Joint Plug

3 in the Care Center Basement: 1) the power distributor at the east end of the Holding Cells; 2) the south-west distributor in Construction Storage, by the forklift; 3) the distributor on the south platform of Processing (reached after using the first two in the Collection Pool).

Each plug powers an area, and light drives The Girl away. Slots: Holding Cells west end (Workshop), Furnace ('L' Passage), Elevator Passage (Fuel Storage), Central Passage (Boiler Room and Bunkroom), Collection Pool (2 at once: Ventilation Passage), Construction Storage (Inspection) and the Processing south platform (Ventilation Passage and Processing Office). With all 3 at the east end of the Holding Cells and the left lever pulled, the cells open and you rescue Emily.

Pulling a plug cuts that area's power. When you pull the Construction Storage one, The Girl drops and blocks the vent: plug it back in, she retreats, then take it again. Before plugging all 3 into the cells, take the Empty Injector from the second north cell: afterwards you can't. The Unlocking the Cells file explains the puzzle.