In order to solve fog flashover problem of the roof insulator on Electric Motor train Unit（EMU）in rainy and foggy winter environment

3D finite element model（FEM）of EMU pantograph support insulator and the water droplets on the insulator surfaces are established on the basis of quasi-static electric field and the finite element method. The influence of separated water droplets

clean water films and contaminated water films on the surface electric field distribution along the roof insulator is discussed. The results indicate that the water droplets have almost no influence on the whole electric field but distort the neighboring electric field of the insulator

while the maximum field intensity has 6.31 times of the original field. Considering the asymmetry of the insulator

the maximum field intensities increase with the rise of circumferential angle

and all of them exceed the air corona inception field strength. Moreover

with loss of hydrophobicity

water films induce corona discharge easily and contaminated water films even lead to fog flashover.