AEMC 4500 User Manual

Also see for 4500: User manual

Contents

Soil Resistivity MeasurementsWhy make soil resistivity measurements?Soil resistivity measurements have a threefold purpose. First, such dataare used to make sub-surface geophysical surveys as an aid in identifyingore locations, depth to bed rock and other geological phenomena. Second,resistivity has a direct impact on the degree of corrosion in undergroundpipelines. A decrease in resistivity relates to in increase in corrosion activ-ity and therefore dictates the protective treatment to be used. Third, soilresistivity directly affects the design of a grounding system, and it is to thattask that this discussion is directed. When designing an extensive ground-ing system, it is advisable to locate the area of lowest soil resistivity in orderto achieve the most economical grounding installation.Resistivity measurements are of two types, the 2-point and the 4- pointmethod. The 2-point method is simply the resistance measured betweentwo points. For most applications, the most accurate method is the 4-pointmethod, which is used in the Model 4500 Ground Tester. The 4-pointmethod, as the name implies, requires the insertion of four equally spaced,and in-line, electrodes into the test area. A known current from a constantcurrent generator is passed between the outermost electrodes. The poten-tial drop (a function of the resistance) is then measured across the twoinnermost electrodes. Model 4500 is calibrated to read directly in ohms.ρ = 4πAR1 + 2A - 2A(A2 + 4B2) (4A2 + 4B 2)Where: A = distance between the electrodes in centimetersB = electrode depth in centimetersIf A > 20 B, the formula becomes:ρ = 2π AR (with A in cm)ρ = 191.5 AR (with A in feet)ρ = Soil resistivity (ohm-cm)This value is the average resistivity of the ground at a depth equivalentto the distance “A” between two electrodes.36 Digital Ground Resistance Tester Model 4500