Four situations turn a mains supply into a hazard

1. Damaged insulation exposes the bare conductor inside a cable.
   1. Touching the exposed metal completes a path from the live wire, through you, to earth, and the current that flows can be fatal.
   2. Flexes fail where they are bent repeatedly, trapped under a door or yanked out by the cable, so it is the flex and not the plug that needs checking.
2. Overheating cables occur when a cable carries more current than it was designed for.
   1. The heating rises steeply with current, since the power wasted in the cable is $P=I^{2}R$.
   2. A hot cable melts its own insulation, which exposes the conductors and can set fire to a carpet or a curtain.
   3. Leaving an extension lead coiled up while a heater runs from it makes this far worse, because the heat cannot escape from the middle of the coil.
3. Damp conditions are dangerous because water conducts electricity.
   1. A wet hand or a wet floor gives the current an easy low-resistance route through you to earth.
   2. That is why mains sockets are not fitted inside a bathroom and why an outdoor socket needs a weatherproof cover.
4. Excess current from overloading happens when too many appliances are run from one plug, extension lead or multi-socket adaptor.
   1. Each appliance draws its own current and they all add together in the one cable, so three heaters on one adaptor push it well past its rating.
   2. The result is an overheated cable or a melted socket, which is why every adaptor is marked with a maximum current.

A mains circuit has three wires, and each has a different job

1. The live wire is brown, and it brings the supply voltage to the appliance.
2. The neutral wire is blue, and it completes the circuit by carrying the current back to the supply, sitting close to earth voltage.
3. The earth wire is striped green and yellow, carries no current in normal use, and connects the metal case of the appliance to the ground.
4. Touching the live wire is dangerous because you are standing at earth voltage, so a large p.d. exists across you and a current passes through you to the ground.
5. Touching the neutral wire is far less dangerous, because it is already at very nearly the same voltage as you are.

The switch belongs in the live wire, or the appliance is never really off

1. A switch in the live wire cuts the appliance off from the high voltage, so nothing inside it is live once the switch is open.
2. A switch in the neutral wire would stop the current, but every part inside the appliance would still be joined to the live supply.
3. Anyone opening that appliance to change a bulb or clear a jam would then get a shock even with the switch off.
4. A fuse follows the same rule and is fitted in the live wire, since a fuse in the neutral would blow and still leave the appliance connected to the live supply.
5. It is also why you switch off at the wall and unplug before working on anything electrical.