Breakdown sequence in SF6at pressures up to 8 bar in a geometry representative of a circuit-breaker

Breakdown sequence in SF 6 of an electrode arrangement which represents the contacts of a circuit-breaker is investigated at pressures up to 8 bar under positive impulse voltages. The Schlieren frames point out the role played by the cathode as a source emitting electrons by field effect emission. Photomultiplier records demonstrated, at P d = 2 bar, that the discharge's formative time strongly decreases: from 1 µs at pressure lower than P d , down to 150 ns at pressure higher than P d . A change in the discharge regime is associated with this transition. An experimental investigation, using photomultipliers and Schlieren devices, allows us to describe both breakdown sequences. At low pressures, there is a critical volume at the anode. Nevertheless, it is a negative discharge which is responsible for the breakdown although the geometrical electrical field at the cathode boundary is about 60 kV cm -1 . At high pressures, there is no critical volume. However, a positive discharge develops across the gap. Physical considerations taking into account the electron energy are proposed to explain the mechanisms involved in both types of discharge.