Static ground testing

[bnewport]

I have to test and document the impedance of the grounding system in a large factory. We have a clamp-on ground tester for this task. The grounds to be tested are for static. The grounds either go to ground bars throughout the plant, or to the building steel, and typically have some kind of ground clamp on the user end. Is this tester the proper way to test this type of ground, or is this strictly for testing impedance at the ground rod? Does anyone understand the theory of a clamp on tester? I was told it induces a voltage on the ground, measures current flow to earth, and calculates impedance. What is an acceptable impedance per the NEC, NFPA, etc...? I was told 2 ohms or less, but I don't know where this standard came from. I have NFPA 77, but didn't see a proper method of testing, or acceptable impedance.

Bryan

 

[dereckbc]

Re: Static ground testing

Originally posted by bnewport:
Is this tester the proper way to test this type of ground, or is this strictly for testing impedance at the ground rod? Does anyone understand the theory of a clamp on tester?

I assume you are using an AEMC loop tester. It simply injects a current into a loop then reads the voltage derived from the impedance @ the test frequency (1689 Hz). There is a certain amount of skill and/or knowledge required to use the instrument.

The main purpose is to get a general idea (verification) of ground rod impedance formed by the loop created by the utility ground rod at the transformer, the grounded service conductor between the transformer and service disconnect, the GEC, earth, and the ground rod under test. However it is only a ball park figure and always reads higher than actual value.

Example: If we clamp around any grounding electrode in a multi-grounded system (more than one rod), the measured value of the electrode under test will be the resistance of that particular rod in series with the equivalent parallel resistance value that the rest of the multi-grounded system represents. If we had 4 grounding electrodes and each a resistance value of 25 ohms and we were to clamp around any electrode in the system, the measured value would be 25 ohms in series resistance of 8.33 ohms giving a total of 33.33 ohms.

Moving on from there the only real purpose you could use the meter for is verifying continuity between points. Once you get past the service and start reading cable inside, all you are reading is hard loops and verifying continuity between points. Example: take a 50 foot piece of wire, lay it out, and join the two ends together. The meter will indicate .7 ohms every time or a shorted loop. It only verifies continuity, but does not tell the impedance because it only goes down to .7 ohms. It takes a very special high dollar DLRO to read any resistance lower than 1 ohm.

So IMO the loop tester is only good for verification of Pole Grounds, Transmission Tower Grounds, Transformer Grounds, Phone Pedestal Grounds, Service Grounds, and verifying continuity of loops.

To actually measure earth impedance of a ground electrode systems you need a three-point, or four-point "fall-of-potential method.

As for what should the ground impedance should be is undefined by NEC. It only requires a single rod, pipe, or plate that does not have a resistance of 25 ohms or less shall be augmented by one additional electrode of any of types specified by 250.52(A)(2). So if one rod produces say 500 ohms, you are only required to add another rod which should give you 250 ohms. The 2 ohms you quoted are specifications used by some industries, not any NFPA requirement.

 

[bnewport]

Re: Static ground testing

Dereck-

Thank you for the quick and knowledgeable reply! You are correct- it is AEMC. I was suspicious last week when we began testing. Some of the readings just didn't seem right, so we put the testing on hold temporarily. Since I wasn't sure how this device derived the impedance, it was difficult to tell if it was accurate or not. The manual does show taking readings only at the point of connection to the grounding electrode. Thank you again!

Bryan

 

[ron]

Re: Static ground testing

Bryan,
As Dereck mentioned, the clamp-on will offer "the measured value of the electrode under test will be the resistance of that particular rod in series with the equivalent parallel resistance value that the rest of the multi-grounded system represents" which is better than nothing. Depending on your project scope, you could at least verify that the connection at the equipment had some relevancy to the rest of the system and wasn't disconnected.

 

[dereckbc]

Re: Static ground testing

You are welcome. Here is a quick follow-up of a real application using the same example as above with four rods.

