Volt-time characteristics of rod-sphere gaps were measured in gas mixtures comprising 59% SF 6 + 1% C-C 4 F 8 + 40% CO (buffer gas) and 99% dry air + 1% SF 6 . The experimental pressures used were 400 KPA and 500 KPA (abs.) and the gap lengths included 5mm, 10mm, 15mm, 20mm and 25mm. The tests were conducted under both polarity standard impulses, 20/1000µs, 80/1200µs, and 250/1700µs switching surges. Under certain pressure and polarity conditions with time lags less than 1 µs and breakdowns occurring on the impulse wave front the breakdown strength decreased with increasing the rate of rise of voltage.