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3.6.4.3.(1)
ApplicationIntent
OS1
ObjectiveOS1 Fire Safety
Attribution
[F02-OS1.2]
Application
Application 1: 
Material within a concealed space used as a plenum within a floor assembly or within a roof assembly, where:
  • the materials have a flame-spread rating of not more than 25 and a smoke developed classification of not more than 50, except for:
    • tubing for pneumatic controls,
    • optical fibre cables and electrical wires and cables that exhibit a vertical char of not more than 1.5 m when tested in conformance with the Vertical Flame Test — Cables in Cabletrough in Clause 4.11.4. of CAN/CSA C22.2 No. 0.3, “Test Methods for Electrical Wires and Cables,”
    • optical fibre cables and electrical wires and cables that are located in totally enclosed noncombustible raceways, and
    • totally enclosed non-metallic raceways conforming to Article 3.1.5.20., and
  • the supports for the ceiling membrane are of noncombustible material having a melting point not below 760°C.
This applies to buildings described in Sentence 1.3.3.2.(1) of Division A.
Application 2: 
This also applies to ceiling assemblies used as plenums, as stated in Sentence 3.1.9.6.(1).
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Intent
Intent 1: 
To supersede the requirements of Sentence 3.1.5.15.(1), which would otherwise permit combustible ducts used in horizontal runs without any other restrictions, and to supersede the requirements of Article 3.6.5.1., which would otherwise require noncombustible materials, and permit combustible materials if certain conditions are met.
This is to limit the probability that:
  • combustible material having inappropriately high flame spread and smoke development properties will be used in concealed spaces used as plenums, which could lead to the development of an inappropriate amount of smoke and the spread of fire across the exposed surfaces of the material, which could lead to the spread of fire and smoke from one fire compartment to another fire compartment by means of the plenum, which could lead to harm to persons in the other fire compartment, and
  • supports for the ceiling membrane will prematurely fail when exposed to fire conditions, which could lead to the spread of fire into the floor or roof assembly, and to other parts of the building, which could lead to harm to persons in the other parts of the building.
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