Patent Application
20060169479
Electrical cables are widely used in telecommunications applications for the transmission of voice, video and data signals. Electrical cables typically include a conductive cable core surrounded by a jacket that provides mechanical strength and protection to the cable core. PVC is commonly used as a cable insulating and jacketing material since it is cheap and, with the addition of various elastomers, can be made extremely flexible, even at lower temperatures.
However, when PVC bums it produces considerable amounts of smoke and releases toxic halogen compounds, which account for many fire-related deaths. To reduce the risk of fire propagating through a building's ductwork, safety codes often require that plenum-rated cables meet industry standards for low smoke generation and low flame spread. Cables obtain the plenum rating upon successfully passing NFPA 262 (UL 910) flame propagation and smoke generation tests, which require that the materials used in conductor insulations and cable jackets be capable of withstanding a specified amount of heat for a specified amount of time without combustion or contributing significantly to the sustenance of a fire.
To successfully achieve a plenum rating, cables are constructed of materials that are more fire resistant and produce less smoke than traditional jacket materials. While there are several versions of PVC with varying characteristics, to Applicants' knowledge none are able to pass the plenum test. Some versions of PVC and polyolefins may attain plenum capability when combined with certain other polymers that are more fire resistant. However, maintaining the safety margins against the plenum flame test is sometimes difficult. Construction must be highly controlled and, in some instances, cable designs that pass the test one time may not pass on another trial.
More successful methods for increasing flame resistance include adding halogens to the jacket material. Fluoropolymers are commonly used to increase the fire resistance of the material. The most common thermoplastic polymer in plenum cables is fluorinated ethylene-1-propylene copolymer (FEP). See, for example, U.S. Pat. Nos. 5,841,072, 5,841,073, and 5,563,377, the disclosures of which are incorporated herein by reference.
Unfortunately, fluoropolymers are much more expensive to manufacture, thus the higher cost of plenum rated cables. Furthermore, fluoropolymers are tougher and more difficult to extrude, resulting in plenum cables that are not as flexible as PVC cables. Some cables include a composite of FEP and other materials. See, for example, U.S. Pat. No. 5,932,847, the disclosure of which is incorporated herein by reference. However, these composite designs often require twist length or expansion consideration to minimize signal propagation delay skew, as well as increase manufacturing complexity and product cost.
There is also a high concern about the true safety of halogen-based cables. When halogen-based cables burn (at whatever level they produce smoke), the smoke is corrosive and contains poisonous gases. Halogen-free polymer materials require complicated self-extinguishing formulations of compounds in order to obtain low smoke cable products. These materials add cost, complexity and may degrade the electrical performance of the cable.
In one of many possible embodiments, a communications cable includes a core having at least one insulated electrical conductor, and a jacket having an inner surface and a plurality of ribs projecting radially inward from the inner surface, such that ribs are separated by adjacent channels.
Another embodiment provides a method of making a cable by forming a plurality of ribs on an inner surface of a cable jacket, wherein the ribs project radially inward from the inner surface and run longitudinally along the length of the cable, and wherein the ribs are separated from neighboring ribs by adjacent channels; and enclosing a cable core within the cable jacket, the cable core having at least one insulated electrical conductor.
The accompanying drawings illustrate various embodiments of the present invention and are a part of the specification. The illustrated embodiments are merely examples of the present invention and do not limit the scope of the invention.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.
The following description includes specific details in order to provide a thorough understanding of the present cable and methods of making and using it. The skilled artisan will understand, however, that the products and methods described below can be practiced without employing these specific details. Indeed, they can be modified and can be used in conjunction with products and techniques known to those of skill in the art in light of the present disclosure.
Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearance of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.
Referring now to the Figures,
The jacket (11) preferably runs along the length of the cable and completely surrounds the cable core (12). The jacket (11) has an inner surface (21) and a plurality of ribs (20) projecting radially inward from the inner surface (21). The ribs (20) can be formed as part of the jacket (11) and thus can be made of the same material as the jacket (11). While there is generally no minimum or maximum number of ribs (20) on the jacket (11), the number used usually depends on their size and shape and the size of the cable (10).
In one embodiment the ribs (20) are rounded, elliptical or smooth-edged at their tips (22). This configuration allows the ribs (20) to more effectively maintain the core (12) position and decrease its movement. In another embodiment of the cable (10), shown in
Generally, the ribs (20) may all be the same size and shape, as shown in
Each rib (20) is separated from a neighboring rib (20a) by an adjacent channel (23) that runs longitudinally along the length of the cable (10) between the two ribs (20, 20a). The channels (23) are defined by exposed portions of the inner surface (21) of the jacket (11) and the side walls (25) of the ribs (20). The channels (23) have a width (WC) ranging up to about 40 mils. In one embodiment the width (WC) is about 28 mils. Typically the channels (23) do not contain any conductors (13) or portions of the core (12).
