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Evolution of IEC 60826 ``Loading and Strength of Overhead Lines''
This paper provides an overview of IEC Publication No. 60826 (old number IEC 826) entitled `Loading and Strength of Overhead lines'. This publication has been a milestone in the introduction of improved structural design criteria of overhead transmission lines based on reliability methods. Originally published by IEC in 1991 as a technical report type II (i.e. a pre-standard phase), this document was then extensively reviewed by CIGRÉ and ISC/TC11/WG08, and a revised version of this document is currently being circulated to National Committees of IEC/TC11 for adoption as an IEC Standard. This IEC publication specifies loadings and strengths requirements of overhead lines derived from reliability-based design principles. It is based on the concept whereby a transmission line is designed as a system made of components such as supports, foundations, conductors and insulator strings. This approach enables to coordinate the strengths of components within the system taking into account the fact that in such a series system, the failure of any component could lead to the loss of power transmitting capability. It is expected that this approach should lead to an overall economical design without undesirable mismatch between strengths of line components. Many improvements were introduced in the revised version of IEC 60926 (IEC 2002) such as: dividing the document into a normative section containing all requirements and another section consisting of a commentary to the document and technical annexes, providing default load and strength factors when statistical data are scarce, improvements/simplifications to some loading requirements that existed in the previous version, but may not control the design, etc. The IEC 60826 has already been used in many national standards (ex. CSA C22.3, ex. IS 802, 2002, CENELEC) and utility practices, and has proven to be an essential tool for those migrating toward the more efficient and economical Reliability Based Design (RBD) concepts.