2005 NEC Proposal for Two Rods Rejected!

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joe tedesco

Senior Member
Please read, and if you disagree with the Panel Action submit a comments to NFPA www.nfpa.org

Also, read the negative vote.

5-128 Log #1856 NEC-P05
(250-52(A)(5))

Final Action: Reject

Submitter: Andre R. Cartal, Princeton Borough Building Dept.

Recommendation:

Add new text to read as follows:

Rod and pipe electrode systems shall provide at least two electrodes spaced not less than 1.8 m (6 ft) apart. These electrodes shall not be less than 2.5 m (8 ft) in length and shall consist of the following materials.

Follow with (a) and (b).

Substantiation:

Requiring at least two electrodes will substantiate the deletion of 250.56.

Please see related substantiation in proposal to delete 250.56.

Panel Meeting Action: Reject

Panel Statement:

The code applies to all geographical areas. There are areas in which a single rod will have a resistance to ground of less than 25 ohms and should be permitted.

Number Eligible to Vote: 16

Affirmative: 15 Negative: 1 Ballot Results:

Explanation of Negative:

BOKSINER: This Proposal should be accepted and 250.56 should be deleted.

In fact, as the submitter states, 250.56 does not make much sense.

There is no technical justification to require a specific resistance values for safety purpose in a premises wiring system served by multi-grounded power distribution system.

The only plausible rationale for this rule is to provide a crude metric to determine the quality of contact between the electrode and the earth. However, it is a poor metric of contact quality since it does not take into account earth resistivity.

A better way to help assure contact quality is to require two rod electrodes under all soil conditions.

In practice, a common way to meet 250.56 is to install two rods in all cases. An accurate measurements of rod resistance is difficult and time-consuming.

It is doubtful that many such measurements are performed accurately. It is easier to meet the requirements by installing a second rod. This is the practice in many telecommunication companies and also appears to be a common practice among installers (see, for example, the article Who Cares About 25 Ohms or Less? in EC&M April 1, 2000 issue).

250.56 causes enormous confusion among the users of the Code. A common misinterpretation is that the Grounding Electrode System must have a resistance to ground of 25 Ohms or less.

Another common misinterpretation is that 25 Ohms has a special significance. The use of 25 Ohms as a target for grounding resistance has spread to other fields of electrical engineering. Thus, deletion of 250.56 and corresponding change in this proposal would eliminate confusion and enhance safety without imposing undue hardship on installers.
 

karl riley

Senior Member
Re: 2005 NEC Proposal for Two Rods Rejected!

I agree with the negative vote. Bennie, did you ever finally decide where 25 ohms came from?

Karl
 

bennie

Esteemed Member
Re: 2005 NEC Proposal for Two Rods Rejected!

Karl: Not exactly. So far I have documents indicating that the ground electrode has nothing to do with lightning strikes.

The sole purpose is to provide a means for primary ground fault sensors to function. A feed through the winding of the distribution transformer would go undetected without the earth path.

The limit of 25 Ohms is the calculated limit for insuring the current will be large enough to be sensed by switching equipment including re-closers.

The voltage drop across the 25 Ohms is not in the equation.
 

travis

Member
Re: 2005 NEC Proposal for Two Rods Rejected!

The counties of Henrico,Chesterfield,and the City of Richmond Virginia require 2 ground rods not less than 6 feet apart. THey claim they have done these 25 ohm test and the 2 rods are neccesary.Sandy soil??
 

karl riley

Senior Member
Re: 2005 NEC Proposal for Two Rods Rejected!

Bennie, sounds like you have found the origin. But are the electrodes you mention at the poles? Does this function need the residential electrode also?

And Travis, a university study of ground rod impedances of over 1,000 rods in 4 midwestern states found an average impedance of about 113 ohms. So the fact that one rod won't meet 25 ohms is very probable, so two have to be used according to NEC. Even two will not usually reach 25 ohms, but as we have been discussing of late, what does it matter? The Code only sites lightning and high voltage dispersal as the only reason for the rods (in terms of safety). They won't clear a 120V fault.

Bennie is bringing in that the 25 ohms has to do with power line fault sensing and not residential safety, even relating to lightning.
Karl
 

bennie

Esteemed Member
Re: 2005 NEC Proposal for Two Rods Rejected!

Karl: So far I have not found any engineering or scientific evidence that the user end electrode has anything to do with low side surges. A low side surge, is a surge impressed on the secondary by capacitance coupling from the primary or high side.

The user end rod appears to serve only as a facillitator of ground fault detecting sensors, and fused cut outs.

The fault detected is a winding to winding fault, or high to low side flash over. This can also be from an insulator breakdown.
 

karl riley

Senior Member
Re: 2005 NEC Proposal for Two Rods Rejected!

Bennie, what about a surge from a primary phase coming down in a storm on the secondary neutral?
Karl
 

bennie

Esteemed Member
Re: 2005 NEC Proposal for Two Rods Rejected!

Karl: You are correct, this can happen miles away, and a portion of the current will flow to the end user service ground. This is why the NESC requires 4 ground connections per mile of line.
 
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