Additional Physics Topic  10 - Household Electricity

AC and DC

The electricity we get from a plug is called alternating current (AC).  The electricity we get from a battery is called direct current (DC).

Mains electricity is supplied to homes in Europe at 230 V AC at 50 Hz.  The bit about the voltage is clear enough.  What is meant by AC and 50 Hz?  Have a look at the picture below.

The graph shows the way the voltage changes with time.  Note the following:

• DC voltage does not change at all.  Its direction is constant;

• The AC voltage is constantly changing from positive to negative.  So its direction is changing all the time.   It does this 50 times a second, so its frequency is 50 Hz.

The time for one complete wave is 1/50th (0.02 seconds or 20 milliseconds [ms]).  A complete wave is from the first peak to the second peak.

We can show this using an instrument called a Cathode Ray Oscilloscope (CRO).  It's a bit of a mouthful, but the shorthand CRO is easier to remember.  A CRO looks like this:

If we show the AC and DC at the same time on the CRO, we get a trace as shown on the right.

Note the following:

• The wave has a shape called a sine wave.

• The y axis (vertical) gives us the voltage.

• The x-axis (horizontal) gives the time.

• The centre red line is the zero volts level.

• The straight green line is the DC equivalent of the alternating voltage.

• There is a peak voltage (V0) both positive and negative.

 Question 1 Which one of these two traces is the AC and which is the DC?

In the USA mains electricity is supplied at 110 V at 60 Hz.

 Question 2 What does 60 Hz mean?

The CRO takes a bit of interpreting. However we can measure AC using a multimeter set to an AC range (much easier).

 Do NOT , under any circumstances, try this at   home

Note that this shows the AC voltage at 244 V. Its range varies from 220 to 250 volts. The average is 230 V.

Interpreting the CRO Trace (HT Only)

We can use the CRO trace to work out the frequency of the wave:

If we set the time base to 5 milliseconds per centimetre, we find that the period (time for one complete wave) is 15 ms = 15 ÷ 1000 = 0.015 s

The frequency of this wave is given by:

frequency = 1/period

frequency = 1 ÷ 0.015 = 67 Hz

For the waveform on this screen to be 50 Hz, the period would need to be 0.020 s, shown as 4 cm on the screen.

 Question 3 The time control is set so that each square represents 5 ms (0.005 s)  Use the equation time = 1/frequency to work out the frequency of the AC waveform.  What do you think the trace will be like if the same CRO were to be used to measure the AC frequency in the USA (60 Hz)?

Peak Voltage and RMS Voltage (HT Only)

If we were to connect a CRO across a mains socket, we would see a pattern like this:

We would see that the 230 volt level was NOT at the peak, but about 70 % of the amplitude. This is called the root mean square voltage (rms). The peak voltage is Ö2 times the rms voltage. The rms voltage give the same energy per coulomb as the same DC voltage.

Peak voltage = Ö2 × 230 V = 325 V

The peak voltage is NOT 230 V.

Wiring a Plug

Most appliances are connected to the mains by a plug that is inserted into a socket.  On the continent, the pattern of plug is different to that in the UK.  We will concentrate on the UK pattern, which is also found in Ireland, and many other countries, e.g. Iraq.

You need to note the following about cables wired to plugs:

• Each conductor is covered in a plastic insulating coating. It is important that there are no nicks in this. A short circuit could occur.

• The three wires a covered in an outside insulating sheath.

• The case of the plug is plastic for insulation. The plug cover must be screwed on tight, otherwise it can come away in your hand, exposing live parts.

• Make sure that the cable grip is tight onto the sheath, not the wires.

• A fuse prevents too much current from damaging the appliance

The standard three-pin plug is wired like this:

Note the following:

• The live wire is brown.

• The neutral wire is blue.

• The earth wire is green and yellow.  The two colours are used for the benefit of people who are colour-blind and may wire a plug wrong as a result, leading to shock.

• There is a cord grip to keep the cable secure.

• Three core cable has three insulated conductors surrounded by a sleeve. Two core cable has live and neutral wires, but no earth.

• The cable has a sleeve to provide an extra layer of insulation.

The Earth wire goes to the ground through a spike in the ground. It used to go through the water pipes, but then plastic pipes were used, which don't conduct electricity. This rather defeats the object of the Earth wire. If a live wire touches the metal case, the current goes to ground, and the fuse blows.

