# Field Quality Control



## Tigger427 (Oct 19, 2012)

More and more projects we are bidding are asking for ground resistance that exceed certain values. These are supposed to be measured on the power and lighting equipment or systems. I have been doing the ground rod testing but do not see anything said about how to exactly take this reading. Has anyone done this testing or is it strictly hired out to third party electrical contractor?


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## HARRY304E (Sep 15, 2010)

Tigger427 said:


> More and more projects we are bidding are asking for ground resistance that exceed certain values. These are supposed to be measured on the power and lighting equipment or systems. I have been doing the ground rod testing but do not see anything said about how to exactly take this reading. Has anyone done this testing or is it strictly hired out to third party electrical contractor?


What value are they looking for?

Welcome to ET..:thumbup:


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## Zog (Apr 15, 2009)

Tigger427 said:


> More and more projects we are bidding are asking for ground resistance that exceed certain values. These are supposed to be measured on the power and lighting equipment or systems. I have been doing the ground rod testing but do not see anything said about how to exactly take this reading. Has anyone done this testing or is it strictly hired out to third party electrical contractor?


3 point fall of potential, you take readings at 52%, 62%, and 72% of your Z probe distance and plot, if you do not see a flat region on your graph you take more readings at further distances. Either invest in the proper test equipment and training or hire someone to do it for you. 

Suggested reading "Getting down to earth" by Biddle.


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## Zog (Apr 15, 2009)

It is important that the ground rod under test is disconnected from the rest of the grounding system prior to conducting this test, otherwise you will be measuring parallel resistances of all rods connected to the system (Other areas of the plant, the facility next door, etc.) 


The test is conducted by driving two ground rods into the earth as shown below in 3.1. The potential difference between rods X and Y is measured by a voltmeter, and the current flow between rods X and Z is measured by an ammeter


The goal in precisely measuring the resistance to ground is to place the auxiliary current electrode (Z) far enough from the ground electrode under test so that the auxiliary potential electrode (Y) will be outside of the effective resistance areas of both the ground electrode and the auxiliary current electrode. The best way to find out if the auxiliary potential rod (Y) is outside the effective resistance areas is to move it between X and Z and to take a reading at each location.


If the auxiliary potential rod Y is in an effective resistance area (photo 3.2) the readings taken will vary noticeably in value. Under these conditions, no exact value for the resistance to ground may be determined.


On the other hand, if the auxiliary potential rod Y is located outside of the effective resistance areas (photo 3.3), as Y is moved back and forth the reading variation is minimal. The readings taken should be relatively close to each other, and are the best values for the resistance to ground of the ground X. The readings should be plotted to ensure that they lie in a “plateau” region, often referred to as the “62% area.”​Attached Thumbnails


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## Tigger427 (Oct 19, 2012)

*Ground resistance.*

I understand and have conducted the three and four point test. My question concerns the ground resistance test for power and lighting equipment or systems with capacity of 500kVa and less. I am seeing this more and more on spec books and am wondering how best to do this test. It is also for the same at 500 to 1000kVa and more than 1000kVa, but my range is mostly the first. Thanks for the help.


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## brian john (Mar 11, 2007)

Are you talking about a ground test for the Equipment Grounding Conductor at the point of use?
Or are you asking about a test that test the resistance of the power conductors to ground as in what is commonly referred to as a megger test?


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## Tigger427 (Oct 19, 2012)

I believe that it is for the grounding at the panels. I do not see a way to test from the panel to the ground rod because of distance and walls in the building.


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## brian john (Mar 11, 2007)

Two point ground test or utilize a ductor/DLRO. A meaningful test would not be taken from the ground electrode but from the main system bond, neutral to ground.


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## Tigger427 (Oct 19, 2012)

Thanks was not sure if that was the proper way or not.


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## brian john (Mar 11, 2007)

Tigger427 said:


> Thanks was not sure if that was the proper way or not.


In a commercial facility it is ALMOST IMPOSSIBLE for the EGC ground resistance to be high.

If they use metallic conduit, all the other metallic components of a typical commercial building, copper water pipes, sprinkler systems, rebar, metal ducts, metal studs and then throw in a copper EGC. 

I really feel this test is being requested by someone that lacks knowledge in the installation and testing of grounding systems.


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## Tigger427 (Oct 19, 2012)

That is apparently a standard item they are putting on spec sheets now days, every job we look at now request this test. Most of the Architects do not know what it is for but every once in a while a General will want all the i's dotted and t's crossed and we have to show how we tested it. Thanks for the helpful reply.


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