26 www.megger.comthe resistance for this distance is 20 Ω. This is the measured resistance of the earth electrode. This rule works well for simple electrodes, such as a driven rod. It also works for a small group of rods. But you must know the true electrical center of the electrode system fairly accurately. Also, accuracy of readings is better if the earth resistivity between the three electrodes is reasonably constant. Finally, C should be far enough away from the earth electrode system so that the 62 percent distance is out of the “sphere of influence” of the earth electrode. (See discussion with reference to Figs. 14 and 15). For the test, the electrode should be isolated from the electrical system that it is protecting; otherwise, the whole system is tested which (depending on local practices) may include the pole ground, system neutral, and transformer ground. This obscures the specific effect of the local ground. Basically, you now have the principle of earth resistance testing. The rest is refinement -- in test methods, use of electrodes or electrode systems, and information about earth resistivity, as covered in later portions of this manual.Basic Test Methods for Earth ResistanceThe earth tester generates an a.c. signal which is fed into the system under test. The instrument then checks the status of the circuits for good connection and noise. If either of these variables is out of specification then the operator is informed. Having checked that the conditions for test are met, the instrument automatically steps through its measurement ranges to find the optimum signal to apply. Measuring the current flowing and the voltage generated the instrument calculates and displays the system resistance in the range of 0.001 to 20,000 ohms, depending on the model chosen.There are three basic test methods as noted below. The first two are shown schematically in Figs. 11 and 12.
