Guidelines for RUS Approval to Use Steel Distribution Poles

| In the August 1997 issue of the Items of Engineering    Interest, RUS summarized information that borrowers need to include with their requests    for RUS approval to use steel distribution poles. These guidelines have been slightly    revised and are provided below.

In order to assist the RUS staff in considering requests to use steel    distribution poles, borrowers should adequately address in writing items 1 through 8    below. Currently, approval to use steel distribution poles is only being granted on a    case-by-case trial basis to gain experience.

Information to include in requests to use steel distribution poles:
- Because approval is only being considered for site specific projects, define the project        and where the steel poles will be installed.
- State the maximum number of steel poles to be used.
- Furnish reasons for using steel poles. If favorable cost is the main reason, then        include an engineering economic analysis of the cost of using steel poles compared with        standard RUS construction with wood poles. This analysis should include the additional        cost (if any) for equipment and material needed to compare both structure types as        equivalent in mechanical strength, raptor protection, and quality of service such as        blinking lights due to lightning flashovers.
- Provide a description of the proposed steel poles and the method(s) of corrosion        protection to be utilized when manufactured, when placed in service and for future        maintenance.
- RUS regulations require that all assembly units must be built according to RUS        construction standards. If nonstandard assembly top construction is being proposed, then        furnish sufficient dimensioned drawings and other technical information for RUS’        evaluation of the design.
- Except for various miscellaneous material items, RUS regulations require that borrowers        use materials that RUS has accepted or technically accepted. A compilation of accepted        materials may be found in Informational Publication 202-1, "List of Materials        Acceptable for Use on Systems of RUS Electrification Borrowers." Contact the Chair,        Technical Standards Committee "A" (Electric) for information on technically        accepted items. If the proposed design uses materials that do not fall into the any of the        acceptance categories above, then furnish RUS sufficient information, data and test        results of all such materials for evaluation and approval determinations.
- Indicate that the steel poles to be used have been selected based on engineering        calculations considering the expected duty and conditions to which they will be exposed.        (See also "Wood Equivalent Steel and Concrete Poles" in this issue.)
- Provide a statement regarding the anticipated impact that the pole-top assembly design        of the steel pole may have on the possible electrocution of raptors. Also state the        mitigation measures that will be incorporated in the design to minimize such        possibilities.

The following design information should also be considered but does not    have to be submitted to RUS:

RUS strongly advocates a minimum lightning impulse withstand strength        (often incorrectly and simply referred to as a BIL level) of 300 kV for distribution        pole top assemblies. A minimum of 300 kV withstand needs to be maintained at        dead-end assemblies. Withstand strengths of less than 300 kV will usually        facilitate flashovers of lightning strikes to or proximate to distribution lines. A        recloser operation, which will cause lights to flicker, is usually required to clear the        resulting arc. Thus, a minimum of 300 kV withstand is required to maintain a        reasonable quality of service. Standard RUS pole type assemblies, with wood poles, have a        minimum withstand strength of 350 to 400 kV. If the steel pole design has a withstand        strength of less than 300 kV, borrowers should consider what additional measures,        such as the installation of surge arresters, might be used to minimize flashovers, or,        what impact a design with a withstand strength of less than 300 kV might have on the        quality of service.

A steel pole may be used as a grounding conductor if the pole meets the        sufficient conductivity and low impedance requirements of the NESC and RUS specifications.        However, a directly embedded steel pole is not recognized in the NESC as a grounding        electrode. Thus, the NESC and RUS requires that separate driven ground rods or grounding        electrodes be used for all equipment, surge arresters and other required system grounds,        including grounding the poles themselves, if needed to provide adequate grounding. (See        NESC Rule 215.C.1.)

Borrowers should use stainless steel or galvanized steel ground rods        and non-copper ground wire in the soil on steel pole lines to mitigate the corrosive        effects of buried dissimilar metals in close proximity.

The design of unguyed angle and dead-end steel pole structures should        consider pole deflection and the possible need for greater embedment depths.

For NESC Grade C construction, there should not be a direct    substitution of wood poles with steel poles of the same designation. Engineers should    calculate the design load and select the standard class steel pole based on these    calculations. Extreme ice conditions and appropriate high winds should be considered in    the design loads. (See also "Wood Equivalent Steel and Concrete Poles" in this    issue.) For steel poles on distribution lines, RUS advocates the use of NESC Grade B    overload factors for angle and deadend (guyed or unguyed) structures.

The information requested above is needed to insure a safe, reliable,    and economical distribution line when using steel poles. Considerations such as these plus    many other design considerations are incorporated into all RUS standard designs for    overhead distribution lines. RUS is developing a guide specification for steel    distribution poles which, when finalized, borrowers could use to purchase and use steel    distribution poles without further RUS approval.

If you would like further information or have any questions, please    call Harvey Bowles, Chief, Distribution Branch, at (202) 720-5082, or Bob Lash,    Chief, Transmission, at (202) 720-0486.
 

[Abbreviations] [Returnto Table of Contents] |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|