Lesson video

Hello, my name's Dr.

George.

This lesson is called, "Short Circuits and Earth Wires," and it's part of the Unit Mains Electricity.

The Outcome for the lesson is, "I can explain how electric circuits are wired in a home with circuit breakers, fuses, and earth wires for safety." Here are the Keywords for the lesson.

I'm not going to read through them all now, I'll introduce them when they come up.

But you can come back to this slide anytime if you want a reminder of the meanings.

The lesson has three parts.

They're called, Short Circuits and Double Insulation, Earth Wires and Domestic Circuits and Circuit Breakers.

Now first, just a quick point about the language we use when we talk about complete circuits.

Sometimes people can get confused by this, because when a switch is closed, it means the connections are pushed together so that the gap between them is closed.

And so a closed switch is a switch that's completing a circuit, allowing current to flow.

That can sometimes cause confusion, because closed sounds as though something is off.

But when a switch is closed, a circuit is actually on.

So take a look at these circuit diagrams. Which of them contain closed switches? And when I ask a question, I'll give you five seconds, but you may well need longer, in which case press pause while you're thinking and press play when you've chosen your answer.

And there were closed switches in A and C.

A, has one closed switch and one open switch, and C, has two closed switches.

Now moving on to short circuits.

A short circuit is a complete circuit with a very small resistance.

In this picture, there's a short circuit across the terminals of this battery, because there's a complete loop and there are no components in the circuit to provide any significant resistance.

And with very little resistance, there'll be a large current through the wire and the battery, and this can make the battery dangerously hot.

In this circuit there is a short circuit.

There's a complete loop which current can flow around without passing through any component.

And the lamp doesn't light up, it looks as though the circuit's broken.

And it's because the short circuit has very little resistance and nearly all of the current flows through it, rather than flowing through the loop containing the lamp.

And again, the large current can make the battery dangerously hot.

So which of these would cause a short circuit? And the correct answer is A, a loose wire, making a new circuit with very low resistance would cause a short circuit.

Now electrical wires used for mains appliances are covered in plastic insulation to prevent electric shock.

The bare metal ends of the wires are screwed or soldered in place, so they only make contact with the correct circuit and nothing else.

And this plastic coating on the wires provides one layer of insulation.

Some appliances also have a plastic outer casing, with no metal on the outside, like this kettle for example.

If a wire inside the kettle came loose, it could touch the outer casing and that could be a live wire as a high voltage.

But because the casing's made of plastic, which is an electrical insulator, the appliance is still insulated and it'll still be safe to touch.

This plastic outer casing is a second layer of insulation in addition to the plastic around the wires.

And appliances with two layers of insulation like this, are described as double insulated.

And there's a symbol for that, one square inside another.

Now a question for you.

True or false, the fridge-freezer shown is double insulated.

And I'd also like you to give a reason why you chose that answer.

It's false, this fridge-freezer has metal on the outside, and metal is an electrical conductor, so it isn't double insulated.

Now can you explain how you know the electric plug in this picture is double insulated and why that's important? Press pause while you're writing your answer and press play When you're ready to check it.

I'll show you an example answer now.

The electric plug is double insulated because it has two layers of insulation.

The wires inside are covered in insulation and the bare ends are screwed down inside the plug.

The plug casing is also made of plastic, which is a second layer of insulation.

If any bare wires came loose and touch the inside of the plug casing, we're still protected from an electric shock.

So well done if you made a lot of the same points.

Now let's move on to the second part of the lesson, Earth Wires.

Inside every mains plug there's a fuse and some mains appliances have fuses inside them as well, and they look a bit like this.

A fuse contains a thin wire that melts if the current through it becomes dangerously large.

And that's a safety feature, because large currents cause heating and that can damage appliances and it can cause a fire.

Fuses are always connected to the live wire, which has a voltage of plus or minus 230 volts.

The voltage swings between high positive and high negative voltage.

If there is a fault that makes the current become dangerously large, the thin wire inside the fuse melts and turns off the electric circuit.

And the fuse is always put into the live wire, not the neutral wire, so that not only does the fuse stop current flowing, but it also stops the appliance from being connected to a high voltage.

The mains lamp here is now connected to zero volts on both sides.

So there's no risk of electric shock from touching the lamp even if there's something wrong with it.

So what happens when there's a fault that causes a dangerously high current in a mains appliance? And the correct answer is a thin wire and the fuse melts.

The whole fuse doesn't melt, just the wire inside, and nothing blows up.

You may hear the phrase, "A fuse blows," and that is a correct phrase to use, but that's not the same thing as blowing up.

Nothing is exploding here.

Now some appliances have a metal casing as we've seen, and if a live wire inside them comes loose, it may touch the inside of that outer casing.

If a person then touches the casing, they could complete a circuit between the live wire and the ground.

So we have the live wire at high voltages and the ground at zero volts.

There's now a large potential difference across the person and current could flow through them giving them an electric shock.

And mains electric shocks are very dangerous.

So any appliance with a metal case should have a third wire inside its cable, an earth wire.

The earth wire is screwed or soldered to the metal case.

And at the other end, the earth wire is connected to the ground, literally connected to the planet Earth, and that's why it's called the earth wire.

And that will be at zero volts.

When the appliance is working properly, the earth wire does nothing.

There's no current through it.

It's not part of the circuit that makes the appliance work.

Now what is the earth wire connected to? The correct answer is A, it's connected to the metal casing of an appliance and to the ground.

Look at this washing machine, the case is made of metal and an earth wire is screwed into the inside of the metal case, as you can see on the right.

And the earth wire is connected to the case, the case is resting on the floor, and the floor is part of the house, which is connected to the earth.

The earth wire completes a circuit, only if a live wire touches the metal case.

