Abstract: The mid span clearance has important influence on tower top geometry and reliable operation of overhead transmission lines. The existing comparison between domestic and foreign design standards mainly focus on wind load on the basis of tower structure, while research on mid span clearance between conductor and conductor or conductor and earthwire are absent. Chinese, Japanese, European and CIGRE design standards are selected to study mid span clearance calculation methods. The analysis and comparison includes different aspects such as design and check conditions, wind load, swing angle calculation and electric clearance requirements. The existing problems in standards are also discussed. An 500 kV line is taken as example to discuss different tower top geometry between phase to phase determining by mid span clearance. Influence of wind velocity, wind direction, conductor arrangement, conductor type and spans on tower top are also studied. It is proposed that, in domestic design and check of mid span clearance, normal distribution should be used for swing angel of conductors and standard deviation should be decided by line importance. Also, it is advised to perform long-term research and observation of swing characteristics of conductors in order to improve overhead lines design.

Key words: overhead transmission lines, mid span clearance, swing angle, standard deviation, electrical clearance