Abstract:
A method of terminating or joining mineral insulated cables with metallic outer tubes does not use brazing to fasten outer sleeves over the outer tubes. 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 outer tubes.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to termination and connection of mineral insulated (MI) heating units and cables. 
     2. Description of the Related Technology 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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; all oxide must be removed, or else the joint will not be good. 
     Brazing involves high temperatures that alter the physical properties of the metal in the tubes, making it 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. 
     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 OF THE INVENTION 
     Accordingly, objects of the present invention are 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 unhealthy fumes. 
     The present invention reduces or eliminates 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. That 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 couplings and end fittings. 
     The present invention may be assembled or made either in a factory or in the field. 
     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 
     FIG. 1 is a plan view of the invention; 
     FIG. 2 is a detailed, partially cut-away, view of the invention according to FIG. 1; 
     FIG. 3 is a detailed, partially cut-away, view of the invention according to FIG. 1; 
     FIG. 4 is a detailed, partially cut-away, view of the invention according to FIG. 1; and 
     FIG. 5 is a cross-sectional view of section V—V of FIG.  1 . 
     FIG. 6 is a view similar to FIG. 4 but showing an alternative grounding strap. 
     FIG. 7 is a view similar to FIG. 4 but showing an alternative grounding cap. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. Two external-power connection wires  105  extend from one end of the mineral insulated cable  100  for connection to a power source. A first coupling  120  couples the wires  105  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 ,  155  are enclosed in them. Their common structure is shown in FIG.  5 . It is noted that the invention does not require similarity of structure. 
     In this disclosure, “termination” includes the structure at the end of a single cable section as well as a joint between two cables. 
     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  190 , and a pair of inner conductors  135  or  155 . The inner conductors may be of low resistance ( 135 , power lead) or of high resistance ( 155 , heating element). 
     FIG. 2 shows in detail the termination or coupling  120  at which the two external power wires  105  are joined to the power lead  130 . The wires  105  may be extensions of the two conductors  135  running through the power lead  130 , or alternatively there may be a brazed joint  123  between each conductor  135  and its respective external connection wire  105 . If the conductors  135  and/or the wires  105  are insulated wires, then an insulating sleeve (e.g., shrink tubing) can be applied to cover the bare joint. 
     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 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. 
     The preferred material for the sleeve is free-machining brass. One example of a preferred crimping tool is Thomas and Betts model TBM-25S. 
     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. The epoxy seeps into any gaps between the tube  132  and the crimped portion  122  of the termination  120 . 
     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. 
     FIG. 3 illustrates the coupling  140  between the power lead  130  and the heating element  150 . A preferably brazed connection  143  joins the low- and high-resistance conductors  135  and  155 . 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 . 
     Preferably, the cable is assembled in a vertical orientation with the crimped portions  122 ,  142 , and  162  downward. The epoxy  180  fills the upper open end of the sleeve  120 ,  140 , or  160 , flows downward to fill the cavity, ani sets. That seals the conductors within and mechanically joins the tubes  132 ,  152  into a solid unit. 
     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 , shown in FIG. 1, is used to connect the two tubes and is preferably brazed to the tubes  132 ,  152 , and optionally to the sleeve  140 . The wire or strap  325  may be spot-brazed as shown, either before or after filling the sleeve  140  with the epoxy  180 . 
     A similar connecting wire or strap may optionally connect the tube  132  to the sleeve  120  or an adjacent metallic structure (not shown). 
     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 with epoxy. 
     FIG. 7 shows an embodiment in which the grounding strap  325  is replaced with a grounding cap  327 . The grounding cap  327  may be crimped onto the sleeve  144  or attached with threads. 
     Here, and in the following claims, “connection strap” includes any wire, strip, clamp, spring, lead, cable, mesh, screw-on or clip-on device, or any other conductive element. 
     FIG. 4 shows the termination  160 . The two heating-element wires  155  are (preferably) brazed together at a joint  163 , and a sleeve  164  is crimped over the tube  152 . 
     Although certain presently preferred embodiments of the present invention have been specifically described herein, it will be apparent to those skilled in the art to which the invention pertains that variations and modifications of the various embodiments shown and described herein may be made without departing from the spirit and scope of the invention. Accordingly, it is intended that the invention be limited only to the extent required by the appended claims and the applicable rules of law.