Explanation: Is this for a nuclear power station? I am familiar with this use of "tranche" for nuclear (not that I'm saying it is not said of other power plants).
If it is, then "unit" is the translation of "tranche". "Armoire" could also be "switchboard" (because the switchboard is housed inside the cabinet (or cubicle)). Is it really on wheels? Have never encountered such a thing!
You probably need to know the functionof this "armoire". The AFNOR nukular dictionary gives:
armoire - cabinet
armoire (appareillage de protection et de coupure) - switchboard
armoire (contrôle-commande) - board, panel
armoire (en applique) - box (wall-mounted)
armoire (sans portes) - chassis, frame, rack
Bourth France Specializes in field Native speaker of: English PRO pts in category: 295
The translation workplace 
peer agreement (net): +1
