Topic  7

What is mains electricity and how can it be used safely?

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.  If it isn't, revise Topic 6!  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 like this:

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

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

Question 2  What does 60 Hz mean?  ANSWER

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.

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.

Question 3  What is wrong with this plug in the picture below?  ANSWER

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

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.

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

Safety with Mains Electricity

Mains electricity can be dangerous.  The very least you can expect from touching a live wire is a severe belt.  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 fusebox.  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.

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

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

Question 5  Can you see anything else that is wrong in the picture above?  ANSWER

Modern electrical installations have circuit breakers instead of fuses in the fusebox.  If these trip in the case of too big a current, they can be easily reset.  This is much easier than replacing a fuse.

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.

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.

Using the CRO to determine the frequency (HT only)

The CRO can be made to display the AC waveform.  The CRO plots a voltage time graph with voltage on the vertical axis and time on the horizontal axis.  The scales are calibrated by adjusting different controls on the CRO.

Here is a picture of the waveform.

Question 6  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)?  ANSWER

Now answer Question 7

Now try the Topic Quiz

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