Ground testing

[Paul W. Abernathy]

P.S.........THOSE THAT DONT LIKE IT....

The site has the same article with the following links on the SAME article...

Who Cares About 25 Ohms or Less?

By Ken Michaels, Power Quality Consultant

Apr 1, 2000 12:00 PM



Being an EDUCATOR.....I am FREE to post it.....so some little girls in the office got scared from some "has been" and removed a good post for you guys to learn from.....because they feared copyright...FYI..EC & M does not own the article....GEESHHHHHS

[Paul W. Abernathy]

Richard,

This is what BOB was refering too:

You should measure the resistance of an electrode with respect to the surrounding soil in the area. You can only do this by using the fall-of-potential method with a three-terminal, earth ground resistance tester. To properly test the resistance of a GES, you must follow some simple rules:
1. Disconnect the electrode under test from the rest of the electrical system. Considering this, it is not possible to test the grounding electrode system in nearly all circumstances.
2. Don't use a meter that injects DC current into the ground rod. Do not use standard VOMs.
3. Don't perform test measurements if the current on a GES is greater than 5A.
Contrary to popular belief, clamp-on earth ground resistance testers can be inaccurate in field applications. These testers require a low-resistance feedback loop with adequate spacing between electrode systems to provide meaningful readings. Many people often add a high resistance (caused by loose connections in the feedback loop) to the displayed value of the meter. Also, inadequate spacing between electrodes results in the meter only making a comparative bonding test, which almost always results in a low-resistance value.

[jbonelli1]

Paul, here's the link. I just put it up..

http://www.nachitools.com/mm5/mercha...ory_Code=ELECT

NACHI members create an account and then e-mail me to let me know you have. I'll code the account for the discount.

Joe Bonelli
info@nachitools.com

[Paul W. Abernathy]

Their ya go my brother...changed the link for ya......

[Richard L. Bennett]

Paul


Trust me I do understand the difference of a Point to Point DC resistance check and a AC three or 4 point check -- Been there and done that but if you have a lost of time to check out your own home DC works. (Untill some one can show me otherwise)

Yes, I understand about noise and other currents. What I like is this thread that Gerry started has caused some of us to go back to the books to understand grounding and ground testing

Thanks Gerry

rlb

[Bob Badger]

Richard here is some info about the fall of potential tests.

Testing is recommended to be by the fall-of-potential method. It should be completed by a trained, experienced technician using test equipment in current calibration and following the equipment manufacture operator’s manual. The fall-of-potential method is often referred to as the "three point method". Utilizing a ground resistance meter (digital is preferred), two auxiliary electrodes are driven into the soil at predetermined distances, per the testing specifications, in a straight line from the ground being tested. Grounding mats and grounding systems, other than rods, often require a modified test method. If you have questions for these test procedures please contact PSI directly. During a normal test of ground rod(s) the meter supplies a constant current between the ground rod(s) under test and the most remote auxiliary electrode. A series of measurements of the voltage drops between the ground rod(s) under test and the remote electrode are made by moving the intermediate electrode in steps away from the ground rod under test. The goal is to reach the actual rod’s resistance and this is most often reached at the 62% distance point.

It is not possible to get a result worth a darn standing in the house with a typical ohmmeter.

As far as going back to the books, well,... grounding is is something I pay a lot of attention to. I am not convinced a good connection of homes electrical system to earth accomplishes much other than a warm fuzzy feeling that we are now much safer. Bonding exposed metal that may become energized is much more important.

The company I work for may install a 1000 amp temp service for the start of a new project, at that time there will be no available electrodes. We drive two rods using 6 AWG (easer than proving less than 25 ohms) and we are now code compliantly 'grounded'.



Go to Mike Holt's and do a search on

Electrodes

Step Potential

Lightning

You find lots of great info if your really interested in electrical connections to earth.

[Paul W. Abernathy]

"warm fuzzy feeling"

lol...I like that Bob...lol........Yes, while some actually may not be 25 ohms or less to ground....by todays minimum standard (2) ground rods will meet the requirement and will make local AHJ's happy.

Trust me........I don't do industrial....BOB does....bonding is probably one of the most important parts of industrial wiring...

[Richard L. Bennett]

Bob

Another good subject to pass around about grounding -- what current can it carry?

