For gas insulation of high voltage electrical equipment

U-t characteristic is able to fully reflect impulse breakdown characteristic of gap. Owing to take it complexly UBD

50%will be commonly used to describe the characteristic in engineering practice. In this paper

a computational method of gas impulse breakdown voltage UBD

50%has been proposed based on un-uniform factor of electric field. Considering dry-air with 0.4 MPa and N2 with 0.5 MPa as examples

a function relationship between electric field un-uniform factor ζ and a small amount of Emax

50%experimental values can be obtained by combining numerical simulation with experiment. UBD

50%can be calculated by the formula with the change of ζ. Finally the error of calculation results is analyzed. The error is less than 7% which verifies validity of the computational method.