CSE Pure Power - Summer 2008 - (Page 21)

grounding story 21 ❮❮ PURE POWER // SUMMER 2008 installed ground rods conﬁguration. Because the earth’s resistivity is neither uniform nor consistent, a simple and direct method of measuring earth resistance is needed. The NEC does address locating the electrodes in moist locations away from the building. The typical speciﬁed distance from the building is beyond the drip line or beyond the gutters. IEEE Standard 142-1991 Table 11, “Effect of Moisture Content on Soil Resistivity,” shows with a low moisture content of 2% by weight sandy loam soil resistivity of approximately 185,000 ohms. At 24% moisture content the same soil resistivity is near 7,000 ohms. This is shown in Table 2 at the right. Table 1 – Resistivity of soils and resistance of single rods SOIL DESCRIPTION Well graded gravel, gravel-sand mixtures, little or no ﬁnes Poorly graded gravels, gravel-sand mixtures, little or no ﬁnes Clay gravel, poorly graded gravel, sand-clay mixtures Silty sands, poorly graded sand-silts mixtures Clay sands, poorly graded sandclay mixtures Silty or clay ﬁne sands with slight plasticity Fine sandy or silty soils, elastic silts Gravelly clays, sandy clays, silty clays, lean clays Inorganic clays of high plasticity GROUP SYMBOL* AVERAGE RESITIVITY (OHMS CM) 60,000–100,000 RESISTANCE OF 5/8 IN X 10 FT. ROD 180-300 GW GP 100,000-250,000 300-750 GC 20,000-40,000 60-120 SM 10,000-50,000 30-150 SC 5,000-20,000 15-60 ML MH 3,000-8,000 8,000-30,000 9-24 24-90 TEMPERATURE EFFECTS ON RESISTANCE TO GROUND IEEE Standard 142-1991 Table 12, “Effect of Temperature on Soil Resistivity,” shows the relationship of soil resistivity and soil temperature. At -5 C, the resistivity can be as high as 70,000 ohms cm. At 50 C, the resistivity is shown for the same soil to be 4,000 ohms cm. See Table 3 on page 22. The resistivity will be seasonal; winter is the worst. Most contractors will provide a second electrode/ ground rod as allowed by code in lieu of doing insulation resistance testing. Master type speciﬁcations appear to be based upon what is allowable by the NEC. CL CH 2,500-6,000† 1,000-5,500† 17-18† 3-16† * The terminology used in these descriptions is from the United Soil Classiﬁcations and is a standard method of describing soils in a geotechnical or geophysical report. † These soils classiﬁcation resistivity results are highly inﬂuenced by the presence of moisture. © IEEE Table 2 – Effect of moisture content on soil resistivity MOISTURE CONTENT (% BY WEIGHT) 2 4 6 8 TOP SOIL no data no data 135,000 90,000 60,000 35,000 25,000 20,000 15,000 12,000 10,000 10,000 RESISTIVITY (OHMS CM) SANDY LOAM 185,000 60,000 38,000 28,000 22,000 17,000 14,000 12,000 10,000 9,000 8,000 7,000 RED CLAY no data no data no data no data no data 180,000 55,000 20,000 14,000 10,000 9,000 8,000 REDUCING RESISTANCE TO GROUND WITH MULTIPLE GROUND RODS IEEE Standard 142-1991 Table 14, “Multiplying Factors for Multiple Rods,” shows how many additional ground rods are required to reduce the resistance to ground. The section and table shows that with two rods the equivalent resistance is approximated with the starting resistance divided by number of rods used and multiplied by the factor in the table. See Table 4 on page 22. 10 12 14 16 18 20 22 24 © IEEE www.purepowermagazine.com http://www.purepowermagazine.com

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CSE Pure Power - Summer 2008 Contents In the News Industry Roundup Risk Assessments for COPS Grounding Requires More Power Systems to Protect Healthcare Important Changes Coming in NFPA 70E A Look at Arc-Resistant Switchgear Agencies and Associations New Products Ad Index

CSE Pure Power - Summer 2008

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