Cabin air inlet Dump solenoid Ground test valve Differential control system Isobaric control system Balance diaphragm Baffle plate Filter True static atmosphere connection Cover Base Outflow valve Head Solenoid air valve Actuator diaphragm Cabin air orifice Pilot Reference chamber Atmospheric vent ports 16-32 Figure 16-56.  Two pressurization safety valves are shown on a  Boeing 747. Figure 16-55.  This outflow valve on a transport category aircraft  is normally operated by an ac motor controlled by a pressure  controller in the electronics equipment bay. A second ac motor  on the valve is use when in standby mode. A dc motor also on the  valve is used for manual operation.  Figure 16-54.  An all-pneumatic cabin pressure regulator and outflow valve. flow from the cabin to prevent internal pressure from exceeding  design limitations.  Figure 16-56  shows cabin air pressure  safety valves on a large transport category aircraft. On most  aircraft, safety valves are set to open between 8 and 10 psid. Pressurization safety valves are used to prevent the over  pressurization of the aircraft cabin. They open at a preset  differential pressure and allow air to flow out of the cabin.  Wide-body transport category aircraft cabins may have more  than one cabin pressurization safety valve.  Some outflow valves incorporate the safety valve function  into their design. This is common on some corporate jets  when two outflow valves are used. One outflow valve  operates as the primary and the other as a secondary. Both  contain a pilot valve that opens when the pressure differential  increases to a preset value. This, in turn, opens the outflow  valve(s) to prevent further pressurization. The outflow valves  shown in  Figure 16-52  operate in this manner.  Cabin altitude limiters are also used. These close the  outflow valves when the pressure in the cabin drops well  below the normal cabin altitude range, preventing a further 