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© 2001-2007 Calix. All Rights Reserved.
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.
