Ground Rod Spacing Requirement

Status
Not open for further replies.
Z

ziggle

Member
Hi folks,

My company has required customers to put a supplementary ground rod in for the telecom equipment we manufacture. They require that this supplementary ground rod be bonded to the service entry ground rod using 6 AWG wire (so far so good). They also say that for long wire runs, additional ground rods should be driven every 20 feet between their equipment and the service ground rod and to bond these ground rods together with 6 AWG wire. I have been told that this 20 foot rule is an NEC requirement. I can't find it in NEC 2005. Can any of you point me to the source of this requirement?

Thanks.
 
They also say that for long wire runs, additional ground rods should be driven every 20 feet between their equipment and the service ground rod and to bond these ground rods together with 6 AWG wire. I have been told that this 20 foot rule is an NEC requirement.
Click to expand...

800.100(A)(4) requies that the primary protector grounding conductor shall be as short as practiable. In one- and two-family dwellings, the primary protector grounding conductor shall be as short as practicable, not to exceed 20 ft in length.

There is an exception to the length requirement for one- and two-family dwellings where it is not practibale to achieve an overall maximum primary protector grounding conductor length of 20 ft, a separate communications ground rod meeting the minimum dimensional criteria of 800.100(B)(2)(2) shall be driven, the primary protector shall be grounded to the communications ground rod in accordance with 800.100(C), and the communications ground rod shall be bonded to the power grounding electrode system in accordance with 800.100(D).

This rule doesn't require additional ground rods every 20 feet, just the one ground rod if the length of the primary protector grounding conductor exceeds 20 feet.

Hope this helps,

Chris
 
This is considered a supplementary grounding electrode; it is not required by the NEC so the NEC does not care how it is installed. See 250.54.
 
20 foot distance

20 foot distance

I wonder where this requirement comes from? I am seeing a 20 foot ground rod requirement in numerous other specifications. For example, I see it in a Sprint grounding specification as well.
 
Ahhhh, now we are getting closer

Ahhhh, now we are getting closer

Ok, I have seen this rule as well. Where does this rule come from? I am looking for a source that I show my customers.
 
ziggle said:
I wonder where this requirement comes from? I am seeing a 20 foot ground rod requirement in numerous other specifications. For example, I see it in a Sprint grounding specification as well.
Click to expand...

Specifications don't always come from a code requirment. Specification can come from an engineer, manufacture ect...

Chris
 
I think I have it now

I think I have it now

You gave the me the clue that I needed. You are right -- the requirement is based on good engineering practice. Basically it looks like you need to space the electrodes far enough apart to ensure that they do not interact. Interaction in this case means that you do not realize lower ground resistance. Here is a quote from MIL-HDBK-419 Section 2.6.2 and 2.6.2.1.

Most of the resistance of a single electrode (ground rod) is obtained within a reasonable distance from the electrode. For a vertical rod, better than 90 percent is realized within two rod lengths. If two or more electrodes are closely spaced, however, the total effective resistance of neither is realized. This interaction prevents the resistance of N electrodes connected in parallel from being 1/N times the resistance of one of the electrodes. For this reason, the crowding of multiple vertical rods is not as beneficial in terms of dollar cost per ohm as is achievable with fewer rods properly spaced. If the electrodes in a multiple electrode installation are separated by adequate distances, the interactive influence is minimized. The separation between driven vertical ground rods in a group of rods should not be less than the length or greater than twice the length of an individual rod. Consider two rods driven into the earth with their tops flush with the surface. The two rods are electrically parallel, but the presence of one rod affects the resistance of the other.

If this statement is correct, the spacing is desireable to ensure that you get the best ground contact for a given number of ground rods. Does this seem reasonable?
 
Yes it is the universally accepted practice developed from many trials and experiments. So just for general rule rods should be spaced a minimum of 2x the buried depth. However the NEC has no such requirement, it is purely a design issue.

Now a personal question to you

I have been a Telecom Power Protection Engineer for about 30 years and have dealt with every Telecom manufacture on the planet. Designed 1000?s of grounding electrode systems, etc. They all require about a 10-ohm GES impedance or less.

Now for the question. WHY?

After 30 years I have only heard one good explanation for it (it was what I expected), and 10,000 dumb looks and lies. What is your story? :cool:
 
Importance of Ground Resistance on Surge Protection

Importance of Ground Resistance on Surge Protection

My company manufactures Fiber-to-the-Home equipment. I have been looking at our lightning test results and our field failures. I have seen very little correlation between ground resistance, lab failures, and field failures. Near as I can tell, 10 ohms is an obtainable value that does not have a direct relationship to lightning protection.