 Question 4 What is wrong with this plug in the picture below?

 Question 5 What is wrong with this plug in the picture below?

The correct wiring of a plug is most important.  Badly wired plugs can lead to bad electrical accidents.  It is important to:

• place the correct wire to the right pin;

• ensure the cord grip is gripping the cable sleeve;

• avoid exposed conductors;

• make sure that there are no slits in the insulation of the wires.  This can easily happen when cutting back the sleeve with a sharp knife.

Use proper wire cutters and strippers; don't use scissors, kitchen knives, or your teeth.  The pictures below show a couple of examples of bad plugs and connectors.

Many appliances now have moulded plugs put on at the factory.  These are safer.

There are different patterns of plugs in use throughout the world. The different patters have evolved according to local custom. The picture below shows moulded plugs.

The plug on the left is used extensively in Europe. The plug on the right is the normal British pattern used in the UK and Ireland.

• The European plug does not have a fuse in it, but it can be gripped more easily (important for elderly and frail people).

• The British plug has a 5-amp fuse in a removable carrier.

• The earth is wired to a socket on the European plug; the pin itself is in the socket into which the plug is inserted. It is possible to plug this into an un-earthed socket (and the wrong way round) which could be hazardous.

• With the British plug this is not possible. The earth in opens shutters on the socket, preventing probing little fingers from touching live parts.

• Close to the plug, the pins are insulated to prevent the risk of shock if the plug isn't put in properly.

The British plug is recognised internationally as being a particularly safe design, and many countries are adopting the pattern.

Safety with Mains Electricity

Mains electricity can be dangerous.  The very least you can expect from touching a live wire is a severe shock.  If you are sweaty, you can get a severe burn, or even be killed.  Big currents heat up wires, so it's essential to have safety features built in.

Fuses

Mains plugs have a fuse in them.  There is also another fuse or circuit breaker in the fuse box.  A short circuit or other fault in an appliance can make too big a current flow, which can melt wires or even set them on fire.

Source not known

Plugging too many appliances into the same socket can also overload the socket, making it hot.  This is asking for trouble.

 Question 6 Can you see anything else that is wrong in the picture above?

A fuse is a weak point in the circuit that gets hot and melts if the current gets too big.  By melting it turns off the current.  Fuses are thinner than the other wires in the circuit and get hotter more quickly, saving costly or catastrophic damage to the appliance in case of a fault.

A fuse repeatedly blowing indicates that an appliance has a fault.  Do not be tempted to open up an appliance for yourself.  Take it to a professional.

You can see the fuse in this plug:

Faulty appliances can be dangerous. They can:

• Give a shock which is at best unpleasant, at worst fatal.

• Electrical burns that can go deep and cause permanent damage.

• Cause a fire.

The danger signs are:

• Scorched plugs and sockets.

• A hot "fishy" smell indicating a plug getting hot.

• Fuses blowing all the time.

• Loose or split wires.

You must never:

• replace a fuse that keeps on blowing all the time with one of a higher value. If the fuse goes for no obvious reason, the chances are that there is a fault.

• use water on an electrical fire; you could get a fatal shock. Use a carbon dioxide extinguisher. If you don't have one, get out, call the Fire Brigade out, stay out.

Fuses are available in four common sizes, 3 amp, 5 amp, 10 amp, and 13 amp (there are other sizes). It is important to chose the correct fuse:

1. Look at the power rating in watts, given on the appliance (on the bottom or the back).

2. Use current = power ÷ voltage to work out the current.

3. Chose the next size up.

 Worked example: An appliance is rated at 1 kW. What fuse should be used? Answer 1 kW = 1000 W. Current = 1000 W ÷ 230 V = 4.3 A Next size up is 5 A

 Use the equation current = power ÷ voltage to calculate the best fuse to use to protect a 1500 W vacuum cleaner.

The Fuse Box

In all houses you will find a fuse box.  It is often under the stairs, or in the garage.  In my house, it's under the kitchen sink!  Domestic fuses have a number of different values, e.g.:

• 5 A for lighting;

• 20 A for the garage;

• 30 A for the ring mains;

• 45 A for the cooker or shower.

