In Article " Ground Resistance Calculations ", I explained the following points:
- Calculating Ground Resistance,
- Calculating Ground Resistance for substations,
- Verifying Ground Grid Conductor Installation,
- Design Guidelines and Requirements.
Also, In Article " Introduction to Earthing System ", I explained the following points:
- Introduction,
- Determining Ground Resistance,
- Soil Resistivity.
Today, I will explain the Ground Resistance Measurements.
Ground Resistance Measurement
A careful measurement of the resistance of the ground installation as constructed is desirable. Extreme precision is not always possible in measurement, but the results should be more dependable than values calculated by design engineering.
Instruments Used For Ground
Resistance Measurements
These are some of the instruments used for
ground resistance measurements:
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Ground Impedance
Measurement Methods
There are many Ground
Impedance Measurement Methods as follows:
But, There are four basic methods of
measuring ground resistance:
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Fall-of-Potential Method
Uses for Fall-of-Potential Method
Use the Fall-of-Potential Slope
Method for determining the resistance of large areas such as substation
yards, ground perimeters of large buildings, and other large areas.
Figure 1 shows the measurement arrangement
for this method of testing.
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Figure 1. Fall-of-Potential method for ground resistance measure
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The Measurement Arrangement for Fall-of-Potential
Method
1- Place the current probe (cp) a minimum of 328 feet (100
meters) from the ground grid boundaries. On large size plants or substations this
distance (c), is longer.
The formula for this distance
is: grid, grounding system, or substation diagonal of the area in meters (DL) equals
the square root of one of the sides squared (a2) plus the other side squared (b2) or
2- Insert the (cp) probe into the earth a minimum of
2 feet (0.6 m) in depth.
3- Insert the potential probe (pp) into the earth in a straight
line with the CP at distance (p) from the ground grid (g) you are measuring.
4- A minimum of 3 different
measurements at 3 different distances must be recorded. They are equal to
0.2C, 0.4C, and 0.6C and are represented as R1, R2, and R3 respectively as in Figure 2.
Pass a test current through the
ground grid through the cp. The
pp measures the voltage produced between the ground
grid and the surface of the ground. The instrument directly measures the
resistance (R) of the ground grid by dividing the measured voltage by the
test current (E/I).
5- Calculate the slope
variation coefficient using the following formula:
µ is a measure of the change of the slope in
the resistance curve.
µ should be less than or equal
to 1.59 (µ ≤ 1.59). If µ is not less than or equal to 1.59, do the whole measuring
process again with a longer (C) distance.
6- Include the key plans of the
location, the weather, and soil conditions when filling out your report.
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Interpretation of the Result
1- Obtain the value, pt/c,
from the table of values in Figure 3, once you calculate the slope variation
coefficient µ.
2- pt is the distance of the potential probe
position to the ground grid where the true resistance value should be
measured.
3- Measure the true resistance
by inserting the potential probe at the corresponding distance pt. Take an average value of the measured
true resistances from at least three different compass directions from the
grid. This result is the ground resistance of the grid.
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VALUES OF PT / C FOR VARIOUS
VALUES OF (µ )
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|||||
(µ )
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pt / C
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(µ )
|
pt / C
|
(µ )
|
pt/ C
|
0.40
|
0.643
|
0.80
|
0.580
|
1.20
|
0.494
|
0.41
|
0.642
|
0.81
|
0.579
|
1.21
|
0.491
|
0.42
|
0.640
|
0.82
|
0.577
|
1.22
|
0.488
|
0.43
|
0.639
|
0.83
|
0.575
|
1.23
|
0.486
|
0.44
|
0.637
|
0.84
|
0.573
|
1.24
|
0.483
|
0.45
|
0.636
|
0.85
|
0.571
|
1.25
|
0.480
|
0.46
|
0.635
|
0.86
|
0.569
|
1.26
|
0.477
|
0.47
|
0.633
|
0.87
|
0.567
|
1.27
|
0.474
|
0.48
|
0.632
|
0.88
|
0.566
|
1.28
|
0.471
|
0.49
|
0.630
|
0.89
|
0.564
|
1.29
|
0.468
|
0.50
|
0.629
|
0.90
|
0.562
|
1.30
|
0.465
|
0.51
|
0.627
|
0.91
|
0.560
|
1.31
|
0.462
|
0.52
|
0.626
|
0.92
|
0.558
|
1.32
|
0.458
|
0.53
|
0.624
|
0.93
|
0.556
|
1.33
|
0.455
|
0.54
|
0.623
|
0.94
|
0.554
|
1.34
|
0.452
|
0.55
|
0.621
|
0.95
|
0.552
|
1.35
|
0.448
|
0.56
|
0.620
|
0.96
|
0.550
|
1.36
|
0.445
|
0.57
|
0.618
|
0.97
|
0.548
|
1.37
|
0.441
|
0.58
|
0.617
|
0.98
|
0.546
|
1.38
|
0.438
|
0.59
|
0.615
|
0.99
|
0.544
|
1.39
|
0.434
|
0.62
|
0.610
|
1.02
|
0.537
|
1.42
|
0.423
|
0.63
|
0.609
|
1.03
|
0.535
|
1.43
|
0.418
|
0.64
|
0.607
|
1.04
|
0.533
|
1.44
|
0.414
|
0.65
|
0.606
|
1.05
|
0.531
|
1.45
|
0.410
|
0.66
|
0.604
|
1.06
|
0.528
|
1.46
|
0.406
|
0.67
|
0.602
|
1.07
|
0.526
|
1.47
|
0.401
|
0.68
|
0.601
|
1.08
|
0.524
|
1.48
|
0.397
|
0.69
|
0.599
|
1.09
|
0.522
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1.49
|
0.393
|
0.70
|
0.597
|
1.10
|
0.519
|
1.50
|
0.389
|
0.71
|
0.596
|
1.11
|
0.517
|
1.51
|
0.384
|
0.72
|
0.594
|
1.12
|
0.514
|
1.52
|
0.379
|
0.73
|
0.592
|
1.13
|
0.512
|
1.53
|
0.374
|
0.74
|
0.591
|
1.14
|
0.509
|
1.54
|
0.369
|
0.75
|
0.589
|
1.15
|
0.507
|
1.55
|
0.364
|
0.76
|
0.587
|
1.16
|
0.504
|
1.56
|
0.358
|
0.77
|
0.585
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1.17
|
0.502
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1.57
|
0.352
|
0.78
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0.584
|
1.18
|
0.499
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1.58
|
0.347
|
0.79
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0.582
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1.19
|
0.497
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1.59
|
0.341
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Figure 3.
Table of values for PT / C related to µ
Example#1:
Plot the measured resistance values against the potential probe spacing found from the curve in Figure 4.
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| Figure 4. Plotted resistance values |
Calculate for µ using the resistance values below. Use the slope variation coefficient formula shown below also.
From Figure 3 for µ = 0.96; pt/C = 0.550
Measure the true resistance by positioning the potential probe at a distance equal to pt or 0.550(C), or from Figure 4, pt = 0.550 (100m) =55 m.
Factors
Affecting Ground Impedance Measurement
These
factors will be discussed in the next Article.
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In the next Article, I will explain the Factors affecting Ground Resistance Measurement Reliability. Please, keep following.
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