Why Have Wire that Lasts 2 Hours in a Fire?
A large building, or especially a tall building can't be evacuated in just five minutes. If some people can't leave a building for nearly an hour, the wire, and its insulation has to last at least that long, even when the fire is all around the wire.
Fire can burn through insulation on life saving communication wires, unless the wire has been tested and rated to withstand a fire for 2 hours, or the wire has been included in an assembly that has been tested and rated to withstand a fire for 2 hours.
By Douglas Krantz
To get out of a tall building, when there's a fire, people use the stairs.
Stairs, though, are tall, narrow, people conduits. They may be the escape route, but they're also a choke point; at any one time to get out of the building, not many people can use the stairs. Once the stairs are full, even if there's a fire right behind them, some people trying to escape the fire have to wait their turn --- just to get into the stairwell.
Staged or Partial Evacuation
To keep the stairways clear for those in immediate danger, the people who aren't in danger are either told to stay in place or not told anything at all... at least while those who are in immediate danger are still trying to escape.
This is staged or partial evacuation. It's is done by making the announcement about the fire only to the fire zone and adjacent zones. In a tall building, the zones are the Floor of Incident, Floor Above, and Floor Below.
At issue here, though, is protection of the wires carrying the evacuation message to those on the upper floors. Often they weren't told right away about the fire, but when it's their turn to evacuate, the wires passing through the fire floors still have to be able to carry the message.
Two Hour Rating of Fire Alarm Wiring
NFPA 72 talks about the fire alarm notification wiring used for partial evacuation of a building in case of fire. Even when a fire is attacking the wire, so the wire will still be usable at the end of 2 hours, NFPA 72 says that this wiring will have a 2 hour fire protection rating.
In essence, while there's still a chance that people are in the building needing to hear the evacuation announcement, the wiring carrying the announcement, even through a raging fire, will be intact.
In other words, the wire has to be protected for 2 hours.
Upper Floor Announcements
For instance, fire alarm signals in a tall building pass from one evacuation zone, through a second evacuation zone, to a third evacuation zone. (Example - the signal goes from floor 5, through floor 6, to floor 7 and above.) Remember, the idea is to eventually have everyone get out of the burning building.
Wire Riser Two Hour Rating
Because of the 2 hour rating, even though a raging fire, when it's time to evacuate the upper floors, those people on the higher floors will still get the evacuation message.
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