A high pressure air gap is used as a fast ’’crowbar’’ switch with the aid of a low pressure gap decoupler. In order to trigger the switch, a special trigger circuit is designed, in which a single pulse breaks down the high and low pressure gaps successively. The jitter of breakdown time of this switch is less than 20 nsec over a wide range of operating conditions, pressures in both gaps, and working voltage. The switch is designed for 40 kV operation. Its inductance is 22 nH, and the resistance at a current of 40 kA is 4 mΩ. Seventy switches are installed in a 40 kV, 210 kJ fast bank. Up to now they have been running for about 20 000 shots with little maintenance.