Abstract:
An electric cable is provided having a plurality of insulated power conductors, a pilot and ground wire assembly and a saddle positioned between the plurality of insulated power conductors with the pilot and ground assembly being positioned within the saddle, where the saddle is constructed from a semi-conductive polymer. An inner sheath surrounds the saddle and the plurality of insulated power conductors, where the inner sheath is constructed from a semi-conductive polymer and where the inner sheath, saddle and the plurality of conductors form a flat cable. An outer insulating sheath disposed around the outside of the inner sheath.

Description:
FIELD OF THE INVENTION 
       [0001]    This application relates to a power cable. More particularly, the present application relates to a power cable for use with mining shuttle cars and the like. 
       DESCRIPTION OF THE RELATED ART 
       [0002]    Mining shuttle cars are typically powered by electric motors, using an attached cable connected to a remotely located power source/generator. These cables are subject to significant mechanical stresses from both the general mining environment as well as the coiling and uncoiling that occurs hundreds of times a day as the shuttle car travels back and forth in the mine. 
         [0003]    As such, these cables need a significant amount of insulation and polymeric armoring to survive the environment while being flexible enough to be continuously coiled and uncoiled. However, more insulation provides better protection but makes the cable heavier and stiffer. 
         [0004]    Moreover, aside from the mechanical considerations, safety concerns in mines may require that the cables not only be fitted with pilot and/or grounding wires but also that the cables have an electrically conductive screen surrounding the insulation of the conductors. Such pilot and/or ground wires as additional components inherently add additional weight and stiffness. Additionally, typical screens for surrounding the conductor are made from braided copper which although flexible cannot endure continued flexing (coiling/uncoiling) over prolonged periods because they can become frayed. Moreover such frayed braided conductors can actually start to cut into the wire insulation again shortening the life of the cable. 
       OBJECT AND SUMMARY 
       [0005]    The present arrangement overcomes the drawbacks associated with the prior art and provides a shielded power cable for use in mining or other related industries that is both robustly protected and is sufficiently flexible to withstand numerous cycles of coiling/uncoiling each day. 
         [0006]    To this end the present arrangement is directed to an electric cable having two insulated power conductors, a pilot/ground wire assembly, a saddle positioned between the two insulated power conductors with the pilot ground assembly being contained within the saddle, the saddle being constructed from a semi-conductive polymer that is flexible. 
         [0007]    An inner sheath surrounds the saddle and the two insulated power conductors, where the inner sheath is constructed from a semi-conductive polymer and where the inner sheath, saddle and two conductors form a flat cable. An outer sheath of high mechanical strength material is disposed around the outside of the inner sheath. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The present invention can be best understood through the following description and accompanying drawing, wherein: 
           [0009]      FIG. 1  illustrates one example of a power cable in accordance with one embodiment; and 
           [0010]      FIG. 2  illustrates another example of a power cable in accordance with one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    In one embodiment of the present invention as shown in  FIG. 1 , a power cable  10  is provided for mining cars and the like. In this arrangement, cable  10  has two power conductors  12 A and  12 B each made of a plurality of stranded and bundled metal wires  14 A/ 14 B, surrounded respectively by conductor screens  16 A/ 16 B and insulation  18 A/ 18 B. 
         [0012]    In one arrangement wires  14 A/ 14 B of conductors  12 A/ 12 B are constructed as seven (7) bundles of stranded tinned copper wires, each bundle having thirty seven (37) wires with a cross sectional area of about 35 mm 2 . However, additional constructions are possible such as 7 bundles with 19 wires each or 7 bundles with 47 wires each. Conductor screens  16 A/ 16 B are preferably constructed from an extruded semi-conductive cross-linked polyethylene (XLPE) of about 0.51 mm thickness. However other materials and thicknesses could be used such as a semi-conductive polymer tape screen, used in lieu of an extruded semi-conductive screen or in addition thereto. In one embodiment conductor insulators  18 A and  18 B are made from 1.52 mm thick ethylene propylene rubber (EPR) with coloring or ink printed (colored) as needed. 
         [0013]    As shown in  FIG. 1 , between conductors  12 A and  12 B, a saddle  26  and pilot/ground wire  20 / 24  assembly is provided for grounding and supporting conductors  12 A and  12 B during repetitive coiling. Such an arrangement additionally has a pilot wire  20 , surrounded by an insulation  22  which in turn is surrounded by a series of grounding wires  24  within saddle  26 . 
         [0014]    In one arrangement, pilot wire  20  is made from a braided tinned copper wire with an exemplary cross-section of about 8 mm 2 . However a larger wire could be used in conjunction with larger conductors  12 A/ 12 B. Insulation  22  surrounding pilot wire  20  is preferably constructed from polypropylene, but other possibilities are available such as EPR, or a thermoplastic elastomer (TPE). The surrounding ground wires  24  are preferably constructed as 17 stranded 1.3 mm 2  copper wires (each of which are made from seven (7) bundled wires totaling 1.3 mm 2  in cross section). In certain instances if pilot wire  20  is increased in size, e.g. owing to larger power conductors  12 A/ 12 B, this may necessitate more strands of ground wires  24  to fully cover pilot insulation  22 . For example a 70 mm 2  in cross-section power conductor  12 A/ 12 B might have a 16 mm 2  cross-section pilot  20  and require twenty (20) or more stranded 1.3 mm 2  copper bundles for ground wire  24 . 
