Fire resistant coaxial cable

Abstract

A fire-resistant coaxial cable is described in which the dielectric between the central conductor and outer coaxial conductor can ceramify under high heat. The dielectric is composed of a ceramifiable silicone rubber, such as that having a polysiloxane matrix with inorganic flux and refractory particles. An outer wrap of ceramic fiber yarn surrounds the outer conductor and continues to insulate it from the outside if a low smoke zero halogen jacket burns away. Embodiments include those with durable corrugated outer conductors or flexible braided outer conductors. Methods of testing and installation are described.

Claims

1. A fire resistant coaxial cable apparatus comprising: a center conductor; a tubular ceramifiable silicone rubber dielectric surrounding the center conductor and having a radial thickness greater than 4.2 millimeters, the ceramifiable silicone rubber dielectric comprising inorganic flux particles and refractory particles in a poly siloxane matrix, the ceramifiable silicone rubber dielectric configured to convert from a resilient elastomer to a porous ceramic when heated above 425° C.; an outer conductor surrounding the dielectric; and a ceramifiable silicone rubber inner jacket surrounding the outer conductor.

2. The cable apparatus of claim 1 wherein the center conductor has a diameter of 4.6 millimeters (0.18 inches).

3. The cable apparatus of claim 1 wherein the ceramifiable silicone rubber dielectric has a radial thickness greater than 7.2 millimeters.

4. The cable apparatus of claim 1 further comprising: a silicone glass tape between the dielectric and the outer conductor.

5. The cable apparatus of claim 1 further comprising: a low smoke zero halogen outer jacket surrounding the inner jacket.

6. The cable apparatus of claim 1 wherein the outer conductor comprises: a metal foil; and a braided metal in direct contact with and surrounding the outer conductor.

7. The cable apparatus of claim 6 wherein the metal foil comprises a copper-metalized tape.

8. The cable apparatus of claim 6 wherein the braided metal comprises tin-coated copper.

9. The cable apparatus of claim 1 wherein the outer conductor comprises a corrugated metal.

10. The cable apparatus of claim 9 wherein the corrugated metal has a wall thickness of 0.53 millimeters (0.021 inches) and corrugations of the corrugated metal have a layer thickness of 1.8 millimeters (0.070 inches).

11. The cable apparatus of claim 1 where in the center conductor comprises a single solid wire or multiple strands of wire.

12. A fire resistant coaxial cable apparatus comprising: a center conductor; a tubular ceramifiable silicone rubber dielectric surrounding the center conductor and having a radial thickness greater than 4.2 millimeters, the ceramifiable silicone rubber dielectric comprising inorganic flux particles and refractory particles in a poly siloxane matrix, the ceramifiable silicone rubber dielectric configured to convert from a resilient elastomer to a porous ceramic when heated above 425° C.; an outer conductor surrounding the dielectric; and a ceramic fiber wrap inner jacket surrounding the outer conductor.

13. The cable apparatus of claim 12 wherein the ceramic fiber wrap inner jacket comprises fiber material selected from the group consisting of refractory aluminoborosilicate, aluminosilica, and alumina.

14. The cable apparatus of claim 13 wherein the ceramic fiber wrap inner jacket comprises fibers having individual fiber diameters of between 7 and 13 microns (μm).

15. The cable apparatus of claim 12 wherein the outer conductor comprises: a metal foil; and a braided metal in direct contact with and surrounding the outer conductor.

16. The cable apparatus of claim 12 wherein the outer conductor comprises a corrugated metal.

17. The cable apparatus of claim 12 where in the center conductor comprises a single solid wire or multiple strands of wire.

18. A fire resistant coaxial cable apparatus comprising: a center conductor; a ceramic fiber wrap dielectric surrounding the center conductor; an outer conductor surrounding the dielectric, the ceramic fiber wrap dielectric configured to maintain a predetermined spacing between the center conductor and the outer conductor, the predetermined spacing fixed at a constant between 3.4 millimeters and 7.6 millimeters; and a ceramifiable silicone rubber inner jacket or a ceramic fiber wrap inner jacket surrounding the outer conductor.

