Electrical safety: fuses, earthing, hazards

Last time

Before we start: last time, in Potential dividers, thermistors and LDRs, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Tiny currents can kill

A current of just a tenth of an ampere through your chest can stop your heart. The mains can push far more than that through you. So every plug, socket and appliance has safety devices built in, and you need to know what they are and how they work.

Electrical hazards

Damaged insulation on a cable can expose the live wire. Touching it gives a shock. Overheating: too many appliances on one socket, or a cable that is too thin, can start a fire. Water conducts electricity. Wet hands, or electrical equipment in a bathroom, are dangerous. Pushing objects into sockets, and using metal ladders near power lines, are hazards too.

The three wires

The live wire is brown and carries the supply voltage. The neutral wire is blue and completes the circuit. The earth wire is green and yellow. It is a safety wire and normally carries no current. The switch and the fuse are always in the live wire, so that when they are off, the appliance is not live.

Fuses

A fuse is a thin wire in a glass or ceramic tube. If the current is too high, the wire heats up, melts, and breaks the circuit. It protects the appliance and the cable from overheating and fire. The fuse rating should be just above the normal current. Standard ratings are 3, 5 and 13 amperes.

Choosing a fuse

Work out the normal current: I equals P over V. Pick the nearest standard fuse above that current. Too low, and it blows all the time. Too high, and it does not protect. A 2.0 kilowatt kettle on 230 volts takes 8.7 amperes, so a 13 ampere fuse is the one to use.