Abstract:
A method of terminating or joining mineral insulated cables having metallic outer tubes that does not use brazing to fasten the outer sleeves of the terminating coupling over the outer tubes of the cables. Instead, the sleeves are crimped onto the tubes. The space within the termination or joint is filled with an epoxy. A conductive strap can be used to maintain ground continuity between the metallic outer tubes and between the outer metallic tubes and the sleeves.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]    The present application claims the benefit of U.S. patent application Ser. No. 09/739,755 filed Dec. 20, 2000, the disclosure of which is hereby incorporated by reference in its entirety into the present application. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of Invention  
           [0003]    The present invention relates to termination and connection of mineral insulated (MI) heating units and cables.  
           [0004]    2. Description of the Prior Art  
           [0005]    Mineral insulated cables are used primarily as heating units and power cables. Those cables have an outer sheathing in the form of a metal tube, one to seven conductors, and insulation of magnesium oxide around the conductors which insulates and also holds the conductors in place inside the metal tube.  
           [0006]    Sections of mineral insulated cable may be terminated, joined to each other, or joined to non-mineral insulated cables. For example, a mineral-insulated section of non-heating cable may be joined to a section intended for heating; these two sections might be identical except that the heating section has more-resistive internal conductors.  
           [0007]    The conventional method of terminating a mineral insulated cable is to slide an open-ended termination fitting over the outer metallic tube of the mineral insulated cable, fill the interior with mineral insulation similar to that in the cable, and then braze a cap onto the open end of the fitting.  
           [0008]    The conventional method of making joints between two sections of mineral insulated cable is to join the conductor wires protruding from the ends of each of the metallic tubes, for electrical continuity, and then complete the joint with a slide-on coupling that is brazed onto the metal tubes. The space inside the coupling is hollow and must be filled. That is conventionally accomplished by drilling a small hole in the side of the coupling, injecting additional mineral insulation to fill the void, and then sealing the hole by brazing. The hole is typically tapped to a 6-32 NC thread into which a mating brass screw is turned. The screw is broken off and the end is brazed over.  
           [0009]    This process is both time-consuming and uncertain: time-consuming because the fill hole should, for obvious reasons, be small, and filling the entire void within is a lengthy process; uncertain because it is nearly impossible to assure that the void is evenly filled and packed. Gaps are liable to be left, and any effort to pack the mineral insulation into place might shift the conductors, putting strain on wire joints and possibly even causing a short circuit.  
           [0010]    In addition, the mineral insulation is usually very hygroscopic and, inevitably, it absorbs water. The filled-in insulation must then be dried prior to sealing the hole, and that requires great care.  
           [0011]    Not only the after-filling, but the brazing of the couplings to the tubes, is likewise time-consuming. Thorough cleaning of the tubes and couplings is needed and all oxide must be removed, or else the joint will not be satisfactory.  
           [0012]    Brazing involves high temperatures that alter the physical properties of the metal in the tubes, making them brittle and leading to increased liability to cracking and a larger permissible bend radius. It also causes new oxidation, which must be removed. In addition, brazing creates unhealthy fumes.  
           [0013]    The prior art does not disclose any method of terminating (capping or joining) sections of mineral-insulated cable which is fast, insures uniform filling of voids between internal conductors, does not require cleaning of oxide or corrosion from metal surfaces, and which does not harm cables through high temperature.  
         SUMMARY AND OBJECTS OF THE INVENTION  
         [0014]    In view of the foregoing, it should be apparent that there still exists a need in the art for an apparatus and method of terminating a mineral insulated cable that overcomes the problems of the prior art. Accordingly, it is a primary objective of the present invention to terminate or join sections of mineral insulated cable quickly and reliably; to insure the absence of voids in joined or terminated sections of mineral insulated cable; to eliminate the cleaning of oxide or corrosion from metal surfaces; to keep metal parts below temperatures at which embrittlement occurs; and to avoid the production of unhealthy fumes.  
           [0015]    Briefly described, these and other objects of the invention are accomplished in accordance with its apparatus aspects by providing one or more mineral insulated cables and a sleeve that is crimped to the termination of one of the mineral insulated cables, the sleeve being filled with epoxy potting. If the termination is between two cables, then the outer metallic sleeves of the cables are electrically grounded together using a connection strap and the conductors are brazed together. The sleeve may be electrically connected to one or both of the outer metallic tubes by a cap or a connection strap. The cap may also be electrically connected to one of the outer metallic tubes using a connection strap. The types of connections straps include, but are not limited to, wires, strips, clamps, springs, leads, cables, meshes, screw-on or clip-on devices.  