19. The cable apparatus of claim 18 wherein the outer conductor comprises: a metal foil; and a braided metal in direct contact with and surrounding the outer conductor.

20. The cable apparatus of claim 18 wherein the outer conductor comprises a corrugated metal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cut-away perspective view of a coaxial cable in accordance with an embodiment.

(2) FIG. 2A is a cut-away side view of a braided coaxial cable in accordance with an embodiment.

(3) FIG. 2B is a cross section of the braided coaxial cable of FIG. 2A.

(4) FIG. 3A is a cut-away side view of a corrugated coaxial cable in accordance with an embodiment.

(5) FIG. 3B is a cross section of the corrugated coaxial cable of FIG. 3A.

(6) FIG. 4 is an illustration of installed cables in a building distributed antenna system in accordance with an embodiment.

(7) FIG. 5 illustrates of a central processing rack in accordance with an embodiment.

(8) FIG. 6 illustrates coax cables connecting distributed antennas to an antenna tap in accordance with an embodiment.

DETAILED DESCRIPTION

(9) Fire resistant coaxial cable is described. Some embodiments of the cable can survive two hours in fire conditions of 1010° C. (1850° F.), maintaining or increasing dielectric spacing and avoiding shorting to allowing radio frequency (RF) signals to pass. This coaxial cable may be suitable for meeting building codes for a distributed antenna system (DAS) without the need for fire-protective soffits, conduits, or other expensive shielding.

(10) Flexible braided cables and durable corrugated cables, among other cable types, are described. Braided cables as described can be suitable for replacing 50Ω LMR®-600 flexible communication cable manufactured by Times Microwave Systems, Inc. of Wallingford, Conn., United States.

(11) A “ceramifiable” material includes a material that turns from a flexible material into a ceramic when exposed to high temperatures, such as over 425° C., 482° C., 1010° C., or as otherwise known in the art. The material can be a composition of component materials that have different melting ranges. The lowest-melting temperature component materials may melt at 350° C. Between 425° C. and 482° C., other component materials of the material my devitrify, passing from a glass-like state into a crystalline state. Additives can bond refractory fillers together, forming a porous ceramic material.

(12) An example ceramifiable polymer may be the peroxidically crosslinking or condensation-crosslinking polymer described in U.S. Pat. No. 6,387,518.

(13) A “ceramifiable silicone rubber” includes silicone polymer (polysiloxane) with additives that cause the material to turn into a fire-resistant ceramic in high temperature fire conditions, or as otherwise known in the art. This may include peroxide crosslinking or condensation-crosslinking high consistency silicone rubber. A silicone polymer matrix can include low-melting point inorganic flux particles and refractory filler particles in a polysiloxane matrix. Example products include, but are not limited to: Ceramifiable Silicone Rubber Compound RCS-821 manufactured by Shenzhen Anpin Silicone Material Col, Ltd. of Guangdong, China; ELASTOSIL® R 502/75 compound manufactured by Wacker-Chemie GmbH of Munich, Germany; and XIAMETER® RBC-7160-70 compound manufactured by Dow Corning Corporation of Midland, Mich., United States of America.

(14) A “ceramic fiber wrap” includes a textile that includes microscopic ceramic fibers and fillers that maintain structural integrity at high temperatures. Example products include NEXTEL® ceramic fibers and textiles manufactured by 3M Corporation of Saint Paul, Minn., United States of America. 3M NEXTEL® textiles include aluminoborosilicate, aluminosilica, and alumina (aluminum oxide Al.sub.2O.sub.3) fibers with diameters ranging from 7 microns to 13 microns. Per the World Health Organization (WHO), fiber diameters above 3 microns (with length greater than 5 microns with a length-to-diameter ration greater than 3:1) are not considered respirable.

(15) A “refractory” material includes non-metallic material having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above 1,000° F. (811 K; 538° C.) (ASTM C71), or as otherwise known in the art.

