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8/13/2019 Ahdab Electrode Resistance
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MEASUREMENT OF GROUND
ELECTRODE RESISTANCE
Professor Ahdab Elmorshedy
Professor Ahdab Elmorshedy
When an electrode system has been designedand installed, it is necessary to measure the
ground resistance between the electrode and
true Earth.
The most commonly used method of
measuring the ground resistance of a ground
electrode is the 3-point measuring technique
shown in Figure 1.
This method is derived from the 4-point
method, which is used for soil resistivity
measurements.
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Professor Ahdab Elmorshedy
1. Arrangement of the Electrodes for the Fall of Potential Method
Professor Ahdab Elmorshedy
The 3-point method, called the fall of
potential method, comprises the Ground
Electrode to be measured and two other
electrically independent test electrodes, usually
labeled P (Potential) and C (Current).
These test electrodes can be of lesser quality
(higher ground resistance) but must be
electrically independent of the electrode to bemeasured.
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Professor Ahdab Elmorshedy
Analternating current (I)is passed through
the outer electrode C and the voltage is
measured, by means of an inner electrode P,
at some intermediate point between them.
The Ground Resistance is simply calculated
using Ohms Law; Rg= V/I.
Professor Ahdab Elmorshedy
The measurement of ground resistance is as
much an art as it is a science, and resistance
measurements can be affected by many
parameters, some of which may be difficult to
quantify.
It is best to take a number of separate readings
and average them, rather than relay on the resultsof a single measurement.
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Professor Ahdab Elmorshedy
The auxiliary test electrode C must be
positioned far enough from the ground electrodeunder test so that the auxiliary test electrode P
will lie outside the effective resistance areas of
both the ground system and the other test
electrode.
If the current test electrode, C, is too close, the
resistance areas will overlap and there will be a
steep variation in the measured resistance as the
voltage test electrode is moved.
Professor Ahdab Elmorshedy
If the current test electrode is correctly
positioned, there will be a flat resistance area
somewhere in between it and the ground system,
and variations in the position of the voltage test
electrode should only produce very minor
changes in the resistance figure.
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Professor Ahdab Elmorshedy
The instrument is connected to the groundsystem under test via a short length of test
cable, and a measurement is taken.
Measurement accuracy can be affected by the
proximity of other buried metal objects to the
auxiliary test electrodes.
Professor Ahdab Elmorshedy
Objects such as fences and buildingstructures, buried metal pipes or even othergrounding systems can interfere with themeasurement and introduce errors.
Often it is difficult to judge, from visualinspection of the site, a suitable location for
the tests stakes and so it is always advisableto perform more than one measurement toensure the accuracy of the test.
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Professor Ahdab Elmorshedy
1. Fall of Potential MethodThis is one of the most common methods
employed for the measurement of ground
resistance and is best suited to small systems that
dont cover a wide area.
It is simple to carry out and requires a minimal
amount of calculation to obtain a result.
Professor Ahdab Elmorshedy
This method is generallynot suited to large
grounding installations, as the stake
separations needed to ensure an accurate
measurement can be excessive, requiring
the use of very long test leads (refer to
Table 1).
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Professor Ahdab Elmorshedy
Normally, the outer test electrode, or current test
stake, is driven into the ground30 to 50 metersaway from the ground system, (although this
distance will depend on the size of the system
being tested - refer to Table 1) and
the inner electrode, or voltage test stake, is then
driven into the ground mid-way between the
ground electrode and the current test stake, and in a
direct line between them.
Table 1: Variation of current and voltage electrode separation
with maximum ground system dimensions, in meters.
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Professor Ahdab Elmorshedy
Let E be the electrode whose resistance toground is required to be measured and let P
and Cbe the auxiliary rods driven into the
ground.
A known value of current I is circulated
between C and E, and the voltage drop V
betweenEandPis measured.
The resistance of the electrodeEto the ground
is V/I.
Professor Ahdab Elmorshedy
The optimum location for the potential
electrodePis0.62 of the distance fromE
to Cwhen the distance D is at least 30
times the depth of the electrodeE.
Let the base of the electrode E be a
hemisphere of (equivalent) radius r and
the other two electrodes designated asshown in the figure.
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The potential atEdue to the entering current
isI/2rand due to the current leaving at Cis -I/2D.
The total potential atEcan be given by
D
I
r
IV
22=
Similarly the total potential at Pdue to the
current entering at E and that leaving at C
can be given by
)(22 HD
I
H
IV
=
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The net potential difference betweenEandPwill be given byV = VE -VP
=
)(2222 HD
I
H
I
D
I
r
IV
+=
)(
1111
2 HDHDr
IV
Professor Ahdab Elmorshedy
If the resistance curve (Figure 2) between Eand Cis observed, it flattens beyond P (if located
optimally between Eand C) and the resistance of
the ground electrode between E and P and that
betweenEandCare nearly same.
