Siemens SIMATIC S7-400 Reference Manual page 383

Danger of damage to equipment.
If you mix up the power supply PCB and the monitoring PCB in the fan subassembly, the fan
subassembly may be damaged.
During maintenance of the unit, make sure you do not mix up the power supply PCB and the
monitoring PCB.
Monitoring function
In the case of a fault (defective fans) the fans are not switched off. Once you have replaced
the defective fan(s), the fault is acknowledged automatically as soon as the fans have reached
the required speed. Any faults that occur are not stored. When you switch on the fan
subassembly, the fans start running. After approximately 10 s the current status of the fans is
indicated via LEDs and relays.
S7-400 Automation System Module Data
Reference Manual, Ausgabe 11/2016, A5E00850736-08
CAUTION
9.4 The 120/230 VAC fan subassembly (6ES7408-1TB00-0XA0)
Cable duct and fan subassemblies
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