         [0015]    It is noted that the above arrangement of pilot wire  20  and ground wires  24  are typically required in some instances for safety standards. Ground wires  24  may be attached at one end to a mining shuttle car frame and at the other end to the coupler at the remotely located power supply. This grounding circuit travels in turn through stationary cables all the way back to a grounding bed located outside of the mine. This bed is to assure that all equipment in the mine is securely grounded in case a power conductor, such as conductors  12 A/ 12 B contacts any machine frame. Pilot wire  20  simply monitors ground wire  24  for continuity. 
         [0016]    As shown in  FIG. 1 , between conductors  12 A/ 12 B and surrounding pilot/ground wire assembly  20 / 22 / 24 , a saddle  26  is formed of an extruded semi-conductive polymer to seat the two conductors  12 A and  12 B. In one arrangement saddle  26  is made using a die-shaped extrusion and is constructed from black semi-conductive chloroprene rubber (CR-generically called Neoprene™) which includes carbon black additive to make the polymer semi-conductive. Other polymers for saddle  26  may include chloro-sulfonated polyethylene (CSPE) which likewise has incorporated carbon black. 
         [0017]    As shown in  FIG. 1 , the shape and material selection for saddle  26 , used to encase ground assembly  24  and pilot wire  20 , provide a distinct advantage over prior art shuttle car cables. For example, the cradle shape of saddle  26  and intimate contact with the centrally located grounding conductor  24  is useful in holding all of the elements of cable  10  in place during the lifetime of coiling the cable, which as noted above could be hundreds of times per day or more. The selection of semi-conductive CR or CSPE provide saddle  26  with flexibility that can also be easily “loaded” with high levels of carbon black, whereas other polymers cannot readily incorporate enough carbon black into the rubber to reach desired conductivity set by some exemplary grounding/safety standards which is 200 Ω·m at 23° C. maximum. 
         [0018]    As shown in  FIG. 1 , surrounding conductors  12 A and  12 B and the entire saddle  26 , ground assembly  24  and pilot wire  20 , is a semi conductive inner sheath  28 . In a preferred embodiment, inner sheath  28  is constructed from the same material as saddle  26  namely semi-conductive chloroprene rubber (CR or Neoprene™) or semi-conductive chloro-sulfonated polyethylene (CSPE) either one of which would include sufficient carbon black to reach the desired conductivity of the rubber which is 200 Ω·m at 23° C. maximum. 
         [0019]    As a result of this construction, both conductors  12 A and  12 B are completely surrounded by a semi conductive polymer sheathing on the inside between conductors via saddle  26  and on the outside of conductors  12 A and  12 B by inner sheath  28 . 
         [0020]    The combination of semi-conductive rubber inner sheath  28  and saddle  26  completely surrounds and protects power conductors  12 A and  12 B from a user who may come into contact with an energized conductor as can happen on a non-shielded shuttle car cable. Additionally, semi-conductive inner sheath  28  is lighter in weight, but has a significant advantage in durability as prior art copper braid shields fatigue rapidly on flat cables since there is no helix that is absorbing and becoming damaged by the mechanical forces as is the case with a round braid shield cable as discussed above in the prior art. 
         [0021]    As shown in  FIG. 1  the exterior of cable  10  is coated finally with an outer isolative sheath  30 . In a preferred embodiment, sheath  30  is constructed of about 3.18 mm thick black chlorinated polyethylene (CPE) or black chloroprene rubber (CR—Neoprene™). 
         [0022]    Cable  10  and the accompanying description of components is intended to be exemplary. Certain additional features may be added to the cable such as colorants, binders or other cable accessories without deviating from the salient features of the invention. Moreover, the above design is for a DC cable with two conductors  12 A and  12 B. However, inventive features of the present application may likewise be applicable to an AC cable design having three conductors. 
         [0023]    For example,  FIG. 2  shows an AC cable  110  that has three conductors  112 A,  112 B and  112 C, each having a similar construction to conductors  12 A and  12 B. Here cable  110  has the same pilot wire  120  and ground wire  124  as well as saddle  126 , again of similar qualities as those described above. In  FIG. 2 , owing to the third conductor  112 C, an additional saddle  150  is placed between conductor  112 B and  112 C and may also include some additional grounding cables  152 , such as 4 mm 2  flexible stranded tinned copper grounding wires, within saddle  150  as required. 
         [0024]    Applicants note that the above described cable  10  and the appurtenant features are exemplary and are not intended to limit the scope of the invention. For example other cables that are similarly constructed with possibly four or more conductors, or with one conductor, but otherwise substantially similarly constructed are intended to be within the purview of this invention. 
         [0025]    While only certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes or equivalents will now occur to those skilled in the art. It is therefore, to be understood that this application is intended to cover all such modifications and changes that fall within the true spirit of the invention.