Need help understand grounding test (Fall of potential)

[Dsg319]

Working at a new facility which is following the IEEE standards of 5ohms or less for the grounding system.


My problem with understanding is when testing why disconnect the rod from the rest of the grounding system? Than won’t you only be reading that one and single rod resistance to earth rather than the whole system?

I can understand the grounding system would need to be isolated away from utility power as you would get super low readings being attached to their MGN. But I can’t understand testing a single rod at a time rather than the whole grounding network resistance to earth.

What value will one rod to earth mean if after test I connected back to a system of 100 rods and Ufer grounds?

Or will the rest of the grounding network while disconnected from rod under test still lower the resistance path back to the single rod.

I just can’t wrap my head around it.

 

[don_resqcapt19]

Think of each connection to earth as a resistor in parallel with all of the other connections to earth. The sum of all of the parallel resistors connected to the earth is the resistance of the grounding electrode system.
Testing of a large grounding electrode system using the fall of potential is almost impossible as the third test probe needs to be around ten times the diagonal measurement from the grounding electrode system being tested. For example a 300 x 300 building with a ground ring or ground rods around the perimeter, would require the third probe to be around 4000' feet from the building,

 

[Dsg319]

Think of each connection to earth as a resistor in parallel with all of the other connections to earth. The sum of all of the parallel resistors connected to the earth is the resistance of the grounding electrode system.
Testing of a large grounding electrode system using the fall of potential is almost impossible as the third test probe needs to be around ten times the diagonal measurement from the grounding electrode system being tested. For example a 300 x 300 building with a ground ring or ground rods around the perimeter, would require the third probe to be around 4000' feet from the building,

Gotcha, thank you. So when doing a test will the rest of the (disconnected) grounding system impact the reading I will get on a single particular rod as in wether the rest of the system was there or not?

 

[don_resqcapt19]

Gotcha, thank you. So when doing a test will the rest of the (disconnected) grounding system impact the reading I will get on a single particular rod as in wether the rest of the system was there or not?

You have to have the third test probe outside the influence of the rest of the grounding electrode system. I have no idea how far that might be when testing a rod that is disconnected from the rest of the system. For just a single rod, you need to have the third probe around 80 to 100' from a 10' rod.

 

[ron]

I'm interested to here what your test results are. Many older systems get cut and bastardized over the years and exist just as nice for decades and have test results >25ohms

 

[Dsg319]

Yes that’s what I have always done. Probe at 100’ than 10’ increments all the way back to 10’ than have a Microsoft excel spreadsheet that you give all the data and it calculates.

I guess I just have a hard time wrapping my head around testing a single rod when I have always thought they are wanting to see their grounding system less than 5ohms.

 

[Dsg319]

I'm interested to here what your test results are. Many older systems get cut and bastardized over the years and exist just as nice for decades and have test results >25ohms

I had one test station come out around 15ohms and a different test station (different electrode) at 7ohms