![]() ![]() Ohm’s law can then be applied to calculate the measured apparent resistance. ![]() At this point, a known current is applied across probes C1 & C2, while the resulting voltage is measured across P1 & P2. C1 & C2 are the outer probes and P1 & P2 are the inner probes. Probes P1, P2, & C2 are driven at 5’, 10’ & 15’ respectively from rod C1 in a straight line to measure the soil resistivity from 0’ to 5’ in depth. In the following Wenner 4-Point Test Setup diagram, a probe C1 is driven into the earth at the corner of the area to be measured. These are the probes that take the actual soil resistance measurement. The inner two probes are the Potential probes, P1 and P2. These are the probes that inject current into the earth. The outer two pins are called the Current probes, C1 and C2. As the “4-point” indicates, the test consists of 4 pins that must be inserted into the earth. The following describes how to take one “traverse” or set of measurements. This raw data is usually processed with computer software to determine the actual resistivity of the soil as a function of depth.Ĭonducting a Wenner 4-point (or four-pin) Soil Resistivity Test The same is true if you space the probes 40’ across the earth, you get a weighted average soil resistance from 0’ down to 40’ in depth, and all points in between. The basic premise of the soil resistivity test is that probes spaced at 5’ distance across the earth, will read 5’ in depth. ![]() This is one “traverse” or set of measurements, and is typically repeated, albeit with shorter maximum spacings, several times around the location at right angles and diagonally to each other to ensure accurate readings. Typical spacings will be 1’, 1.5’, 2’, 3’, 4.5’, 7’, 10’, etc., with each spacing increasing from the preceding one by a factor of approximately 1.5, up to a maximum spacing that is commensurate with the 1 to 3 times the maximum diagonal dimension of the grounding system being designed, resulting in a maximum distance between the outer current electrodes of 3 to 9 times the maximum diagonal dimension of the future grounding system. The soil resistivity test spaces four (4) probes out at equal distances to approximate the depth of the soil to be tested. This soil resistivity test is commonly performed at raw land sites, during the design and planning of grounding systems specific to the tested site. It is expressed in ohm-meters and represents the resistance measured between two plates covering opposite sides of a 1 m cube. Electrical resistivity is the measurement of the specific resistance of a given material. ![]()
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