Groud Rod Testing

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DRMOELLE

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The problem we are having is about how to do testing on ground rods. We have many on our site and we currently are using an Amprobe model DGC-1000. The problem comes from some of our staff that they feel itr is not good enough to use just that. This is my questiuon is using that tester good enough and is using it the proper way to test the ground rods.

Thank you Don Moeller
 
Re: Groud Rod Testing

Don I am not familiar with the Amprobe model DGC-1000, I assume it is a clamp-on type like my AEMC. If that assumption is correct, if used properly the meter will always read higher than the actual impedance. The reason is it reads the UUT in series with all the other parallel sources.

For example lets assume we have four rods buried in the ground and connected together. Lets also assume each rod has an impedance of 100-ohms. If we clamped any one rod we would measure the one rod of 100 ohms plus the series impedance of the 3-rods in parallel and our meter would read 133-ohms.

Now the kicker is the 4-rod system I just described is really 25 ohm system because 4 rods of 100 ohms in parallel is 25 ohms. However lets take the example one step further. Lets say we have the same 4-rod system in place connected to the AC service properly and we clamp the service grounded conductor. Here are the assumptions:

1. the 4-rod system as a whole is the same and a value of 25-ohms (4-rods @ 100-ohms each).

2. The service neutral is a MGN system and has an earth impedance of say 2-ohms.

In this example the meter would read 27-ohms. Not bad, close enough for government work!

To be really accurate requires us to disconnect the service grounded conductor and use a 3-point fall of potential test. Howevwer it is still a calculated measurement because it has to be done from several locations, none of which will measure the same, and has to be averaged out.

[ October 06, 2005, 05:42 PM: Message edited by: dereckbc ]
 
Re: Groud Rod Testing

"Well, that's all I have to say about that." - Forrest, Forrest Gump
 
Re: Groud Rod Testing

After reading Dereck's post, you'll know why it's easier to sink a second ground rod.
 
Re: Groud Rod Testing

Derek correct me if I am wrong, but is would seem that if you could accept approximately an 8% error to the high side, it would seem to not be so bad. The inconvienence of disconnecting the grounding from the service to use the 3 point measuring method would seem to offset the difference for the sake of convienence for the consumer and the tester.
 
Re: Groud Rod Testing

pierre, you are absolutely correct. The clamp-on is a great meter for acceptance testing or verification. For example is you had a contract to meet lets say a 10-ohm ground impedance. You clamp the meter on the service grounded conductor and it read 9-ohms. Good enough because you know the reading is higher than the true impedance. ACCEPTED :cool:

I know of no reason why anyone doing residential or comercial work would ever need to know the actual impedance. The only reaon I know how is because I work for telephone, broadcast, data, military, and aerospace industrial contracts that specify a maximinum earth impedance.
 
Re: Groud Rod Testing

Thanks for the compliment, I appreciate it. I would really like to teach someday. I even tried getting into it a coulpe of years ago when I was unemployeed. Unfortunatley either I was not qualified (MS accreditied degree required) or the pay was too low. Maybe after I retire something will come about when money isn't a factor to me anymore.
 
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