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Lesson 10: Domestic electricity

Introduction

In this lesson we'll look at AC and DC electricity, plugs, earth wires and fuses, and paying for electricity.

The difference between AC and DC electricity

Mains electricity is alternating current, or AC.  Batteries are DC, or direct current devices.

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With AC the current rapidly changes direction all the time.  It's like a bicycle wheel being rocked rapidly back and forth.  Just like the wheel, the current changes direction everywhere at the same time.

AC still transfers energy.  If you rock a wheel with your hand whilst keeping the brake on, then energy is transferred from you to the brake.

With DC the direction of the current doesn't change.  It's like a bicycle wheel being spun in one direction.

AC voltage is there even if there is no current

An AC current changes direction because the voltage is constantly changing between positive and negative values.  The mains has an alternating voltage.  The voltage is still there even if there is nothing connected so there's no current.

Why AC voltage is different in different countries

AC voltage is different in different countries.  In Europe it's about 240 V and repeats itself 50 times a second (i.e. it has a frequency of 50 Hz).  In the US it's about 110 V at a frequency of 60 Hz.

The difference is because of a violent disagreement between two men a hundred year ago.  Thomas Edison, a businessman and prolific inventor, claimed 110 V DC was better because it was safer.  Nikola Tesla, a brilliant scientist, said Edison was wrong because 240 V AC wasted less energy.

Though Edison was the better businessman, Tesla was correct, and the US still uses an inefficient 110 V AC.  In fact, most new houses in the US are wired for both 110 V AC and 230 V AC.

What's the advantage of AC over DC?

The advantage of AC electricity is that you can change the voltage using a transformer.  Transformers only work with AC voltage.  But why would we want to do that?

Think of all the machines and lights in a city demanding energy from a power station.  The power station has to provide energy at the same rate as the city is demanding it.  If it doesn't all the lights dim and nothing works properly.

How quickly energy is transferred is called 'power'.  This depends both on voltage (amount of energy per charge) and current (how many charges pass per second).  So if the city demands a certain power you can provide it at high voltage and low current or low voltage and high current (or medium voltage and medium current).

It turns out that the higher the current the more energy is wasted in the long power lines connecting the power station to the city.  So if you want them to carry a low current they have to provide power at a very high voltage.

So what happens is that electricity is generated at a high AC voltage.  A transformer is used to step this up to a very high voltage for the long power lines to the city.  In the city other transformers reduce the voltage to the 240 V (or 110 V in the US) that people's lights, TVs and machines are designed to work at.

The basics of AC electricity

AC electricity can be very complex so we’ll just look at the ideas that are similar to DC.

In the UK mains voltage is actually nearer 230 V but 240 V is an easier number to do sums with so we'll use that instead.  To get an average of 240 V the peak voltage has to be around 320 V but you don't need to worry about the peak voltage for the moment.

The voltage changes between an average of +240 V and -240 V.  There's nothing special about negative voltage.  It just means the current tends to go in the other direction.

Domestic appliances are connected in parallel

Each of your electric lights, your electric hob, your washing machine, your TV and all your other electrical appliances are connected in parallel with each other.

This means they have the full mains voltage across them and you can turn one thing off without turning everything off.

The more appliances you turn on the more current is drawn.  This is like a wheel making bigger oscillations at the same rate.  If nothing is connected then the voltage is still there even though there is no current.

The live and the neutral wires

The wire where the voltage changes all the time is called the 'live'.  The wire where the voltage stays the same is called the 'neutral'.

As long as there's no fault the current in the live is exactly the same as the current in the neutral.  The neutral shouldn't give you a shock because it's at 0 V but DON'T TRY THIS.  It might be wired incorrectly.

Wiring a plug

The first thing you notice when you take the back off a plug is that the wires are different .  Wiring a plug incorrectly is dangerous so the ukusother("colours","colors","colours"); help us get the wiring right.  We'll use the European system.

The live is brown.  Remember brown bread means dead in Cockney rhyming slang.  The neutral is blue and the earth wire is green with yellow stripes.

The were chosen so that even -blind people could tell them apart.

The fuse and the earth wire work together

If you follow the conducting path through a plug you'll see that the fuse is part of the live part of the plug.  If the fuse blows then the circuit is broken and no current flows anywhere.  The appliance is now completely isolated from the live.

There are two things you need to remember about the earth wire

  1. The earth wire only does anything if there is a fault – otherwise it’s not involved in the circuit at all.
  2. The earth wire is there to help the fuse blow and so isolate the appliance from the live.

