Ground Rod or Pipe Spacing

[bphgravity]

1. Section 250.53(A)

2. New Text

3. Add to the end of the end of the section: Where more than one rod or pipe electrode are used for the same system, the distance between electrodes shall not be less than the length of the longest rod or pipe and not exceed four times the length of the longest rod or pipe.

4. Substantiation: 1. I feel section 250.56 should be deleted completely. (proposal already provided for this) 2. The spacing requirements in this section only applies to rod, plate, and pipe electrodes installed to meet this section. There is no requirement for electrode spacing under any other purpose for the same system. 3. 250.53(B) spacing requirements only apply to electrodes of different systems. 4. One of the primary purposes of electrical system grounding is to limit the voltage imposed by lightning as indicated in 250.4(A)(1). Per the NFPA 780 section A.4.13.2.4, "No benefit is gained from the second ground rod if placed closer than the length of the longer rod. No additional benefit is gained if the second rod is placed over four times the length of the longer rod."

 

[charlie]

Re: Ground Rod or Pipe Spacing

No additional benefit is gained if the second rod is placed over four times the length of the longer rod.

Bryan, I can't agree with the idea of not permitting more than four times the length of the rod. If someone wants to install two rods spaced five times the length apart, what harm would be done?

In my opinion, " and not exceed four times the length of the longest rod or pipe" should be stricken from your proposal for it to be viable.

 

[bphgravity]

Re: Ground Rod or Pipe Spacing

Thanks Charlie. I have been trying to research were this statement came from in the NFPA 780. There must be data or a study that indicated this limitation in electrode spacing.

 

[mc5w]

Re: Ground Rod or Pipe Spacing

Having ground rods that are more than 4 times the driving depth really only affects the ability of an additional rod to act as a sink for lightning current.

The driving depth does not really affect the 3 foot radius in which most electrode resistance occurs. Driving depth does affect contact with permanent soil moisture particularly during a drought.

Your proposal would also discourage people from driving 2 rods 16 to 20 feet deep using ground rod couplings. A Colorado electrical utility reports ( rummage around over at www.eng-tips.com ) that on the newer portions of their system that have 17 foot ground rods they have ZERO lightning damage. Maybe in 10 years they will have a Big Lightning Stroke that will toast a 17 foot rod.

One of the top electrical inspectors ( John Labriola ) in northeast Ohio says that the top 6 feet of a rod that is driven from the surface does NOT act as a grounding electrode. In Army manual 5-690 a ground ring for a lightning rod system has to be buried in a 6 meter ( 10 foot ) deep trench for this reason. John Labriola became a little infamous for what was called a Labriola Loop because back when he was an electrical contractor he like to drive a ground rod beneath a basement floor BEFORE the floor was poured. He would run the GEC unbroken through the ground rod clamp and then run under the floor to the street side of the water service. This required an additional electrical inspection before the floor was poured.

 

[bphgravity]

Re: Ground Rod or Pipe Spacing

My proposal has nothing to do with the actual length of the rod itself, just the spacing of the rods. Thanks for the reply though...