The size of a tower on an ultra high voltage（UHV） AC transmission line relates to the air gap determined by a switching overvoltage. According to the results of air gap switching impulse（SI） tests

the longer is the time to crest of a switching impulse

the higher is the discharge voltage in the air gap. Five definitions were used to calculate the front time of the switching overvoltage（SOV） impulses either measured in the field test or calculated by the EMTP software

and to convert the impulses into the double exponential switching pulsed voltages for the air gap tests at the full scale UHV AC transmission line towers and other apparatus in a substation. The results show that the SI impulse with 1 000/5 000 μs can be specified as the test impulse. According to the results of gap clearance tests which adopted 1 000/5 000 μs SI impulses

with the assumption that the middle phase air gap of the UHV single-circuit tower is 7.7 m and the altitude for the whole lines is 1 500 m

the calculated flashover rates of energizing overvoltage and single phase faults are 1.23×10-10 per year and 8.63×10-4 per year

respectively; both of the rates are less than 0.01 per year and hence meet the requirements of the national standards in China.