250.56

[bkludecke]

I am sending in a proposal to delete all of 250.56 which is the whole 25 ohms or less or else drive the 2nd rod.

Substantiation:

"Our research has failed to turn up any substatiation for this requirement. There is no stated method for determining ground resistance or whether the testing needs to be by a 3rd party. In most soils the ground resistance changes with the seasons and moisture content of the soil. No other types of grounding electrodes are subject to this requirement (ie. concrete encased electrode or 10' of 1/2" buried metal water pipe), nor is this requirement of less than 25 ohms resistance applied to multiple electrodes which seems to imply that the requirement is unneccesary to begin with."

I'm sure this has been proposed in the past many times and I'm sure that the proposal will be DOA but I'd at least like to know what the code panel's reasoning is on this subject.

 

[chris kennedy]

"Our research has failed to turn up any substatiation for this requirement.

I really don't have an opinion on this but where are the results of your research?

 

[bkludecke]

Our research consists of asking the question on this and other forums, asking a member of code panel #5, reading an article on the subject on the MH website which seemed to be mostly speculation, and asking 2 electrical construction engineers who both also tried to speculate on the reason.

If you know why this code rule is in place, please tell me.

 

[chris kennedy]

If you know why this code rule is in place, please tell me.

No clue here. I was just pointing out the question the CMP would ask when faced with this proposal.

 

[bkludecke]

Yeah, I was kind of in a rush to get this one out. When it gets tossed out I'll take another run at it for the 2114 cycle but I will try to get the SW section IAEI to endorse it first. The brains in that outfit could put language in it that may work better than mine.

Thanks

 

[George Stolz]

Bryan Holland proposed alternately in two different cycles to require 25 ohms out of ground rods (i.e. drive more than two) and then to delete the requirement. The first was rejected along the lines of "there is no substantiation to increase the requirement" and the second was rejected along the lines of "the existing language was used successfully for many years", IIRC.

If we keep chipping away at it, they might just buckle one of these days. It's a nonsensical requirement.

 

[joebell]

I'll take another run at it for the 2114 cycle

Man thats along time from now

I do agree with you however that the 25 ohms to ground requirement is not needed. The only time I have seen this inspected is when I was in Japan working at a munitions depot and we were placing the overhead distribution system undrground. All exposed metal parts had to be connected to a grounding electrode,(we used ground rods) and we had to acheive less that 10 ohms to ground. Needless to say we drove alot of ground rods.

Joe

 

[rbalex]

Well you have until November 7 to "flesh it out." Then, if it is rejected, you have October 23, 2009 to beef up the substantiation.

I would agree with Chris and George that documenting your "research? as much as possible in the initial Proposal is essential. I would also throw in a bit of research on why "bonding" is more critical than "grounding" in most applications commonly subject to the NEC.

For years CMP5 was dominated by utilities, especially telecom utilities. For the telecoms grounding was essential from a signal degradation/interference point of view. A great deal of that thinking is also still present in Arts 725 and Chapter 8; it really isn?t a general safety issue. In cases where signal degradation/interference is genuinely a safety issue, there are other adequate safeguards already in the NEC such as 725.11.

For the power utilities, ?step & touch? were the original concerns. However, step and touch, are only real concerns when the earth IS a/the primary ground return path such as a downed overhead line. If bonding is done per Art 250 then the earth is NOT a ground return path and step and touch concerns go away. For large industrial complexes, where overhead lines may be common, the NEC actually directs you toward the NESC. See 90.2(A)(2) FPN to 2.

I would address all of the above in the Proposal so that they couldn?t just throw it out in the Comment stage as being ?new material.?

 

[bkludecke]

All good ideas, thanks. My proposal has already been sent in but maybe if a few more of us sent in the same proposal with a variety of logical reasons to substantiate the proposed deletion of 250.56 perhaps the panel members would be inspired to have another look at it.

 

[brian john]

Does anyone have any insight as to when the 25 ohm foirst appeared in the NEC?

 

[frenchelectrican]

Brian John.,, I am not sure which spot but pretty much either 1999 or 2002 NEC code did show up that era.

Anything before that I really can't be sure due some of my older NEC books were damaged.

IMO it kinda confusing with the requirement and some case I am sure you will agree with me is the soil testing to verify it if can get 25 ohm reading but a catch there is that when you do the test ya have to put down the date , weather during the test etc etc.,,

And what I heard { take this with grain of salt } some do reqired two ground rods and some only need one depending on the area it covered.

Merci,Marc

 

[iwire]

Does anyone have any insight as to when the 25 ohm foirst appeared in the NEC?

Bryan Holland has done a lot of research about this and if I remember correctly he found the 25 ohm requirement going all the way back to telegraph system requirements.

 

[George Stolz]

Yeah, I remember him narrowing it down between 1900 and 1917, but that's a very dim memory.

 

[George Stolz]

Okay, I just looked at the old NEC pdf's I have on my computer, and saw no mention in the 1915 NEC, but there is mention in the 1918 NEC:

So, apparently, the 25-ohm/2 ground rod requirement dates back to 1918.

 

[frenchelectrican]

Thanks George for the info for going back on the NEC history I learn something on that


Merci,Marc

 

[brian john]

Thank you George.

 

[tryinghard]

?No other types of grounding electrodes are subject to this requirement (ie. concrete encased electrode or 10' of 1/2" buried metal water pipe), nor is this requirement of less than 25 ohms resistance applied to multiple electrodes which seems to imply that the requirement is unneccesary to begin with."...

Bob, I?m glad to see your challenging this. These type of inconsistencies sure do cause confusion, yet it?s a simple fix to clean it up and be consistent with all the other electrodes.

 

[bkludecke]

It's going to take plenty of us to submit proposals like this if we are going to get the CMP to look seriously at removing the requirement. Just as well would be to simply require the 2nd electrode with no mention of resistance thresholds.

 

[sandsnow]

If you are an NFPA member you can ask that they research the ROP/ROC on that 1918 code section. I do not know if they go back that far. it might be helpful to know why it was put in.

 

[currin]

25 Ohm grounding

25 Ohm grounding

The 1925 Code first mentioned the 25 ohm requirement. The 1923 code first mentioned driven rods.

I have a problem with the requirement because it does not insure proper grounding. Many municipalities simply require two ground rods with no regard to actual effective grounding. Someone could drive one 20' deep rod and have 26 ohms while another could drive two 8's and have 100 ohms and guess which one meets code? It is almost like lawyers determine it rather than electricians and engineers.