Soil resistivity measurement
Soil resistivity is 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. Poor soil conditions can be overcome with more elaborate grounding systems. The soil composition, moisture content and temperature all impact soil resistivity. Soil is rarely homogenous and its resistivity will vary geographically and at different depths. Moisture content changes seasonally, varies according to the nature of the sublayers of earth and the depth of the permanent water table. It is recommended that the ground rods be placed as deep as possible into the earth as soil and water are generally more stable at deeper strata.
Calculating soil resistivity
The measuring procedure described here uses the Wenner method and uses the formula:
ρ = 2 π A R
ρ = the average soil resistivity to depth A in: ohm-cm.
π = 3,1416.
A = the distance between the electrodes in cm.
R = the measured resistance value in ohm from the test instrument.