Two rods

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jxofaltrds

jxofaltrds

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Mike P. Columbus Ohio
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I did a search on this site but could not remember where the diagram of the fields of two grounds was posted. Does anyone remember?

I need this diagram to show why the spacing is required.

Thanxs
Mike P.
 
Re: Two rods

i know if no diagram that shows the ground rod "layout", only that the second rod must be six feet away from the first rod. you don't need a second rod if you can determine that the resistance to ground on the first rod is 25 ohms or less.... some areas of the country you can put ten ground rods in parallel and still not have less that 25 ohms to ground (back to the return source). so what does that tell you??? ground rods are really not that important other than possibly reducing lightning damage....
 
Re: Two rods

Mike, If I remember right there is a fine print note in the NEC (somewhere around 250.20-250.70) that tells you the intent. I know this isn't as good as a diargram, but it may help having something printed from NFPA to help argue a point.
 
Re: Two rods

Mike you might try www.aemc.com they sell ground resistance testing equipment. They have a file "Understanding ground resistance testing" that goes into details about ground resistance. They talk about overlapping of effective resistance areas which is what I think you are looking for.
 
Re: Two rods

Mike, read the last paragraph on this page.

Roger
 
Re: Two rods

Bob, like some other situations, it could be that they don't consider this a direct electrical connection, so the correct way doesn't need to be adhered to. :D


Roger
 
Re: Two rods

The most effective use of driven rods requires a space between the rods equal to double the length. There is not a lot of improvement in the effectiveness after you are 6 to 8 feet apart, so I think that is the basis for the code rule.
Don

[ January 10, 2004, 01:44 PM: Message edited by: don_resqcapt19 ]
 
Re: Two rods

PS

Why I ask this was that I said that two ground rods (not being used as two ground rods) could cause a problem.

Here is what happened: Two different electrodes, an old pipe and a ground rod were 1' apart. I said that they "both" could not be used.

The response was that they are or act like one electrode. I thought they "interfered" with each other???

Mike P.
 
Re: Two rods

Mike,
are you talking about driving 2 rods at the same location, but driving them at 45degree angle away from each other?
 
Re: Two rods

Mike,
What are you trying to do? Are you just replacing the pipe with a rod, or are you needing to add a second electrode to comply with 250.56?
Don
 
Re: Two rods

mIKE:

Try the AEMC link, the spacing has to do with the effectivness of the electrodes and their spheres of influence. Test have shown that rods are more effective when seperated by at least the 2 times the length of the rod rod. But as with all electorde, this can be affected by soil conditions and temperature.

I have wondered why the NEC had the 6 foot rule and not a twice the length rule? If one rod does not achieve the necessary resistance why not drive a second rod in a location, that testing has shown is ineffective.
 
Re: Two rods

"Mike,
What are you trying to do?"

Don

Doing (did) an inspection on an exsisting service. Tenant complaint on the inside work and I saw this on the outside:

Outside was an old "pipe" used as an electrode. At sometime the service was increased and a ground rod was installed. Both are used as electrodes. Both are within 1-2' of each other.

I think that the old "pipe" should not be used.

Am I correct?

Mike P.
 
Re: Two rods

IMO:

If your area does not require a second rod, the first rod meets the 25 ohm requirement, then having a second rod next to the first should not be determental. If the pipe is the first electrode and a second rod was added to meet the two electrode requirement, then this it is a no no.
 
Re: Two rods

The effectiveness of the new rod is not reduced by being connected to an old rod next to it. In fact there is a slight improvement by having the second rod. The rule for separation is to increase the effectiveness of multiple rods. If this installation does not require a second rod there is no code or electrical problem.
Don
 
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