To be honest with you, I even question whether the measurements we get of ground resistance are relevant because of inductance. When I apply fast surge voltages to our systems, the inductive impedance tends to dominate over the ohmic resistance. Again, I do not see the importance of ground resistance for fast transient protection.

Our products have ethernet, voice, RF video, and a low voltage DC power ports. While I have never seen an RF video failure (the coaxial core usually shorts over to the braid before it gets into the hardware), I see an equal number of failures on all the other ports. All the ports have lightning protection.

What was the one good explanation you heard?
 
ziggle said:
What was the one good explanation you heard?
Click to expand...
What I always suspected like you said impedance is not important. The only logical reason and what just one very old Bell Engineer told me a long time ago was Not to Leave It To Chance, or Left To A Electricians Discretion. In other words spec it out.

Now I do agree with your assessments about lightning, which is the usual canned answer by most factory reps and is BS. Lightning protection and the Ground Electrode Systems are two separate things IMO, they just have some common infrastructure, the GES.

In my business I rely heavily on a ground ring because it easily facilitates as the GES and the anchor for lightning protection. The reason is simple it makes it very easy to protect services as they enter and leave a building with multiple entry/exit points.

Since I have Verizon FOIS in my home I suspect I know what you make and have one more question for you if you make the fiber modems for the customer premise. Why would any supplemental ground be needed for such an application? They are AC powered and only have glass connected to them from the outside world.

Seems to me it would be much more cost efficient to spend a few more dollars add a few good SPD on the AC line from L-N, N-G, across the Coax and RJ-45’s inside the unit.. Am I missing something? The EGC should be more than adequate.
 
Last edited:
Why the Supplemental Ground?

Why the Supplemental Ground?

Ahhh .... Therein lies a tale ...

Not all ONTs ("fiber modems" as you called them) have supplemental ground rods. Indoor ONTs ground to an outlet safety ground and outdoor ONTs within 20 feet of the service ground rod tie to the service ground rod.

For outdoor ONTs more than 20 feet from the service entry, I hired a large consulting firm (better not to name them) to develop a grounding strategy for ONTs. Their recommendation was to have a supplemental ground rod for ONTs mounted outdoors. Their reasons were:

  • 50% of the customers use armored cable and this must be grounded for safety reasons. We then ground the ONT to that local ground rod as well.
  • We have encountered many very poorly grounded homes. There were concerns about poor home grounds and people having contact with phone and ethernet and video coming from outdoors that could have large voltages induced on them. We have surge suppression devices on all ports that tie to our local ground rod.
  • We have encountered many telcom outside plant personnel who demand a local ground rod. They feel it is necessary to ensure adequate safety. One particularly large telco said they wanted a ground rod at the ONT and a tie back to the home's service entry ground with additional ground rods every 20 feet in between.

I have always been uneasy with these recommendations, but I went with them because I felt they were conservative. I am now rethinking the whole thing.
 
ziggle said:
Ahhh .... Therein lies a tale ...

Not all ONTs ("fiber modems" as you called them) have supplemental ground rods. Indoor ONTs ground to an outlet safety ground and outdoor ONTs within 20 feet of the service ground rod tie to the service ground rod./QUOTE]

Ok, I will buy that. Was stuck in my little box having only my own system to look at and it is indoors using a 120 VAC outlet, so an outside unit slipped right by me. FWIW, now that I have fiber I will never go back to copper services. I get mean data soeed, phones and HDTV on demand for less than all the other providers cab do.

Just a thought and passing on what one of us big telcos do in our facilities is dead end the metal sheath outside at the last vault, and then run glass only inside? Wouldn't that alleviate the issue and be cost effective. In other words run the metal sheath to the nearest splice/distribution box, then go to the customer with a fiberglass strength member.

Using a separate rod isolated from the customer AC service ground electrode is a no-no and can cause a lot of problems that the Telco can be held liable for. I see this a lot with Telco, CATV, and SATV.

Really appreciate chatting with you so don't take my questions or cynicism personally. I hope to learn from manufactures and have worked with them for many years. Even right a lot of grounding practices for a large Telco, so I have a vested interest. Our path might of crossed at some point like a conference with Tom Croita or Bill Bush.
Click to expand...
 
Last edited:
I would love to meet with some outside plant experts

I would love to meet with some outside plant experts

If there are some shows where you and others interested in this subject frequent, I would love to meet with you and discuss this subject. Send me a list of shows you attend (they probably are different than the Fiber to the Home shows I go to). I will put at least one of your shows on my list.

Thanks for an excellent conversation.
 
Status
Not open for further replies.
Top