# How do clamp on earth testers make measurements?

## QClamp on earth testers measure grounding resistance by clamping the ground line. How do they calculate the resistance value?

A

The clamp part of the Clamp On Earth Tester FT6380/FT6381 has a two-layer core that includes a voltage injector and a sensor to measure current. The clamp is applied to the measurement target’s grounding resistance Rx, and the voltage V is injected from the voltage injector. Using the constant voltage and measured current, the overall resistance of a loop with multiple groundings can be calculated with the following formula: Rx + 1/(1/R1 + 1/R2 + 1/R3 + 1/R4…) = V / I. *If the second term on the left side of the formula is sufficiently less than the first term (grounding electrodes are connected in parallel; when there are many grounding electrodes, the second term decreases the more grounding electrodes there are), Rx can be expressed as follows: Rx = V/I. The FT6380 and FT6381 can measure multiple groundings in which a single wire is grounded multiple times. Multiple groundings include structural groundings and groundings used at chemical plants and for communications. These instruments cannot be used to measure single or independent groundings such as those for which residential grounding electrodes are used.