Patent Publication Number: US-2004055780-A1

Title: Combined suspension cable and electrical conductor

Description:
[0001] This application claims priority to provisional application serial No. 60/395,818 filed Jul. 11, 2002. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates a combined suspension cable and power conductor for a suspended electrical apparatus, and more particularly, to combined structure having a weight bearing outer portion and an internal electrical conductor for supporting and providing electrical power and data signals to the suspended apparatus, which eliminates the need for an unsightly external wire running along the suspension cable. The invention also relates to a combined suspension cable and power conductor which can be used with a mounting assembly that allows for gross adjustment of the position of the suspended apparatus at the time of wiring, and convenient fine adjustment of the position at a later time.  
       [0004] The invention is illustrated in the context of a suspended lighting fixture, maybe used equally well with other electrical apparatus such as signage, computer displays, telecommunications equipment, switches, controls, powered connection devices, theft control devices, sensors, medical equipment and any other equipment or device which requires both suspension and electrical connections for data signals and/or operating power.  
       [0005] 2. Related Art  
       [0006] A suspended ceiling fixture or luminaire is typically mounted using a horizontal crossbar structure of some type attached to an electrical junction box, which in turn, is attached to a suitable weight-bearing structural member such as a ceiling joist. A canopy in the form of an inverted dome may be used to cover the mounting structure and the junction box to provide a finished appearance.  
       [0007] On such mounting assembly is of the type illustrated in copending U.S. provisional patent application No. 60/324,888, filed Sep. 26, 2001, and which is incorporated by reference herein as if fully set forth.  
       [0008] The fixture itself can be suspended from the mounting assembly in several different ways, depending on the size and shape of the fixture and/or the esthetic effect desired. For example, rigid hollow tubes or rods, threaded at the ends can be attached to the mounting assembly and the fixture by suitable locking nuts. Alternatively, decorative chains suitably attached to the mounting assembly and the fixture may be used. As a further alternative, stranded “wire rope” cables, connected by conventional cable grippers, can be employed.  
       [0009] When hollow tubes are employed, the required electrical conductors can pass through the tubes from the junction box to the fixture, and are thus not visible. When chains or cables are employed, however, there is no place to hide the wires, so they are generally threaded in the chain links, or secured to the cable and, in either case, are visible as they extend from the ceiling to the fixture.  
       [0010] Attempts have been made to avoid the use of separate wires and suspension cables by constructing composite cables having one or more insulated electrical conductors surrounded by a weight-bearing sheath. These, however, have met with limited success because of the need to satisfy conflicting requirements.  
       [0011] For example, the composite cable should have approximately the same diameter as conventional stranded wire rope for esthetic reasons, for convenient handling during installation and to accommodate standard wire rope grippers or other suitable gripping hardware. At the same time, the cable must have sufficient strength to support the weight of the fixture even though its interior is occupied by the non-weight bearing electrical conductor. Also, the design must permit mechanical connections to be made easily and reliably at both ends, and in particular, must minimize the risk that the conductor insulation will be damaged by attachment of the gripping hardware during installation or in normal use. Further, because of the close proximity of the conductor to a weight-bearing sheath that may be metallic, the risk of insulation failure due to manufacturing defects or handling must be minimized. In addition, the cable must be easy to cut and must not unravel excessively at the cut end. Further, the conductor material must be soft enough to minimize kinking and to straighten easily when the cable itself is straightened.  
       [0012] Further, all conducting parts of the fixture and mounting assembly must have a common ground and all of the conductors must have strain relief to satisfy typical local electrical code requirements, and safety standards of organizations such as Underwriter&#39;s Laboratories.  
       [0013] Another desirable feature would be that the cables can be used with a mounting assembly that permits gross adjustment of the position of the fixture while it is being wired, and fine adjustment afterwards without the need for disassembly or removal of parts already installed.  
       [0014] No satisfactory solution to these conflicting requirements appears has been available up to now.  
       [0015] It would be extremely desirable to have a composite cable structure which can be used both to suspend a lighting fixture and to provide electric power to the fixture from an overhead junction box without a separate visible electrical conductor, and that satisfies the above-described requirements.  
       SUMMARY OF THE INVENTION  
       [0016] It is accordingly an object of the present invention to provide an improved mounting cable for a lighting fixture or other electrical apparatus.  
