83 omhs to ground??

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Roger Desi

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We have purchased the Fluke 1625 ground test meter and was testing/learning how to use it on our grounding system here at the shop.
Let me explain the best I can on where we conducted our test.
We have a computer room, and in this computer room we have a 208 volt 3 phase lighting panel, 1 computer, 1 small UPS system, 1-208v xformer (powering the UPS system) and phone service. In the corner of this room is a 10 foot ground rod. This ground rod has a #4 solid copper wire attached to a multi hole ground bar. There is a #6 green copper wire coming from the 208volt 3 phase panel ground bar and is attached to the multi hole ground bar. We have the 208volt xformer the UPS and phone service grounds attached to the multi hole ground bar as well. We have two clamps on the 1625 fluke meter, one clamps around the ground rod itself and the other clamp, clamps to the #4 soild at the multi hole ground bar with both clamps facing the proper direction (which is the stackless method)and the results of the test is 83 omhs. We know that is a very high reading, so we selected another ground wire (#8 green) about 4 inches away that is grounded to the same ground bar but goes to the UPS system our results were 0.061 ohms.Keep in mind that the #4 bare copper wire that goes to the ground rod was never disconnected. How can the two test be so different when they are using the same ground loop and the length of the wires are the same? Also the 208volt 3 phase panel is a sub panel for the computer room. I could use some help on understanding what is going on or what I might doing wrong.

Thanks,
Roger
 
An 83 ohm reading for a single ground rod is entirely reasonable. Soil is simply not a very good conductor.

As to why you got such vastly different readings, you have to read the manual to understand how 'stakeless' ground resistance testing works. The 'stakeless' mode _cannot_ measure the resistance of a single ground rod. Instead it measures the resistance of a continuous circuit. If you arrange your test probes so that this circuit has a single ground rod on one side of your probes, and _all the other grounding electrodes_ on the other side of the probes, then the measured resistance is an approximation to the resistance of that single rod.

But if you place your test probes on some continuous metallic loop, then your measurement will be the resistance of that metallic loop, not the resistance of any ground rods incidentally connected to that loop.

-Jon
 
83 ohms is an excellent reading for a single rod system, life does not get any better. Add a second rod and you should get about 40 to 50. Your instrument is a loop tester, and when you switch it to another ground wire you are reading a loop resistance of copper wire inside your facility. You have to know how to use the meter.

I sure hope the single ground rod is not isolated from the facility ground serving the building or you can expect SMOKE comming out of your phone switch, UPS, or computers at some point in the future . Sure sounds like it is from your description. :confused: If it is you need to run a bonding jumper from the single ground rod to the AC sevice ground electrode system, or at least to the nearest AC ground buss in a panel.

FWIW ground resistance to earth means nothing in your application. 5 ohms is just as good as 500.
 
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dereckbc said:
I sure hope the single ground rod is not isolated from the facility ground serving the building or you can expect SMOKE comming out of your phone switch, UPS, or computers at some point in the future . Sure sounds like it is from your description. :confused: If it is you need to run a bonding jumper from the single ground rod to the AC sevice ground electrode system, or at least to the nearest AC ground buss in a panel
Click to expand...

I was thinking the same thing
 
dereckbc said:
I sure hope the single ground rod is not isolated from the facility ground serving the building or you can expect SMOKE comming out of your phone switch, UPS, or computers at some point in the future . Sure sounds like it is from your description. :confused: If it is you need to run a bonding jumper from the single ground rod to the AC sevice ground electrode system, or at least to the nearest AC ground buss in a panel.
Click to expand...



Funny how great minds think alike...:grin:

Whenever I am on a site for inspection, the first thing I look at is the grounding/bonding scheme. Amazing how often it is not properly installed.

What precedes the smoke? :wink:
 
I read somewhere that a 8ft rod in average soil, just barely will get 25ohm, that's why Im a believer in Ufer grounds, the surface area rivals a single rod.......however, some folks are scared of a lightine strike busting a foundation.....never seen it, but I guess it could happen
 
