Non-shielded, fire-resistant plenum cable

DETAILED DESCRIPTION The foregoing needs have been met by the cable of this invention which includes a core comprising at least one insulated conductor that may be a metallic or a lightguide fiber conductor.
The cable is protected by a non-metallic sheath system having a relatively low thermal conductivity.
Such a sheath system is effective to provide a predetermined time delay before any thermal decomposition of the conductor insulation when the cable is subjected to relatively high temperatures.
More particularly, the sheath system includes an inner layer of an inorganic cellular material which has a relatively low air permeability and which encloses the core.
At least one tape comprising a flame retardant thermosetting material is wrapped helically about the layer of cellular material.
In a preferred embodiment, the sheath comprises a layer of woven glass material and two tapes each comprising a Kapton.
RTM.
film material which are wrapped helically in opposite directions about the core.
These components of the sheath system cooperate to delay the transfer of heat energy during a fire for at least a predetermined time.
Either the cellular layer or one of the tapes may be made to have a relatively high heat absorptivity, such as, for example, by providing an outer face of the cellular layer or an inner face of the one of the tapes with a heat absorptive material.
The cable of this invention is particularly useful in providing a desired degree of flame retardance for small pair size cables which includes less than twenty-five pairs of insulated metallic conductors.
Inasmuch as the cables does not include a metallic shield, it is more flexible, thereby facilitating installation.
Another advantage of this cable relates to its manufacture.
In the preferred embodiment, each of the two outer tapes is coated on at least one of its major faces with an adhesive.
After the tapes are wrapped helically in opposite directions with overlapping adjacent turns about the layer of cellular material, the cable is subjected to heat to cause the adhesive to melt and seal the overlapping seams