Siemens SIMATIC S7-400 Reference Manual page 386

Cable duct and fan subassemblies
9.5 The 24 V DC fan subassembly (6ES7408-1TA01-0XA0)
Danger of damage to equipment.
If you insert the monitoring PCB in the wrong position in the fan subassembly, the fan
subassembly may be damaged.
During maintenance of the unit, make sure you do not insert the replacement monitoring PCB
in the wrong position.
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.
386
CAUTION
S7-400 Automation System Module Data
Reference Manual, Ausgabe 11/2016, A5E00850736-08