Electric shocks and safety
In a nutshell
Faulty appliances can cause electric shocks and fires. There are a number of safety features of appliances and household electrical circuits that are designed to reduce these risks.
Electric shocks
Your body is at 0 V, just like the earth. If you touch a live wire, a large potential difference is generated across your body and a current flows through you. This causes a large electric shock which could injure or even kill you if it strong enough.
Even if a plug socket is turned off, there is still a danger of an electric shock. There is no current flowing, but there is still a potential difference in the live wire. If you touched the live wire, your body would provide a link between the supply and the earth, which would cause a current to flow you.
Any connection between live and neutral could be dangerous. If the link forms a low resistance path to earth, a huge current will flow, which will result in a fire.
Curiosity: Birds that sit on power lines do not get electrocuted. This is because each of their feet is at the same potential difference along the same cable. The birds are not touching the earth and so not current is drawn through them.
Safety
Current surges can occur because of faulty electrical appliances. This can lead to the circuits and wiring in the appliances melting or causing a fire. Faulty appliances can cause fatal electric shocks. Earthing and fuses prevent electrical overloads. Here is how they work:
1.
| If a fault develops in which the live wire somehow touches the metal case, then because the case is earthed, a very large current flows through the live wire, the case and the earth wire.
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2. | This surge in current melts the fuse when the amount of current exceeds the fuse rating. Fuses are connected to the live wire, so that melting the fuse breaks the circuit and cuts off the live supply. |
3. | The melting of the fuse isolated the whole appliance, making it impossible to get an electric shock from the case. It also prevents the risk of fire caused by the heating effect of a large current.
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4. | Fuses should be rated as near as possible but just higher than the normal operating current.
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5. | The larger the current, the thicker the cable you need to carry it. This is to stop the cable from getting too hot and melting. This is why the fuse rating needed for cables usually increases with cable thickness.
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6.
| As well as fuses in plugs, there are also household fuses. These work in the same way, but protect the wiring in the house, not just in the appliance.
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Circuit breakers
Circuit breakers can be used in place of household fuses. Instead of melting a fuse, a large current may instead 'trip' (turn off) a circuit breaker.
Circuit breakers turn off quicker than the time taken for a fuse to melt. They can also be reset, which is much more convenient than having to replace a fuse. However, circuit breakers are more expensive than fuses.
Insulating materials
All appliances with metal cases are usually earthed to reduce the potential of an electric shock. If an appliance has a plastic casing and no metal parts showing then it is said to be double insulated.
Any appliance that is double insulated does not need an earth wire, just a live and neutral. Cables that only carry the live and neutral wires are called two-core cables.
Example
Some appliances, such as vacuum cleaners or desk fans, do not have an earth wire. This means they are double insulated. They are both made with plastic casings. This means that they cannot pass an electric shock to the user, as even if the wires inside become loose they will touch a plastic case, which insulates the electricity.
