Abstract:
A fuse cartridge is provided having improved tensile strength via the provision of a mechanical strain wire that is in parallel with a fusible element mechanically but not electrically, thereby increasing the tensile strength of the fusible element without altering the electrical time-current characteristics of the fusible element. In a preferred embodiment, the strain wire is provided by looping a fusible element wire from a first terminal around a second terminal and back near the first terminal but electrically insulated therefrom.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 60/432,560 filed on Dec. 12, 2002. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to fuse cartridges for circuit-interrupting devices such as fuses, cutouts and the like and more particularly to a low current fuse cartridge having improved tensile strength.  
           [0004]    2. Description of the Related Art  
           [0005]    Various types of circuit interrupting devices in the electrical power distribution and transmission field utilize fuse cartridges having fusible elements that are designed to provide desirable circuit interrupting characteristics, i.e. time-current characteristics. The available time-current characteristics include protection against fault currents and other overload currents. For example, ANSI standard C37.42 describes various time-current characteristics. The circuit-interrupting devices are utilized at different locations in the electrical systems to provide different functions along with coordinating with one another to provide the most reliable system and minimize outages and the number of affected power users on the system. In order to provide desirable low current time-current characteristics, small diameter fusible elements are required. However, these fusible elements do not have the necessary tensile strength to withstand the spring force of circuit interrupters such as is required for the Fault Tamers Fuse Limiter available from the S&amp;C Electric Company, Chicago, Illinois. A circuit interrupter of this type is shown in U.S. Pat. No. 5,502,427. Accordingly, heretofore, it has not been possible to provide a suitable low current fuse cartridge for such devices. While some prior art fuses provide strain wires in parallel with the fusible element, this is not suitable to achieve low-current fusible elements due to the deleterious effects of the strain wire on the time-current characteristics.  
           [0006]    While the fusible elements of the prior art may be generally suitable for their intended uses, it would be desirable to provide a low current fuse cartridge having improved tensile strength.  
         SUMMARY OF THE INVENTION  
         [0007]    Accordingly, it is a principal object of the present invention to provide a low current fuse cartridge having improved tensile strength.  
           [0008]    These and other objects of the present invention are efficiently achieved by a fuse cartridge having improved tensile strength via the provision of a mechanical strain wire that is in parallel with a fusible element mechanically but not electrically, thereby increasing the tensile strength of the fusible element without altering the electrical time-current characteristics of the fusible element. In a preferred embodiment, the strain wire is provided by looping a fusible element wire from a first terminal around a second terminal and back near the first terminal but electrically insulated therefrom. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0009]    The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in conjunction with the accompanying drawing in which:  
         [0010]    [0010]FIG. 1 is an elevational view, partly in section and with parts cut away for clarity, of a fuse cartridge in accordance with a first embodiment of the present invention;  
         [0011]    [0011]FIG. 2 is a top plan view of portions of the fuse cartridge of FIG. 1;  
         [0012]    [0012]FIG. 3 is a partial view of an alternate embodiment of the fuse cartridge of FIG. 1; and  
         [0013]    [0013]FIG. 4 is a partial view of yet another alternative embodiment of the fuse cartridge of FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0014]    Referring now to FIGS. 1 and 2, there is illustrated a fuse cartridge  10  in accordance with a first illustrative embodiment of the present invention that is suitable for use as the fuse cartridge  52  of U.S. Pat. No. 5,502,427. While the fuse cartridge  10  of the illustrative embodiment includes certain features for specific adaptation to the fuse limiter of the 5,502427 patent, it should be understood that the principles of the improved tensile strength low-current fuse cartridge of the present invention are applicable for use with other types and forms of circuit interrupters as well. The fuse cartridge  10  includes a fusible element  12  having an element wire  14  that defines the time-current characteristics of the fuse cartridge  10  and that is disposed between an upper terminal  16  and a element terminal  18 , e.g. being electrically connected to a lower portion  16   a  of the upper terminal  16  by a swaging operation or the like.  
         [0015]    In accordance with important aspects of the present invention, the element wire  14  is looped around the element terminal  18  and is mechanically secured to an insulating portion  20  of the upper terminal  16 , e.g. the insulating portion  20  being a circular member affixed to and carried by the upper terminal  16  and also including a lower extending portion  20   a . In the specific arrangement as shown in FIG. 2, the wire element  14  is looped through passages  21 ,  23  in the insulating portion  20  and tied back upon itself.  
