QUESTION:
After Power Out/On, my S7-400-CPU only starts up, when the power failure does not exceed a certain duration. The diagnosis buffer displays an event ID 16#4555 that states that a hot restart is not possible since the monitoring time limit has been exceeded.

ANSWER:  
Check the hardware configuration to see if hot restart is configured as startup option after power on and if a monitoring time for hot restart has been defined.

Monitoring time means that if the duration of the state Power off exceeds the time entered in this field the CPU remains in stop mode and displays above message.

If you do not want to monitor the time, choose the setting "0". Now the duration of the state power off will not be monitored.

Figure 1: Settings window for configuration of CPU properties.

Note:
The numerical value entered is multiplied with factor 100 ms, which for example results in a monitoring time of 1,2 seconds when entering a value of 12 (12*100 ms = 1,2 s).