Mains electricity is useful but dangerous. Safety devices and correct wiring protect people from electric shock and stop faults from causing fires.
The three wires
A UK-style three-pin plug uses colour codes so wires are connected correctly:
| Live | brown | carries the alternating supply |
| Neutral | blue | completes the circuit |
| Earth | green and yellow | safety wire to the metal case |
Protective devices
A fuse is a thin wire that melts and breaks the circuit if the current becomes too large. A circuit breaker does the same job but can be reset. The earth wire, together with a fuse, protects a metal-cased appliance: if the live wire touches the case, a large current flows to earth and blows the fuse, cutting off the supply.
Key idea
Choose a fuse rated just above the normal current. Normal current I = P / V.
Worked example
A 920 W heater runs on a 230 V mains supply. Normal current I = P / V = 920 / 230 = 4 A. The correct fuse is the next standard value above 4 A, i.e. a 5 A fuse (not 3 A or 13 A).
Remember
- A fuse and switch are always fitted in the live wire.
- Double-insulated appliances need no earth wire.
- Water lowers resistance, so wet hands greatly increase shock risk.