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Reference
FiberPoint Structured Wiring Enclosure Installation Guide
Form 361015-0001A
Full-Of-Potential Testing Method
The “Full-of-Potential Method” sometimes referred to as
the “Three Terminal Method”, is the most commonly
accepted way to measure the total resistance of a
grounded earth electrode. Providing that the measured
resistance at the conclusion of the test meets the specified
requirements, then no additional testing should be
required. The test procedure measures the total resistance
of the earth ground electrode, the contact resistance of the
electrode to the earth and the earth near the grounding
system.
An earth resistance megger is a test set used to make
resistance measurements of earth electrodes. An AC
voltage is applied across two electrodes that have been
placed into the ground and at a distance apart from each
other. As current passes through the ground between the
electrodes, a voltage can be measured at a point between
the electrodes. The test set provides an internal voltage
meter to accommodate this measurement. The voltage
reading taken by the test set is converted to Ohms of
resistance by using Ohms Law. (R=E/I).
The test starts with connecting the megger to the earth
electrode that is to be tested. A second test electrode is
placed into the ground at a distance out and away from the
outer current electrode that is under test. The distance
between the electrode being tested and the outer current
electrode may be limited to the physical characteristic of
the surrounding area or the length of the test conductor
being used. It is not uncommon for the outer electrodes to
be spaced at 100 feet. A third test electrode is used as a
probe and placed into the ground at a number of points that
are along a line between the outer electrodes. Each time
the test probe is placed into the ground, a resistance
reading is taken and recorded. A simple resistance curve is
then plotted (See ‘Figure R-3: Plotted Curve’). At about
60% of the distance between the earth electrode under test
and the outer current electrode, the resistance over
distance does not continue to increase. It is at this point
that the resistance measurement taken is most accurate.
In cases when the outer current electrode is placed too
close to the earth electrode that is under test, the earth
shells that surround each electrode will overlap each other.
Resistance readings taken when earth shells overlap will
be very inconsistent and for the most part erroneous. As
resistance measurements are taken and plotted, a curve of
increasing resistance over distance will accumulate. The
resistance curve will not flatten out.
Most earth megger test sets will indicate the presence of
stray electrical currents in the ground. Resistance
measurements taken under such a condition should not be
relied on as accurate. The source of the electrical currents
should be identified and eliminated if possible. The
presence of electrical currents in or near a ground system
will compromise the overall integrity of the grounding
system.
Optical Solutions recommends that the testing of grounds
should be placed on a preventative maintenance schedule
to insure the maximum safety possible. Maintenance
records should be kept of each CPE installation. In some
locations, extremely high earth ground resistance is
present. In these locations, chemicals that lower earth
resistance can be added to the earth that surrounds the
electrode. This type of treatment must be done on a
scheduled maintenance routine complete with resistance
records.
Figure R-3: Plotted Curve
10
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100
0
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Distance (feet)
Resist
ance (
ohms)
1400