           [0016]    The method of the present invention is carried out by reducing or eliminating brazing from the processes of joining two sections of mineral insulated cable or of terminating an end of a section of mineral insulated cable. In the present invention, couplings are joined to tubes by crimping, and voids are filled with epoxy, the internal conductors are brazed together and the outer metallic tubes of the two cables are electrically coupled. Further, a cap may be screwed-on or clamped on the open end of the sleeve after the epoxy sets and a connection strap electrically connecting the cap to one of the outer metallic tubes may added. That method of terminating mineral insulated cables is faster and more certain than the prior-art methods, does not harm the metal of the tubes, requires a lower level of skill, and eliminates the need for drilling holes in coupling sleeves and end cap fittings.  
           [0017]    The present invention may be assembled or made either in a factory or in the field.  
           [0018]    With these and other objects, advantages and features of the invention that may become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims and to the several drawings attached herein.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a plan view of the invention showing two mineral insulated cables and terminations at the ends and between the cables;  
         [0020]    [0020]FIG. 2 is a detailed, partially cut-away, view of the invention according to FIG. 1;  
         [0021]    [0021]FIG. 3 is a detailed, partially cut-away, view of the invention according to FIG. 1;  
         [0022]    [0022]FIG. 4 is a detailed, partially cut-away, view of the invention according to FIG. 1;  
         [0023]    [0023]FIG. 5 is a cross-sectional view of section V-V of FIG. 1;  
         [0024]    [0024]FIG. 6 is a detailed, partially cut-away, view of the invention shown in FIG. 3 but with an alternative grounding strap shown;  
         [0025]    [0025]FIG. 7 is a detailed, partially cut-away, view of the invention shown in FIG. 3 but with an alternative screwed on grounding cap shown;  
         [0026]    [0026]FIG. 8 is a detailed, partially cut-away, view of the invention shown in FIG. 3, but with an alternative clamped on mesh grounding cap shown; and  
         [0027]    [0027]FIG. 9 is a detailed, partially cut-away, view of the invention shown in FIG. 3, but with an alternative spring-fit grounding cap shown.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]    Referring now to the drawings wherein like elements are identified by like reference numerals, there is shown in FIG. 1 a mineral insulated cable  100  of the “D” design type, the type which is exemplary in this application. Multiple external-power connection wires  105   a ,  105   b , . . .  105   f  . . .  105   n , where n is greater than 1, extend from one end of the mineral insulated cable  100  for connection to a power source. There may be any number of conductors  105   n  but it is preferred that the number of conductors  105   n  be less than eight (that is, n&lt;8) and ideally less than four (that is, n&lt;4).  
         [0029]    A first coupling  120  couples the wires  105   n  to a power lead  130  of the cable  100 , which is joined by a second coupling  140  to a heating element  150  terminated by a cap  160 . The power lead  130  and the heating element  150  are similar in structure, differing mainly in what type of conductive wires  135   n ,  155   n  are enclosed in them (see FIG. 3). Their common structure is shown in FIG. 5. It is noted that the invention does not require similarity of structure.  
         [0030]    In this disclosure, “termination” includes the structure at the end of a single cable section as well as a joint between two cables.  
         [0031]    [0031]FIG. 5 is a nominal cross section of the power lead  130 , but includes reference numerals for elements of the heating element  150  to simplify the drawing. Each section includes a metallic outer tube  132  (of the power lead) or  152  (of the heating element), a filling of mineral insulation  180 , and inner conductors  135   n  or  155   n . The inner conductors may be of low resistance ( 135   n , power lead) or of high resistance ( 155   n , heating element).  
         [0032]    [0032]FIG. 2 shows in detail the termination or coupling  120  at which the external power wires  105   n  are joined to the power lead  130 . The wires  105   n  may be extensions of the conductors  135   n  running through the power lead  130  (as best seen in FIG. 5), or alternatively there may be a brazed joint  123  between each conductor  135   n  and its respective external connection wire  105   n . If the conductors  135   n  and/or the wires  105   n  are insulated wires, then an insulating sleeve like shrink tubing (not shown) can be applied to cover the bare joint.  