(16) A “low smoke zero halogen” or “low smoke free of halogen” (LSZH or LSOH or LSOH or LSFH or OHLS) is a material classification typically used for cable jacketing in the wire and cable industry. LSZH cable jacketing is composed of thermoplastic or thermoset compounds that emit limited smoke and no halogen when exposed to high sources of heat.

(17) FIG. 1 is a perspective view of a coaxial cable 100 that has layers cut away. The cable has a round cross section and is radially symmetric around an axial centerline.

(18) Center conductor 116 includes nineteen strands of individual wires 118 that are bundled and twisted together. Each individual wire is nickel-plated copper.

(19) Radially surrounding the center conductor is ceramifiable silicone rubber dielectric 114 in a cylindrical, tubular form. Center conductor 116 is centered in dielectric.

(20) Because silicone rubber can be difficult to extrude in the thicknesses needed for proper spacing between the center and outer conductor, dielectric 114 may exhibit multiple layers that are partially cured with each other. To create the large thickness shown in the figure, ceramifiable silicone rubber layer 114A was extruded in a first batch process around the center conductor and partially cured. Ceramifiable silicone rubber layer 114B was then extruded in a second batch process around layer 114A and partially cured, forming some cross-links between the layers. Finally, ceramifiable silicone rubber layer 114C was extruded in a third batch process around layer 114B and cured, forming cross-links between the layers.

(21) If the entire thickness of ceramifiable silicone rubber dielectric were extruded all in one batch, the silicone rubber may not harden to the point where it can support the weight of the inner conductor. If that were the case, then the inner conductor could sag or otherwise move within the soft silicone rubber, becoming uncentered. Multiple passes through an extrusion machine, each pass increasing an extrusion orifice diameter, helps prevent this problem. A tunnel of ultraviolet (UV) lights can shine onto the layers as they exit the orifice, helping to speed curing.

(22) Ceramifiable silicone rubber layer 114C can be surrounded by wrapping it with silicone glass separator tape 112. As shown in the figure, separator tape 112 has a 25% overlap. Silicone glass separator tape 112 helps hold the thick outer layer 114C of silicone rubber dielectric 114.

(23) Outer conductor 108 was formed from copper metallized mylar tape wrapped around separator tape 112. The metallized tape was formed with copper over mylar substrate 110.

(24) Copper braiding 106 surrounds and is in direct contact with the copper metal of outer conductor 108. The braiding includes 36 AWG (American Wire Gauge) tin plated copper woven in a continuous fashion at a coverage of at least 85%.

(25) Inner jacket 104 is another layer of ceramifiable silicone rubber. It surrounds braiding 106, enclosing it in a fire resistant shell.

(26) Outer jacket 102 surrounds inner jacket 104. Outer jacket 102 is a low smoke zero halogen jacket, which protects the pliable silicone rubber of the inner jacket and slides more easily through walls and conduits. The outer jacket can be made of cross-linked irradiated polyolefin and can be colored in order to stand out from other non-emergency cables.

(27) Example dimensions of a coax cable are shown in the following tables. These dimensions are not limiting.

(28) TABLE-US-00001 TABLE 1 Ceramifiable Silicone Dielectric, Braided 1″ Coax Cable Structure Outer Diameter Layer thickness Material Center 4.7 mm (0.185 in.) 4.7 mm diameter nickel plated conductor copper, 19 strands of 0.0372″ DIA Dielectric 19.8 mm (0.779 in.) 7.5 mm ceramifiable silicone rubber Separator 20.4 mm (0.804 in.) 0.3 mm silicone glass Tape with 25% nominal lap Shield #1 20.5 mm (0.809 in.) 0.06 mm copper mylar foil tape with 25% nominal lap, copper side up Shield #2 21.2 mm (0.834 in.) 0.3 mm 36 AWG tin braid plated copper braid, 85% min. coverage inner 24.2 mm (0.954 in.) 1.5 mm ceramifiable jacket silicone rubber outer 27.4 mm (1.078 in.) 1.6 mm low smoke jacket zero halogen

