Fall of Potential Test - Voltage/Resistance Rises Dramatically

[RabbleRabble]

Good question. The sudden rise in resistance at the end of the curve is probably due to a decrease in voltage drop between the two current probes. As for the "effective resistance area," I found this excerpt from "Getting Down to Earth" by Megger:

Resistance of Surrounding Earth: An electrode driven into earth of uniform resistivity radiates current in all directions. Think of the electrode as being surrounded by shells of earth, all of equal thickness.

Attachment 77

The earth shell nearest the electrode naturally has the smallest surface area and so offers the greatest resistance. The next earth shell is somewhat larger in area and offers less resistance. Finally, a distance from the electrode will be reached where inclusion of additional earth shells does not add significantly to the resistance of the earth surrounding the electrode. It is this critical volume of soil that determines the effectiveness of the ground electrode and which therefore must be effectively measured in order to make this determination. Ground testing is distinct when compared to more familiar forms of electrical measurement, in that it is a volumetric measurement and cannot be treated as a “point” property.

Generally, the resistance of the surrounding earth will be the largest of the three components making up the resistance of a ground connection. The several factors that can affect this value are discussed in Section II on Earth Resistivity. From Section II, you’ll see that earth resistivity depends on the soil material, the moisture content, and the temperature. It is far from a constant, predictable value ranging generally from 500 to 50,000 ohm-cm³.

Getting Down to Earth is a good read, I found this on page 12/13, here is a PDF you can download if you want to read more: http://www.biddlemegger.com/biddle-u...ToEarth-MC.pdf

Thanks for this post. It helped clear some things up for me.

[russgentile]

Thanks for this post. It helped clear some things up for me.

I could not load www.biddlemegger.com , IP not found message.

-RG

[SecondGen]

Thanks for this post. It helped clear some things up for me.

I could not load www.biddlemegger.com , IP not found message.

-RG

This question was addressed a while back in this thread: https://testguy.net/threads/6939-How...2839#post12839

I have not been able to find this in hard language (specifically IEEE 81 or FOP procedures), but most specifications call for fall-of-potential testing to only be performed after 48 hrs following any generous rainfall to allow the ground to dry.

Check out this thread for all of the Megger guides with up to date links: https://testguy.net/threads/7503-The...ide-Collection