Domestic Uses of Electricity and Electrical Safety — AQA GCSE Physics
Mains electricity is supplied to homes as alternating current, delivered safely through a three-core cable.
Mains supply
UK mains electricity is alternating current (a.c.) at a frequency of 50 Hz and a potential difference of about 230 V. (Alternating current repeatedly changes direction, unlike the direct current from a battery, which flows one way.)
The three-core cable
Most appliances are connected to the mains with a cable containing three wires, each with a coloured insulating coating:
| Wire | Colour | Role |
|---|---|---|
| Live | brown | carries the alternating potential difference (~230 V) from the supply |
| Neutral | blue | completes the circuit, stays near 0 V |
| Earth | green and yellow | a safety wire that stops the case becoming live |
Why the live wire is dangerous
The live wire is at a high potential difference compared with your body (which is at earth potential, 0 V). Touching it would cause a current to flow through you to earth, even if a switch is off. A connection between live and earth can cause a large current and a fire.
The earth wire and fuse
The earth wire and a fuse (or circuit breaker) work together: if a fault makes the metal case live, a large current flows to earth, which melts the fuse and breaks the circuit, protecting the user.
Exam tips
- Mains = a.c., 230 V, 50 Hz.
- Learn the three wires by colour and role.
- The live wire is dangerous because of the large p.d. to earth.
- The earth wire + fuse protect the user by breaking the circuit in a fault.