Investigations of the lightning impulse breakdown characteristics of a SF/sub 6//N/sub 2/ gas mixture, N/sub 2/ gas and CO/sub 2/ gas have been carried out. In the case of the SF/sub 6//N/sub 2/ gas mixture the 50% breakdown (electric) field strength for the positive impulse voltage was slightly lower than that for the negative impulse voltage, and the pressure of the SF/sub 6//N/sub 2/ gas mixture must be increased to approximately twice the pressure of SF/sub 6/ gas to obtain the same dielectric strength as that of SF/sub 6/ gas. For CO/sub 2/ gas, the 50% breakdown field strength for the negative impulse voltage was lower than that for the positive impulse voltage in the case of SF/sub 6/ gas. However, the dielectric strength of CO/sub 2/ gas, even at twice the pressure of SF/sub 6/ gas, was approximately 60% of the strength of SF/sub 6/ gas, so that the CO/sub 2/ gas-insulated busbar may be approximately 1.6 times as large as the SF/sub 6/ gas-insulated busbar. In the case of N/sub 2/ gas, the 50% breakdown field strength for the positive impulse voltage was lower than that for the negative impulse voltage, and the dielectric strength of N/sub 2/ gas, even at twice the pressure of SF/sub 6/ gas, was less than 50% of the strength of SF/sub 6/ gas, so that the N/sub 2/ gas-insulated busbar may be approximately 2.1 times as large as the SF/sub 6/ gas-insulated busbar. A remarkable difference between CO/sub 2/ and N/sub 2/ gas was found from the observation of discharge forms and partial discharge properties.