The information previously posted "Article ID: 10819" is wrong, at least for B3, C3 (fw: 06.61.16.0002) !!!
'Fan 1' - are the fans located on the rear of the unit near the stacking cables
'Fan 2' - are the fans located on the sides of the SecureStack unit
... and not the opposite as described in Article ID: 10819.
Tomaz Mag. Crespo.