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Electrician

An Introduction to Ground Testing by Megger

of utility power, through the soil to the ground under test. It is only this current that contributes to the measurement, leaving interfering transients out. Likewise, the voltage probe enables measurement of the drop over the soil to its critical point of placement.

Both circuits are completed by connection of a second pair of terminals (or a common in the case of three-terminal testers) to the ground under test. (See Figure 1.)

Understand the Test Environment Providing the most applicable and highest quality tester, however, is only the first part of the operator’s responsibility. No ground tester can perform a successful test all by itself since a ground test is never routine. The operator’s knowledge and skill must always be an essential element of a proper test.

The proper placement of the probes is critical and defies standardization of procedure. A degree of trial and error cannot be avoided, because the earth is not a defined circuit, like a piece of equipment. The experience and ability of the operator are valuable in reducing this process to an efficient level, and no instrument can substitute for this factor.

The resistance environment with which the ground electrode is surrounded, whether it be a single rod or complex grid, is determined by a critical volume of soil. This volume may be thought of as an area of electrical field influence around the electrode. It has at the same time both a fixed nature, determined by soil type, structure of the electrode, electrical demands upon it and other factors, and a variable component, determined by transient factors like moisture and temperature.

Put simply, this entire critical volume must be measured, for it is what influences the flow of fault current from the ground electrode into the earth. For the proper volume to be measured, probes must be sufficiently spaced. Only the operator’s knowledge can accomplish this placement properly and efficiently. Because soil conditions are never precisely the same, there is no set method to predict spacing in advance, and no instrument design can eliminate the operator.

Space the Probes Properly If the potential probe is too close, measurements are taken within the electrode’s sphere of influence, and different readings would be obtained with other placements. Indeed, a quality tester will give an accurate measurement to that point, but it is not taking into account all the resistance that a fault current will meet.

SOIL RESISTIVITY TESTING (4 point method) The most common method utilized for measuring soil resistivity is the Four Point Method using the equally spaced Wenner Arrangement. This method is commonly referred to as the Four Pin Method. This method is the most accurate method in practice for measuring the average soil resistivity of large volumes of earth. This method utilizes a specialized ground test instrument that has a four-terminal arrangement. Small electrodes are driven into the earth, all at depth B and spaced (in a straight line) at equal distance intervals A. The test current (I) is passed between the two outer electrodes (C1 and C2) and the voltage (V) is measured between the two inner electrodes (P1 and P2). The instrument knowing the voltage and current calculates the resistance from Ohms law (V/I) and gives the resistance (R) in ohms.


Four Point Method using Equally Spaced Pin Arrangement The average soil resistivity ρ (ohm•cm) is calculated by the following formula where R is the resistance measured in ohms, (A) is the pin spacing expressed in centimeters and (B) is the depth that the pins are inserted into the soil also expressed in centimeters and π is the constant 3.1416. 


This soil resistivity (ρ) value is an indication of the average soil resistivity to a depth which is equal to the pin separation distance (A). If the condition whereby the pin separation distance (A) is more than 20 times the pin depth (B) then the formula for resistivity above can be simplified to the following: 


In practice pin separation distance A is measured in feet, so the following formula can be used to calculate the average soil resistivity when A is expressed in feet. The constant takes into account the conversion from feet to centimeters.