       [0017] Another object of the invention is to provide a composite cable for a suspended electrical apparatus which permits both suspension and delivery of electricity to the apparatus from an overhead junction box.  
       [0018] It is another object of the invention to provide a composite mounting cable for a suspended lighting fixture or the like which provides reliable mechanical support and invisible electrical wiring.  
       [0019] It is a further object of the invention to provide a composite mounting cable for a suspended lighting fixture or the like having one or more internal insulated electric wires and an external weight-bearing sheath.  
       [0020] It is also an object of the invention to provide such a composite mounting cable that can accommodate conventional stranded-wire cable grippers or other gripping hardware with minimized risk of damage to the insulation of the internal wire during installation or normal use, or electrical leakage due to cracks or other insulation defects.  
       [0021] It is a further object of the invention to provide a composite mounting cable and a mounting system that allow all conducting parts to have a common ground.  
       [0022] It is another object of the invention to provide such a composite mounting cable and a mounting system in which all conductors have strain relief.  
       [0023] It is yet another object of the invention to provide such a composite mounting cable which is easy to cut.  
       [0024] It is a still further object of the invention to provide such a composite mounting cable in which the weight-bearing element will not unravel excessively when cut.  
       [0025] It is also an object of the invention to provide such a composite mounting cable and mounting system that will allow for gross adjustment at time of wiring and fine adjustment later.  
       [0026] It is also another object of the invention to provide such a composite mounting cable where simple electrical connections can be made without piercing the conductor.  
       [0027] It is yet a further object of the invention to provide such a composite mounting cable that can accommodate conventional stranded-wire cable grippers or other gripping hardware where the conducting material is soft enough to minimize kinking and allow the ability to straighten the cables with minimal effort.  
       [0028] The above-stated objects are achieved according to the invention by providing a cable having at least one insulated internal conductor formed of fine stranded solid wire, a discrete second layer of electrical insulation surrounding the insulated electrical conductor, and a braided metal sheath surrounding the second layer of electrical insulation.  
       [0029] The two insulating layers are preferably formed of a material such as a fluoropolymer that has good dielectric properties and a low coefficient of friction. The insulation surrounding the conductor fits tightly so there is no relative movement between the two. Preferably, however, the discrete second insulating layer fits more loosely around the insulated conductor, and also within the surrounding sheath. so that relative movement between the two insulating layers and between the second insulating layer and the outer sheath is possible when the gripping hardware is attached to the cable. Alternatively, the second insulating layer may be formed of a tough material such as an aramid or fiberglass, and the inner layer may be formed of any suitable insulator. In either case, the outer insulator may be fitted snugly around the insulated conductor, or even bonded to it, depending on the dielectric and/or mechanical properties of the insulating material.  
       [0030] The external metal sheath is preferably formed of braided or woven metal such as stainless steel, but may also be formed of fibers of a braided or woven aramid material.  
       [0031] Other features and advantages of the present invention will become apparent from the following description of the invention and the accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0032]FIG. 1 is schematic drawing of a suspension assembly for a suspended fixture employing four suspension cables according to the invention.  
     [0033]FIG. 2 is a cut-away view of the composite cable according to a first embodiment of the invention.  
     [0034]FIG. 3 is a cross-sectional view of the cable shown in FIG. 2.  
     [0035]FIG. 4 is a schematic side elevation showing the manner in which the cable can be connected to a conventional cable gripper.  
     [0036]FIG. 5 is a cut-away view of the composite cable according to a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0037] Referring to FIG. 1, a lighting fixture mounting assembly, generally denoted at  10 , may comprise a horizontal crossbar  12  secured in any suitable or desired manner to an electrical junction box (not shown) located in the ceiling above the fixture and attached to a joist or other structural member. A canopy  14  fits over and is attached to crossbar  12 . Conductive straps  19  may be used to provide a common ground between canopy  14  and crossbar  12 .  
     [0038] In the illustrated configuration, electrical apparatus (not shown), which for convenience will be referred to herein as a fixture, is to be suspended by four cables  16  which are anchored to crossbar  12  by suitable gripping hardware  18 , then pass through protective and decorative grommets  19  in canopy  14 . The lower ends of cables  16  are secured to the fixture itself by suitable grippers (not shown) which may be like or similar to gripping hardware  18 .  
