Using Electricity In The Home

A torch and a table lamp both light up, and the supplies behind them could hardly be less alike. The torch cell pushes charge around its circuit one way and keeps pushing that way until it is flat. The lamp is fed by a supply that reverses itself many times a second, is powerful enough to kill, and is trusted anyway because three wires, a strip of thin metal and a switch that flicks itself off stand between it and the person holding the plug.

This lesson is about telling those two supplies apart and about the arrangements that make the dangerous one safe to live with. Telling them apart takes a single question, asked about the current rather than about anything else: does it always pass the same way round the circuit, or does it keep turning round? Everything after that is protection. You will follow one fault through a metal-cased appliance, from the loose live wire that makes the case dangerous, through the earth wire that gives the current somewhere harmless to go, to the fuse that melts a fraction of a second later, and you will see why every one of those protective devices belongs in the same one of the three wires.