Plug fuses usually come in 3 A, 5 A and 13 A. The right fuse is the one just above the normal current of the appliance. Copy the method:
📝 Copy into your book
Choosing a fuse:
1. Work out the current: I = P ÷ V (from P = V I)
2. Choose the fuse rating just above that current.
Q1 A 2300 W kettle runs on the 230 V mains. Calculate the current and choose a fuse.
Show answer
I = 2300 ÷ 230 = 10 A → 13 A fuse
Q2 A 92 W TV runs on the 230 V mains. Calculate the current and choose a fuse.
Show answer
I = 92 ÷ 230 = 0.4 A → 3 A fuse
Q3 A 920 W hairdryer runs on the 230 V mains. Calculate the current and choose a fuse.
Show answer
I = 920 ÷ 230 = 4 A → 5 A fuse
✅ Score out of 3.
Step 4: Main task – Plug Detective (15 minutes)
You’re an electrical safety inspector checking a student house. You find these five problems. For each one, write: What’s wrong? · Why is it dangerous? · How should it be fixed?
- A lamp plug has the brown and blue wires connected the wrong way round.
- A metal toaster has no earth wire connected.
- A kettle cable is split, and bare copper wire can be seen.
- A 2300 W kettle has a 3 A fuse, and someone has replaced it with a piece of kitchen foil because “the fuse kept blowing”.
- A hairdryer is kept on a shelf above the bath.
Stuck? Ask yourself: could someone touch something connected to the live wire? Could too much current flow without anything stopping it?
Show the inspector’s report
1. Wires swapped: the fuse and switch are now on the neutral side, so parts of the lamp stay connected to 230 V even when it’s switched off or the fuse has blown. Rewire so brown goes to live and blue to neutral.
2. No earth: if a live wire comes loose and touches the metal case, the case becomes live and anyone touching it gets a shock. Connect the green and yellow earth wire to the case.
3. Bare wire: someone could touch the live wire directly and get a shock, or the wires could touch and cause a fire. Replace the whole cable – don’t just tape it.
4. Foil instead of a fuse: the kettle takes 10 A, so a 3 A fuse was always going to blow – it was the wrong fuse. Foil won’t melt, so a fault could let a huge current flow, overheating the cable and starting a fire. Fit a 13 A fuse.
5. Hairdryer by the bath: water conducts electricity. If it falls in, current could flow through the water and the person. Keep mains appliances away from water.
✅ Check: did you explain why each fault is dangerous, not just what’s wrong?
Step 5: Exam practice (14 minutes)
Look at the marks – they tell you how many points to make. Answer in full sentences, then mark yourself.
Q1 (3 marks) Give the colour and the potential difference of the live, neutral and earth wires.
Show model answer
Live – brown, about 230 V (1). Neutral – blue, at or close to 0 V (1). Earth – green and yellow, 0 V (1).
Q2 (3 marks) Explain why a person touching the live wire gets an electric shock, even if the appliance is switched off.
Show model answer
The live wire is still at a high potential (about 230 V) when the switch is open (1). The person’s body is at 0 V, so there is a large potential difference across them (1). So a current flows through their body to earth (1).
Q3 (3 marks) A 1.15 kW microwave runs on the 230 V mains. Calculate the current in its cable.
Show model answer
1.15 kW = 1150 W (1)
I = P ÷ V = 1150 ÷ 230 (1)
I = 5.0 A (1)
✅ Add up your marks out of 9 and write the score in your book.
Step 6: Exit ticket (10 minutes)
Home detective: without touching anything, look at the plugs and cables in one room at home. Write down anything that could be unsafe. Then finish:
- The earth wire keeps us safe by…
- You should never replace a fuse with foil because…
- One thing I’m still not sure about is…
🎉 Well done – lesson complete! Your 📝 boxes are your revision notes for this topic. Want more? Try the Circuits and mains electricity questions.
