Apparently there is a maintenance key in the furnace room connected to wall of light (on step stool). But it's not there. I found the one on the balcony (also on step
Dishonored 2 walkthrough level 4: The Clockwork Mansion - Polygon
Dishonored 2 M04: The Clockwork Mansion part 2 - Clockwork Mansion | VG247
Dishonored 2 walkthrough level 4: The Clockwork Mansion - Polygon