Water rises in the tank up to level 1, the level detector triggers the pump, water goes down to level 2 and the level detector stops the pump.

The pumping station works on the same principle, but the level detector alternates the pump starts, 1st cycle = pump 1; 2nd cycle = pump 2, etc... Where one of the pump is faulty or where the input flow is in excess of the flow rate of the on-duty pump (level 3 reached), the level detector will trigger the second pump.