An upward bend in the resistance curve above,
near the point C is due to the resistance of the
auxiliary rod C itself and does not affect
measurement of the resistance of the main
electrode E.
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Therefore the potential difference
between E and Cand between E and Pwill also be the same.
Hence, the measured resistance of the
electrodeEwill be
I
VR =
+=
)(
1111
2 HDHDrR
wherec = D/randp = H/r.
But the resistance of the ground electrode is
R= /2r.
If the measured value Ris to be equal to R,
the condition to be satisfied will be
+=
pcpcrR 1111
2
0111
=
+pcpc
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Professor Ahdab Elmorshedy
ccccc
p
cpcp
pcpc
618.02
1-5
2
)4(
0
0111
22
22
=
=
+=
=++
=
+
Professor Ahdab Elmorshedy
From this, it follows thatH = 0.618D will
satisfy this condition.
This indicates that for any separation of
the current electrodes (E and C), the true
resistance of one of them is obtainable
when the potential electrode (P) is 61.8%
of the distance toward the other.
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Professor Ahdab Elmorshedy
The Fall of Potential method incorporates a
check to ensure that the test electrodes arepositioned far enough away for a correct readingto be obtained.
To perform a check on the resistance figure, twoadditional measurements should be made; thefirst with the voltage test electrode (P) moved10% further away from the ground under test,and the second measurement with the (P)electrode moved 10% closer to the ground undertest.
Professor Ahdab Elmorshedy
Professor Ahdab Elmorshedy
If these two additional measurements are in
agreement with the original measurement,
within the required level of accuracy, then the
test stakes have been correctly positioned and
the DC resistance figure can be obtained by
averaging the three results.
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Professor Ahdab Elmorshedy
If there is disagreement amongst any of
these results, then it is likely that the stakes
have been incorrectly positioned, either by
being too close to the ground system being
tested, too close to one another or too close
to other structures that are interfering with
the results.
Professor Ahdab Elmorshedy
The stakes should be repositioned at a larger
separation distance or in a different direction
and the three measurements repeated until a
satisfactory result is achieved.
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Professor Ahdab Elmorshedy
2. The 62% Method
3. Other Test Methods
(a) The Slope Method
(b) The Star-Delta Method
(c) The Four Potential Method
Professor Ahdab Elmorshedy
2. The 62% Method
The Fall of Potential method can be adapted slightly foruse with medium sized grounding systems.
This adaptation is often referred to as the 62% Method,as it involves positioning the inner test stake at 62% ofthe ground electrode-to-outer stake separation (recallthat in the Fall-of-Potential method, this figure was50%).
All the other requirements of test stake location - thatthey be in a straight line and be positioned away from
other structures - remain valid.When using this method, it is also advisable to repeat
the measurements with the inner test stake moved 10%of the ground electrode-inner test stake separationdistance, as before.
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Professor Ahdab Elmorshedy
The main disadvantage with this method is that
the theory on which it is based relies on theassumption that the underlying soil is
homogeneous, which in practice is rarely the
case.
Care should be taken in its use and a soil
resistivity survey should always be carried out.
Alternatively, one of the other methods should
be employed.
Professor Ahdab Elmorshedy
3. Other Test MethodsMany other methods exist for taking ground
resistance measurements.
Many of these methods have been designed in
an attempt to alleviate the necessity for
excessive electrode separations, when
measuring large ground systems, or the
requirement of having to know the electrical
center of the system.
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Professor Ahdab Elmorshedy
(a) The Slope Method
This method is suitable for use with large grounding
systems, such as substation grounds.
It involves taking a number of resistance measurements atvarious ground system to voltage electrode separationsand then plotting a curve of the resistance variationbetween the ground and the current.
From this graph, and from data obtained from tables, it ispossible to calculate the theoretical optimum location for
the voltage electrode and thus, from the resistance curve,calculate the true resistance.
Professor Ahdab Elmorshedy
(b) The Star-Delta MethodThis technique is well suited to use with large
systems in built up areas or on rocky terrain,where it may be difficult to find suitable locationsfor the test electrodes, particularly over longdistances in a straight line.
Three test electrodes are used, set up at thecorners of anequilateral trianglewith theground
system in the middleand measurements are madeof the total resistance between adjacentelectrodes, and also between each electrode andthe grounding system.
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