The flashover accident of equipment in transformer substation caused by the hanging object became oneof the most important of the accident of the power system. It is the serious threat for the security and stability of thepower system. Because of the process of the flashover is difficult to be monitored. In this article the 3 D emulationalmannequin is modelled for the jumper of busbar of 220kV in the transformer substation of 500kV. From the researchof the electric field distribution among the different position

the different shape and the different material while thehanging object dropped

we got the characteristics and mechanism of breakdown of long air gap. Then

weresearched the relation of the dropping position and the shortest length of the hanging object. Through simulation andcorresponding analysis show that: affect two or more pieces to paint a fall busbar jumper near field distortion causedflashover breakdown busbar trip the main reason. In particular

the longer the paint film is

the more beneficial tothe distortion of the electric field

thus leading to the breakdown of the arc channel. Finally

through the study guideto the substation line hanging provide protection theory.