Siemens SIMATIC S7-400 Reference Manual page 42

Power supply modules
3.2 Redundant power supply modules
● A redundant battery concept (backup concept) is only provided when two backup batteries
are used in each of the power supply modules. If only one battery is used in each module,
only non-redundant backup is possible, since both batteries are being used at the same
time.
● The failure of a power supply module is registered via a plug and remove interrupt (default
STOP). If used in the second segment of the CR 2 no message is sent if the power supply
module fails.
● If two power supply modules are inserted but only one is switched on, there is a power-up
delay of up to one minute when the line voltage is switched on.
Note
The check box "Startup if preset configuration not equal to actual configuration" should be
activated in the "Properties" dialog box of the CPU.
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S7-400 Automation System Module Data
Reference Manual, Ausgabe 11/2016, A5E00850736-08