48 www.megger.comAll elements of the loop are measured in series. The method benefits from low return path resistance, i.e. where the ground resistance is large compared to the effective return loop resistance.Figure 22 shows the basic methodology of the stakeless test. The tester is clamped over RX, the ground being measured. All test current travels through the ground being measured (RX) and divides between the remaining parallel resistances (R1 to Rn). In a multiple ground system, the circuit can be considered a loop consisting of the individual ground electrode, a return path via all other electrodes and the mass of earth. The single electrode will have a higher resistance than the remainder of grounds connected in parallel. Figure 23 shows a practical example of where the clamp-on method is highly effective because of the low return path loop resistance. The application is an interconnected parallel ground, like a lighting string. Importantly, ground forms part of the loop. The resistance-to-ground measure at R6 is:Rloop = Vloop/Iloop = R6 + (1/(1/R1 + 1/R2 + 1/R3 + 1/R4 + 1/R5))For six similar electrodes with a resistance of 10 Ω, the loop resistance measured when testing each electrode would be:Fig 22: Basic clamp-on ground testing methodology
