Abstract:
A current limiting device for protecting electrical circuits has a case and a pair of separable electrodes disposed within the case. Each electrode has a plurality of openings with an ablative member abutting the openings at an outer surface of the electrode. A spring is disposed between each ablative member and the case for urging the electrodes together. When the electrical current exceeds a predetermined setpoint the electrodes separate and an arc is created between the electrodes. The arc heats the ablative member causing expulsion of gasses which further increase the gap resistance and cool the arc to thereby quenching the arc. In a second embodiment of the current limiting device, one of the electrodes is a fixed electrode. The ablative member is disposed about a surface of the moveable electrode with a plurality of legs passing through a plurality of openings of the moveable electrode and in contact with an inner surface of the fixed electrode. A plurality of ablative member springs urges the ablative member against the fixed electrode and a plurality of electrode springs urge the movable electrode against the fixed electrode. In the second embodiment the efficiency of the expulsion of gasses is increased because the legs of the ablative member are positioned within the arc.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the field of high power voltage, circuit interruption devices and more particularly to arc quenching expulsion current limiting devices. 
     Current limiting devices require the rapid development of arc voltage. Prior art shows the use of conductive material filled polymers as contact materials (Ref. U.S. Pat. No. 4,778,958). Such contact materials, while showing good arc quenching capability, show high contact resistance and high erosion rate. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the invention, a current limiting device for protecting electrical circuits includes a pair of separable electrodes disposed within a case. Each electrode has at least one opening with an ablative member abutting the opening at an outer surface of the electrode. A spring is disposed between each ablative member and the case for urging the electrodes together. 
     In another embodiment of the present invention, a current limiting device includes a first and second separable electrodes disposed in the case. The second electrode has at least one opening for receiving a member formed of ablative material. The ablative member includes a leg portion that passes through the opening of the second electrode to contact the first electrode. An ablative member spring urges the ablative member against the first electrode, and an electrode spring urging the second electrode against the first electrode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of one embodiment of the current limiting device of the present invention; 
     FIG. 2 is a partial perspective view of an electrode of the current limiting device of FIG. 1; 
     FIG. 3 is a cross-sectional view of an alternate embodiment of the current limiting device of the present invention; 
     FIG. 4 is a partial top plan view of a movable electrode of the alternate embodiment of the current limiting device of FIG. 3; 
     FIG. 5 is a partial cross-sectional view of a second alternative embodiment of a current limiting device of the present invention, wherein the current limiting device is shown in the closed position; and 
     FIG. 6 is a partial cross-sectional view of the current limiting device of FIG. 5 wherein the current limiting device is shown in the open position. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an exemplary embodiment of an arc quenching expulsion current limiting device is shown generally at  10 . The current limiting device  10  is located within a current carrying loop of an electric circuit (not shown). The current limiting device is coupled in series with a power source and a load via leads  12  to provide short circuit protection. Any current in the current carrying conductor therefore will pass from the power source, through the current limiting device and to the load. 
     The current limiting device  10  comprises two opposing separable electrodes  14  disposed within a generally rectangular case  16 . Each electrode is substantially planar having a generally rectangular shape. The electrodes  14  comprise an electrically conductive material. Examples of suitable conductive materials include copper, silver, silver plated copper and any of the electrical contact materials such as silver tungsten, silver cadmium-oxide and silver tin-oxide. In the alternative, the electrodes may also be formed of thermal-electric heating materials, such as a bimetal, to aid the electromagnetic force urging the contacts apart. Furthermore, other magnetic arrangements may be added to aid in faster and greater contact separation. 
     The case  16  is constructed from a non-conducting material, such as a polymeric material. Preferably, the case includes vent holes  40  to permit the release of gases produced during operation of the current limiting device. Each wire lead  12  is attached to a respective end  20  of each electrode  14  that passes through the case  16 . By surrounding at least one of the conductors with a magnetic material such as steel and attaching steel to the electrode(s) to form in effect a solenoid, the electromagnetic force urging the contacts apart can be enhanced. 
     Referring now to both FIGS. 1 and 2, each electrode  14  has an inner contact portion  22 , opposing each other. The inner contact portion  22  includes a plurality of openings  24  disposed therein. The inner contact portions may be formed of a meshed material. The inner contact portions further extend inwardly to provide a trough  26  for receiving a strip  28  formed of ablative material, which will be described hereinafter in greater detail. The inner contact portions  22  of the electrodes electrically contact each other when disposed in the closed position to permit conduction of the current from one lead  12  to the other. The openings  24  of the inner contact portions  22  further permit the heat and gasses of a gap created arc to rapidly interact with the ablative strip. It can be appreciated that other porous material or structures having a plurality of openings  24 , such as wire mesh or grate, are also suitable. 
