The earth is the zero-volt reference for power distribution
systems. In North America and other countries it’s called ground, and in
Europe, Australia, and other countries it is called earth.
Voltage is only meaningful when it is referenced to another
point. When a bird lands on a high-voltage wire, it doesn’t get electrocuted
because it does not complete a circuit to a zero-voltage reference or to
another point in the circuit with a different potential.
If the bird happens to straddle the gap between the
high-voltage line and the metal transmission tower or another line, sparks will
fly. That’s because the voltage needs a reference.
We typically take zero volts as the absolute reference for
voltage measurement. The exception is when we want to know the voltage drop
across a particular component such as a resistor or a transistor.
But normally, for example, a 12-volt DC power supply means
that the positive terminal is 12 volts higher than a zero-volt reference. When
we say a voltage rail is at 5 volts, that implies that is has been referenced
against zero volts and it is 5 volts below the reference.
Without some reference point with which to compare, voltage
measurements are meaningless. But what is our zero-volt reference based on?
The zero-volt reference is the earth, the largest current
sink available to us. The earth is actually a conductor, although various parts
of it are better conductors than others. Soil composition, moisture content,
mineral content, and other factors influence the impedance of the soil at any
given location.
But the earth is a very large current sink, and as long as
we can establish good contact with it, we have a good zero-volt reference.
Every power distribution system has at least one point that is electrically
connected to the earth, usually by means of a copper rod driven into the
ground.
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