As before we clamp a rod and measure the 33.33-ohm. Then we clamp the GEC and measure 9-ohms. So now we get confused right? Now we go and get a 3 or 4-point ground tester, disconnect the GEC and we measure 6.25 ohms.

So what the heck is right? Well 6.25-ohms is the impedance of the installed ground electrode system.

So why does the clamp on tester give two different readings and neither matches the 3-point? Because when we clamped a single ground rod, it reads the single rod in series with the parallel resistance of the three other rods as previously discussed. When we clamped the GEC we read the 6.25-ohm resistance of the 4-rod ground electrode system in series with the utility MGN system (not discussed so far) of 2.75-ohms to give us 9-ohms.

This all leads me back to my statement that using the clamp-on ground tester takes some skill and knowledge to interpret the readings.

Now to really confuse you, none of the readings (6.25, 9, 33.3, or 2.75) are the actual impedance of the ground. Theoretically the impedance is 6.25-ohms of the ground electrode system in parallel with the 2.75-ohms of the utility MGN for a total of 1.9-ohms. Interesting, huh?

 

[brian john]

Re: Static ground testing

It has been my experience that most readings taken with the clamp on tester have been done improperly.

But the three point test is often done improperly. In a facility like this trying to measure an approximate earth resistance is difficult, due to the large grid (assuming everything is tied to building steel and the concrete slab) the sphere of influence of the grounding electrode is huge and trying to get outside this sphere takes LONG LEADS and a long straight run of earth (highways and roads are a problem due to lead damage and going onto other property can be tricky.

What we have found is do a four point test over a large area (if the area is available) and are able to state for that area the soil can meet the required specifications and then do a two point test between select ted items to verify low resistance between the items.

 

[don_resqcapt19]

Re: Static ground testing

The title of the original post is "static" grounding. Is the connection to earth and the grounding electrode very important in controlling static? I think that bonding all of the parts where static can accumulate is more important. I have read a number of specs for static control that specified 10 ohms or less between all of the points. The IEEE green book says that in most cases 1,000,000 ohms is enough to prevent static buildup.
Don

 

[brian john]

Re: Static ground testing

Don;

It has been my expierence that folks worry more about the reisistance of the STICK in the ground and not enough on taking care taht everything is properly bonded and grounded on the inside.

I have been to sites with major neutral ground issues, screen shake, hum bars and EMF issues. The fix was that was given by another contractor or engineer was to drive more ground rods.

 

[bnewport]

Re: Static ground testing

Don-

Your question about the connection to earth being important is a good one... I am not new to grounding, but as far as static control, I am as green as a ground screw. Grounding in general seems to be the most misunderstood subject in the electrical trade. The grounding system in this facility is already in place, and we are just supposed to test the integrity. We recommended some changes already that were obvious. When you say "bonding all of the parts where static can accumulate", do you mean something like a static dissipative mat that is bonded to the equipment (drums, solvent tanks, etc.)? I have heard of the IEEE green book, but have not read it. Does this address the subject better than NFPA 77? Thanks to everyone for the information.

Bryan

 

[don_resqcapt19]

Re: Static ground testing

Bryan,
The Green Book doesn't have a lot of information about static control. The bonding would be between all of the conductive parts of the material transfer system. If the material flow rates are low, this bonding will prevent the static build up that could become a source of ignition. I understand that with high flow rates of some liquids, that there can be static build up even with in bonded and grounded steel piping systems. I think that the air force had some problems with this when transferring jet fuel at high flow rates.
Don

 

[bnewport]

Re: Static ground testing

The static control is not only for systems that transfer liquid, but also for bonding production equipment, parts washer tanks. etc. I think you guys answered my main question concerning the tester, and the manufacturer also gave me detailed information concerning their product and its application. Like I mentioned earlier, the ground system is in place, so we dont have to design/install it, only test it.

Bryan