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ELECTRICAL MEASUREMENT LAB
Department of Electrical Engineering
Supervisor
Submitted by
Dr. Girish Parmar
Control & Instrumentation
(Associate Professor)
Department of Electronics Engineering
University College of Engineering,
Rajasthan Technical University, Kota
February, 2015
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Experiment No.: 8
OBJECT: Measure Earth’s resistance using fall of potential method.
THEORY:
Introduction
The resistance to remote earth of the grounding system needs to be at a minimum in order to sustain its
effectiveness. A few of the components that make up this resistance are the physical properties of
the material used to make the electrode and conductor, all connections made, contact resistance between
the electrode and the soil, and the soil resistivity.
A complete grounding system might include
only one earth electrode, an entire group of electrodes with a grounding grid, or anything in between
and beyond. The earth electrodes from any of these types of systems can have their resistance to
remote earth determined. Theoretically, the resistance to remote earth of an earth electrode can be
calculated. This calculation is based on the general resistance formula:
R = (r x L) / A
where:R = resistance to remote earth (W)
r = soil resistivity (W-cm)
L = length of conducting path (cm)
A = crossectional area of path (cm)
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Fig. 8.1 Single Electrode Measuring Method
Fig. 8.2 Single Electrode Measurement Setup
After applying a voltage at the designated location in the circuit, the current between the EUT and
auxiliary probe is measured, and the voltage between the EUT and the reference probe is measured.
Using Ohm’s Law, the resistance can now be calculated. This routine is completed by the earth tester. It
has been determined that the distance D be approximately 100 feet, and the distance between the EUT and
reference probe be 0.62 times the value of D. These values have been tested using the fall of potential
method to give optimal. But due to the large amounts of uncertainty in all properties of probes and
soil, the value of D should be varied for each individual test until reasonably consistent values
appear; however, it is recommended that the value of D stay greater than 80 feet.
It is also necessary
to take a second set of readings at 90o to the original in case of interference from overhead power
lines or any underground electrical equipment or metal objects.
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Rs = 1 / [ (1/R1) + (1/R2) + (1/R3) + …(1/Rn) ]
where
Rs = resistance to remote earth for entire grounding system (W)
R1,2,3… n = resistance to remote earth for each individual electrode (W)
Fig. 8.3 fall of potential method
Fig. 8.4 Waveform of fall of potential method
Working: First, the earth electrode of interest must be disconnected from its connection to the site. Second,
the tester is connected to the earth electrode. Then, for the 3-pole Fall-of-Potential test, two earth stakes are
placed in the soil in a direct line—away from the earth electrode. Normally, spacing of 20 meters (65 feet) is
sufficient. For more detail on placing the stakes, see the next section.A known current is generated by the
Fluke 1625 between the outer stake (auxiliary earth stake) and the earth electrode, while the drop in voltage
potential is measured between the inner earth stake and the earth electrode. Using Ohm's Law (V =IR) , the
tester automatically calculates the resistance of the earth electrode .Connect the ground tester as shown in
the picture. Press START and read out the RE (resistance) value. This is the actual value of the ground electrode
under test. If this ground electrode is in parallel or series with other ground rods, the RE value is the total value
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of all resistances. To achieve the highest degree of accuracy when performing a 3–pole ground resistance test,
it is essential that the probe is placed outside the sphere of influence of the ground electrode under test and
the auxiliary earth.If you do not get outside the sphere of influence, the effective areas of resistance will
overlap and invalidate any measurements that you are taking. The table is a guide for appropriately setting the
probe(inner stake) and auxiliary ground (outer stake).To test the accuracy of the results and to ensure that the
ground stakes are outside the spheres of influence, reposition the inner stake (probe) 1 meter (3 feet) in either
direction and take a fresh measurement. If there is a significant change in the reading (30 %), you need to
increase the distance between the ground rod under test, the inner stake (probe) and the outer stake (auxiliary
ground)
until
the
measured
values
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remain
fairly
constant.
Factors That Can Change Your Minimum Earth Resistance:
We will discuss later what value of earth resistance is considered low enough. You’ll see that the re
is no general rule usable for all cases. First, however, consider three factors that can change the
earth electrode requirements from year to year:A plant or other electrical facility can expand in
size. Also, new plants continue to be built larger and larger. Such changes create different needs in
the earth electrode. What was formerly a suitably low earth resistance can become an obsolete
“standard.”As facilities add more modern sensitive computer-controlled equipment, the problems
of electrical noise is magnified. Noise that would not effect cruder, older equipment can cause
daily problems with new equipment.As more nonmetallic pipes and conduits are installed
underground, such installations become less and less dependable as effective, low resistance
ground connections. In many locations, the water table is gradually falling. In a year or so, earth
electrode systems that formerly were effective may end up in dry.
RESULT: A number of tests are made at different spaces of P and the resistance curve is plotted.
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