     [0039] A suitable construction for mounting assembly  10  may be as illustrated in copending U.S. patent application No. 60/324,888, referred to above, but the present invention may also be used with other types of mounting assemblies.  
     [0040]FIGS. 2 and 3 illustrate the construction of the composite cable according to the invention. The cable, generally denoted at  30 , is comprised of a central insulated conductor  31  having a core  32  of stranded or solid copper wire, and a layer of electrical insulation  34  surrounding wire  32 , a discrete second layer of electrical insulation  36  surrounding insulated conductor  31 , and an outer braided-metal or woven-metal layer  38 , formed preferably of stainless steel, all substantially coaxial with each other. The insulating layers  34  and  36  are preferably formed of a fluoropolymer such as Teflon® manufactured by E. I du Pont de Nemours and Company. Use of a fluoropolymer material for insulation layers  34  and  36  has several advantages. These include toughness, good insulating properties, high heat-resistance which permits use of thin layers, ease and low cost of manufacture, and ease of handling during wiring. Use of a fluoropolymer insulating material is also advantageous because of its very low coefficient of friction, as discussed below.  
     [0041] Inner insulation layer  34  fits tightly around conductor  32  so there is substantially no slip or relative movement between the two. Preferably, however, insulation layer fits more loosely around central conductor  31  so that slip between the two insulation layers is possible due to the low coefficient of friction. This slip or give between insulation layers  34  and  36  has been found to help prevent the stainless steel sheath  38  from penetrating the insulation and short circuiting wire  32  due to forces applied to it by the gripping hardware. Also, the second layer of insulation  36  minimizes the risk of electrical leakage to the sheath  38  in case of small defects in the insulating layer  34 .  
     [0042] It is also possible, depending on the mechanical properties of the insulating material, for the outer insulation layer  36  to fit tightly around inner insulation layer  34 , or even to be bonded to it. In such a construction, the second insulating layer may, if desired, be formed of a tough material such as an aramid or fiberglass to minimize the risk of damage to the conductor. The inner insulating layer may be formed of any suitable material.  
     [0043] Inner insulation layer  34  may be applied to conductor  32  in any suitable conventional manner, and maybe at least about 0.001 inch thick, and preferably at least about 0.05 inch thick, and more preferably, at least about 0.01 inch thick, depending on the dielectric and mechanical properties of the insulating material.  
     [0044] Outer insulating layer  36  maybe at least about 0.001 inch thick, preferably at least about 0.05 inch thick, more preferably, at least about 0.01 inch thick, and even more preferably, at least about 0.07 inch thick, again, depending on the dielectric and mechanical properties of the insulating material.  
     [0045] As will be appreciated by persons skilled in the art, use of thinner insulating layers, to the extent permitted by the dielectric and mechanical properties of the insulating material, will result in a smaller overall size for the cable.  
     [0046] Internal conductor  32  is preferably a stranded wire formed of many strands of soft, fine copper. Such wire tends to exhibit little or no memory if the cable is bent, e.g., during shipping or installation, and is accordingly easier to straighten during installation. For an 18 gauge conductor, good results can be obtained using at least 32 strands of 33 gauge soft copper wire, and preferably 168 strands of 40 gauge wire. For a 16 gauge conductor, at least 40 strands of 32 gauge copper should be employed.  
     [0047] It will be understood, however, that solid wire, rather than stranded wire may also be used.  
     [0048] The thickness of outer sheath  38  is determined by the desired maximum diameter of cable  30  in relation to the gauge of conductor  32  and the thickness of the insulating layers. In addition, the rigidity of sheath  38  must not be so great that it is hard to handle or cut or that it has a tendency to unravel excessively when cut. Thus, in the case of stainless steel wire, the gauge must be small. At the same time, weight-bearing requirements must also be taken into account in determining outer diameter. Cables capable of supporting weights of one-half pound and up can be provided.  
     [0049] In a preferred embodiment, the outer diameter of cable  30  is 0.096 inch. For a 25 pound rating, outer sheath  38  may be formed of at least 160 strands of 44 gauge braided or woven stainless steel.  
