Abstract: The shut-off system according to the invention for large conduit cross-sections has a swing flap which can be swung to and fro between two end positions about a pivot axis by means of a drive device, wherein the drive device has a drive shaft which is connected to the swing flap via an elbow lever, and at least one lever arm rigidly connected to the drive shaft is provided which is in operative connection with at least one operating cylinder for rotation of the drive shaft. The drive device has a motion link which is fastened so as to be rotatable about a pivot bearing, wherein the operating cylinder is connected to the motion link to pivot the same, and a transmission element is provided between the lever arm and the motion link in order to transmit the pivoting motion of the motion link to the drive shaft.