Short Answer: The FOP method, described in the IEEE Std 81 standard, is used for all types of ground impedance measurements to determine a grounding system’s ability to dissipate electrical energy into the ground.
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Long Answer: This method is used for all types of ground impedance measurements to determine a grounding system’s ability to dissipate electrical energy into the ground. Grounding systems are designed to provide safety around electrical equipment and associated structures. They are also used in lightning protection systems along with a lightning arrester to dissipate the energy from lightning strikes. The lower the resistance between the arrester system and the grounding grid/rod and the ground, the better protection it provides.
Newly installed grounding systems may have low resistance to the ground, but this may deteriorate with time due to corrosion by electrolysis and other corrosive processes in the ground. Resistance depends on corrosion of the grid/stake, breaks in connecting wires, and variations in soil such as moisture levels. Therefore, it is important to test the grounding system regularly to make sure that it still provides a safe, low resistance.
Ground resistance is dependent on varying conditions like soil moisture, soil temperature and dissolved electrolytes, etc. Repeat measurements at different times of the year are therefore not exactly repeatable.
The grounding system should make as low resistance to the ground as possible so that in the event of an electrical short, the electrical current can flow to the ground as unobstructed as possible to be safely dissipated. All grounding systems have some ground resistance, but ideally it should be as low as possible. Typical resistance values are in the 2 - 5 Ω range for large installations, even though the National Electrical Code requires resistance to ground not to exceed 25 Ω.