Ground Resistance Measurements

  • 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:
  • Direct-indicating megohmmeter with self-contained hand- or power-driven internal generator,
  • Direct-indicating megohmmeter with self-contained battery,
  • Direct-indicating megohmmeter with a rectifier using an external alternating-current supply,
  • Resistance Bridge with a galvanometer and batteries.





Ground Impedance Measurement Methods
There are many Ground Impedance Measurement Methods as follows:
  • The 2-Point Method,
  • The 3-Point Method,
  • The Fall of Potential Method,
  • The 62% Rule,
  • The Ratio Method,
  • The Tag Slope Method,
  • The Intersecting Curve Method,
  • Staged Fault Tests,
  • Driving Point Impedance,
  • The SGM Method.

But, There are four basic methods of measuring ground resistance:

  • Fall-of-Potential Method,
  • Two-Point Method,
  • Three-Point Method,
  • Ratio Method.

In this ground inspection course, I will explain only the most used method for Ground Impedance Measurement which is the Fall-of-Potential Method.





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.



Figure 1. Fall-of-Potential method for ground resistance measure


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.



Figure 2. Resistance values related to distance




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.



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.


VALUES OF PT / C FOR VARIOUS VALUES OF (µ )
(µ )
pt / C
(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
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
1.17
0.502
1.57
0.352
0.78
0.584
1.18
0.499
1.58
0.347
0.79
0.582
1.19
0.497
1.59
0.341

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.




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


  • Difficulty reaching true remote earth reference voltage,
  • Effect of Auxiliary Electrode Location (Earth Current Return),
  • Size and location of voltage probes,
  • Interaction Between Instrumentation Wires,
  • Interference from Overhead Lines and their Grounding,
  • Background 60 Hz Voltage and Harmonics,
  • Ground Impedance Magnitude.

These factors will be discussed in the next Article.



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