(29) TABLE-US-00002 TABLE 2 Ceramifiable Silicone Dielectric, Corrugated 1″ Coax Cable Structure Outer Diameter Layer thickness Material Center  4.7 mm (0.185 in.) 4.7 mm diameter nickel plated conductor copper, 19 strands of 0.0372″ DIA Dielectric 19.8 mm (0.779 in.) 7.5 mm ceramifiable silicone rubber Outer 23.4 mm (0.920 in.) 1.8 mm corrugated copper Conductor inner jacket 26.4 mm (1.039 in.) 1.5 mm ceramifiable silicone rubber outer jacket 29.6 mm (1.165 in.) 1.6 mm low smoke zero halogen

(30) FIGS. 2A-2B are views of braided coaxial cable 200 in accordance with an embodiment. The cable has a round cross section and is radially symmetric around a centerline CL.

(31) Similar to the embodiment shown in the previous figures, center conductor 216 is in direct contact with and surrounded by tubular ceramifiable silicone rubber dielectric 214.

(32) The center conductor can copper or other electrically conductive metals, and it can be solid or multi-stranded. The ceramifiable silicone rubber dielectric can be replaced by ceramic fiber wrap material.

(33) Unlike the embodiment shown in the previous figure, outer conductor 208 is in direct contact with the dielectric. It includes an aluminum or copper foil, which is in direct contact with and surrounded by copper braid 206. Foil 208 presents a smooth, constant inner diameter of conductive metal across the dielectric from the inner conductor, whilst metal braid 206 offers additional conductive pathways for electrons to flow.

(34) Ceramic fiber wrap inner jacket 204 is in direct contact with and surrounds metal braid 206. It is woven continuously around the outer conductor such that it completely covers the outer conductor.

(35) Alternatively, the ceramic fiber wrap inner jacket can be replaced with ceramifiable silicone rubber.

(36) Low smoke zero halogen jacket 202 surrounds ceramic fiber wrap inner jacket 204.

(37) FIGS. 3A-B are views of corrugated coaxial cable 300 in accordance with an embodiment. The cable has a round cross section and is radially symmetric around a centerline CL.

(38) Center conductor 316 is composed of copper or another electrically conductive metal. The center conductor can be a single solid wire (as shown) or multiple smaller strands of wires twisted and bundled together.

(39) Center conductor 316 is in direct contact with and surrounded by tubular ceramifiable silicone rubber dielectric 314. The silicone rubber dielectric can be continuously extruded around the center conductor or extruded in layers as described above.

(40) In some embodiments, the dielectric can be a ceramic fiber wrap with dimensions to maintain a predetermined thickness depending on the dielectric constant of the ceramic fiber wrap material and desired electrical impedance (e.g., 50Ω, 75Ω) of the cable.

(41) Corrugated metal outer conductor 320 is in direct contact with and surrounds tubular ceramifiable silicone rubber dielectric 314. The corrugated metal outer conductor is composed of a relatively thin metal wall with regularly spaced undulations. The metal can be copper or another electrically conductive metal. An infinitesimal radial cross section of the undulations may radially symmetric, or undulations may be helical.

(42) In the exemplary embodiment, the undulations have a constant wall thickness 326 of 0.533 mm±0.076 mm (0.021 inches±0.003 inches). An amplitude-plus-wall-thickness dimension, or layer thickness 324 of the undulations is 1.78 mm (0.070 inches). A peak-to-peak wavelength 322 of the undulations is 2 corrugations per centimeter (5 corrugations per inch).

(43) Ceramic fiber wrap layer 304 directly contacts and surrounds corrugated metal outer conductor 320. Ceramic fiber wrap layer is woven from a ceramic fiber yarn around the outer conductor such that it completely covers the outer conductor.

(44) Alternatively, the ceramic fiber wrap layer can be replaced with ceramifiable silicone rubber.

(45) Low smoke zero halogen jacket 302 surrounds ceramic fiber wrap layer 304. The jacket protects the cable from damage when it is fed and pulled through conduits. It also offers a relatively slippery surface to minimize force needed to push or pull the cable along conduits and raceways.

