SUMMARYGas-insulated switchgear (GIS) is subjected to very fast transient overvoltages such as lightning surges or disconnector switching surges. Therefore, the sparkover voltage and time (V-t) characteristics of SF 6 in a very short time range of less than 1 µs are of great interest from the viewpoint of insulation design and coordination for a GIS. This paper describes the V-t characteristics of SF 6 at a gas pressure of 0.5 MPa using a steep-front square impulse voltage under a quasi-uniform field gap and presents a quantitative evaluation of the V-t characteristics for a nonstandard lightning impulse voltage. In the case of a square impulse, the V-t characteristics of positive polarity were observed to be almost flat over a long time range from 80 ns to 10 µs, and rose steeply over a short time range from 80 ns down to 20 ns. For negative polarity, the V-t characteristics exhibit a gentle rise from 200 ns down to 40 ns. In the estimation of V-t characteristics, the equal-area criterion parameters were quantitatively estimated using the square impulse. For a nonstandard lighting impulse, we found that application of the equal-area criterion with these parameters for the nonoscillating impulse and oscillating impulse of up to 5.3 MHz as a model of lightning surge and disconnector switching surge is possible.