Revision of the 1997 National Electrical Safety Code

| Work on the revision of the 1997 NESC National    Electrical Safety Code (NESC) (to create the 2002 NESC) has been ongoing for the past    year. The seven NESC subcommittees have completed their meetings and review of proposed    changes. The preprint of the Code Revisions developed by the subcommittees will be    published September 1, 1999, and will include a request for input from the public    concerning the proposed changes. Of major concern to cooperatives and consultants are the    changes being considered for Sections 25 and 26, Safety Rules for Overhead    Lines - Loadings for Grades B and C and Strength Requirements, respectively.

Some of these proposed changes are summarized below:
- In Rule 250C, the subcommittee may remove the 60-foot (18 m) height limit. The        present code requires one to consider extreme winds for structures over 60 feet        (18 m). NESC Subcommittee 5 is considering removal of this exclusion, thus        requiring designers to consider extreme winds for all structures regardless of their        height. If this proposal passes, distribution lines may have to be the same strength as        transmission lines if extreme winds control the design. 
- A new combined ice and wind 50 year map may be included in section 25,        Loadings for Grades B and C. The subcommittee is proposing to allow utilities to meet        current requirements of Light, Medium, or Heavy Loading District loads or to meet the new        combined ice and wind map.
- The current 50-year extreme wind map will be replaced with the new ASCE 7, 50-year        wind map based on a 3-second gust wind speed. Initially this may appear as a rather benign        change. However, use of this wind information involves equations to calculate the wind        pressure on an overhead line which are fairly cumbersome.
- Fiber reinforced composite structures and crossarms are being added to the strengths and        loading sections.
- One proposal concerns the Grade C transverse overload factor on steel or        prestressed concrete structures. The proposal is to change the overload factor from 2.2 to        1.75.
- Subcommittee 5 is also considering a complete rewrite of sections 24, 25, and 26.        This proposal eliminates light, medium, and heavy loading district loads and replaces        these loads with construction, extreme wind, and extreme wind and ice loads.

This summary is not an attempt to detail all proposed changes as there    are many others being proposed. Individuals involved in the design of transmission and    distribution lines should closely review the preprint when it is issued on    September 1, 1999, and make their opinions known by submitting comments by    May 1, 2000, to:

Secretary    National Electrical Safety Code    IEEE Standards Department    445 Hoes Lane    P.O. Box 1331    Piscataway, NJ 08855-1331

NESC Subcommittee 5 will also be holding a session to briefly explain proposed changes    and to entertain questions at the winter meeting of the Tower, Poles, and Conductor    subcommittee of the IEEE Transmission and Distribution Committee at Las Vegas (early    February, 2000), and perhaps at other events as well.

If you would like more information or have any questions, please call    Don Heald, Structural Engineer, Transmission Branch, at (202) 720-9102.
 

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