One of the things that causes the most confusion when people start learning about wiring up and using electronic circuits is the Earth. This is really the fault of engineers and practical scientists who tend to use jargon instead of clear explanations. They also tend to leave some of the most important bits of information off many diagrams of electronic systems!

The real Earth (the planet you are probably standing on) is electrically neutral. This means that it has the same number of electrons and protons, so their charges cancel out overall. Scientifically, we describe this by saying that the Earth has an Electric Potential of zero. The earth wire of a mains plug is connected to the actual Earth. As you may have noticed, the Earth is quite a big place - not as big as ‘Space’ of course, but big enough to get lost in after a few pints... Because of this (the size, not the pints) it is almost impossible to charge up anything wired to the Earth.

This inability to charge items connected to the Earth is the reason that many items have their boxes wired to the Earth. It means that any fault inside the equipment cannot produce a dangerous voltage on the box, so you should not get an electric shock from touching the outside of the box even if something inside goes really wrong!. In order to give you a shock it would have to charge up the entire planet!

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Various symbols are used on circuit diagrams to represent earth or ground potential. This can be confusing, but when in doubt you can assume that they all usually mean ‘zero volts’ - i.e. the place from which all other voltages in the circuit are measured. In practice the meanings of the symbols are slightly different. Just to confuse things a bit more, some engineers use these symbols in different ways to others. Don't worry, just assume that they all mean zero volts.

In fact,
• The earth symbol indicates a place actually wired to the planet earth via the mains wires provided or using a wire to a metal plate buried in the Earth.

• The ground symbol usually indicates a connection back to a place in the power supply which provides the energy required by the circuit in order to work. It is usually assumed that this place in the power supply is connected to the Earth.

• The chassis symbol means a connection to a metal box enclosing the circuit. So far as the circuit is concerned, this metal box can be as good a place as the Earth for referencing voltages. From the point of view of most electronic circuits this works just like an earth connection, however it need not actually be connected to the earth. Hence the chassis of some equipment can sometimes be charged up to a high voltage. (With respect to the Earth.)

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In most cases, the ground and chassis connections are just indirect ways to reach to earth. However, in some cases, e.g. a portable radio using batteries, or the electrics in a car, the ground or chassis represent a sort of ‘local’ or ‘floating’ version of the earth used as the zero volts reference point. In most cars, the elctrical equipment is powered from a 12V battery. This provides 12V with respect to the metal bodywork (the chassis). So far as all the car electronics is concerned, they just see 12V. However this doesn't stop you from sometimes getting an electric shock when stepping in or out of the car! This is because the chassis may sometimes become charged up to a high voltage with respect to the Earth due to movement of the (insulating) rubber tyres.

Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.