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#46325 - 12/18/04 12:28 PM ground resistance ?
mj Offline
Member

Registered: 06/10/02
Posts: 188
Loc: meriidian, ms
does any know how to check the ground resistannce to comply with the 25 ohms or less , without the use of the special $2000. meter. I can not get the city to buy a ground test device. thanks

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#46326 - 12/18/04 01:17 PM Re: ground resistance ?
Roger Offline
Member

Registered: 05/18/02
Posts: 1779
Loc: N.C.
Mj, PaulUK posted a method he used a while back and I'll see if I can find it, if I can't, maybe he'll see this and post it again.

In the mean time look at this meter, http://www.electrical-contractor.net/The_Store/EX/382152.htm that can be purchased in the store here, and would be greatly appreciated.

Roger

[This message has been edited by Roger (edited 12-18-2004).]

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#46327 - 12/18/04 02:00 PM Re: ground resistance ?
mj Offline
Member

Registered: 06/10/02
Posts: 188
Loc: meriidian, ms
Roger, it was a ohms law way to calculate the voltage ,by using a 100 watt bulb connected in a circuit... whereas if the reading is 102 volts or more the resistance to ground is 25 ohms or less. I just can not remember how to do the connections to preform the test.

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#46328 - 12/18/04 02:47 PM Re: ground resistance ?
Roger Offline
Member

Registered: 05/18/02
Posts: 1779
Loc: N.C.
Mj, this is the one I remember, I can't find the link, but here is the text.

 Quote:

The general method that is often used here in the U.K. is to pass a current through the ground and then measure the potential using a auxiliary electrode.
Call the existing grounding system X. Drive a ground rod, Y, into the earth at some distance from any of the existing ground rods. The reistance areas must not overlap.

Connect an adjustable low-voltage source (we usually use about 25 to 40V AC) between X and Y with an ammeter in series to monitor the current.

Now drive an auxiliary electrode Z at a point roughly mid-way between X and Y, and connect a voltmeter between X and Z. The ground system reistance is then simply E/I by Ohm's Law.

To check that the rod resistance areas are not overlapping, move rod Z about 10% of the X-Y distance closer to X and repeat the test. Do the same with Z moved about 10% closer to Y. If all three readings are approx. the same, then take this as the system earth resistance. If the readings differ substantially, then you will need to move ground rod Y farther away and start over.

To obtain accurate readings, the original ground system X must be disconnected, otherwise any other interconnected paths to ground will affect the reading.
Paul posted this a couple of years ago.

Roger



[This message has been edited by Roger (edited 12-18-2004).]

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#46329 - 12/18/04 04:38 PM Re: ground resistance ?
Tom Offline
Member

Registered: 01/01/01
Posts: 1069
Loc: Shinnston, WV USA
From doing a lot of reading on the internet, there are very few places in the U.S. that would pass the 25 ohm test with one rod.

I don't think the light bulb test would be generally accepted in the U.S. Performing the test with the correct equipment would take more time then the cost of the second rod & labor to drive it.

Of all the electricians, contractors & government agencies I deal with, only one inspector from the Mine Safety & Health Administration in West Virginia actually has the correct equipment to perform the test.

As far as MJ's question is concerned, he is the AHJ. He should require the installer to prove 25 ohms or less, it is not up to MJ to do the test.

Tom
_________________________
Few things are harder to put up with than the annoyance of a good example.

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#46330 - 12/18/04 05:01 PM Re: ground resistance ?
Roger Offline
Member

Registered: 05/18/02
Posts: 1779
Loc: N.C.
Tom, for sake of conversation, if I show a rod meets the 25 ohm requirement, what does it mean to you as an inspector, and besides the NEC requirement, why do you care?

This is not trying to put you on the spot, or confront inspectors in general, but I don't know if all inspectors are up to speed on everything they are asking for.

Roger



[This message has been edited by Roger (edited 12-18-2004).]

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#46331 - 12/19/04 06:30 AM Re: ground resistance ?
Tom Offline
Member

Registered: 01/01/01
Posts: 1069
Loc: Shinnston, WV USA
Roger,

If you demonstrate that a single rod has a resistance of 25 ohms or less, that means I don't have to ask you to drive a second rod as you are already in compliance with 250.56 The only other thing it means is since you are in compliance, I'm not incurring any liability by accepting a single rod.

The thing I would like to know was how 25 ohms became the benchmark. Is this science or is it just an arbitrary number like so many others in the NEC?

Tom
_________________________
Few things are harder to put up with than the annoyance of a good example.

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#46332 - 12/19/04 07:28 AM Re: ground resistance ?
trekkie76 Offline
Member

Registered: 06/19/04
Posts: 219
Loc: baileyville, maine, usa
I didn't know the code members just made up numbers for the sake of filling pages in a book. I would think most rules exist because someone got hurt or property was lost because of the existing rule.I doubt they put 25 ohms in the NEC just because 10 was taken.

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#46333 - 12/19/04 08:56 AM Re: ground resistance ?
PCBelarge Offline
Member

Registered: 06/08/03
Posts: 657
Loc: Dobbs Ferry, NY, USA
Roger
That is a good question and one I am asked all of the time.

I do not know how the 25 ohms to ground was established, and I am not too sure what it will do for safety.

As for an answer to your question, the NEC requires it, so that is what I look for. As an inspector, I do not always agree with the NEC, but it is not up to me to change any of the requirements (even though this may be one of them ).

Pierre
_________________________
Pierre Belarge

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#46334 - 12/19/04 10:57 AM Re: ground resistance ?
Roger Offline
Member

Registered: 05/18/02
Posts: 1779
Loc: N.C.
Tom, it is actually cheaper for a contractor to drive a second rod than to spend a couple hours or more performing a fall of potential test. Another flaw to testing ground resistance is that a 25 ohm measurement today can be 200 tomorrow.

Trekkie, the reason for the 25 ohm standard is somewhat elusive, and has been the subject of lengthy discussions on a number of forums.

Think about it, 25 ohms @ 120 v equates to 4.8 amps and would not trip a standard 15 or 20 amp breaker in a residential setting.

Now for one of its two main functions, (the first being a simple ground reference for the Neutral) the same 25 ohms probably would not mean much more than 200 ohms at the voltage level of a lightning strike, it will help disperse it in any case. On the other hand, it will impose a dangerous voltage gradient for a distance around it in the case of a lost neutral.

Hello Pierre, FWIW, I agree with you and Tom as far as an inspector requiring code compliance, my question was simply for conversation.


Roger




[This message has been edited by Roger (edited 12-19-2004).]

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