So only if something goes wrong in that particular way.

If the live wire does touch the metal case, there's a danger of electric shock, because the metal case is now live, it's now at high voltage.

Here's a loose live wire touching the casing, and there's now a direct connection between the high voltage live wire and zero volts of the earth.

There's a large potential difference across the wires, and so a large current will flow.

This is a short circuit.

And the large current melts the fuse, turning off the electricity and making the washing machine safe.

So is this true or false? A loose live wire is touching the metal case of a fridge.

The fuse melts when someone touches the fridge.

And can you also explain why you chose the answer that you did? Correct answer is false.

And the reason is a fuse melts as soon as the wire touches the metal case, before it's touched by a person, because of very large current flows through the live and earth wires.

So it's not someone touching the case that makes the fuse melt.

It's the large current that makes the fuse melt, as soon as the live wire touches the case.

Now this diagram shows a model of a main circuit connected to a washing machine.

The tin can here represents the metal case of the washing machine.

So look carefully at the way the wires are connected and then answer the two questions.

What do you think will happen if the loose wire touches the metal case? That's the red lead that's at the top right.

And explain why you think this will happen.

Press pause while you're writing your answers.

And when you press play, I'll show you example answers.

So here are some example answers.

What will happen is the fuse wire will melt and turn off the lamp.

And the reason for that is when the loose wire touches the metal can, there's a short circuit through the live wire, the fuse, the tin can and to the earth wire.

So that's the upper loop in this picture.

A very large current flows through the short circuit and melts the fuse wire so the circuit with the lamp is turned off.

So we have a large current in the upper circuit, the upper loop here, that fuse wire then melts and that breaks not only the upper loop, but the lower loop that contains the lamp.

So no current flows through the lamp anymore.

Well done if you've realised that.

And now let's look at domestic circuits, that's home circuits and circuit breakers.

Electricity enters a house at one point and it passes through an electricity metre and a consumer unit.

All the electrical wiring in a house is connected to the consumer unit.

Here we have the metre on the left and the consumer unit on the right.

And all lights and electric sockets in a house are wired up in parallel circuits to a consumer unit.

There's usually one circuit for downstairs lights, one for upstairs lights, circuits for groups of electric sockets, and a separate circuit for an electric oven, which needs a particularly large current.

With parallel circuits, lights or switches, can be used independently of each other.

If one light, for example, is switched off, other lights can still be switched on, because they're part of separate loops.

So here are three pictures, each with a consumer unit at the bottom.

How are the electric circuits in a house wired up? The correct one is C, parallel circuits for groups of lights or groups of sockets.

If there are too many appliances plugged into one main circuit, then a very large current can flow through it.

The wires can become very hot and this could even cause a house fire.

Circuit breakers in the consumer unit, stop this happening.

Here's a circuit breaker, there's a separate one for each of the circuits in the house.

Which of the following statements about circuit breakers in the consumer unit are correct? The correct answer is B.

Each circuit breaker turns off several lights or sockets.

And that's because each circuit breaker turns off one circuit in the house and that will be a parallel circuit to more than one light or more than one socket.

Circuit breakers do a similar job to fuses, but they do it in a different way.

The type of circuit breaker found in a consumer unit, shown in this photo, contains a switch and an electromagnet.

The larger the current flowing through the circuit breaker, the stronger the electromagnet inside it becomes.

If the current goes over a certain size, the electromagnet becomes strong enough to pull the switch and that turns the current off.

Circuit breakers work faster than fuses and they can be turned back on once the fault has been fixed.

So you can just go and flip the switch back when the problem has been solved.

There's also another kind of circuit breaker called a Residual Current Circuit Breaker or RCCB.

And that uses electronics to turn off the live wire very quickly if the current through it is different to the current through the neutral wire.

So it's a higher level of safety, it doesn't simply turn off if the current's too high, but it turns off if there's anything wrong that could make the live wire current different from the neutral wire current.

And one of these can be plugged into a single socket and they should always be used when plugging in lawnmowers or any other device that could accidentally cut through a wire.

Why are circuit breakers used in consumer units rather than fuses? There are two answers here.

One is that they turn off more quickly when there's a fault, and the other is they can be turned back on when the fault has been fixed.

Whereas with fuses, you need to replace with a new fuse.

And now a longer written task for you.

I'd like you to compare the features of circuit breakers and fuses.

So write down what they both have in common, and then write down features that only circuit breakers have or only fuses have.

You don't have to write this in full sentences, as long as you make your points clearly.

So press pause when you do this and press play when you're ready to check your answers.

And here are points that you could have made.

Let's look at both first.

They both turn off a circuit if there's too much current, they both stop wires from overheating and causing a house fire, and they both can be used in a consumer unit, although fuses aren't often used that way.

Circuit breakers only turn off very quickly, can be turned back on once the fault is sorted out, and they're more expensive than fuses.

And then points about fuses only, they turn off more slowly than a circuit breaker.

They need replacing once the fault is fixed and they're much cheaper than circuit breakers.

So well done if you made a lot of those same points.

And this is the end of the lesson.

So I'll finish with a short summary.

A short circuit happens when a wire becomes loose and touches the incorrect part of a circuit or makes a new circuit.

It is a complete circuit with very small resistance.

If there's too much current in an appliance, a fuse on the live wire in its plug melts and turns it off.

If a live wire touches the inside of a metal case, the metal case may become live.

But if an earth wire is connected to the case, a large current flows through it, melts the fuse, and turns the electricity off.

Domestic circuits are connected to circuit breakers in a consumer box.

If the current in the circuit is dangerously large, the circuit breaker turns it off to prevent fire.

I hope you enjoyed the lesson, which contained a lot of key points that anyone ought to know.

I hope to see you again in a future lesson.

So bye, for now.