If it takes a lighting hit 25 ohms will equal a LOT of voltage for a short time

One will have a warm and fuzzy feeling

Well I think we all have enjoyed Garry's QOD and have learned a little bit more than we knew when we started

Now on to some bigger and better projects like Florida HI Bill and maybe a new chapter project. Of course there is always a project or two around the house and dogs to walk

Have a good day

rlb

[Richard L. Bennett]

I have been on the phone about my statement about using CHEAP DVOM meter in place of more expensive test equipment and feel some additional information is in order

Remember this is my opinion and as of yet the "experts" have not challenged my thoughts

1. DC and low frequency AC resistance is the same on a ground system
2. RF impedance is a subject that we are not talking about
3. The reason AC is used is because test people do not understand physics
4. Some times a little "battery action" will be seen on a DC test
5. Battery action can be computed out by reversing the test leads

Once again I have enjoyed the phone calls and the extra thought that I have had to endure

Gerry you started this

NOW is the subject dead or are there some experts out there that are going to cut me a new ***** again?

If I am wrong -- show me -- I don't want to pass wrong info

rlb

[Bob Badger]

Originally Posted by rbennett

NOW is the subject dead or are there some experts out there that are going to cut me a new ***** again?

A little sensitive?

So your standard ohmmeter has three leads and compares the readings between all of them?

You must shop at different places than I do.

I am not going to go into all the BS surrounding the importance of 'earth' connections. A lot of it is simply untrue, I invited all to Holt's to find some more points of view.

However if you are going to charge for an additional service and your going to call that service "Ground Resistance Testing" or something like that IMO it would be fraud to perform the test with a standard ohmmeter.

Engineers would never accept that. (A ground resistance testing report is often required by our contracts and 95% of the time the contract requires the fall of potential test, no clamp on meters accepted.)

NEC Electrical inspectors would never accept 25 ohms on a standard ohmmeter as proof of NEC compliance.

They will expect a clamp on ground resistance tester or the fall of potential test.

But to each their own.

[jtedesco1]

Here's the rest of the story:

http://www.nachi.org/forum/showpost....58&postcount=5

[Richard L. Bennett]

Bob

No I am not sensitive. Just looking for someone to help me out with some information here. Joe’s post from were ever does not answer the questions of why one can not use a VOM.

Ground testing can be 2 point, 3 point, and 4 point. The reason that AC is used is because of possible battery (galvanic) action might influence the readings when using DC. DC still works.

The extra terminals on the ground testers are not to compare different resistance reading at the same time – One set is a current source and the other set is a volt meter. By knowing the voltage and the current in a circuit the resistance can be computed.

By passing a known current through the ground and measuring the voltage at different points, the ground system resistance can be found by Ohm’s law. In the case of a ground testing meter the resistance is computed in the meter.

Also by going to a step down type of test the resistance of the test leads does not enter into the test and does not have to be subtracted out like in a 2 point test. As far as certifying a ground system or testing to a standard that is another subject.

If the person wants a test to be done as per his MOP then that is how it must be done.

The point that I am trying to make is that a DC resistance check between two ground points about 6 feet apart is a valid ground test and is affordable.

Now here is where I have shot myself in the foot. THE GROUND TESTING METERS have come down in price to the point that one does not have to go through all the work associated with using a standard multi test meter.

If SureTest were to build a ground system tester into their test box and it was a push button – red green light test I would think it would be a good deal.

Just looking for knowledge here not a bar fight.

Bob, Joe, Paul and yes Gerry thanks for the posting and the sharing of knowledge. Too bad we are sort of out of the HI SOP

rlb

[jtedesco1]

Look at the following items: I want the first one advertised, would make life easier!

PS: See 250.56 in the NECH and Soares Book on Grounding

Ground Resistance
AEMC Ground Resistance Test >>>>>> Look!!

AEMC Accessories

Megger Ground Testers

Megger Accessories

Amprobe Ground Resistance Testers

Amprobe Accessories

[Bob Badger]

rbj.

I will not pretend to know why the right meter is needed.

I will stand behind my statement that the way you describe will produce a meaningless reading.

There is a reason that no reputable testing agency uses a ohmmeter.

There is a reason that electrical inspectors will not accept your method.

Because it is not a true ground resistance test.

Follow all the links and see if you think your results will match the accepted testing methods.

Originally Posted by rbennett

Just looking for knowledge here not a bar fight.

I am not looking for a fight either, but you seem to think you have found some secret info that everyone else is blind to.

I have posted the question at Mike Holt's as I am interested in the 'why' as well.

http://www.mikeholt.com/codeForum/vi...1f4ced#1187416

[Paul W. Abernathy]

great Bob....I am going over to see how the replies are going...