# Mains Electricity and Safety — GCSE Physics
Mains electricity powers our homes — but at 230 V, it can be extremely dangerous. Understanding the wiring of a plug, the difference between AC and DC, and safety features like fuses, circuit breakers, and earthing is essential knowledge.
1. AC vs DC
Direct Current (DC):
- Current flows in one direction only
- Produced by batteries and solar cells
- Constant voltage
Alternating Current (AC):
- Current reverses direction repeatedly
- Produced by generators (power stations)
- UK mains: 230 V, 50 Hz (reverses 50 times per second)
AC is used for mains because it's easier to transmit over long distances using transformers.
2. The Three-Pin Plug
| Wire | Colour | Function | Voltage |
|---|---|---|---|
| Live (L) | Brown | Carries the current to the appliance | Alternates ±325 V peak (230 V rms) |
| Neutral (N) | Blue | Completes the circuit, carries current back | ~0 V |
| Earth (E) | Green/yellow | Safety wire — carries current to earth if there's a fault | 0 V |
The Live Wire Is Dangerous
- Even when the appliance is switched off, the live wire is at 230 V
- Touching it can cause a fatal electric shock
- A shock occurs when current flows through your body to earth
3. Safety Features
Fuses
- A thin wire that melts if too much current flows
- Breaks the circuit, stopping current
- Connected in the live wire
- Must be replaced after blowing
- Standard ratings: 3 A, 5 A, 13 A
Circuit Breakers (MCBs)
- Electromagnetic switches that trip automatically when current is too high
- Faster than fuses
- Can be reset (don't need replacing)
- More expensive than fuses
Residual Current Device (RCD)
- Detects a difference in current between live and neutral wires
- If current is "leaking" (e.g., through a person), the RCD cuts the circuit in milliseconds
- Much faster than fuses — can save lives
Earthing
- The earth wire connects the metal case of an appliance to the ground
- If a live wire touches the metal case:
- Current flows through the earth wire (low resistance path)
- Large current causes the fuse to blow
- Circuit is broken — appliance is safe to touch
- Without earthing, the case would become live = electrocution risk
Double Insulation
- Some appliances have plastic casings and no exposed metal
- Marked with ⧈ symbol
- No earth wire needed — the plastic casing cannot become live
- Examples: hairdryers, electric drills, phone chargers
4. Dangers of Mains Electricity
- Electric shock: Current through the body can stop the heart
- Burns: Electrical energy converted to heat in body tissues
- Fires: Overloaded circuits or damaged wires can cause fires
Safety Precautions
- Don't touch appliances with wet hands
- Don't overload sockets
- Use RCDs, especially outdoors
- Regular inspection of wires for damage
- Never open electrical appliances while plugged in
Worked Example: Example 1
Problem
Question: A toaster uses 1000 W at 230 V. Which fuse should be used?
Choose a 5 A fuse (next standard rating above 4.35 A).
Solution
Worked Example: Example 2
Problem
Question: Explain how the earth wire and fuse work together to protect a user if a live wire touches the metal case of a washing machine.
- Live wire touches the metal case
- Case becomes live (230 V)
- Current flows through the earth wire to the ground (low resistance)
- Very large current flows through the live wire
- The fuse in the live wire melts/blows
- Circuit is broken — no more current flows
- The case is now safe to touch
Solution
6. Practice Questions
- State the colour and function of each wire in a three-pin plug. (3 marks)
- Explain the difference between AC and DC. (2 marks)
- Why is the fuse always placed in the live wire, not the neutral? (2 marks)
- What advantage does a circuit breaker have over a fuse? (2 marks)
- Explain why an appliance with double insulation does not need an earth wire. (2 marks)
Answers
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Summary
- UK mains: 230 V, 50 Hz AC
- Three wires: live (brown), neutral (blue), earth (green/yellow)
- Fuses melt when current is too high; always in the live wire
- Earth wire + fuse: protect against live-case faults
- RCDs detect current leakage and cut circuits in milliseconds
- Double insulation: plastic casing means no earth wire needed