Mains electricity and safety · Grade 5

Mains electricity is very useful but can kill, so homes are designed with safety features: insulation, earth wires, fuses, circuit breakers and RCDs. The mains is alternating current at about 230 V (50 Hz) in South Africa and the UK, and 120 V (60 Hz) in the USA – far more than the 1.5 V of a cell.

Key facts

  • International (IEC) colours: live = brown, neutral = blue, earth = green and yellow.
  • The live wire carries the alternating voltage; the neutral completes the circuit; the earth only carries current if there is a fault.
  • A fuse melts if the current is too big. Choose the smallest fuse just above the normal current: I = P ÷ V.
  • Earthing: if a metal case becomes live, a large current flows to earth and blows the fuse.
  • Double-insulated appliances (plastic case) need no earth wire.
  • An RCD cuts the supply in milliseconds if the live and neutral currents differ (typically by 30 mA).

Worked examples

Example 1

A 2,300 W kettle runs on 230 V. Fuses come in 3 A, 5 A and 13 A. Which should it have?

Answer: 13 A

Other years