Siemens S5-135U/155U System Manual page 128

Central Controllers and Expansion Units Power Supply Units
Setting the Backup
As of Version 6 of PSU 6ES5 955-3NA12, you can set jumper RR-LL to
Battery Monitor
choose whether the signaling relay (monitor output) should switch in the
event of battery failure as well as fan failure:
If the backup voltage fails or a fan is at standstill, or in both cases, the
signaling relay will drop out (monitor output). The "Batt. Low" LED will
also light up.
Note
The signaling relay in the power supply unit will drop out in the event of a
fan fault or backup battery fault. The user's circuitry for the signaling relay
must therefore be suitable for both types of fault.
If the signaling relay drops out because of a backup battery fault and the
programmable controller is therefore switched off, the program in the main
memory may be lost. Loss of program can be avoided if an external backup
voltage (3.4 V) is present at the sockets on the front plate of the power
supply unit during shutdown of the programmable controller.
4-62
Jumper RR-LL open (state when delivered): relay will signal only a fan
failure
Jumper RR-LL closed: relay will signal a fan and battery failure.
Relay contacts 2-1 closed: battery backup voltage in order and fan
running
Relay contacts 2-3 closed: backup voltage under 2.7 V or fan failure
System Manual
C79000-G8576-C199-06