Short-Circuit Forces, Load Factors, and Ultimate Strength: Clearing Up Confusion

There is ambiguity and confusion within the power industry related to codes and standards that define structural design criteria for electromotive forces due to a short-circuit condition. The lack of cohesive guidance throughout the industry for reliable and efficient design of substation structures subject to short-circuit loads typically results in conservative structural design. The intent of this paper is to explore the effects of short-circuit events on rigid bus support structures in substations, to recommend modification factors to the short-circuit force used in ASCE 113 (2008), suggest new strength reduction factors for insulators, and propose new load factors for short-circuit loads in ASCE 113 (2008). The final recommended design short-circuit force for use with ASCE 113 (2008) should be defined as a product of a Structural Response Factor, an Electrical Reliability and Resiliency Factor, and the IEEE 605 (2008) short-circuit force. Another recommendation is the adjustment of the safety and strength reduction factors for porcelain insulators. The strength reduction factor for ultimate strength design should be increased from 0.5 to 0.75 and the safety factor used for the design of insulators according to allowable strength design should be reduced from 2.5 to 2.0. Finally, the load factors for the short-circuit load effect used in allowable strength design should be changed to 0.67 and 0.45, for short-circuit-only and combined load combinations, respectively.