Getting Down to Earth 45APPENDIX I Nomograph Guide to Getting Acceptable Earth Resistance12Dr. L.E. Whitehead of the DuPage Laboratories developed a nomograph (Fig. 21) which can be a helpful guide in meeting the established standard for a minimum earth resistance. If you have a given earth-electrode system and find that your Megger instrument reading is too high, the graph can be used to show what you must do to lower the value. Note that it covers three variable conditions that affect earth resistance of the electrode: earth resistivity, length of rod, and diameter of rod.To illustrate use of the nomograph, let’s take an example. Assume you have a 5/8-in. rod driven 10 ft into the soil. Your Megger instrument indicates an earth resistance of 6.6 Ω. But let’s say your specification for this resistance is “no more than 4 Ω.” To get this, you can change one or more of the three variables -- the simplest and most effective being depth of the driven rod. To find the required depth to give you a 4-Ω earth resistance, proceed as follows: With a ruler, draw a line from the 10-ft point in the L line to the 5/8-in. point in the d line; this gives a reference point where the line crosses the q line. Connect this reference point with 6.6 Ω-the measured resistance on the R line, as shown in Fig. 21; read the value of earth resistivity when this line crosses the p line. The value is 2000 ohm-cm.To determine the required rod depth for a 4-Ω earth resistance, draw a line from this point on the R line through the 2000 point on the p line until you cross the q line. The dashed line on Fig. 21 shows this step. Now, assuming you keep rod diameter unchanged, connect the 5/8 point on d line through your new reference point on q and extend the line to L. This gives you the required rod depth for the 4-Ω resistance value. Finally, take a new instrument reading to check the value, because earth resistivity may not be constant (as the nomograph assumes).12 Reference 21 (page 77)
