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Why Live, Neutral and Earth wires?

In most homes there are three wires: live (or hot), neutral and earth (or ground). They are colour-coded so you know which is which. Different countries may use different colours. In Britain in the cable (cord) connecting the kettle or whatever to a plug live is brown, neutral is blue and earth is green and yellow. In the cord in the United States hot is black, neutral is white and ground is left bare. (The cables under the floor and in the walls may be different colours but only a qualified electrician should ever touch these.)

1 The Earth (or Ground) Wire

If we empty an egg-cupful of water into a test tube the water level in the test tube will go up a lot, but if we empty it into a 5 litre beaker the water level will hardly go up at all. The larger the container (actually the larger the surface area) the less the water level will go up: emptying a child's bucket into the sea will not change sea level. To change sea level a measurable amount we will need to add a lot of water, for example by melting all of the Greenland and Antarctic ice caps. So we measure the height of the mountains or the depth of the oceans from (mean) sea level - the mean because we need to take the tides into account. The tides are discussed on their own Page.

If we have a metal sphere, and place it on a metal surface, because they are both conductors there will be no potential difference between them: they will be at the same voltage.

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If we fix the sphere onto an insulating rod and put an electric charge onto it there will now be a potential difference between the sphere and the surface: the potential difference between them will depend upon the charge and how big the sphere is.

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We measure charge in coulombs (C) and potential difference in volts (V) and capacitance in farads. (F). Something has a capacitance of one farad if a charge of one coulomb raises its potential by one volt. The Earth is very big and has a very high capacitance, millions of farads, which means that we can add big charges to it without significantly changing its voltage, so in the same way that we measure heights above sea level we measure voltages above the voltage of the Earth: we say the Earth is at zero volts, and earthing (also called grounding, a point makes its voltage relative to the Earth zero. Earthing in the home was originally done just by connecting the point to the nearest metal gas or water pipe, which were all linked together to metal pipes in the ground. Today many gas and water pipes are now made of plastic, so in modern homes earthing electric sockets is done by running the earth cable right back to the distribution board, and then connecting it to the metal sheath which covers and protects the mains cable which brings the electricity into the house. But all the metal gas and electricity pipes must still be earthed, so if a metal pipe is connected to a plastic pipe you must always bond it to another metal pipe.

Your body is very much smaller than the Earth, and so it has a very much lower capacitance, a few “puffs” (picofarads or million millionths of a farad) which means that a very tiny charge may produce a very high voltage, several thousand volts just by walking a few metres on an an untreated nylon carpet, enough to give you a tiny shock when you touch a metal doorknob - but a current of less than a millionth of an ampere so not dangerous (to you! - but it might destroy the silicon chip which you have in your hand - see the Page on Static Electricity). This was quite a problem forty years ago, but today most carpets are treated with an antistatic spray so if you are under forty you may never have experienced it.

But you might have used a Van der Graaf generator at school, ten thousand volts is fairly normal.

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Of course you must be well insulated: ordinary footware is not good enough, a current will pass through your body to earth.

2 - The Live (or Hot) Wire

This is the wire at mains voltage - if you touch it you may get an electric shock, a dangerous one, if electricity passes from the live wire to earth through your body: ordinary footware will not be sufficient protection, see the photograph above! But you can protect yourself using the special type of circuit breaker described in The Neutral Wire.

3 - The Neutral Wire

In a swimming pool at a holiday centre there may be a water slide: water is pumped from the pool to the top of the slide and then runs down the slide and back into the pool. The running water makes the slide slippery so you can have fun. The water circulates and the water in the pool is an important part of the circuit.

If we pour a cup of water onto a flower bed in the garden it will soak away almost immediately, but if we pour a bucket of water onto the flower bed it will not all soak away immediately, for a time there will be a small puddle.

The potential of the Earth is taken to be zero but the resistance of the soil is most certainly not zero, so we cannot usually use the soil as a part of a circuit - if we tried to pass a large current to earth the soil near the wire might be at a dangerously high voltage. Instead we use a neutral wire: this runs all the way from your home back to the power station. It is earthed at each electricity sub-station, where, among other things, there are transformers which change the mains voltage in the live wire to 230 V before it comes into your home. This neutral cable carries same current as your live cable, but because it is earthed it will be at approximately zero volts throughout its length: it does have some resistance so there will be a small voltage difference along it between your home and the first point at which it is earthed, and between that point and the next earthing point, and so on, depending upon how much electricity you are using. So in your home there can be up to 5 V difference between neutral and earth. (But do not try to measure it yourself!)

An ordinary 13 A fuse or circuit breaker is designed to prevent the current in the live wire from exceeding 13 A. But if the same current were to pass through your body from live to earth as passes through an electric fire from live to neutral the fuse would not blow/the circuit breaker would not trip but you would almost certainly die: fuses and circuit breakers are designed to prevent electricity cables from overheating not you! This is further explained on another Page.

The special circuit breaker designed to protect you from electric shock is measuring not the current in the live wire but the difference between the current in the live wire and the current in the neutral wire: if there is any difference between them it means that some of the electricity is going where it should not be going, for example through you, and the circuit is broken - instantly! You should always use this sort of circuit breaker out of doors, in a bathroom or on power tools. But it would not protect you if you grabbed the live wire with one hand, the neutral wire with the other and happened to be standing on a perfect insulator!

© Barry Gray October 2019