Isolate facility ground to check?

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Radar Doc

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All of our facilities at work require a ground of 10 ohms or less. All facilities have the neutral tied to ground at the main power entry panels. If I'm checking a ground at one facility, is it necessary to remove power and disconnect the ground from the facility to get an accurate reading of that particular ground rod for the facility?

It seems to me if the ground is left connected to the facility, it is tied to the neutral at the power entry panel, and that neutral is tied back through the power grid to all the other facilities (which have neutral tied to ground). I would think these parallel circuits would just lower the ground reading. I ask because we have some seriously low ground readings in our records, on the order of 0-.1 ohms for most facilities, but as high as 4 ohms at others.

We can use a Vibroground to perform fall-of-potential, but have access to the AEMC clamp-on checker also. I would like the most accurate reading of the actual ground resistance at each facility. Is my thought-process correct about measuring with power/ground wire disconnected? If so, do you have a reference/procedure for such a measurement? Thanks, Mike
 
The clamp-on type ground testers do work without removing the conductor from the system.
For other methods, the ground conductor should be removed from the system. Remember, one does not just go and remove the conductor for testing. AEMC and Megger have very good information on their sites for this type of testing procedure.
 
Pierre C Belarge said:
The clamp-on type ground testers do work without removing the conductor from the system.
Click to expand...

So, as I understand, the clamp-on testers are reading the entire grid of neutral-to-ground lines from the other facilities, as well as the ground rod at the site under test, correct? So, if something was wrong with the ground rod(s) at the site under test(ie high resistance), wouldn't I still read a fairly low resistance on the meter due to the connection with the other facilities?

I forgot to mention these are 208VAC/3-phase connections (3 phase wires, a neutral and a ground connection at each power entry panel).
 
Just clamp the AEMC tester to the Ground Electrode Conductor. As long as it reads 10 or less you are good to go, because it will read the resistance of your Groune Electrode System in series with the MGN resistance. In other words the true resistance is lower than what the meter will read.

For example if your GES is 5-ohms, and the MGN is 3-ohms your meter will read 8. Otherwise the only other way is to open the GEC and use a 3-point FOP test which requires to shut off the service before you can safely disconnect the GEC...
 
One issue to be concerned with if you have down stream grounds on your neutral conductors you will either get an erroneous reading or an error message from the tester (depending on the tester manufacture and age). In my expierence (Just a SWAG) about 70%+ maybe higher of sites we test have neutrals grounded down stream from the main service.
 
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