     The strip  28  comprises an ablative material such as cellulose filled melamine formaldehyde, nylon, and epoxy. The ablative material is a material which ablates and emits gas at temperatures greater than 200C. The material can be a polymer material such as a thermoplastic (for example, polytetrafluoroethylene, poly(ethyleneglycol), polyethylene, polycarbonate, polyimide, polyamide, polyoxymethylene, polymethylmethacrylate, polyester, etc.); a thermoset plastic (for example, epoxy, polyester, polyurethane, phenolic, alkyd); or an elastomer (for example silicone (polyorganosiloxane), (poly)urethane, isoprene rubber, neoprene, etc.). 
     In addition, the polymer material can be filled with a filler to improve specific properties such as the mechanical properties, dielectric properties, or to provide enhance arc-quenching properties or flame-retardant properties. Materials which could be used as filler include: a filler selected from reinforcing fillers such as fumed silica, or extending fillers such as precipitated silica and mixtures thereof. Other fillers include titanium dioxide, lithopone, zinc oxide, diatomaceous silicate, silica aerogel, iron oxide, diatomaceous earth, calcium carbonate, silazane treated silicas, silicone treated silicas, glass fibers, magnesium oxide, chromic oxide, zirconium oxide, alpha-quartz, calcined clay, carbon, graphite, cork, cotton sodium bicarbonate, boric acid, alumina-hydrate, etc. Other additives may include: impact modifiers for preventing damage to the material such as cracking upon sudden impact; flame retardant for preventing flame formation and/or inhibiting flame formation in the current limiter; UV screens for preventing reduction in component physical properties due to exposure to sunlight or other forms of UV radiation. 
     The ablative strip  28  is generally rectangular having a predetermined size generally equal to the dimensions of the trough  26  of the electrodes  14 . More specifically, the ablative strip is disposed over the plurality of openings  24  of the inner contact portions  22 . The thickness of the ablative strip  28  is greater than the depth of the trough  26  such that the strip extends beyond the trough. 
     A pair of leaf springs  30  are disposed in the case  16  to urge and compress the strips  28  of ablative material, disposed in the electrodes  14  together. Each leaf spring  30  is set between an inner surface of the case  16  and an outer surface  32  of each ablative strip  28 . Ends  34 ,  36  of each leaf spring  30  are mounted onto the case. A central portion  38  of each spring engages each respective ablative strip  22  to springably compress the ablative strips and the electrodes  14  together. 
     When the current limiting device  10  is connected in series with the load, the leaf springs  30  maintain the raised inner contact portion  22  of each electrode  14  in contact during normal operation. The electric current flowing through the electrodes  14  creates an electromagnetic force urging the electrodes apart. The electromagnetic force urging the electrodes open is directly proportional to the current flowing through the wires. Opposing the electromagnetic force are leaf springs  30 , each spring urging its respective electrode  17  towards the opposing electrode  14  and maintaining the electrodes closed as described hereinbefore. The electrodes part when the force of the current overcomes the force of the leaf springs  30 . One skilled in the art would appreciate that the stiffness of the spring changes the set point of the device  10 , i.e., a stiffer spring results in a higher setpoint. A resistor  13  may also be electrically connected in parallel with the device  10 , such as between the leads  12 , may be used to minimize gas pressure and promote rapid arc quenching. 
     When an overcurrent or ground fault condition occurs, the electrodes  14  separate, creating a gap between the electrodes that results in a high voltage arc forming therebetween. The arc rapidly generates heat and ionizing gasses. The plurality of openings  24  on the electrodes  14  facilitates the transfer of heat from the arc and promotes the intermixing of the evolved gases from the ablative strips  28  with the plasma created by the arc. 
     The heat further causes the strips  28  of ablative material to gasify. The gasses from the ablative strip decrease the conductivity within the gap, cool the electrodes  14  along the arc length and also create a high-pressure region to further force the electrodes open. The disposition of the ablative strips on the openings  24  results in a rapid and high gap voltage build up terminating the overcurrent condition. As described hereinabove, the vent  40  permits expulsion of the gasses to limit the high-pressure in the case  19 . 
     Referring to FIG. 3, an alternative embodiment of the current limiting device is shown generally at  50 . The device comprises a case  52 , having a vent  53  that houses a fixed electrode  54  and an opposing movable electrode  56 . One end  58  of each electrode  54 ,  56  passes through the case  52  and is attached to wire leads  60  respectively. The fixed electrode  54  and movable electrode  56  is formed of an electrically conductive material as described hereinabove. The fixed electrode  54  is supported within the case  52  by a bottom surface  57  of the case  52 . The fixed electrode  54  is generally a solid rectangular strip. An inner end  62  of the movable electrode  56  is in electrical contact with an inner surface  64  of the fixed electrode  54 . The inner end  62  of the movable electrode  56  includes a plurality of through openings  66 , as best shown in FIG. 4, for receiving an ablative member  68 . At an intermediate portion  70  of the movable electrode  56 , the movable electrode steps upward, away from the fixed electrode  54  to separate the ends  58  of the electrodes a predetermined distance. 
