In a 3-point fall-of-potential earth resistance test, how far from the ground rod should the C2 probe be placed?

In a 3-point fall-of-potential earth resistance test, the C2 probe should be placed at a distance that ensures accurate measurement of the resistance to ground in relation to the ground rod being tested. The optimal distance for positioning the C2 probe is typically around 100 feet to 130 feet from the ground rod.

Choosing 131 feet aligns with standard practices in the field, which recommend placing the C2 probe far enough away to avoid any interference that could affect the resistance measurements. This distance is necessary to ensure that the voltage gradient is stable and the current can be accurately measured without significant distortion. It helps ensure that the testing results reflect the true resistance of the grounding system you are evaluating, leading to more reliable and usable data for assessing electrical safety.

The other distances do not typically conform to established recommendations for fall-of-potential testing, which is why they may lead to erroneous measurements or insufficiently accurate results.