Now we'll see how to do a ground resistance measurement, using the same tester shown in the first picture in this post. For a theoretical summary just look literature for the Wenner method about soil resistivity, the operative scheme is shown in the following figure, where G is the alternate current generator, A the ammeter and V the voltmeter.
When the generator G leads a current in the soil, the ammeter A measures the current value. At the same time the voltmeter V measures the potential difference between electrodes number 2 and 3. Resistivity ρT of the soil is:
ρT = 2 π a RT (2)
where RT is the soil resistance shown on the Unilap Geo and a is the distance (in meters) between the electrodes. Using a Earth/Ground tester, different instruments like G, A and V are replaced by just one and the soil resistance RT is directly shown to you.
To do an accurate measurement you should repeat it in several different places around the ground where the grounding system is located.