Fall of potential testing

I

Isaiah

Senior Member
Location
Baton Rouge
Occupation
Electrical Inspector
My understanding is the fall of potential test is really intended for the grounding electrode system at transformers, generators or service entrance.
At this construction site they’re checking every ground rod - which amounts to hundreds.
Isn’t this overkill?


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If you test the complete system, the far electrode needs to be 5 to 10 times the diagonal measurement of the complete grounding electrode system. This can be thousands of feet for a large building with a ground ring and multiple ground rods.

Testing each rod by itself, requires the far electrode to be 5 to 10 times the length of the rod, away from the rod. A much more manageable distance.

Often the ground rods are required to be placed in test wells and connected with a mechanical connection so the rod can be isolated from the rest of the grounding electrode system. If they don't isolate the individual rods, you are testing the complete system, and must locate the far test electrode appropriately.

Form this this site
For it to work correctly, the current return electrode must be placed at a distance from the system under test that is at least five times the diagonal dimension of that system. Otherwise, it may not be outside of the sphere of influence of the grounding system under test, which can lead to inaccurate results. On large sites, this can literally be a mile or more away (so make sure you bring a really big spool of wire).
 
don_resqcapt19 said:
If you test the complete system, the far electrode needs to be 5 to 10 times the diagonal measurement of the complete grounding electrode system. This can be thousands of feet for a large building with a ground ring and multiple ground rods.

Testing each rod by itself, requires the far electrode to be 5 to 10 times the length of the rod, away from the rod. A much more manageable distance.

Often the ground rods are required to be placed in test wells and connected with a mechanical connection so the rod can be isolated from the rest of the grounding electrode system. If they don't isolate the individual rods, you are testing the complete system, and must locate the far test electrode appropriately.

Form this this site
For it to work correctly, the current return electrode must be placed at a distance from the system under test that is at least five times the diagonal dimension of that system. Otherwise, it may not be outside of the sphere of influence of the grounding system under test, which can lead to inaccurate results. On large sites, this can literally be a mile or more away (so make sure you bring a really big spool of wire).
Click to expand...

Thank you very much Don for this detailed info
The site has installed an equipotential plane (ground loop) #4/0 bare copper below grade which in code terms, doesn’t qualify as a ground ring electrode since it’s buried only about 18”. Periodically they’ve placed ground rods/wells but most of them are not being used for GECs in a separately derived system or service entrance- they are simply part of the equipotential plane.
These are the rods that they’re testing - most of them are getting very high readings like 150-200 ohms due to rocky soil conditions.


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So the 4/0 is really a grounding electrode jumper. You can connect anything that requires a connection to a grounding electrode to the 4/0 ring.

You can use the parallel resistance rule and add the resistances of the individual rods to get an approximate resistance of the total system.
 
don_resqcapt19 said:
So the 4/0 is really a grounding electrode jumper. You can connect anything that requires a connection to a grounding electrode to the 4/0 ring.

You can use the parallel resistance rule and add the resistances of the individual rods to get an approximate resistance of the total system.
Click to expand...

The client wants 5 ohms or less on the overall grounding grid to include grounding jumpers and associated electrodes.
The individual rod readings I’ve seen thus far indicate it will exceed that amount. The only thing I know to do is drive more rods or connect exposed rebar if available to reduce ohmic value. Do you have any additional ideas to reduce the resistance?


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Isaiah said:
Do you have any additional ideas to reduce the resistance?
Click to expand...
Some use chemical ground rods, others drill deep holes, 30' or deeper, and use sectional ground rods with ground enhancement material for the fill in the drilled hole.

Is there a real reason for needing a 5 ohm grounding electrode system, or is it just a boiler plates spec? Few installations really need a low resistance grounding electrode system.
 
don_resqcapt19 said:
Some use chemical ground rods, others drill deep holes, 30' or deeper, and use sectional ground rods with ground enhancement material for the fill in the drilled hole.

Is there a real reason for needing a 5 ohm grounding electrode system, or is it just a boiler plates spec? Few installations really need a low resistance grounding electrode system.
Click to expand...

Thanks Don
This is for a very obstinate client who knows nothing about grounding and doesn’t care to learn
They go strictly by a specification that was written probably before I was born…and that’s been quite a while LOL


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tortuga said:
Its a telco requirement going back to ma bell days I bet it's for a -48VDC system or carried over from that.
Click to expand...

Just found out
They’re using electrolyte mixture inside of tubes to reduce ohmic values
But it has nothing to do with the 48VDC system you mentioned above


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