36 www.megger.comSupplementary TestsThere are related tests which can be performed to supplement the information gained from the ground test and to augment the protection provided by the ground electrode. One of these is a continuity test to assure that it is complete and adequate throughout the grounding conductors and down to the point of contact with the electrode. Either a three-terminal or four-terminal tester can be used in a two-terminal configuration by shunting together the appropriate pairs. The two leads can thus be connected across a bond, weld, joint, or length of conductor, and the resistance measured. An earth tester, however, provides only a convenient backup check, not a fully rigorous continuity test. The reason for this is that, for safety’s sake, the test current is limited to values below a level harmful to the human body. A fully rigorous proof of a bond, however, must stress the connection at current levels capable of revealing corrosion, cracks, loose connections, and the like. For this reason, a dedicated low resistance ohmmeter capable of 10 A or more of test current is preferred.To protect personnel about to perform a ground test, as well as to identify the presence of electrical problems in the system, the ground electrode can first be checked for the presence of fault current. It is not uncommon, in an unbalanced or faulted electrical system, for the electrode to be carrying a fault current, more or less constantly, to ground. This may be only a few milliamps or several amps, and occurring undetected. A sufficiently sensitive clamp-on milliammeter can reveal the problem, and protect the testing crew from possible shock, in just a few seconds.The total impedance of the system can be measured at once by using a loop tester. This instrument simulates a fault between a phase conductor and ground, and thereby measures the total impedance of the entire ground loop, including conductors and the earth return path back to the transformer and its winding. If any of these elements have too high a resistance, protective devices may be inhibited from operating properly, even though the ground electrode itself is maintained at a sufficiently low resistance.
