1 members (Scott35),
184
guests, and
13
robots. |
Key:
Admin,
Global Mod,
Mod
|
|
|
Joined: Jan 2003
Posts: 4,391
Moderator
|
The NEC does not require the GES to be less than 25 ohms to ground. 250.56 Resistance of Rod, Pipe, and Plate Electrodes. A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 250.52(A)(2) through (A)(7). Where multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall not be less than 1.8 m (6 ft) apart. All this says is if you have only one electrode of rod, pipe or plate that it must either have less than 25 ohms of resistance to ground or be aided by other electrodes. Once you have two electrodes there is no resistance requirement. If you put in two rods and the resistance to ground is 50, 100, or 200 ohms the NEC is satisfied. [This message has been edited by iwire (edited 01-12-2006).]
Bob Badger Construction & Maintenance Electrician Massachusetts
|
|
|
|
Joined: Jan 2004
Posts: 1,507
Member
|
Re-read your post Bob and maybe I took your quote out of context. I should have said that if the system Grounding Electrode is a rod, pipe or plate it needs to be 25 ohms or less. If the System Electrode is Building Steel, a water pipe electrode, or anything other that a rod, plate or pipe the resistance is not stated in the NEC. I also agree- add another electrode and walk away from it. This was a test Bob Did I pass?
George Little
|
|
|
|
Joined: Jan 2003
Posts: 4,391
Moderator
|
You always pass. I should have said that if the system Grounding Electrode is a rod, pipe or plate it needs to be 25 ohms or less. I agree if it is one rod, if it is more than one electrode an infinite resistance to ground would be acceptable.
Bob Badger Construction & Maintenance Electrician Massachusetts
|
|
|
|
Joined: Mar 2001
Posts: 345
Member
|
Stacked rod? Que?
Oh by the way... A tree fell on the drop that fed the garage, ripped whole service off building, and being replaced new en toto. (With new ground) I did mean a sectional ground rod. I use the tapered rod couplers so that I only need to carry the less expensive copper weld rods that are not threaded. You just drive the first rod, place the rod coupler on top, insert the second rod into the rod coupler, and drive it to near flush, test and repeat until the desired resistance is reached. I use an electric jack hammer and a ground rod cup to do the driving. In answer to Iwire's why. The reason that I go for an actual measured resistance of twenty five ohms or less is in the hope of reducing the likelihood of surge and spike damage. I began doing that after a summer camp hired me to stop the destruction of their telephones, answering machines, fax machine, and so forth. I ordered protective devices and while waiting for them to be shipped I worked on the grounding. After getting the affected buildings grounded to less then twenty five ohms all of the equipment damage ceased and the staff declared me a hero. The camp is located on a ridge line in Virginia and gets lightning during most of the camping season. They weren't experiencing direct strokes to the buildings or wiring but they were going through telephone and electric control devices at several a week. I was not able to drive deep rods there so I installed ground rings and interconnected all of the separate building grounding electrodes using buried bare number two copper trenched to a five foot depth. I got my sectional rod experience back when I was in the Air Force. I drove ground rods for telegraphic fire alarm signalling. The grounds had to be good enough to signal over with a 100 milliampere signal current in the event of a broken wire. Some of the alarm station rods went to forty feet in order to get a good ground return signal. There we used threaded sectional rods and the base had a nifty hydraulic drill to drive them. I haven't seen anything like it since. It actually drilled the threaded rods into the earth turning them as they were hammered. I wonder if a Jacobs chuck adapter on a large hammer drill would accomplish the same thing. -- Tom Horne
Tom Horne
"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison
|
|
|
|
Joined: Jul 2004
Posts: 9,935 Likes: 34
Member
|
" interconnected all of the separate building grounding electrodes "
From my experience with lightning in the Florida sand box I will say THAT is what fixed your lightning problem. It is not as important what potential "ground" is, only that it is consistant throughout the system. In theory ground electrodes are supposed to do that but in places with poor soil conductivity that local ground can be a lot different than the building several feet away.
Greg Fretwell
|
|
|
|
Joined: May 2003
Posts: 2,876
OP
Member
|
Tom, I too have also used sectional rods in the Marines, but only in 3X3'sections for a total of nine. Whats worse than driving them, is pulling them back up! Move a generator, you get to move the same rod too.
Anyway, you would be hard pressed to get a rod 20' here, for that matter 2' will put you at bedrock on the hills. And often can not drive 8' unless at an angle.
Mark Heller "Well - I oughta....." -Jackie Gleason
|
|
|
Posts: 99
Joined: August 2003
|
|
|
|