Earth Surface Voltage Gradients
Experienced As Stray Voltage |
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Stray voltage may be perceived by livestock on a farm where voltage gradients of sufficient
magnitude exist in the earth. These gradients result from the operation of electric systems
on or near the farmstead and will be present not only on the earth's surface, but also between
the earth (barn floor) and conductive objects connected to the earth. |
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A voltage difference may be measured between any two points chosen on a voltage
gradient plane, just as a difference in height may be measured between any two
steps on a staircase. Generally, the greater the distance between
the points on the voltage gradient plane, the greater the voltage difference in
uniform gradients. |
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The two points may be two points on the earth's surface, a point on the earth and a
metallic piece (e.g. a stanchion pipe) connected to the earth, or two metallic objects
connected to different points on the earth's surface. |
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The voltage to which a cow can be exposed in a gradient situation will depend on the points
at which the cow makes contact with the voltage gradient plane. Since the voltage experienced
tends to be higher the greater the distance between the two points, the way to determine the
amount of voltage to which a cow may be exposed is to identify the two farthest parts of the body
of a cow that can, in a practical situation, result in a contact with the gradient plane. If the
cow cannot contact two points on a voltage gradient plane at the same time (e.g. two points
separated by 20 feet), it cannot be exposed to the voltage between the two points. |
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The stray voltage that a cow experiences varies as the cow moves around,
because the two contact points of the cow will be touching a different pair
of points on the voltage gradient plane as it moves. In the
following figure, cows stand on various parts of a voltage gradient plane
formed at the surface of the earth by a power line ground rod at the base
of a pole. |
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One way to visualize the variability of stray voltage on a voltage gradient plane is
to imagine a probing wooden pitchfork in the place of the cow. The two tines of the fork are spaced
about five feet apart similar to the distance between a cow's
front and hind hooves. With the tines covered with metallic thimbles and a meter
connected between them, it can be moved around the voltage gradient surface
to probe for different stray voltages.
This shows that stray voltage depends on
two contact points; contacting the earth at one point does not result in a voltage.
According to the pitchfork model, stray voltage is a situational
experience; it varies as the two points are moved as a set from location to location. |
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