         [0033]    The first termination or coupling  120  is preferably a sleeve of generally constant diameter prior to being crimped onto the outer tube of the cable  130 , after which the smaller-diameter crimped portion  122  grips the tube  132  of the power lead  130 . The un-crimped portion  124 , which contains the tube  132  (if any), is of larger diameter.  
         [0034]    The preferred material for the sleeve is free-machining brass. One example of a preferred crimping tool is Thomas and Betts model TBM-25S.  
         [0035]    There is no need for any additional mineral insulation to be inserted into the un-crimped portion  124 . That space is filled with an epoxy  180 , that seals the end of the mineral insulated cable  100  without the need for any cap, tamping, or brazing. However, a cap could be used, if desired. The epoxy seeps into any gaps between the tube  132  and the crimped portion  122  of the termination  120 .  
         [0036]    The preferred type of epoxy  180  is potting epoxy which is capable of withstanding high voltages and high temperatures. One example of a preferred epoxy is DURALCO 4525 made by Cotronics of Brooklyn, N.Y.  
         [0037]    [0037]FIG. 3 illustrates the coupling  140  between the power lead  130  and the heating element  150 . A preferably brazed connection  143  joins each pair of corresponding low- and high-resistance conductors  135   n  and  155   n . A large-diameter sleeve  144  is crimped at one end to form a reduced-diameter portion  142 , which grips the tube  152  of the heating element  150 . The gap between the tubes  152  and  132  and the space inside the sleeve  144  is filled with epoxy  180 .  
         [0038]    Preferably, the cable is assembled in a vertical orientation with the crimped portions  122 ,  142 , and  162  (FIG. 4) downward. The epoxy  180  fills the upper open end of the sleeve  120 ,  140 , or  160  (FIG. 4), flows downward to fill the cavity, and sets. That seals the conductors within and mechanically joins the tubes  132 ,  152  into a solid unit.  
         [0039]    However, in many cases it is preferable to connect the two tubes  132  and  152  electrically and well as mechanically, for example where the tube  152  acts as a ground element. To do that, a connection strap or grounding wire  325  (FIG. 1) is used to connect the two tubes and is preferably brazed to the tubes  132 ,  152 , and optionally to the sleeve  140  as shown in FIG. 1. The wire or strap  325  may be spot-brazed as shown, either before or after filling the sleeve  140  with the epoxy  180 .  
         [0040]    A similar connecting wire or strap may optionally connect the tube  132  to the sleeve  120  or an adjacent metallic structure (not shown).  
         [0041]    Here, and in the following figures and claims, “connection strap” includes any wire, strip, clamp, spring, lead, cable, mesh, screw-on or clip-on device, or any other conductive element.  
         [0042]    [0042]FIG. 6 shows that the grounding strap  325  may also be internal to the tubes  132 ,  152 . In this embodiment the strap  325  may be brazed in place before filling the sleeve  144  with the epoxy  180 .  
         [0043]    [0043]FIG. 7 shows an embodiment in which the grounding strap  325  is replaced with a grounding end cap  327 . The grounding cap  327  may be crimped, clipped, or clamped onto the sleeve  144  or screwed on with threads. As shown in FIG. 7, one end of the end cap  327  is screwed on the sleeve  144  and the other end is crimped on the tube  132 .  
         [0044]    [0044]FIG. 8 shows an embodiment in which the grounding cap  327  is replaced with a electrically-conducting mesh  328  which is clamped on the sleeve  144  and tube  130  using clamps  330 . The clamps  330  could also be used to secure the grounding cap  327  shown in FIGS. 7 and 9 to the sleeve  144  and tube  130 .  
         [0045]    [0045]FIG. 9 shows an embodiment in which the grounding cap  327  is clipped on or held on using a spring  329 . In this case, the cap  327  would clip to the sleeve  144  at one or more suitable friction contact points in opposition to the force of the spring pushing against the cap  327  or, the spring could be biased to hold the cap  327  onto the sleeve  144 .  
         [0046]    [0046]FIG. 4 shows the termination  160 . The two heating-element wires  155   n  are (preferably) brazed together at one or more joints  163 , and a sleeve  164  is crimped over the tube  152 .  
         [0047]    Although only preferred embodiments of the invention are specifically illustrated and described herein, it will be appreciated by one of ordinary skill in the art that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.