Stray Voltage vs. Time |
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Currents on electric power systems are in a constant state of change.
Electrical loads like motors and lights are being turned on and off in a seemingly
random fashion on all parts of these systems at all times. This gives rise to one
of the most significant features of stray voltage—its variability over time.
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Loading on a power system
is analogous to traffic on a highway: the number of cars and trucks
on any one section of the highway at any time is like the amount
of current in the wires at a particular location on the electrical system.
Electric load variability is similar to the variability of highway traffic,
and like traffic, will change with time.
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When the electrical load increases,
the amount of neutral conductor current may also increase.
And, as we have learned, an increase in the amount of current
on a farm or utility-grounded neutral conductor can result in an
increase in voltage gradients
[Combined Gd]
and related stray voltage levels.
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Current (load) variability can be described according to
the length of the time frame: instantaneous changes,
those that happen more slowly over several hours, those that repeat from day to day,
and those that characterize year-long trends.
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Variability over long time frames stems primarily from electrical loading, the drawing
of more current as power systems are put to use. This variability tends to be cyclical,
mirroring the labor and social activity level from night to day and over the seasons.
Short-term variability stems from rapid changes in electrical loading, the most significant of which
occur during the starting or stopping of large motors of the type found on farms.
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In the next few pages, we will investigate long-term changes
[Long-Term] to see how long-term electric load
variability affects stray voltage levels; next we will look at brief events
[Transients] to learn more about changes of current
and voltage that occur in the blink of an eye and what their impact might be on stray voltage.
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Before moving on, though, it is important to clarify some common misunderstandings
concerning stray voltage as it relates to non-power signals and stray voltage measurements. |
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Measurement and Other Signals |
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Separating power-related stray voltages from other signals in a measurement
is complicated, especially when time factors are involved.
A stray voltage recorder may measure and record a variety of electric signals.
However, it is difficult and sometimes physically impossible to separate the
extraneous signals from the power signals, since they are all present at the same
time and are intermingled in the measurement data.
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As a result, many distinct electrical phenomena and observations are mistaken for stray voltage.
For example, certain electric signals in the air,
in the earth, or on electric wires may be included with or mistaken for stray voltage,
when they have nothing to do with the stray voltage that we have been discussing.
These other signals include radio signals and higher-frequency
signals used in modern telecommunications systems.
There may be signals in the earth caused naturally (telluric currents), by
other utilities, or by pipeline or railroad systems.
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Much of the equipment used for stray voltage analysis today is extremely sensitive
and sophisticated, and capable of capturing both transients and long-term profiles.
Some of the recording equipment in use has the ability to measure electric signals
up to many millions of times per second and at microscopic levels, much lower than our level of concern.
This makes proper instrument set-up and test protocol a critical issue,
and the analysis of the data very difficult.
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When properly isolated, these other signals
may be rightly called stray voltages as well, because "stray" describes
unexpected and undesirable voltages, independent of their origin.
But these other signals are unrelated to the operation of a power system,
to the the ensuing stray voltage phenomenon, and to this discussion.
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Separating the power-related stray voltage signals from these other electric signals
is a difficult measurement issue as well as a problem in the use of the term stray voltage.
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Next, we focus on the way in which stray voltage varies over time.
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