AEMC 4620 User Manual

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AEMC 4620 User Manual Manual pdf 19 page image

Digital Ground Resistance Tester Model 4620 and 4630 17NEC® 2014 250.52 (A)(5) requires a minimum of 8 ft (2.4m) of the elec-trode to be in contact with the soil. The most common of electrode is a10 ft (3m) cylindrical rod which meets the NEC® code, which requires aminimum diameter of 5/8” (1.59cm).4.1.2 Effects of Soil Resistivity on Ground Electrode ResistanceDwight’s formula, cited previously, shows that the resistance to earth ofgrounding electrodes depends not only on the depth and surface area ofgrounding electrodes but on soil resistivity as well.Soil resistivity is the key factor that determines what the resistance of agrounding electrode will be, and to what depth it must be driven to obtainlow ground resistance.The resistivity of the soil varies widely throughout the world and changesseasonally. Soil resistivity is determined largely by its content of electro-lytes, consisting of moisture, minerals and dissolved salts. A dry soil hashigh resistivity if it contains no soluble salts.SoilAshes, cinders, brine, wasteClay, shale, gumbo, loamSame, with varying proportionsof sand and gravelGravel, sand, stones withlittle clay or loam590340102059,0002370406015,80094,000700016,300135,000458,000Resistivity, Ω-cmMinimum Average MaximumTable 14.1.3 Factors Affecting Soil ResistivityTwo samples of soil, when thoroughly dried, may become in fact very goodinsulators, having a resistivity in excess of 109 ohm-centimeters. The resis-tivity of the soil sample is seen to change quite rapidly until approximatelytwenty percent or greater moisture content is reached.