The shape of the lesson
By the end of the lesson, learners can
- Wire it up safely: Connect the live, neutral and earth to their correct terminals and see how a fault is made safe.
- How the fault is made safe: If a fault makes the case live, the earth wire gives the current a safe path and the fuse blows.
- Colours: live brown, neutral blue, earth green and yellow.
- Placement: the fuse and switch go in the live wire.
- Protection: a fault to the case sends a large current through the earth, blowing the fuse.
- Fuse rating: use I = P ÷ V, then pick the smallest fuse just above that current.
One picture every idea returns to
The mains supply uses a live wire (brown), a neutral wire (blue) and an earth wire (green and yellow). A fuse or trip switch in the live wire breaks the circuit if too large a current flows, and the earth wire keeps a metal case safe if a fault makes it live.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Live, neutral, earth.. Three wires run into most mains appliances, each with a job and a colour. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Wire it up safely | Connect the live, neutral and earth to their correct terminals and see how a fault is made safe. Project tab 2 of the lab (The mains circuit) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: How the fault is made safe | If a fault makes the case live, the earth wire gives the current a safe path and the fuse blows. | Mix-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 16 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in How the fault is made safe, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The fuse and switch go in the live wire, so the appliance is cut off from the supply when they break. | The earth wire (green and yellow) connects the metal case to earth, so a fault current flows safely away and blows the fuse. |
The mains circuit, fuses and earthing: worksheet
6 questions, 16 marks. Written in the style of the papers; no past paper question is reproduced.
Name the three wires in a mains cable and state the colour of each.
State in which wire the fuse and switch are connected, and explain why this is the safe choice.
A metal-cased appliance develops a fault in which the live wire touches the metal case. Explain how the earth wire and the fuse work together to make the appliance safe.
A kettle is rated at 2300 W and runs from the 230 V mains. Fuses of 3 A, 5 A and 13 A are available. Calculate the operating current and choose a suitable fuse.
A plastic-cased hairdryer has only two wires, live and neutral, and no earth. Explain why it does not need an earth wire.
State one advantage of a circuit breaker compared with a fuse for protecting a circuit.
Answer key and marking notes
- Live: brown. ✓
- Neutral: blue. ✓
- Earth: green and yellow. ✓
- Both go in the live wire. ✓
- So that when they break the circuit, the appliance is cut off from the dangerous live side rather than left connected to it. ✓
- The earth wire gives the fault current a low-resistance path from the case to the ground. ✓
- A large current flows through the live wire and the fuse. ✓
- This current is above the fuse rating, so the fuse melts and breaks the circuit. ✓
- The appliance is disconnected and the case is no longer live, so it is safe to touch. ✓
- I = P ÷ V = 2300 ÷ 230 = 10 A
choose the 13 A fuse
just above the 10 A operating current; 3 A and 5 A would blow at once
- The plastic case is an insulator and cannot become live. ✓
- So there is no metal body for a user to touch, and no earth is needed (the appliance is double insulated). ✓
- A circuit breaker can be reset by switching it back on after a fault, rather than being replaced. ✓
- It also trips faster and at a more precise current than a fuse. ✓
2 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
In a mains plug, which wire should the fuse and switch be connected in?
- Athe live wire
- Bthe neutral wire
- Cthe earth wire
- Dit makes no difference
What is the purpose of the earth wire and fuse together in an appliance with a metal case?
- AIf a fault makes the case live, a large current flows to earth and blows the fuse, cutting off the supply.
- BThey make the appliance run more efficiently.
- CThey reduce the voltage supplied to the appliance.
- DThey store charge in case of a power cut.
Quiz key
The fuse and switch go in the live wire, so that when they break the circuit the appliance is isolated from the dangerous live supply. Placing them in the neutral would leave the appliance live even when switched off.
If the live wire touches the metal case, the earth wire provides a low-resistance path so a large current flows and melts the fuse, disconnecting the supply and making the case safe. They are a safety system, not an efficiency or voltage feature.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.