Mains electricity and safety · Grade 7

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 13 A extension lead on 230 V is used for a 2,000 W heater and a 1,200 W hairdryer at the same time. Is this safe?

Answer: No – the total current is about 13.9 A, more than 13 A.

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