Why Determine the Soil Resistivity?
Soil Resistivity is most necessary when determining the design of the grounding system for new installations (green field applications) to meet your ground resistance requirements. Ideally, you would find a location with the lowest possible resistance. But as we discussed before, poor soil conditions can be overcome with more elaborate grounding systems. The soil composition, moisture content, and temperature all impact the soil resistivity. Soil is rarely homogenous and the resistivity of the soil will vary geographically and at different soil depths. Moisture content changes seasonally, varies according to the nature of the sub layers of earth, and the depth of the permanent water table. Since soil and water are generally more stable at deeper strata, it is recommended that the ground rods be placed as deep as possible into the earth, at the water table if possible. Also, ground rods should be installed where there is a stable temperature, i.e. below the frost line. For a grounding system to be effective, it should be designed to withstand the worst possible conditions
How do I Calculate Soil Resistivity?
The measuring procedure described below uses the universally accepted Wenner method developed by Dr. Frank Wenner of the US Bureau of Standards in 1915. (F. Wenner, A Method of Measuring Earth Resistivity; Bull, National Bureau of Standards, Bull 12(4) 258, p. 478-496; 1915/16.) The formula is : Divide ohm—centimeters by 100 to convert to ohm—meters. Just watch your units. Example: You have decided to install three meter long ground rods as part of your grounding system. To measure the soil resistivity at a depth of three meters, we discussed a spacing between the test electrodes of three meters. To measure the soil resistivity start the Fluke 1625 and read the resistance value in ohms. In this case assume the resistance reading is 100 ohms. So, in this case we know: A = 3 meters, and R...
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