Abstract: The sensitivity of spark test apparatus is affected by environmental factors, tungsten wire electrode and cadmium disk electrode. In this paper, the intersection / separation trajectories of tungsten wire electrode and cadmium disk electrode in a test cycle of IEC spark test device are analyzed. Two special cases of tungsten wire concave in and concave out from cadmium disk slot are distinguished. The separation angle of tungsten wire electrode in the groove of cadmium disk is analyzed. It is pointed out that the separation angle range is from 9° to 86°, and only 50% of them are close to vertical separation. The difference of separation time caused by different separation angles is 6.38 times, which affects the initial separation speed of tungsten wire and leads to the obvious difference of spark sensitivity in each open circuit of spark test apparatus.