Why touching a live wire is dangerous

1. The human body conducts electricity, mainly because it contains a lot of salty water.
2. When you stand on the floor or the ground, your body is at about $0V$, the same potential as the earth.
3. If you touch the live wire, there is a potential difference of about $230V$ across your body.
4. That potential difference drives a current through you, from the point of contact down to the ground.
5. The current makes your muscles contract uncontrollably, which can stop you letting go of the wire.
6. It also heats the tissue it passes through, causing deep burns.
7. A current of only a few tens of milliamperes passing through the chest can upset the rhythm of the heart and kill.
8. Wet skin has a much lower resistance than dry skin, so the same $230V$ drives a much larger current, which is why water and mains electricity are such a dangerous mix.

Danger when a switch is open

1. Opening a switch makes a gap in the circuit, so no current flows through the appliance and it turns off.
2. The gap does not disconnect the wiring between the switch and the supply, so that wiring stays connected to the live supply.
3. The live pin of the plug, the live contact in the socket and the live wire up to the switch therefore all stay at about $230V$ relative to earth.
4. A wire does not need to be carrying a current to be dangerous, because the danger comes from its potential difference relative to earth.
5. If you touch a live part while the switch is open, your body completes a new circuit to earth, and a current flows through you.
6. If a switch is wrongly fitted in the neutral wire, opening it turns the appliance off but leaves every part inside it connected to the live wire.
7. This is why the switch is always placed in the live wire, so that opening it cuts the appliance off from the live supply.
8. Even so, the safe way to work on an appliance or a light fitting is to unplug it or switch off the supply at the consumer unit, rather than relying on the wall switch.

Connecting the live wire to earth

1. Any conducting connection between the live wire and earth gives a current a path to flow along, driven by the $230V$ potential difference.
2. If the connection is a person, the current flows through their body and causes an electric shock.
3. If the connection is a metal object or a bare wire, its resistance is very low, so a very large current flows.
4. A very large current heats the wiring rapidly, which can melt insulation, cause sparks and start a fire.
5. Connections like this can happen when a drill bit cuts into a cable hidden in a wall, when a metal tool is pushed into a socket, when water gets inside an appliance or when worn insulation lets a live wire touch a metal pipe.
6. No connection of any kind should ever be made between the live wire and earth, except through the protective earth wire of an appliance during a fault.

Protection by insulation

1. Every core in a mains cable is coated in plastic, an electrical insulator that stops charge passing from the live wire into anything it touches.
2. The outer sheath of the cable and the casing of the plug add a second layer of insulation around the wires.
3. Many appliances, such as hairdryers, vacuum cleaners and phone chargers, are double insulated, so their outer casing is made entirely of plastic with no exposed metal.
4. A double insulated appliance has only two cores in its cable, live and neutral, because there is no metal case that needs an earth wire.
5. Double insulated appliances carry a symbol of one square drawn inside another.
6. Insulation protects you only while it is undamaged, so frayed cables, cracked plugs and scorched sockets must be replaced, and appliances should never be handled with wet hands.
7. Bathrooms use pull-cord light switches and have no ordinary sockets, so that wet hands cannot come close to a live part.