         [0016]    Thus, the fuse cartridge  10  exhibits the electrical properties of one wire and the mechanical properties of two parallel wires. In this manner, the electrical time-current characteristics of a single wire element are provided along with the increased mechanical tensile strength of two wires.  
         [0017]    Specifically, in the illustrative embodiment, the element terminal  18  of the fuse cartridge  10  includes a forked portion  33  defined by times  30 ,  32  and a pin  34  that is passed through an aperture  36  of the element terminal  18  that also communicates with the forked portion  33 . The element wire  14  is looped around the pin  34 . The element terminal  18  is electrically and mechanically connected to a lower terminal  40  via a flexible cable arrangement  42  that is affixed at either end to the terminals  18  and  40  respectively. It should be noted that to achieve appropriate operation, the wire element  14  upon melting must also release the terminals  16 ,  18  for relative separation. In the arrangement shown, this is achieved by the wire element  14  being freely looped about the pin  34 .  
         [0018]    In the illustrative embodiment adapted for use with the fuse of the aforementioned U.S. Pat. device 5,502,427, the upper terminal  16  of the fuse cartridge  10  includes a threaded portion  46  for assembly into the fuse and the lower terminal  40  includes a retention arrangement  48  in the form of a clip for loading and assembly into the fuse. The flexible cable arrangement  42  collapses upon operation of the fuse cartridge  10  thus providing desirable space within the fuse tube assembly  15  of the aforementioned patent. This also establishes a relatively short length of the element terminal  18 . The forked portion  33  serves additional functions as a corona shield and mechanically protects the element wire  14  during handling and assembly by functioning as a stop that abuts the upper terminal  16 . The fuse cartridge  10  includes an outer sheath  50  affixed over the upper and lower terminals  16 ,  40  and attached to the upper terminal  16 . In preferred arrangements, the outer sheath  50  is fabricated from arc-extinguishing material. Upon assembly of the fuse cartridge  10  into a fuse, the tension is applied to the fuse cartridge  10 , i.e. across the upper terminal  16  and the retention arrangement  48  of the lower terminal  40 .  
         [0019]    Considering other aspects of the present invention and referring now to FIG. 3, an alternate arrangement for securing the wire element  14  to the insulating portion  20  includes passing the wire element  14  through a passage  27  in the insulating portion  20 , the wire element  14  at the upper end thereof including a mechanical stop  50 . Also illustrated is a bend  52  in the wire element  14  at the other end thereof for improved attachment to the upper terminal  16 , i.e. after being looped around the pin  34  as shown in FIGS. 1 and 2. In one specific arrangement, the mechanical stop  50  is readily achieved by the end of the wire element  14  being formed in a multi-turn coil  54  and the other end being looped through the coil  54  and pulled tightly to secure the wire element and define the stop  50 .  
         [0020]    Referring now to FIG. 4 and considering additional aspects of the present invention, an alternative embodiment of a fuse cartridge  100  provides the electrical properties of one wire and the mechanical properties of three parallel wires. In this manner, the electrical time-current characteristics of a single wire element are provided along with a mechanical tensile strength triple that of a single wire, i.e. three wires. Specifically, the wire element  114  including a mechanical stop  150  passes through a passage  136  of a member  138  carried by the element terminal  118 . Then the wire element  114  is looped around a pin  122  of an insulating portion  120  carried by the upper terminal  116  and back to loop around a pin  134  of the element terminal  118 . Then, the wire element  114  is electrically connected and mechanically secured to the lower portion  116   a  of the upper terminal  116 . When the wire element experiences an overcurrent and melts, the element wire  114  is released and the terminals  116 ,  118  separate. As discussed hereinbefore, it should be understood that the wire element  114  upon melting must be capable of freely moving under spring tension about the pins  134  and  122  to permit the appropriate separation of the terminals  116 , 118 .  
         [0021]    While there has been illustrated and described a preferred embodiment of the present invention, it will be apparent that various changes and modifications will occur to those skilled in the art. Accordingly, it is intended in the appended claims to cover all such changes and modifications that fall within the true spirit and scope of the present invention.