The earth wire starts at the metal case of an appliance.  It's just screwed on so there's an electrical contact.  You can follow the earth path through the plug and ultimately to the ground outside your house.

Say you drop the toaster and the live wire comes loose.  It may come to rest on the case of the toaster.  Because the case is made of metal it would give a shock to anyone who then touched it.

But there's now a conducting path from the live, through the metal case of the toaster and out to the ground outside the house.  This conducting path has a very low resistance (the body of the toaster acts like a very thick wire) so the current is big everywhere.  The fuse in the plug is part of this conducting path and so the current is also big in the fuse.  It melts and isolates the toaster case from the live, making it safe.

MCBs and RCDs

The disadvantage of a fuse is that you have to replace it if it blows.  This normally means fiddling around with a screwdriver, perhaps going out and buying a new fuse of the right rating.

If your plug doesn't have a fuse (as in the US) or if the fault happens at an appliance without a plug (e.g. a lightswitch) then you need to have another place where the circuit can be broken. 

This tends to be at the junction box where the big mains cable enters your house and is split up to provide lots of live and neutral points for all your lighting, electric showers, electric ovens, plug sockets etc.  In each live part there is an MCB - mechanical (or "miniature") circuit breaker.  The MCB acts as a fuse but the beauty is you can just reset it by flicking it back if it trips.

An MCB works by using an eletromagnet to break the circuit.

The problem with an MCB is that it's not very good at protecting you from shocks.  For this you need an RCD or 'residual current device'.

RCDs are often used when there's a chance that you might accidentally cut the live, for example with a saw or a hedge trimmer.  An RCD can be incorporated into a plug.  It trips out if any current leaks to ground - for example through you!

RCDs work by comparing the live and neutral currents and tripping out if they are different.

Choosing the right fuse for your plug

Fuses are rated by current.  In the UK they come in standard values like 3 A, 5 A and 13 A.  But how do you know what fuse to use?

You choose a fuse just above an appliance’s operating current.  Most appliances that are designed to heat things, like electric kettles and toasters, will draw a big enough current to need at least a 5 A fuse.

Most lighting and electronic equipment need a 3 A fuse.

Say a toaster is rate 960 W, 240 V how can you calculate what fuse to use?

You need to find its operating current.  So we use P = IV.  Rearranging, we get I = P/V = 960/240 = 4 A.

A 3 A fuse would blow as soon as you turned the toaster on so you need to use a 5 A fuse in the plug.

Double insulation

Some modern devices, like a laptop charger, don't have an earth wire because they are 'double insulated'.  This means the case is made of plastic and so doesn't conduct electricity.

If the live came loose inside and touched the case you wouldn't get a shock.

Double insulation is not so much about making things extra safe.  It's more to do with the fact that lots of things nowadays are made of plastic rather than metal so they'd be no point in connecting an earth wire even if you wanted to.

The danger of overloading an adaptor

So many things need to be plugged in that you often use an adaptor to be able to plug several things into the same socket.  This isn't a problem if none of the devices draw a high current, for example a desk light, your mobile phone charger and a stereo.

But if you wanted to plug in your toaster, a kettle and your microwave to the same adaptor then there's a fire risk.  This is because if all the devices are switched on then the currents add up in the adaptor.  This high current can cause the it to get very hot, possibly melting or catching fire.

Paying for electricity

You pay for energy rather than power or current.

You need lots of energy if your appliance converts energy quickly or you leave it on for a long time.

For example, a 2400 W electric fire converts energy twice as quickly as a 1200 W hair dryer.  So using the hair dryer for 30 minutes costs the same as using the electric fire for 15 minutes.

Calculating electrical energy in kWh

To dry your hair using a hair dryer might take 2 million joules of energy.  This would make your electricity bill have lots of confusingly long numbers so we use a much bigger unit of energy called the kilowatt-hour.  1 kWh is the same as 3 600 000 J.

A kilowatt-hour is a unit of energy, not power.  This is because you're multiplying power by time.  P = E/t so E = Pt.

If we boiled a 1 kW kettle for 1 hour it would take 1 kWh of energy.  The key thing with doing calculations like this is that you have to use kilowatts and hours even if you're given watts and minutes.

So a 2400 W fire left on for 30 minutes would need 2.4 kW x 0.5 h = 1.2 kWh.

1 kWh of electrical energy costs about 5-10p so this would cost you about 10p, which is pretty cheap.

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