Roger Desi said:
We have purchased the Fluke 1625 ground test meter and was testing/learning how to use it on our grounding system here at the shop.
Let me explain the best I can on where we conducted our test.
We have a computer room, and in this computer room we have a 208 volt 3 phase lighting panel, 1 computer, 1 small UPS system, 1-208v xformer (powering the UPS system) and phone service. In the corner of this room is a 10 foot ground rod. This ground rod has a #4 solid copper wire attached to a multi hole ground bar. There is a #6 green copper wire coming from the 208volt 3 phase panel ground bar and is attached to the multi hole ground bar. We have the 208volt xformer the UPS and phone service grounds attached to the multi hole ground bar as well. We have two clamps on the 1625 fluke meter, one clamps around the ground rod itself and the other clamp, clamps to the #4 soild at the multi hole ground bar with both clamps facing the proper direction (which is the stackless method)and the results of the test is 83 omhs. We know that is a very high reading, so we selected another ground wire (#8 green) about 4 inches away that is grounded to the same ground bar but goes to the UPS system our results were 0.061 ohms.Keep in mind that the #4 bare copper wire that goes to the ground rod was never disconnected. How can the two test be so different when they are using the same ground loop and the length of the wires are the same? Also the 208volt 3 phase panel is a sub panel for the computer room. I could use some help on understanding what is going on or what I might doing wrong.

Thanks,
Roger
Click to expand...

I highly recommend you get some training on the proper use and limitations of your new toy, clamp on ground testers are good for some situations but limited in thier applications, they are no substitute for a 3 or 4 point fall of potential test. AMEC and AVO both have good programs on your clamp on meter.
 
Roger Desi said:
We have two clamps on the 1625 fluke meter, one clamps around the ground rod itself and the other clamp, clamps to the #4 soild at the multi hole ground bar with both clamps facing the proper direction (which is the stackless method)and the results of the test is 83 omhs.
Click to expand...

This doesn't sound like a measurement to "ground" - it sounds like a measurement of the bonding from the #4 solid "grounded" conductor to the ground rod. 83 ohms sounds like improper bonding to me. Thoughts?
 
brian john said:
Why do you say this?
Click to expand...

Because the only connections to the 1625 that are referenced in the description are those to the ground rod and the #4 conductor that connects the ground rod to the terminal block - and that measurement is the 83 ohms. Sounds like a "resistance measurement". I've never used one, but doesn't the 1625 require connections beyond those two in order to make a ground impedance measurement? Maybe the description was incomplete or incorrect?
 
I'm sorry for the late response guys. Right after I posted this question I became really sick and havn't had the time to check back until now. Thanks for all the help. I will check into a class on this new meter. We choose the stakless method because of the location of the ground rod being in a small room in the center of the building. We couldn't use the fall of potential test, there is no way to run the ground wires to an outside source.

On a side note:
The ground rod in question is connected to the main ground loop from the service pole via a 208v 3 phase panel. I was told that this ground rod shouldn't even be there and that it really serves no purpose because the service ground is the main protection. According to the manual that came with the 1625 ground tester the stakless test using the two clamp ons will measure the full ground loop, from service pole to all grounding attached to that grid/loop resulting in an over all ohm reading for the full loop. I need more training on this.

Again thxs for the help. I will continue to investigate this more and figure what's going on.:grin::grin::cool::cool:
 
Roger I am guessing here but from the sound of it you clamped the bonding conductor of the new rod? If that is correct what you seen was the impedence of the new rod you added in series with the facility ground rod. So for example let's say the new rod you added is really 75 ohms., then your existing facility ground is 83-75 = 8 ohms, and now with the new rod is slightly less than 8 ohms overall. Make since?

Best place I find to use the clamp-ons assuming you can turn off the main so no current is flowing on the service is around the service grounded conductor, or if you cannot turn off power is around the main bonding jumper. Then what you will see is your total GES plus the utility ground rod at the transformer. So if you read 10 ohms, then you know your system is something less than 10 ohms.

Just remember the clamp-on ground testers are reading a loop resistance. So when you measure the reading is going to be the UUT (unit under test) impdence, plus the series resistance that forms the loop.
 
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Roger this may help you see what is going on. Or it may confuse you. :D

Look at the drawing and examine it a minute.

Ground.jpg


Ok you can see the utility transformer and its ground on the left, and the facility with a 3-ground rod GES. For simplicity all ground rods are 100-ohms. If you were to measure the facility GES with the Grounded Service Conductor disconnected with a 3-point test it would measure 33 ohms, and 25 ohms with the Service Gropunded Conductor connected. Agree?

Now if you were to clamp your meter anywhere in the drawing it would read 133 ohms. Can you see that? The reason is because you would see 1 rod at 100 ohms in series with 3 parallel 100-ohm rods equalling 33 ohms. So even though we know the facility GES is actually 33 ohms, the clamp-on tester will read 133.

Hope that helps.
 
dereckbc , I understand now with the drawing you provided on what is going on . One thing I dont understand is the math and how you have 33 omhs when in series you have 133 ohms? Could you show me a math example on this, or maybe this is to much to ask, seeing I am new to the forum? I just want to understand this because I am sure I will run into this again in the future, and knowing and understanding how to solve the problem is gold to me.


Again thankyou for helping me on the issue.:)
 
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