With the increasing scale of overhead transmission line

human awareness of the protection to the natural environment is gradually strengthened as well.It is more widespread in wild

and the threat to the safety and stability of transmission lines is even more serious

because of the bird habitant overlaps transmission corridor

because of the bird habitant overlaps transmission corridor.Bird flashover faults have been defined that the transmission lines trip accident caused by bird streamer（bird excrement）or falling objects in case of bird activities nearby transmission line or tower.For the sake of decreasing the rate of flashover faults caused by bird streamer

a Г type iron tower been used to simulate transmission line tower

used a kind of liquid to simulate bird streamer which conforms to the real bird excrement and studied on flashover probability of the 110 kV composite insulator withstand tests at different distances.According to the test results

flashover probability of bird streamer is 50% when it is 25 cm away from the center of the insulator

and flashover probability declined sharply when distance increase.Based on a large number of experimental data analysis

this paper puts forward suggestions for optimization of the structure of bird-preventing helmet

which means the radius should be more than 25 cm.