These fuses can be re-wired.  You can get a card of fuse wire from most DIY shops.

Replacing a fuse in a fuse box is a pain. It means having to scrabble for the fuse wire and, if you find it, re-wire the fuse by torch light (as the battery goes flat). Also there is the temptation to replace the wire with thicker wire (purely as a temporary measure, of course). This can be highly dangerous.  Some installations have cartridge fuses (a bigger version of the fuse in the plug), but replacements can be difficult to get.

In more modern houses, the fuse box is replaced by miniature circuit breakers (MCB). When too big a current flows, the switches turn off, breaking the supply. All you need to do, having dealt with the fault, is to turn the MCB back on again. If it turns off again, the fault is still there.

The MCB does NOT protect against shock. If you use an appliance outside, you should use a residual current device (RCD).  A fixed RCD is placed at the top left of the fuse box in the picture above.  It supplies sockets in the outside shed.  A portable RCD is shown in the right-hand picture.

Ring Main

Once past the fuse-boxes, the house is wired using a special form of parallel circuit called a ring main:

• For the lights upstairs;

• For the lights downstairs;

• For the sockets upstairs;

• For the sockets downstairs.

Some appliances like the cooker are wired using a single wire called a spur.  The ring main is shown in the diagram below:

There are several advantages to the use of a ring main:

·         It is easier to add another socket.

·         The current has two ways to get to each socket; as a result, there is little voltage drop compared with a socket on a long spur.

It is easy to run a spur off the ring main.  Although each socket in this diagram is shown as a short spur, in practice, the ring main is directly wired into the back of the socket.

The permanent wiring in a house is called hard wiring.  The cables have a live wire, which is insulated in a red sleeve, an earth wire, which is not insulated, and a neutral wire, which is kept at about 0 volts, by having a bond to the ground at the substation.  When the earth wire is connected to the back of a socket, a loose insulating sleeve coloured green and yellow is used.

Hard wiring using the code brown for live and blue for neutral is available.

Flexible wiring used to have red as live, black as neutral, and green as earth.  However there are people who are colour-blind, and not able to tell between green and red.  This was potentially very dangerous when they wired a plug.  Therefore the modern code is brown for live, blue for neutral, and green/yellow for earth.

Earthing the Appliance

Look at this picture of a fault with a metal cased appliance.

A fault has occurred in which the live wire has become exposed and is touching the metal case.  Since this appliance has only two-core cable, there is no earth wire.  The appliance will work.  However as soon as you touch it you will get a severe shock.  The current involved in a shock (60 mA) will not blow the fuse, but may well kill you.

Now look what happens if there is three-core cable.

The earth wire is connected to the metal case of the appliance.  The fault leads to a short circuit which will blow the fuse.  The appliance will make a loud bang, but the case will not be live.

Earth leakage circuit breakers act as an extra safety device if you are using electrical appliances outside, for example an electric lawnmower.

Many appliances have plastic cases.  Metal parts are not accessible in normal use.  Therefore there is no need for an earth wire and they are connected by a two core cable.  These appliances are called double insulated.  The symbol for a double insulated appliance is shown below.

Can you see the symbol on the picture below?

Such appliances only need twin-core cable.

How the Earth wire works (HT only)

The Earth wire causes a short circuit because the neutral wire is connected to the ground at the substation.  Although the ground is not a brilliant conductor, there is enough of it to provide a low resistance alternative pathway.  The earth wire is connected to the ground by a long metal rod called an earthing spike.  Therefore there is an alternative route back to the neutral wire.  This is shown in the diagram below.

Because the neutral terminal is connected to the ground, the voltage at the neutral terminal is almost zero volts.  The voltage at the live terminal goes from positive to negative relative to the neutral terminal.  230 volts is the effective voltage.  This is about 70 % of the peak voltage, about 330 V.  So the voltage at the live changes from +330 V to -330 V and back 50 times a second.

 Question 8 Complete the interactive gap-fill exercise

 Summary Mains electricity is supplied to our homes at 230 V AC at 50 Hz The current from a battery is DC.  Its direction is one way only. Mains AC has a direction that is changing all the time from positive to negative. Plugs must be wired up properly. Appliances are protected from too big a current by a fuse. The earth wire prevents the case of a metal appliance from becoming live if there is a fault.