Lem……..( and the doubting alexban ! )
this straight from the horses mouth……..
Boeing 737 –300/400/500 Maintenance manual Ref. 21-31-00 (page 7)
“Pressurization control is automatically transferred from automatic mode to standby if the pressure rate of change exceeds 1.0 psi per minute or cabin altitude exceeds 13,895 feet. In the event of a power failure to the automatic system, control will automatically be transferred to the standby system provided power is available to that system. Upon transfer, the cabin pressure will change to the selected standby cabin altitude at the selected standby rate of change. During standby control the cabin altitude is limited to 14,625 feet by a pressure switch in the controller. This signals the DC actuator, which modulates the outflow valve closed. The Auto Fail and STANDBY lights come on when the automatic system has been transferred to the standby system and the mode selector is in AUTO. The automatic mode of operation may be reselected after an automatic transfer to standby. If any fault remains in the automatic mode, the system will automatically transfer back to the standby mode “.
Hope this makes it clearer
