Influence of Steel Pole and Davit Arm’s Deflection on Clearance to Structure Calculations

Tubular steel poles with davit arms are used extensively as high-voltage transmission line support structures—a double-circuit tubular steel pole with davit arms and I-string insulators is one of the quintessential tangent pole configurations. A davit arm’s length in a suspension application is often dictated by the combination of the I-string insulator’s swing and the required clearance from energized conductor or hardware to the structure under the conditions that produced the swing. To maximize design efficiency of a transmission line, the line designer should be aware of all factors that influence system performance. The pole shaft and arm deflections are factors often overlooked for clearance to structure calculations. National Electrical Safety Code (NESC) Rule 235E states that a flexible structure’s deflection shall be considered in clearance calculations to the supporting structure. Since most davit arms are not designed considering arm deflection limits, arm deflection could result in significant variability in the actual clearance values. With limited design information available during the early phases of a project, most transmission line designers must make reasonable allowances over required clearance values to accommodate the anticipated pole shaft and arm deflection’s impact. This paper will study how the additional clearance allowances established during the initial phases of design to accommodate pole shaft and arm deflection compare to the final installed condition. The study will include analysis of typical configurations of double-circuit tubular steel poles with davit arms and I-string insulators. This study will also examine some of the controls that could be established during various phases of the design process, such as using a deflected structure shape for allowable insulator swing calculations, limiting arm deflection, and communicating an arm rise value to counteract the arm deflection. These controls, among others, may limit the variability between the allowable swing angles established during the early phases of the design and the actual clearance to structure observed.