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Voltage Gradients in/on the Earth |
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Voltage gradients are the "footprints" that a power system, whether utility- or customer-owned,
leaves in the earth. They are the basis for stray voltage. It is, therefore,
important to investigate and learn about voltage gradients. |
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Most electric power systems are connected to the earth (grounded) to ensure people's
safety. Grounding allows power systems to detect equipment malfunctions and
automatically shut down power before harm comes to people and equipment. Another
function of grounding is to divert to the earth any lightning that strikes the power
system, minimizing damage and risk to people. |
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Grounding installed for safety creates pathways for currents to flow through the earth
between power system connections to the earth. The earth has a natural resistance that
opposes the flow of electricity. The work done by the current in overcoming the earth's
resistance results in voltage gradients along the path of the earth current. |
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The earth is not a perfect conductor of electricity.
It is made up of many materials that
resist the flow of current.
Materials with low resistance, such as metals,
develop small voltage gradients. Materials with high resistance,
such as ceramics, have very high gradients that practically
stop all electricity from flowing. The earth can be thought of as
a mixture of these materials, with an electric resistance
that falls between these two extremes. |
The voltage gradient created by the neutral current as it flows into and through the
earth is primarily inside the earth itself, out of the reach of animals.
The gradient, however, does extend to the earth's surface
where dairy cows may be exposed. |
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Because voltage gradients are inevitable and common in grounded power systems, it would
seem that stray voltages would be widespread and significant. Under normal operating
conditions, voltage gradients are generally very small. Any gradients between the front
and rear hooves of a cow are almost always imperceptible. Occasionally, however, these
earth voltage gradients become significant enough to be perceived by cows.
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Voltage gradients cannot be eliminated totally in safely grounded power systems;
that would be contrary to physical laws. However, these gradients
can be reduced to very
low levels—in
most cases, below the perception level of humans and animals. |
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Understanding electric power gradients is the key to understanding
and dealing with stray voltage. In the following pages we further explore
the concept of gradient. We then investigate the variety of gradients
in power systems by first examining a simple study case: current flowing
between two rods in the earth. At the end of this section we look at how earth surface
gradients become stray voltage and the two-point
contact requirement for cows to experience stray voltage from gradients in the earth. |
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