A use of a fluid bypass line in a water extinguishing system for a pump test run of a pump, wherein the fluid bypass line has a reduced cross-section in comparison with a test line branching off from a supply line and comprising an opening element, which is configured to be movable between a locking position, in which the opening element closes the test line, and an unlocking position, in which the opening element opens the test line, and wherein the fluid bypass line is configured to conduct a predefined portion of the extinguishing fluid around the opening element into a fluid reservoir of the water extinguishing system.