You will fall into the hallway that was behind the locked double red doors. Keep following the vent shaft until you get to another grate. Make sure you hold the crouch button to stay crouched because as soon you stand up, a fan will take a little too much off the top. Shoot all three of the headcrabs that are hiding in the vent, then crouch jump into the shaft. Smash the grate with your crowbar and switch to your pistol. Walk up to the vent shaft nearby and turn on your flashlight ( F is the default). Hide behind this blue crate and keep pushing it forward until you reach the turret's switch. We won't worry about the other rooms for now.
Go to the very end of hallway, past the storage closet. Crouch jump through the window, and take a left. Now that the power is off, walk past the hanging wire and to the end of the hall where you'll have to destroy the window with your crowbar. Go back the way you came, exiting out into the hallway. Shoot the two headcrabs from behind the cage with your pistol, and take the ammo near the dead guard. Keep following the path and it will lead to a fan, where a dead security guard with two pistol magazines lie. Before we go to the next area, go back into the vent shaft and take the left passage. Go through the double red doors and back into the hallway. Before you leave this room, murder both of the scientists with either your crowbar or pistol (a single shot to the head is sufficient) to spare them the grisly end they would inevitably face later. This disables power within the local area, including the dangerous light. Go through the doorway labeled 'DANGER: HIGH VOLTAGE' and flip the switch inside. Kill all three headcrabs from behind this cage and then collect the ammo. Barnacles will then begin to come through the ceiling, so watch your step. If your conscience allows, you can let him get killed by the barnacle, or you can save him. Drop down into the room below to find two scientists, one of which gets dragged up by barnacle after he gets scared. This will bring you to another grate, which you must destroy. Crouch down and take a right from the vent shaft entrance. The electricity from the fallen light will destroy the vent shaft, opening a passage. If you go past it, you will meet the same fate as those poor creatures. Watch as the electricity kills the headcrabs, then walk up to the vent shaft. Second is the small vent near the live wire. If you even attempt to walk past the wire, you will get fried. You need to turn off the power before you head this way. The first is the broken ceiling tile and the live electric wire that is hanging down. There are several things to take notice of here. Take the battery and the medkit that are nearby. When the elevator reaches the ground floor, the doors will open. Go into the room with the danger sign and press the switch inside to disable the power.