The outer main engineering isolation door aboard the USS Enterprise-D.
An isolation door is a special reinforced door designed to separate a hazardous area from the rest of a starship or space station. Isolation doors are often employed in Main Engineering, specifically to seal off the warp core and dangerous antimatter reactants in the event of a warp core breach or loss of antimatter containment.
The isolation door on the Enterprise during the Reliant's attack
In 2285, an isolation door automatically activated in Engineering aboard the original USS Enterprise following an attack by the USS Reliant. (Star Trek II: The Wrath of Khan)
The Enterprise-A isolation door
While the USS Enterprise-A was under attack by General Chang's Klingon Bird-of-Prey, the isolation door in Engineering rolled down. (Star Trek VI: The Undiscovered Country)
Main engineering aboard the Galaxy-class USS Enterprise-D was equipped with at least two isolation doors. The outer isolation door served to separate the warp core and engineer's office from the rest of Engineering, the inner door only separated the warp core from the rest of engineering, in an emergency, the engineer's office would still be accessible.
The inner main engineering isolation door aboard the USS Enterprise-D.
In 2366, the outer door was brought down after a Borg attack caused a hull breach in engineering. (TNG: "The Best of Both Worlds")
The inner door was brought down after a faulty dilithium chamber hatch ruptured causing an explosion in the dilithium chamber in late 2367. (TNG: "The Drumhead").
The outer door was used again, sometime between 2366 and 2368, during an accident in Engineering which claimed the life of Ensign Keller. (TNG: "Violations")
The outer door also came down when the Enterprise hit a quantum filament in early 2368. It was opened by Commander Riker and Data whose head had been detached for that purpose. (TNG: "Disaster")
In mid-2369, Captain Picard sabotaged the warp core in order to escape a group of terrorists who used the evacuation of the Enterprise for routine maintenance as an opportunity to steal highly valuable, and highly dangerous, trilithium resin from the core. Picard triggered a coolant leak which caused the outer door to close automatically. (TNG: "Starship Mine")
Isolation doors were also used in the nacelle tube control rooms aboard Galaxy class starships. The isolation doors could be opened with the protection of a one-way force field. The doors, however, could only remain open for 90 seconds in order prevent force field degradation and the release of drive plasma. (TNG: "Eye of the Beholder")
- In an alternate timeline created when the USS Enterprise-C disappeared from the Battle of Narendra III, the isolation door was used in a vain attempt to contain a coolant leak during a Klingon assault. (TNG: "Yesterday's Enterprise")
In 2371, the door was used in a last-ditch effort to contain the warp core breach which eventually destroyed the stardrive section. (Star Trek Generations)
The isolation door on the Defiant-class USS Defiant was used to seal the engine room when the auto-destruct sequence is activated in 2371. (DS9: "The Adversary")
The isolation door on the Sovereign-class USS Enterprise-E was seen when two Son'a battle cruisers fired a subspace weapon at the ship. (Star Trek: Insurrection)
Some starships, like the Intrepid-class lack isolation doors as their Engineering is extremely open and are separate from the rest of the ship with no open corridors leading to the area. (VOY: "Day of Honor", "Waking Moments")
Apocrypha
In Star Trek: Elite Force II, the isolation door can be seen closing as the player character approaches the engineering section during the Idryll attack.