The ribbed jacket configuration reduces the amount of jacket material around the core (12) and insulated conductors (15), and minimizes the contact between the core (12) and the jacket (11). This results in reduced burning and production of smoke. In typical telecommunication cables the core burns and produces smoke more easily than the jacket material. The ribbed jacket configuration increases the distance between the core (12) and fires exterior to or involving the jacket (11), thereby reducing the likelihood that the core (12) will burn.
The ribbed jacket configuration also improves the electrical performance of the cable (10). Instead of surrounding the core (12) with jacket material, the core (12) is surrounded with channels (23) containing air. This reduces the dielectric surrounding the core (12) and insulated conductors (15), thus reducing the amount of attenuation experienced by the electric signal traveling in the core (12). The ribbed configuration also serves to reduce crosstalk in the cable (10). Crosstalk increases significantly when twisted pairs (14) with like pair lay lengths come in close proximity to each other. The ribs (20) hold the core (12) in position and prevent twisted pairs (14) with like pair lay lengths from moving and coming in close proximity to each other.
To further decrease cross talk, the ribs (20) may also be made of a semi-conductive filled or unfilled polymer. Useful semi-conductive filled polymers include polyethylene, polypropylene, polystyrene and the like containing conductive particles, such as carbon black, graphite fiber, barium ferrite, and metal flakes, fibers or powders. Other useful semi-conductive polymers include intrinsically conductive polymers such as polyacetylene and polyphthalocyanine doped with gallium or selenium.
The jacket (11) is also electrically insulating, even though its main purpose is to provide mechanical and environmental protection to the core (12). Thus, the cable jacket (11) can be fabricated from a wide variety of materials serving this function, including thermoset and thermoplastic polymers and polyolefins. In one embodiment, a low-smoke PVC material is used in the jacket (11). In another embodiment, such as where the cable (10) is used in a riser application or cables with twisted pair counts greater than four, the jacket (11) can be made with different PVC materials, LSPVC, PVDF, PVDF/PVC polymers, ETCFE, and other fluoropolymers. These materials can be solid or foamed.
In one embodiment, the jacket (11) is fabricated without any fluoropolymer-based materials, such as ethylene chlorotrifluoroethylene copolymer (ECTFE) and fluroinated ethylene propylene (FEP). Rather than FEP, other fire-resistant polymers, such as polypropylene and polyethylene, may be used. The types and amounts of the fire-resistant polymers that are used depend on the cable transmission requirements, safety standards, physical performance, the desired insulation properties and cost considerations.
The jacket (11) and/or ribs (20) of the cable (10) may also include elongated strength members (24). Strength members (24) can include discrete reinforcing particles, metal rods, or continuous fiber bundles of glass, nylon, graphite, oriented liquid crystalline polymers or aramid (e.g. KEVLAR). For example, in one embodiment the jacket may be extruded over one or more aramid fiber strength members (24) such that the strength members (24) extend along the longitudinal axis of the cable (10) within the ribs (20) of the jacket (11). In another embodiment, the strength members (24) may be metal rods extending radially inward from the jacket (11) within the ribs (20). The jacket (11) and ribs (20) may also comprise extruded oriented liquid crystalline polymers. Discrete reinforcing particles may also be used to add strength to the jacket (11) and ribs (20). Useful reinforcing particles include metal shavings, glass fibers, aramid fibers, graphite fibers, carbon black, clays, and nucleators such as talc or sodium benzoate.
As shown in
The communication cable (10) may also contain a ripcord (42). The ripcord (42) serves to provide access to the core (12) of the cable (10) by separating the jacket (11). For example, one can grasp an end of the ripcord (42) and pull it outward away from an outer surface of the jacket (11), thereby splitting the jacket (11) and exposing the core (12). Any configuration for the ripcord (42) that achieves this function can be employed in the cable (10), and is not limited to the embodiment depicted in the figure.
The cable described herein can be made as known in the art. Briefly, the conductor (15) is obtained and then the insulation (16) is provided on the conductor by any number of techniques, such as a polymer extrusion process. The desired pairs of conductors (13) are then twisted together, and the twisted pairs (14) are bundled together. Finally, the jacket (11) is then provided around the bundle of conductors (13) and twisted pairs (14). The jacket can be formed by extrusion, pultrusion, molding, or other techniques known to those of skill in the art.
The preceding description has been presented only to illustrate and describe embodiments of the invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching.
