Structural-equivalent mean recurrence intervals for NESC Rule 250B.

Overhead lines in the United States are currently designed based on both deterministic and probabilistic loading concepts for ice and wind. These load cases are specified in the National Electrical Safety Code (NESC) (Institute of Electrical and Electronics Engineers, Inc. (IEEE), 2023). The deterministic loads for the continental United States are based on three loading districts: light, medium, and heavy, and the corresponding loads based on the specified weather criteria are to be used with specified structural load factors. In practice, the factored deterministic loads often control the design of a structure compared to the probabilistic loads which apply a load factor of 1.0. Furthermore, the deterministic loads are not associated with a Mean Recurrence Interval (MRI), thereby making the assessment of the reliability index cumbersome for structures controlled by these loads. A method to determine an equivalent MRI for these deterministic loads is presented, which involves associating the factored deterministic loads with a probability of exceedance based on location for ice and wind loads. The method is applied to a 230 kV transmission line including seven structure types and line angles, and for 26 locations across the continental United States, and a comparison of the equivalent MRI based on the deterministic loads is made to the probabilistic loads. Trends are presented which illustrate that the resulting equivalent MRI vary significantly among location and structure types. The findings highlight that different structure types of the same transmission line are associated with a wide range of equivalent MRI. Discussion is provided on the trends observed in case study and the inconsistency in structural reliability that could result from the application of deterministic ice and wind load criteria in NESC Rule 250B. It is not the intent of this paper to state what design MRIs are needed to ensure adequate reliability. The intent of this paper is to determine what range of structural equivalent MRIs overhead lines in the United States are currently designed to withstand. • For transmission line structures designed in the United States, what sort of probabilistic weather conditions are these lines designed to resist? • The structural design for many structure types is controlled by the deterministic load case NESC Rule 250B. Since this load case is deterministic the likelihood of exceeding this load (MRI) is not available. • The proposed method provides a way to determine the equivalent mean recurrence intervals (MRIs) for the deterministic structural load case NESC Rule 250B. • The results show that the equivalent MRIs highly vary based upon location. • The results show that structures with small line angles have lower MRIs and structures with large line angles have higher equivalent MRIs. Tangent structures exhibit a median equivalent MRI of approximately 100-yrs. Deadend structures exhibit a median equivalent MRI of approximately 1000-yrs. [ABSTRACT FROM AUTHOR]