Direct and alternating potential difference

Mains electricity is an ac supply. In the United Kingdom the domestic electricity supply has a frequency of 50 Hz and is about 230 V.

Students should be able to explain the difference between direct and alternating potential difference.

Mains electricity

Most electrical appliances are connected to the mains using three-core cable.

The insulation covering each wire is colour coded for easy identification:

live wire – brown
neutral wire – blue
earth wire – green and yellow stripes.

The live wire carries the alternating potential difference from the supply. The neutral wire completes the circuit. The earth wire is a safety wire to stop the appliance becoming live.

The potential difference between the live wire and earth (0 V) is about 230 V. The neutral wire is at, or close to, earth potential (0 V). The earth wire is at 0 V, it only carries a current if there is a fault.

Students should be able to explain:
- that a live wire may be dangerous even when a switch in the mains circuit is open
- the dangers of providing any connection between the live wire and earth.