AEMC 4500 User Manual

22 Digital Ground Resistance Tester Model 4500Grounding NomographGround RodResistance-OhmsSoil Resistivity(Ohm-centimeters) Rod DepthFeet Rod DiameterInchesRPDDIA20304090506070801001510152435/81/41/23/411.587654321000015000100000500004000030000200003000500100020004000500020301009080706050401512345678910KFigure 81. Select required resistance on R scale.2. Select apparent resistivity on P scale.3. Lay straightedge on R and P scale, and allow to intersect with Kscale.4. Mark K scale point.5. Lay straightedge on K scale point and DIA scale, and allow to intersectwith D scale.6. Point on D scale will be rod depth required for resistance on R scale.4.2 Ground Resistance ValuesNEC® 2008 article 250.56 regarding the resistance of rod, pipe and plateelectrodes states that if the rod, pipe, or plate does not have a resistanceof 25Ω or less to ground shall be augmented by one additional electrode ofany of the types specified by 250.52 (A)(4) through (A)(8). Where multiplerod, pipe or plate electrodes are installed to meet the requirements of thesection, they shall not be less than 6 feet apart.