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Ground Testing Procedure (by Megger)

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Last Updated
6th of October, 2018

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An Introduction to Ground Testing by Megger®:-

Before performing a ground test, it is advisable to develop a working familiarity with two important concepts: how the tester accomplishes the measurement and what the operator must do to assure a proper test.

Choose the Proper Instruments:-

The first consideration is helpful in the selection of an instrument and the fundamental application of the test. Ground tests are frequently attempted with a variety of ohmmeters that happen to be conveniently at hand. This practice is doubly damaging because it may result in an incorrect measurement that is accepted by the operator.

Two-point measurement with a multimeter will give a loop resistance of the circuit that is defined by the arbitrary points of connection, and this will include in its path the soil between those points. But, so what? This is not necessarily an indication of the electrical condition that the ground electrode has established with the surrounding soil. Furthermore, the measurement itself can be made inaccurate by the influence of transient currents that travel in the soil from a variety of sources.

The current is traveling 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)

rep01.jpg


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.

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