This International Standard specifies the loading and strength requirements of overhead lines derived from reliability based design principles. These requirements apply to lines 45 kV and above, but can also be applied to lines with a lower nominal voltage. This standard also provides a framework for the preparation of national standards dealing with overhead transmission lines, using reliability concepts and employing probabilistic or semi probabilistic methods. These national standards will need to establish the local climatic data for the use and application of this standard, in addition to other data that are country specific. Although the design criteria in this standard apply to new lines, many concepts can be used to address the design and reliability requirements for refurbishment, upgrading and uprating of existing lines. This standard does not cover the detailed design of line components such as supports, foundations, conductors or insulators strings.