Puffer-type gas-blast circuit-breaker

In a puffer-type gas-blast circuit-breaker including an interrupter comprising;

• a gas-blast arc-extinguishing mechanism having a fixed cylinder (6) with a top (16) and bottom (14) joined by a cylindrical end wall, fixed main contact fingers (2) and arcing contact fingers (1), movable main contact fingers (3) and arcing contact fingers (4), a mechanism of said gas-blast arc-extinguishing assembly for moving said movable main contact fingers (3) and arcing contact fingers (4) relative to said fixed main contact fingers (2) and arcing contact fingers (1), the improvement wherein the gas-blast assembly includes two concentric pistons (7, 8) mounted within the cylinder (6) for axial movement therein between said top and bottom and including a first piston (7) driven by the mechanism and serving to move the moving contacts (3, 4);
• and a second free piston (8), said pistons (7, 8) and said cylinder (6) delimiting a volume of blast-gas in a current loop used by the opening current which passes via the arcing contacts (2, 4), said opening current passing therethrough in the same radial direction as it passes through the bottom (14) of the cylinder;
• resilient return means (23) placed inside the cylinder (6) between the bottom (14) and the second piston (8) to urge said second piston (8) against the top (16) of the cylinder in such a manner as to maximize the volume of the blast-gas;
• and coupling means (30) for locking the second piston (8) to the top (16) when it reaches the top (16) of the cylinder under the action of the resilient return means 923) and means responsive to movement of said first piston (7) when it leaves the vicinity of the top (16) of the cylinder for releasing said coupling means (3) at opening of the interrupter.