(46) Further example dimensions of a braided coax cables are shown in the following tables for ⅞″ and ½″ embodiments. These dimensions are not limiting.

(47) TABLE-US-00003 TABLE 3 Ceramifiable Silicone Dielectric, Braided ⅞″ Coax Cable Structure Layer Type Outer Diameter thickness Material Center 4.57 mm (0.180 in.) 0.180 in. diameter annealed conductor copper Dielectric 15.24 mm (0.60 in.) to 0.21 in. to ceramifiable 19.81 mm (0.78 in.) 0.30 in. silicone rubber Outer 15.5 mm (0.61 in.) to 0.005 in. aluminum conductor 20.1 mm (0.79 in.) tape Overall 15.6 mm (0.64 in.) to 0.015 in. tinned braid 20.8 mm (0.82 in.) copper Fire jacket 15.7 mm (0.66 in.) to 0.0035 in. ceramic 21.0 mm (0.82 in.) fiber wrap Jacket 15.9 mm (0.72 in.) to 0.003 low smoke 21.2 mm (0.83 in.) zero halogen

(48) TABLE-US-00004 TABLE 4 Ceramifiable Silicone Dielectric, Braided ½″ Coax Cable Structure Type Outer Diameter Material Center conductor  4.57 mm (0.180 in.) annealed copper Dielectric 11.43 mm (0.450 in.) ceramifiable silicone rubber Outer conductor 11.68 mm (0.460 in.) aluminum tape Overall braid 12.45 mm (0.490 in.) tinned copper Fire jacket 14.22 mm (0.560 in.) ceramic fiber wrap Jacket 15.75 mm (0.620 in.) low smoke zero halogen

(49) FIG. 4 is an illustration of installed cables in a building distributed antenna system in accordance with an embodiment.

(50) Building 400 has a cellular distributed antenna system (DAS) and/or Emergency Responder Radio Coverage System (ERRCS) DAS installed. That is, a fire resistant coax cable as described above has been pulled or pushed through conduit and affixed inside and outside of the building, connecting to antennae and other systems.

(51) Head-end rack 438 has been installed in an equipment room on the ground floor of building 400. Within head-end rack 438 is housed an optical master unit and other rack-mounted devices. Fiber optic cable 440 connects the head-end rack 438 to remote access units, including optical signal splitters 436 on each floor and remote access unit 432 on the top floor. Optical signal splitters 436 and remote access unit 432 provide the functions of converting and amplifying optical to electrical signals and back again for their respective floor's antenna units. Signal splitters 436 pull off and repeat optical signals from optical cables 440.

(52) On each floor are indoor antennas 434 that wirelessly connect with users' cellular telephones. Antennae 434 are connected to optical signal splitters 436 and remote access unit 432 by coax cables 443, in accordance with an embodiment.

(53) Coax cables 443 are fire resistant in accordance with embodiments herein. Coax cables 443 can maintaining operation for over two hours at high temperatures. Therefore, building codes may not require coax cable 443 to be shielded from open air. That is, no additional drywall soffits, fire proof conduit, or other expensive structures may be needed to comply with building codes.

(54) Within the head-end rack 438, fire resistant coax cable 441 can connect different rack-mounted devices. Although the equipment room in which head-end rack is situated may be fire proof, this additional cabling may incrementally harden the system to fire damage.

(55) Fire resistant coax cable 442 runs from head-end rack 438 up the side of the building to roof mounted donor antenna 430. Donor antenna 430 is pointed at local cell tower 446 for an optimal signal.

(56) In operation, communications from end users' cell phones goes to indoor antennae 434 and are then fed to optical splitters 436 through fire resistant coax cables 443. Fiber optic cables 440 bring the communications signals to the head end unit on the ground floor, which then sends the signals through fire resistant coax cable 442 to the roof. At the roof, donor antenna 430 sends the signals from coax cable 442 to cell tower 446. Opposite direction communication signals follow a reverse path.

