Proper grounding is essential to maintaining good power quality. Do you know the essential steps of doing a ground test?
The best way to become proficient with ground testing is to test often. You'll learn to recognize patterns of inconsistency and become adept at recognizing factors that produce inaccurate results. You'll learn to disregard test results skewed by environmental influences. Before you begin ground testing, make sure you understand these three essential elements.
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Use a dedicated ground tester,
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Understand the test and test environment, and
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Place the probes properly.
Choosing the proper instruments. The digital multimeter (DMM) is a good tool, but not for measuring ground resistance. Two-point measurements with a DMM will give you the loop resistance as defined by the arbitrary points of connection, including all soil between those points. It also won't correct the presence of transient currents. So, use a device specifically designed to measure ground resistance.
Understanding the test is crucial to doing it right. The Figure (of original article) shows how a dedicated ground tester works. The instrument uses two precise measuring circuits (voltage and current), and combines their values through Ohm's Law to produce the desired measurement (resistance).
Understanding the test environment is crucial to getting a useful measurement. A ground test is never routine. Remember, the earth is not a defined circuit. However, it does provide a resistance environment surrounding the ground electrode. This resistance environment has a fixed nature, determined by the soil type, the structure of the electrode, and the electrical demands upon it. It also has a variable nature, determined by transient factors like moisture and temperature. Thus, you must measure the entire critical volume of soil surrounding the electrode: This is what influences the flow of fault current into the earth.
You establish the test circuits in the soil by strategically placing probes. Proper placement of the probes (in relation to moisture pockets, buried objects, etc.) is critical. You will probably need to experiment with probes in several locations to get a truly representative reading. Pay attention to probe spacing. If the potential probe is too close to the grounding electrode, your readings won't reflect all the resistance a fault would meet. If the current probe is too close, you'll be trying to make readings in conflicting electrical fields.
The current circuit has a square waveform that sets it apart from utility power harmonics. Thus, the ground tester can measure current without interference from transients. Likewise, the voltage (or potential) probe enables the tester to measure the drop over the soil to a probe's critical point of placement. A second set of terminals allows the instrument to complete both circuits.
The "Fall of Potential," or "three point method" references three points of earth contact set up by the ground electrode and the two probes. IEEE Standard #81 describes this method. This test is the recognized basis for ground testing in the United States.
You need such accuracy so you know your path to ground has low enough resistance to allow circuit protection devices to work properly. Further, to do fault calculations, you must know your earth resistance.
Consider these essentials the starting point to a successful ground testing program. The IEEE, ground tester manufacturers and grounding equipment vendors have more information. You can also visit www.ecmweb.com and www. ecmbooks.com.
