$SF_6$(sulphur hexafluoride) is the gas most commonly used in high voltage power equipments. Due to the fact that $SF_6$ is a greenhouse gas with a very high global warming potential, alternatives have been searched for over the last years. This paper presents the fundamental insulation characteristics of $SF_6/N_2$ mixtures(50% $SF_{6}50%N_2$, $20%SF_{6}80%N_2$) in sphere-plane and rod-plane configurations. The electrode gap spacing and the relative gas pressure were varied within the range of 10~50mm and of 3~7bar respectively. To get the 50% breakdown value under lightning impulse of both polarity and AC, the up and down method was applied. In addition, both mixtures were tested in earthing switch and disconnecting switch sections of GIS model. The test result is applied to make design criteria for mixtures.