     The ablative member  68  is composed of an ablative material, similar to that described hereinabove. The ablative member has a rectangular planar portion  72  from which a plurality of cylindrical legs  74  depends downwardly therefrom. The legs  74  of the ablative member  68  have a diameter less than the diameter of the openings  66  of the movable electrode  56  to permit passage of the legs  74  through the openings  66  and to permit free movement of the movable electrode  56  about the legs (to be described hereafter). The legs  74  are of a predetermined length longer than the thickness of the movable electrode  56  to permit the legs to contact an inner surface  64  of the fixed electrode  54  and allow movement of the moveable electrode. A space  76  disposed between the moveable electrode and the rectangular portion of the ablative member  68  defines the arc quenching gap. 
     A plurality of electrode springs  78  are interposed between the case  80  and the outer surface  82  of the moveable electrode  56 . The electrode springs  78  pass through openings in the ablative member  72  to engage the movable electrode  56 . The springs are coil springs and compressively urge the movable electrode  56  downward against the fixed electrode  54 . The setpoint of the current limiting device  50  is dependent on the compressive force of the electrode springs  78 . 
     In addition to the electrode springs  78 , a plurality of ablative member springs  84  are interposed between the case  80  and an opposing surface of the rectangular portion  72  of the ablative member  68 . The springs  84  urge the cylindrical legs  74  against the inner surface  64  of the fixed electrode  54  by the springs  84  to maintain the legs  74  against the fixed electrode during the operation of the current limiting device  50 . 
     During normal operating condition, the springs  78 ,  84  urge, respectively, the ablative member  68  and the movable electrode  56  against the fixed electrode  54  to conduct current to the protected load. When an overcurrent or ground fault condition occurs, the movable electrode  56  is repelled upward and away from the fixed electrode  54 . As described hereinbefore, the electrode springs  78  define the setpoint of the current trip level of the current limiting device. As the movable electrode repels from the fixed electrode, the ablative member  68  is maintained continually in contact with the fixed electrode during the operation of the current limiting device  50 . The ablative member acts to quench the arc created between the electrodes  54 ,  56 . 
     FIGS. 5 and 6 illustrate a further embodiment a current limiting device  90  of the present invention, which is similar to the embodiment  50  of FIGS. 3 and 4. The current limiting device  90  includes a fixed or stationary electrode  92  disposed intermediate a movable electrode  94  and an ablative member  96 . The movable electrode  94  is a solid planar member similar to the fixed electrode  54  of FIG.  3 . The ablative member  96  of similar construction as the ablative member  68  of FIG. 3 is formed of ablative material. The ablative member  96  has a plurality of cylindrical members  98  extending downward from a planar portion  100  that engage the movable electrode  94 . The ablative member  96  and movable electrode  94  are urged together by an ablative member spring  102  that urges the ablative member downward and an electrode spring  104  that urges the movable electrode  94  upward. 
     A portion of the fixed electrode  92  may be formed of a wire mesh that includes a plurality of openings  106  for receiving the cylindrical members  98  of the ablative members  96 . One will appreciate that the fixed electrode  92  may be similar to the movable electrode  54  of FIG.  4 . 
     FIG. 5 is illustrative of the current limiting device  90  during normal operation when no fault condition is present. During normal operation, the force of the electrode spring  104 , which is greater than the force of the ablative member springs  102 , urges the movable electrode  94  upward against the fixed electrode  92  to permit current to pass therebetween to the protected load. 
     During an overcurrent or ground fault condition, the movable electrode  94  repels from the fixed electrode  92  as shown in FIG.  6 . As described hereinbefore, the electrode spring  104  defines the setpoint of the current trip level of the current limiting device  90 . As the movable electrode  94  repels from the fixed electrode  92 , the ablative member  96  is maintained in contact with the fixed electrode during the operation of the current limiting device  90 . The ablative member  96  acts to quench the arc created between the electrodes  92 ,  94 . 
     In this alternate embodiment efficient mixing of the expulsion gasses occurs because the ablative material of the ablative member  96  comprising the cylindrical legs  98  is inserted into the middle of the arc, which is generated during the opening of the electrodes  92 ,  94 . 
     An advantage of the current limiting device as illustrated is to provide a device having low contact resistance between the electrodes and low erosion rate and faster interruption by separating the electrode from the ablative material. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.