General electrical safety
Electricity can be dangerous in the following situations:
If an electrical wire is damaged. This might expose the live wire meaning a person received an electrical shock if they touched it.
Damp conditions. Water that is not pure can conduct electricity. In damp conditions a current could flow outside of a live wire and cause an electrical shock.
Overloading of plug sockets or extension cables. Excess current could flow if more than the recommended number of appliances is plugged in. This could cause a fire if the cables overheat.
Overheating cables. Hot cables could start a fire, or melt through the insulating plastic coating, exposing the live wire.
Podcast: Domestic uses and safety
In this episode, Ellie Hurer and James Stewart explore the domestic uses of electricity and dissect the key features of a three-pin plug. They also discuss direct and alternating potential difference.
ELLIE: Hello and welcome to the BBC Bitesize Physics Podcast.
JAMES: The series designed to help you tackle your GCSE in Physics and combined science. I’m James Stewart, I'm a climate science expert and TV presenter.
ELLIE: And I’m Ellie Hurer, a bioscience PhD researcher. Before you listen, just a reminder that you can listen to the whole series or find an episode that you want to focus on.
JAMES: Yeah, absolutely. Let's get started on today's episode, where we're going to be talking about the domestic uses of electricity and the three pin plug.
I don't know where you are right now, but there are probably, I don't know, two wall sockets in your room?
ELLIE: Yeah, there's loads in the room we're in. There's one socket that's plugged into the microphone that I'm using right now. My phone is charging on a wall socket, and there's a kettle on the other side of the room I plugged in earlier to make my tea.
JAMES: Exactly, because electricity is all around us. If you boiled a kettle this morning, or you used the computer at school, or your phone to listen to this podcast conveniently, you use electricity because it's a vital part of our everyday lives.
ELLIE: But what has this got to do with the physics exam?
JAMES: Alright, let's talk through some key facts about domestic electricity to prepare for your GCSEs.
ELLIE: Right, so most of the things we use at home are connected to the mains electricity, which is what wall sockets are connected to.
JAMES: Mains electricity is supplied using alternating current, which we usually call the AC supply. For a quick recap, an AC supply is an electric current that regularly changes its direction.
ELLIE: Whereas a direct current, or DC, only flows in one direction. But be sure to go back and listen to episode one of this series, it's brilliant, where we dig deeper into those different types of electric current.
JAMES: Here in the UK most homes and domestic buildings have an electric supply with a frequency of about 50 Hertz, that's H Zed, and a voltage of 230 volts that's measured in V if you see it. The frequency of 50 Hertz means that the current changes direction and back again 50 times per second.
A wall socket is what you put your plugs and chargers into. As you know, in the UK, most plugs have three pins, one at the top, two at the bottom. That's because most electrical appliances are connected to the mains using three core cables.
ELLIE: So if you're on your phone or on your laptop, now would be a great time to head to BBC Bitesize to see a diagram of a three pin plug.
JAMES: So a plug has three main copper wires, and you can identify them by the colour of the plastic insulation they are covered in.
ELLIE: The brown wire is the live wire. It's the wire that electric current travels through. The live wire carries the alternating potential difference from the electric supply.
JAMES: Yeah, and that live wire is at 230 volts, so it can be really dangerous.
ELLIE: The wire with green and yellow stripes is called the earth wire.
JAMES: Which you can remember by thinking about how so much of nature on earth is green.
ELLIE: Also, the earth wire is a safety wire to stop the appliance becoming live. The earth wire is at zero volts as it only carries a current if there is a fault.
JAMES: And finally, the blue wire is called a neutral wire.
ELLIE: Which you can remember by thinking about how a lot of blue things like the ocean and the sea are calming colours.
JAMES: That’s right. The neutral wire connects the cable in the wall and completes the circuit. The neutral wire is at or pretty close to earth potential at zero volts.
ELLIE: So, just to help you remember, earth is green and yellow, like so much of nature. Blue is neutral, like the sea and sky, and the live wire is brown.
JAMES: All the wires in a plug play a key role, but the earth wire is extra important because it keeps us safe. The earth wire provides a path for current to flow from the case of the device to the ground if there is a fault.
JAMES: Let's say we were looking at the plug for an electric hob. If the live wire were to become loose, it would be really dangerous if it were to touch the casing of the hob, as anyone touching the appliance would be electrocuted.
ELLIE: So what the earth wire does is direct the electric current to the ground instead of to the person touching the appliance.
JAMES: Exactly, so always be careful when it comes to electrical appliances and if you ever see or feel a spark, stop using that appliance and tell a responsible person, not me or Ellie, that there's a fault because no one wants to get an electric shock.
ELLIE: Alright guys, it's time to recap the key things that we've learnt. So in the UK, the domestic electricity supply has a frequency of 50Hz and is about 230 volts. A three pin plug includes a live wire, an earth wire and a neutral wire.
JAMES: And finally that earth wire is essential to making an appliance safe as it earths the electric current that flows through an appliance if there's a fault.
Thank you for listening to Bitesize Physics. If you found this helpful please go back and listen again and make some notes along the way and come back here whenever you want to revise. There's loads more resources available as well on the BBC Bitesize website, so be sure to check those out too.
ELLIE: And in the next episode of Bitesize Physics, we're going to dig into energy transfers in everyday appliances, so be sure to have a listen.
BOTH: Bye!
Earthing
A mains circuit consists of a live wire (line wire) which carries electricity to the appliance, a neutral wire which carries it away from the appliance and an earth wire. Because the live wire carries electricity to the appliance, a switch must be connected here to switch the circuit off safely in case there is a fault with it.
Without the earth wire, if a fault occurs and the live wire becomes loose, there is a danger that it will touch the case. The next person who uses the appliance could get Cause injury or death by an electric shock..
The earth wire is therefore connected to the case and is attached to a metal plate or water pipe underground. As the wire is made of copper, the earth wire provides a low The opposition in an electrical component to the movement of electrical charge through it. Resistance is measured in ohms.path to the ground. In the event of a fault, the live current passing through the case will follow this path to the ground instead of passing through a person.
Some electrical appliances are double-insulated because they have plastic cases not metal ones. These do not need to have an earth wire because they cannot become live.
Fuses
A fuse provides a built-in fail-safe to the electrical circuit for a device. The fuse contains a thin wire that will melt if the current gets too high. If there is a fault that causes the casing of the device to become live, a large current will flow through the low-resistance earth wire. This high current will cause the fuse to melt.
Fuses for appliances must match the expected current for that appliance. For example, a desk lamp will have a small current flowing through it, so a fuse of 3 amps is appropriate. A kettle or toaster will have a much larger current flowing through it, so a fuse of 13 amps is appropriate. The rating of the fuse should be slightly greater than the expected current.
Once the fuse has melted, the circuit is broken and no more current flows through the device. This means the case of the device is no longer live and there is no risk of electrocution.
A circuit breaker can serve the same function as a fuse but can be reset without the need for replacement if it trips.
The fuse or circuit breaker must be connected in the live wire side of a domestic circuit to ensure that it keeps high voltage from reaching the user, or surroundings, if a fault develops.
A fuse protects the appliance but does not protect the person using it.
It can take up to two seconds for a fuse to melt which is long enough for the user to be electrocuted.
Currents as small as 50 mA (milliamp) can cause electrocution and a fuse would not prevent this from flowing along the live wire.
Quiz
Test your knowledge of electrical safety with this quiz.
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