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, just to get into the stairwell, some people trying to escape the fire have to wait their turn.
Staged or Partial Evacuation
In staged or partial evacuation buildings, to keep the stairways clear for those in immediate danger, the people who aren't in immediate danger are either are told to stay and shelter in place, or they're not told anything at all. at least while the stairs are still full of those who are in immediate danger.
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. Usually, in a tall building, the zones are the Floor of Incident (where the fire is located), the Floor Above, and the Floor Below.
When people are supposed to stay in place during a fire, 2 hour wire is needed to carry signals through the fire to the people above. Remember, when it's time for the people above the fire to evacuate, the wires passing through the fire still have to be able to carry the message.
Two Hour Rating of Fire Alarm Wiring
The NFPA 72 talks about the fire alarm notification wiring used for partial evacuation of a building. Even when a fire is attacking the wire, so the wire will still carry the evacuation notice, the NFPA 72 says that this wiring will be rated to last 2 hours.
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 remain intact.
In other words, the wire itself 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.
The thinking behind the 2 hour rating is, if there is a fire on floor 6, evacuation notices will still be able to pass through floor 6; because of the 2 hour rating, the fire would not be able to destroy the signal carrying wire.
Wire Riser Two Hour Rating
With that 2 hour rating, when it's time to evacuate the upper floors, even though a raging fire, those people on the higher floors will still get the evacuation message.