(57) During a building fire, explosion, or other emergency, coax cables 443, 442, and 441 may be exposed to an inferno of high temperatures. The low smoke zero halogen jacket may burn away. Yet while the insulation of other wires may burn and sublimate and allow their conductors to short out, an embodiment's ceramifiable silicone rubber or ceramic fiber wrap surrounding the outer conductor largely maintains its strength and structural integrity. The ceramic matrix from the ceramified silicone rubber, or the ceramic fiber wrap, does not allow the outer conductor of the coax to electrically short against metal conduit or other wires.

(58) Further, the dielectric, so important in coaxial cables for its impedance and maintaining spacing between an inner conductor and coaxial outer conductor, merely ceramifies under the intense heat. Its polysiloxane matrix melts away while inorganic flux particles flow and join refractory particles. This leaves a microporous ceramic material. Although the resulting ceramic material may be brittle, nothing should move the cable because it is already installed an in place. At least until first responders can rescue victims and put out the blaze, their communications can depend on the wires.

(59) After the fire is out, the ceramified coax cables may be replaced.

(60) FIG. 5 illustrates of a central processing rack 538 in accordance with an embodiment. Fiber optic cable 540 extends from optical master unit (OMU) 550 to the DAS field (of indoor antennae). Bi-directional amplifier (BDA) 551 is connected to OMU 550 by fire resistant coax cable 541. Fire resistant coax cable 542 connects BDA 551 to the roof antenna. Uninterruptable power supply (UPS) 552 maintains battery power when power is cut. Power supply 553 supplies electricity during normal, day-to-day operation.

(61) FIG. 6 illustrates fire resistant coax cables connecting distributed antennas to an antenna tap in accordance with an embodiment. Note that the cable may run on the ceiling where the heat may be most intense during a fire. Indoor antennae 634 are connected with optical splitter 636 via fire resistant coax cables 643. Fiber optic cable 640 connects optical splitter 636 with the head-end unit.

(62) As will be apparent to one of skill in the art, embodiments of the fire resistant coax cable can be used in different configurations of the DAS field, such as those with no fiber optic cables, or where the top floor of a building houses the bi-directional amplifier. The fire resistant coax cable can be used in non-DAS systems, as in anywhere a coax cable is needed to survive high temperatures. For example, such cables may be used in aircraft and other vehicles, mines and tunnels, power plants, etc.

(63) Testing fire resistant coaxial cable in accordance with embodiments are envisioned. Such testing can include providing a coaxial cable having a center conductor surrounded by a ceramifiable silicone rubber dielectric or a ceramic fiber wrap dielectric, which is surrounded by an outer conductor, which is surrounded by a ceramifiable silicone rubber layer or a ceramic fiber wrap layer, which is surrounded by a low smoke zero halogen jacket. The cable can be subjected to high temperatures, such as 400° C., 425° C., 482° C., 500° C., 750° C., 850° C., 950° C., 1000° C., 1010° C., or as otherwise known in the art. The heat causes ceramification of the ceramifiable silicone rubber layer. The ceramic fiber wrap can withstand the heat. The heat may burn at least a portion of the jacket from the cable.

(64) In order to test the cable, one can pass an electric voltage or current signal through the coaxial cable during or after the ceramifying and the burning. The cable can be tested up to and including destruction.

(65) Although specific embodiments of the invention have been described, various modifications, alterations, alternative constructions, and equivalents are also encompassed within the scope of the invention. Embodiments of the present invention are not restricted to operation within certain specific environments, but are free to operate within a plurality of environments. Additionally, although method embodiments of the present invention have been described using a particular series of and steps, it should be apparent to those skilled in the art that the scope of the present invention is not limited to the described series of transactions and steps.

(66) Further, while embodiments of the present invention have been described using a particular combination of hardware, it should be recognized that other combinations of hardware are also within the scope of the present invention.

(67) The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that additions, subtractions, deletions, and other modifications and changes may be made thereunto without departing from the broader spirit and scope.