Getting Down to Earth 47APPENDIX IIClamp-On MethodFall-of-potential testing, and its modifications, conforms to IEEE 81. It is extremely reliable, highly accurate and can be used to test any size ground system. Additionally, the operator has complete control of the test set-up and can check or proof his/her results by testing at different probe spacing. Unfortunately, the Fall of Potential method also comes with several drawbacks: n It is extremely time consuming and labor intensive.n Individual ground electrodes must be disconnected from the system to be measured.n There are situations where disconnection is not possible.The clamp-on ground tester performs a ‘stakeless’ test which is a ground resistance test performed without disconnecting the ground. Based on Ohm’s Law (R=V/I), the stakeless test induces a known voltage in a loop circuit that includes ground, measures resultant current flow and calculates the loop resistance of the circuit. Four pole earth testers also perform a stakeless test; however they use two clamps, a voltage clamp (V clamp) and a current clamp (I clamp), and keep the clamps separate to prevent interaction between the two. The operator must be certain that earth is included in the return loop and be aware that the tester measures the complete resistance of the path (loop resistance). The Megger DET14C shown measuring a ground spike in a typical ground well.
