PART III. GROUNDING ELECTRODE SYSTEM AND GROUNDING ELECTRODE CONDUCTOR
250.56 Resistance of Ground Rod Electrode.
When the resistance of a single ground rod is over 25 ohms, an additional electrode is required to augment the ground rod electrode, and it must be installed not less than 6 ft away. Figure 250.105
No more than two ground rods are required, even if the total resistance of the two parallel ground rods exceeds 25 ohms.Measuring the Ground Resistance
A ground resistance clamp meter, or a three-point fall of potential ground resistance meter, can measure the resistance of a grounding electrode.
Ground Clamp Meter. The ground resistance clamp meter measures the resistance of the grounding (earthing) system by injecting a high-frequency signal via the grounded neutral conductor to the utility ground, and then measuring the strength of the return signal through the earth to the grounding electrode being measured. Figure 250.106
Fall of Potential Ground Resistance Meter. The three-point fall of potential ground resistance meter determines the ground resistance by using Ohm's Law: R=E/I. This meter divides the voltage difference between the electrode to be measured and a driven potential test stake (P) by the current flowing between the electrode to be measured and a driven current test stake (C). The test stakes are typically made of 1⁄4 in. diameter steel rods, 24 in. long, driven two-thirds of their length into earth.
The distance and alignment between the potential and current test stakes, and the electrode, is extremely important to the validity of the ground resistance measurements. For an 8 ft ground rod, the accepted practice is to space the current test stake (C) 80 ft from the electrode to be measured.
The potential test stake (P) is positioned in a straight line between the electrode to be measured and the current test stake (C). The potential test stake should be located at approximately 62 percent of the distance that the current test stake is located from the electrode. Since the current test stake (C) is located 80 ft from the grounding (earthing) electrode, the potential test stake (P) will be about 50 ft from the electrode to be measured.
Question: If the voltage between the ground rod and the potential test stake (P) is 3V and the current between the ground rod and the current test stake (C) is 0.2A, then the ground resistance is _____. Figure 250.107
(a) 5 ohms (b) 10 ohms (c) 15 ohms (d) 25 ohms
Answer: (c) 15 ohms
Resistance = Voltage/Current
E (Voltage) = 3V
I (Current) = 0.2A
R = E/I
Resistance = 3V/0.2A
Resistance = 15 ohms
The three-point fall of potential meter can only be used to measure one electrode at a time. Two electrodes bonded together cannot be measured until they have been separated. The total resistance for two separate electrodes is calculated as if they were two resistors in parallel. For example, if the ground resistance of each electrode were 50 ohms, the total resistance of two electrodes bonded together is about 25 ohms.
CAUTION: If the electrode to be measured is connected to the electrical utility ground via the grounded neutral service conductor, the ohmmeter will give an erroneous reading. To measure the ground resistance of electrodes that aren't isolated from the electric utility (such as at industrial facilities, commercial buildings, cell phone sites, broadcast antennas, data centers, and telephone central offices), a clamp-on ground resistance tester would better serve the purpose.
The resistance of the grounding electrode can be lowered by bonding multiple grounding (earthing) electrodes that are properly spaced apart or by chemically treating the earth around the grounding (earthing) electrode. There are many readily available commercial products for this purpose.
The earth's ground resistance is directly impacted by the soil's resistivity, which varies throughout the world. Soil resistivity is influenced by the soil's electrolytes, which consist of moisture, minerals, and dissolved salts. Because soil resistivity changes with moisture content, the resistance of any grounding (earthing) system will vary with the seasons of the year. Since moisture becomes more stable at greater distances below the surface of the earth, grounding (earthing) systems appear to be more effective if the grounding electrode can reach the water table. In addition, having the grounding electrode below the frost line helps to ensure less deviation in the system's resistance year round.
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