Limitations of the ruling span method for overhead line conductors at high operating temperatures

This report summarises the work by the Task Force to review the accuracy of the ruling span method for conductors operated at high temperatures. The basics of the ruling span approximation method have been examined. The traditional ruling span approach can be used with little or no error for a typical overhead line crossing a rolling terrain to predict sags in suspension spans for conductor operating temperatures in the range of 50°C to 70°C. Sensitivity studies were performed using conductors “Lapwing” and “Tern” in order to quantify such ruling span assumptions as the effect of the longitudinal swing of suspension and line post insulators on conductor sags at high temperatures, and the effect of the suspension insulator string length on the equalization of conductor tensions in adjacent spans. Significant errors in estimating the sag at conductor temperature above 100°C may occur if the tension differences are not taken into consideration in line sections consisting of a series of spans of nonequal lengths. It was confirmed that the ruling span method is the most practical way to string conductors in multi-span line sections