Determining Soil Resistivity w/ Fall of Potential Method

[johnhos]

Since the fall of potential method (used for measuring ground grid resistance) and the 3 point method (used for measuring soil resistivity) are basically the same process, except one drives a ground rod and one uses existing grid; Is it possible to measure a substation’s soil resistivity using the fall of potential method on the existing grounding system?

 

[trosszilla]

Fall of potential method measures the voltage of the substation earth mat with respect to remote earth. It does not measure the resistance. If you know the current into the earth mat and the voltage you can then work out the resistance of the earth mat using ohms law.

You could then potentially work out the soil resistivity by modelling the earth mat in software and playing with the soil res until it obtained the correct mat voltage.

A better way is to measure the soil res directly using a standard test method.

 

[jghrist]

Resistivity is measured with four probes (two current and two potential), not three. Current flows between the two current probes and the instrument measures the voltage between the two voltage probes that are spaced between the current probes.

You don't want to use an existing grounding system as one of the probes because its zone of influence will mess up the measurements. In fact, you want to stay away from existing grounding systems (at least ones that cover a significant area) because they may provide a partial metallic path for current between current probes, bypassing the soil.

trosszilla, you may be technically correct about the fall of potential method not directly measuring resistance, but the ground tester does the ohms law calcs and gives a reading of resistance directly in ohms.