Mains electricity and safety

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 2300 W kettle runs on 230 V. Fuses come in 3 A, 5 A and 13 A. Which should it have?

  1. Use P = IV, so I = P ÷ V I = 2300 ÷ 230 = 10 A
  2. The fuse must be just above 10 A: 13 A.

Answer: 13 A

Example 2

The metal case of a washing machine becomes live because of a loose wire. Explain how the earth wire and fuse protect the user.

  1. The earth wire joins the metal case to the ground with very low resistance.
  2. A big current flows from live, through the case, down the earth wire.
  3. This current is far above the fuse rating, so the fuse melts and disconnects the live wire.
  4. The case is no longer live, so touching it is safe.

Answer: A large current flows through the earth wire, the fuse melts and the supply is cut off.

Example 3

A 13 A extension lead on 230 V is used for a 2000 W heater and a 1200 W hairdryer at the same time. Is this safe?

  1. Total power 2000 + 1200 = 3200 W
  2. Current I = 3200 ÷ 230 ≈ 13.9 A
  3. 13.9 A > 13 A, so the fuse would blow (or the lead overheat).

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