joe tedesco
Senior Member
- Location
- Boston, Massachusetts
Please read, and if you disagree with the Panel Action submit a comments to NFPA www.nfpa.org
Also, read the negative vote.
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.