Electrical power and safety

Electrical safety in the home: this page gives short notes explaining typical electrical wiring in the home and some notes on avoiding electrical hazards.

Two cables from the local substation or transformer carry AC. In Australia, their potential difference is 240 Vac. In Europe, it is often 220 V, and 110 V in the US. In all these examples, the voltage given is the root-mean-square value. See Power, RMS and Three-phase circuits for details. Or else just remember that 240 VRMS (usually) gives same power as 240 VDC. It varies like this:

Why 350 V at peak? Remember that the voltage is zero 100 times per second so, to give the same power as 240 VDC, it must have higher voltage at peak.

Earthing

Domestic power sockets and plugs look like this in Australia (and Argentina, China, New Zealand and for high power sockets in the US):

The three contacts are called:
  • 'live' at 240 V with respect to earth
  • 'neutral' is the return to the substation—near 0 V
  • 'earth' connects to the earth near the building (eg via water pipes or a rod in the ground).

Appliances with a metal case usually have that case connected to the earth pin. When plugged in, this provides a low resistance pathway to ground. See below.

Domestic circuit

A domestic circuit may look like this:

Note that the switches are in the active line. The master switches should be off before changing fuses. The earth pin really is connected to the earth—e.g. via water pipes.

Electrocution often occurs via the floor/chair/bath etc. Wet floors and baths are dangerous because they provide a low resistance pathway from body tissues to the ground.

An earthed case provides low R return path and blows a fuse, or preferably triggers an earth leakage detector. (The ELD turns off very quickly and reduces danger.)

Here, in point form, are some suggested safety hints.
  • Never fool around with the mains. Leave it to qualified persons.
  • Earth leakage detector detects quite small currents to earth and turns of the supply very quickly - they may prevent electrocution.
  • One is hand safer than two, because two hands allows a current path across the chest.
  • The back of the hand is safer than the front, because the muscles that contract the hand are stronger than those that flatten it.
  • Dry, rubber soled shoes can insulate you from the floor.
  • Even small shocks can be dangerous to a person on a ladder.
  • Power saws and electric mowers may cut their own cable and thus electrocute the user.
  • Water is a conductor. Beware the wet.

Clinical precautions

Dry skin is not a very good conductor, but people with electrodes attached to them, or metal catheters in their veins, or salty gels or bathing solution: have a low R pathway to their tissue and so are vulnerable. Even static electric charges might be dangerous to them.

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