The weather protection grid, System 553C, from MLL Hamburg, is a ventilation grid made of aluminium with a free ventilation cross section of 60%. The MLL overpressure valves protect in areas where rapid pressure relief must be provided. The air current caused by the excess pressure can now escape through the overpressure valve. The system 553C is equipped with a S-slat of the heavy series of type 050C in the clamped Frame 503. The MLL overpressure valve is a pressure relief valve that opens automatically when pressure builds in the room and closes again. The valve size is calculated based on the space volume and the air to be vented.