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Fire Alarm -- Description

Elevator Floor Buttons inside the elevator car showing the Star that indicates the primary floor.
The star shows you which floor is the primary escape floor. You can choose which floor you want to go to in an elevator, except when smoke captures it to the primary or alternate floor.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

Why Does a Smoke Detector Stop the Elevator?

By Douglas Krantz

  • Elevators take people up and down
  • Elevators save the effort of climbing
  • Elevators are shortcuts
  • Elevators move past floors people want to skip
  • Elevators are dangerous closed rooms in a fire

Fire Floor

Back in the early '70s, the disaster movie "Towering Inferno" depicted a fire in a high rise office building. Trapping partygoers at the top, the fire broke out in the middle of the building.

In an effort to escape down past the fire, some partygoers tried using the elevator. It stopped on a fire floor, opened its doors, and... Well, just leaving the results up to your imagination, the movie quickly changed scenes.

Elevator Capture or Elevator Grounding

When escaping a burning building, sometimes people try to use the elevator. For their safety, though, the elevator may be stopped on the main floor with doors open; it has been captured and not usable.

This action (or lack of it) prevents the elevator from stopping on a fire floor.

When an elevator is captured (or grounded), it:
  1. Stops transporting people to wherever they were going
  2. Returns to the primary escape floor (unless the smoke is detected there, then the elevator goes to the alternate floor)
  3. Opens its doors to let the people out
  4. Just sits there

Primary Floor

That's the floor people go to escape a burning building. In the United States, a star next to the button in the elevator shows the primary fire escape floor. When smoke is detected in an elevator lobby, this floor is the first choice of elevator capture.

Usually the first floor is primary escape floor, but, depending on the building, it can be the second floor, third floor, basement, or other floor designated as the escape route from the building.

Alternate Floor

If smoke is detected on the primary floor, as a second choice, the alternate floor is where the elevator lets people out.

Sometimes people find the alternate route harder to use, but, considering what happened in the Towering Inferno, that's better than the primary choice.

Smoke in the Elevator Machine Room

Smoke in the elevator machine room captures the elevator to the primary escape floor.

There is an exception to capturing the elevator to the primary floor. The fire may soon expand out of the machine room and into the halls outside. If the machine room is located between the primary elevator lobby and the main egress out of the building, the machine room smoke will capture the elevator to the alternate floor, keeping the passengers away from the room.

Smoke in the Elevator Hoist-way

Smoke should never be in the elevator shaft. If, however, a detector senses smoke there, because it can't tell what floor the fire is on, it captures the elevator to the primary floor.

All Other Smoke Detectors

These are many elevator lobby smoke detectors not on the primary escape floor. When smoke is detected in those elevator lobbies, the elevator is sent to the primary floor, allowing the passengers to escape in the best way possible.

Elevator Capture

This article only shows the bottom line of what happens to an elevator when it is captured by smoke.

The elevator capture smoke detectors may or may not be connected to the common building fire alarm system. They may be wired to an addressable fire alarm system, conventional fire alarm system, or using relay bases on the detectors, wired directly to the elevator control cabinet.

The actual mechanism and wiring detail is different for each building, and isn't important to the discussion of the need for elevator capture. This is because when there is a fire, anyone caught inside the elevator is concerned only with what happens to them inside the elevator.
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Bottom line, where there is smoke, there possibly will be fire. When the elevator equipment detects smoke, the elevator quits letting people enter, and it lets people off in a place without smoke, where they can escape the building.

In the end, elevator capture is all about safety; when there is a fire, how the capture equipment prevents people from being caught in the enclosure of the elevator car.


Douglas Krantz

Describing How It Works

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Electrical Flow

On this website, most references to electrical flow are to the movement of electrons.

Here, electron movement is generally used because it is the electrons that are actually moving. To explain the effects of magnetic forces, the movement of electrons is best.

Conventional current flow, positive charges that appear to be moving in the circuit, will be specified when it is used. The positive electrical forces are not actually moving -- as the electrons are coming and going on an atom, the electrical forces are just loosing or gaining strength. The forces appear to be moving from one atom to the next, but the percieved movement is actually just a result of electron movement. This perceived movement is traveling at a consistent speed, usually around two-thirds the speed of light. To explain the effects of electrostatic forces, the movement of positive charges (conventional current) is best.

See the explanation on which way electricity flows at