Multiple Ground Rods

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Grevco

Member
Location
Louisiana
I am testing ground rods on a location with multiple portable buildings fed from a single panel. When the rods are isolated they may show a resistance of anywhere from 30 to as high as 150 ohms. When they are connected to the circuit they will test at 5 ohms or lower in most cases. My question is, can I consider the reading with the rod connected to the circuit as a good reading? Or do I have to add rods and attempt to lower the isolated reading.
 

tom baker

First Chief Moderator
Staff member
With a 3 or 4 pole earth tester the rod is always isolated from the system-note that I am not 100% certain on this, what do the instructions say?
With a ground loop clamp on tester the rod is always connected into the system (neutral).
 

Grevco

Member
Location
Louisiana
Well the instructions aren't real clear and my two resident know-it-alls are giving me different answers that are fuzzy. In my mind if the resistance is low it should mean that there is a good path even if the path is spread out beyond the 6 foot rule. When the buildings are close enough we are tying the existing rods together. Otherwise we are adding one rod and calling it good.
 

brian john

Senior Member
Location
Leesburg, VA
The rods should be connected as a complete electrode isolated from the neutral ground bond.

Then you should go out a minimum of 10 times the diagonal, for your C2 probe take readings at 10% increments and it 62% do more than a single set of readings typically we take s sets of readings for a total of 30 readings. then graph the readings.

You should see a slight rise to 62% readings then a steep increase after that.

There are a slew of web sites with more detailed complete information.
 
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Grevco

Member
Location
Louisiana
By the way my goal here is to find out if the reading I take on the grounding system as opposed to an isolated rod is legitimate. I will be and filing these measurements for record keeping purposes. You can understand that I don't want any backlash in the future because I recorded the wrong measurement, or didn't add rods when needed. Also if you guys could steer me to a written rule or something like that. Thanks
 

zog

Senior Member
Location
Charlotte, NC
By the way my goal here is to find out if the reading I take on the grounding system as opposed to an isolated rod is legitimate. I will be and filing these measurements for record keeping purposes. You can understand that I don't want any backlash in the future because I recorded the wrong measurement, or didn't add rods when needed. Also if you guys could steer me to a written rule or something like that. Thanks

What method are you using to do the tests? # point, 4 point FOP? We cant help you if you dont tell us the methods and test equipment you are trying to learn how to use.

If this is important I suggest you get a certified testing company to do the test for you, they will do the test the right way using the right (And calibrated) equipment and give you a test report, all done by a certified test tech for a couple hundred bucks. Well worth it.

You can find one in your area at www.netaworld.org
 

Grevco

Member
Location
Louisiana
We are using an EXTECH meter, model number 382152. We connect a lead to the rod, then as per the instructions we the drive the 2 provided t-stakes into the ground at 20' to 33', there are 2 stakes. We have chosen to perform all of our test with the stakes at 20 feet apart and inline. We then allow the meter to tet for three minutes as per the instructions. I have been recording both readings (isolated and connected). As far as calling someone else in to do the test, those in charge of the spending believe that the in-house guys can do a fine job, which basically means forget it. I agree that we can do a fine job it's just that nobody on our crew (3 guys) has ran into this type of situation before.
 

zog

Senior Member
Location
Charlotte, NC
We are using an EXTECH meter, model number 382152. We connect a lead to the rod, then as per the instructions we the drive the 2 provided t-stakes into the ground at 20' to 33', there are 2 stakes. We have chosen to perform all of our test with the stakes at 20 feet apart and inline. We then allow the meter to tet for three minutes as per the instructions. I have been recording both readings (isolated and connected). As far as calling someone else in to do the test, those in charge of the spending believe that the in-house guys can do a fine job, which basically means forget it. I agree that we can do a fine job it's just that nobody on our crew (3 guys) has ran into this type of situation before.

How did you come up with 33'? You need to calculate the minimum didtance to Z based on the charteristics of your ground grid as Brian mentioned.

Also you cant just take 1 reading, you need to take multiple reading (And plot them) to prove you are outside the spheres of influnce of the other rods. At a minimum you should take 3 readings, with Y at 52%, 62%, and 72% of Z, but it is better to take like 30 readings (Also as Brian mentioned, hmmm, he seems to know what he is doing :) ) to get a more accurate measurement on your plot, you are looking for the plateau region for your true reading.

Forget about the connected readings, unless you care about your neighbors grid too.

Tell thise in charge to send one of your guys to a ground testing course. AEMC and Megger both have excellent courses all across the country.
 

mivey

Senior Member
How did you come up with 33'? You need to calculate the minimum didtance to Z based on the charteristics of your ground grid as Brian mentioned.

Also you cant just take 1 reading, you need to take multiple reading (And plot them) to prove you are outside the spheres of influnce of the other rods. At a minimum you should take 3 readings, with Y at 52%, 62%, and 72% of Z, but it is better to take like 30 readings (Also as Brian mentioned, hmmm, he seems to know what he is doing :) ) to get a more accurate measurement on your plot, you are looking for the plateau region for your true reading.

Forget about the connected readings, unless you care about your neighbors grid too.

Tell thise in charge to send one of your guys to a ground testing course. AEMC and Megger both have excellent courses all across the country.
Believe it or not, those are the instructions that came with the Extech. It seems they left out a few of the finer details.

Grevco: Go to the Megger site and get the "Getting Down To Earth" publication. It will explain what Zog & Brian are talking about and you will have a better understanding of what you are actually doing. Unfortunately, the Extech manual left you hanging.
 
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