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:

Ground Impedance Measurement Methods There are many Ground Impedance Measurement Methods as follows:
But, There are four basic methods of measuring ground resistance:

FallofPotential Method
Uses for FallofPotential Method Use the FallofPotential 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. FallofPotential method for ground resistance measure 
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
These factors will be discussed in the next Article. 
In the next Article, I will explain the Factors affecting Ground Resistance Measurement Reliability. Please, keep following.
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