     [0050] Any suitable gripping hardware  18  may be employed. One such gripper, specifically designed for use with stranded wire suspension cables is manufactured by Arakawa Hanging systems of 1020 SE Harrison, Portland Oreg. As illustrated in FIG. 4, such a gripper comprises a threaded body  20  terminating at one end in an enlarged shoulder  22 . A tubular extension  26  protrudes from shoulder  22 . A cable  16  extends through tubular extension  26  and out through the opposite end  20   a  of body  20 .  
     [0051] In use, gripper  18  may be oriented so that upper surface  22   a  of shoulder  22  rests against the underside of crossbar  12 , and is secured in place by a nut and a lock washer (not shown). Cable  16  extends upward to permit connection of electrical conductor  32  within the cable  16  as described above to the building wiring in the junction box.  
     [0052] The Arakawa gripper referred to above employs spring biased gripper balls (not shown) which surround cable  16  within a housing defined by body  20  and lock the cable in place against the weight of the suspended fixture. Tubular extension  26  is movable upwardly against the biasing spring to release the pressure of the balls. This allows cable  16 , which fits loosely within tubular extension  26 , to be repositioned in the “downward” direction (i.e., with gravity). The position of the cable may be adjusted in the upward direction (i.e., against gravity) simply by pulling it up through the housing formed by body  20 .  
     [0053] From the foregoing description, it will be appreciated that the cable according to the invention conveniently and reliably addresses the conflicting requirements for a mechanism to support a suspended lighting fixture and also provide electric power to the fixture without the need for an unsightly visible wire.  
     [0054] When formed of thin braided or woven stainless steel wire, the sheath is easy to cut to expose the interior insulating layers using a conventional tool, and will not unravel excessively when cut. The two layers of insulation may also be easily cut so that the conductor may be dressed in a normal manner when wiring the fixture. The outer sheath and cooperating gripper assures reliable weight-bearing, while the two discrete insulating layers reduce the risk of short circuiting of the conductor. At the same time, the metallic sheath provides a common ground between the fixture and the mounting assembly. Also, because it is attached rigidly at both ends, strain relief for the conductor is assured.  
     [0055] As more than one cable will generally be used to suspend a fixture, at least two conductors will always be available to provide a complete electrical circuit. Sometimes, however, four wires maybe needed, e.g., for phase control dimming, or even five wires, e.g., for digital/0-10V dimming, or, in the case of other types of electrical apparatus, for signaling or data. In such instances, or if a single suspension cable is used, more than one conductor may be provided in the cable.  
     [0056] A multi-conductor cable according to the invention is illustrated in FIG. 5. Here, the cable, generally denoted at  40 , is comprised of first and second insulated conductors  42  and  44 , each comprised of a multi-strand or solid wire  46  and a tightly fitting insulation layer  48 , disposed in substantially side-by-side relation. Surrounding conductors  42  and  44  is a discrete insulating layer  50  and an outer braided metal sheath  52 . As in the first embodiment, insulation layer  50  fits more loosely around conductors  42  and  44  to permit relative movement. between these elements. Thus, even with a single-cable installation, two conductors will always be available, and with a four-cable installation, any combination of up to eight wires can be provided.  
     [0057] In a dual conductor cable as illustrated in FIG. 5, the side-by-side layout of the conductors may result in a somewhat elliptical or flattened cross-section which may not be held reliably by some grippers. In that case a dummy tube of insulation can be added to provide three internal elements within the second insulating layer to restore the round cross-section.  
     [0058] As will be appreciated by those skilled in the art, other variations are possible within the scope of the invention. For example, insulating layers  34  and  36  can be formed of aramid materials such as Kevlar®, (manufactured by E. I. du Pont de Nemours and Company), or other suitable polymers, fiberglass reinforced materials, etc. having the necessary properties.  
     [0059] Outer sheath  38  can also be manufactured of Kevlar. In that case, an outer jacket or coating may be provided over the Kevlar sheath so it does not unravel excessively when cut and to provide protection from UV radiation. Other suitably strong woven materials such as tin or fiberglass might also be employed. When the weight bearing sheath is not electrically conductive, a thin ground wire may also be included within the sheath, or may be provided in any other suitable manner.  
     [0060] In addition, a thin conventional wire rope (with our without a surrounding insulation layer) can be provided inside the second insulating layer to provide additional weigh-bearing capability.  
     [0061] Although the present invention has been described in relation to a particular embodiment thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is intended, therefore, that the invention not be limited by the specific disclosure herein, but that it be given the full scope indicated by the appended claims.