52 www.megger.comearth connected to ground via an earth-ground link at the distribution panel. The stakeless method relies on having a low resistance return path in the loop, so this is an ideal point for clamp-on testers to measure ground resistance. Because the utility earth is very low, the loop resistance measured at the distribution panel ground by the clamp-on tester will be very close to the resistance a fall-of-potential measurement would yield. Note here that a fall-of-potential test would have to take into account the size of the total site ground system and ensure that long enough leads are available to correctly measure the ground resistance outside the sphere of influence of the ground system.Figure 24 is an application where the clamp-on method is often misused at communication towers. This example will help show why knowledge of the system is critical to making the correct test. The illustration shows the problems with trying to use a clamp-on ground tester on a cellular tower. Cellular towers are grounded at the base, with each guy wire grounded and all of them connected together in a ground ring. If the operator clamps around the head of one of the guy wire grounds, the test current will simply complete the circuit in the ground ring and not through the soil. Note that the test current circulates through the conductor that connects the individual elements (ground rods) that comprise the ring. As such, the clamp-on ground tester will not be measuring the quality of the ground system. The reading will actually be a reading of the resistance of the “loop.”Fig 24: Cellular tower — example of a misused application
