Patent Publication Number: US-6665157-B2

Title: Apparatus for interrupting an electrical circuit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of electrical circuit interrupting devices adapted to complete and interrupt electrical current carrying paths between a source of electrical power and a load. More particularly, the invention relates to a novel technique for rapidly interrupting an electrical circuit and dissipating energy in a circuit interrupter upon interruption of a current carrying path. 
     2. Description of the Related Art 
     A great number of applications exist for circuit interrupting devices which selectively complete and interrupt current carrying paths between a source of electrical power and a load. In most conventional devices of this type, such as circuit breakers, a movable member carries a contact and is biased into a normal operating position against a stationary member which carries a similar contact. A current carrying path is thereby defined between the movable and stationary members. Such devices may be configured as single-phase structures, or may include several parallel mechanisms, such as for use in three-phase circuits. 
     Actuating assemblies in circuit interrupters have been developed to provide for extremely rapid circuit interruption in response to overload conditions, over current conditions, heating, and other interrupt-triggering events. A variety of such triggering mechanisms are known. For example, in conventional circuit breakers, bi-metallic structures may be employed in conjunction with toggling mechanisms to rapidly displace the movable contacts from the stationary contacts upon sufficient differential heating between the bi-metallic members. Electromechanical operator structures are also known which may initiate displacement of a movable contact member upon the application of sufficient current to the operator. These may also be used in conjunction with rapid-response mechanical structures such as toggle mechanisms, to increase the rapidity of the interrupter response. 
     In such circuit interrupters, a general goal is to interrupt at current close to zero as rapidly as possible. Certain conventional structures have made use of natural zero crossings in the input power source to effectively interrupt the current through the interrupter device. However, the total let-through energy in such devices may be entirely unacceptable in many applications and can lead to excessive heating or failure of the device or damage to devices coupled downstream from the interrupter in a power distribution circuit. Other techniques have been devised which force the current through the interrupter to a zero level more rapidly. In one known device, for example, a light-weight conductive spanner is displaced extremely rapidly under the influence of an electromagnetic field generated by a core and winding arrangement. The rapid displacement of the spanner causes significant investment in the expanding arcs and effectively extinguishes the arcs through the intermediary of a stack of conductive splitter plates. A device of this type is described in U.S. Pat. No. 5,587,861, issued on Dec. 24, 1996 to Wieloch et al. 
     While currently known devices are generally successful at interrupting current upon demand, further improvement is still needed. For example, in devices that do not depend upon a natural zero crossing in the incoming power, back-EMF is generally relied upon to extinguish the arcs generated upon opening, which, themselves, define a transient current carrying path. The provision of spaced-apart splitter plates establishes a portion of this transient current carrying path and represents resistance to flow of the transient current, producing needed back-EMF. However, depending upon the level of power applied to the device, such sources of back-EMF may be insufficient to provide sufficient resistance to current flow to limit the let-through energy to desired levels. In particular, splitter plates, as one of the sources of back-EMF, may fail at higher voltage levels (current tending to shunt around the plates, for example), imposing a limitation to the back-EMF achievable by conventional structures. As a result, depending upon the nature of the event triggering the circuit interruption, the excessive let through energy can degrade or even render inoperative the interrupter device. 
     There is a need, therefore, for an improved circuit interrupting device which can provide efficient current carrying capabilities during normal operation, and which can rapidly interrupt current carrying paths, while limiting let through energy to reduced levels by virtue of rapid arc extinction. There is a particular need for a new device structure which is economical to manufacture and can be packaged in various sizes and ratings. 
     SUMMARY OF THE INVENTION 
     The invention provides a novel technique for interrupting an electrical circuit and for dissipating energy in a circuit interrupter designed to respond to these needs. The technique may be employed in a wide variety of circuit interrupting devices, such as circuit breakers, motor controllers, switch gear, and so forth. Moreover, the technique may be incorporated with various interrupter structures, such as interrupters employing a light-weight spanner displaced under the influence of an electromagnetic field generated by a core, as well as various other triggering mechanisms which initiate circuit interruption. 
     In accordance with the technique, a transient current carrying path includes at least one variable or controllable resistance element. The element establishes a preferred current path during an initial phase of circuit interruption to cause current flow through the transient current carrying path and thereby to interrupt flow through a normal or main path through the interrupter. The element then changes a conductive state to enhance the energy-dissipating capabilities of the transient current carrying path. In a preferred configuration, a variable resistance structure is positioned adjacent to incoming and outgoing conductors, and is in a relatively conductive state during the initial phase of circuit interruption. Current through arcs during this initial phase of interruption is conveyed into the transient current carrying path by virtue of the resistance of the element. A rapid change in the resistive state of the element then ensues, contributing to rapid interruption of the transient current by contributing additionally to the back-EMF through the device. The change in resistive state may be a function of heating by the transient current. The novel structure may be employed in both single and multi-phase circuit interrupters. The elements which establish the transient current carrying path, and which change their resistive state, may be static components, such as a polymer in which a dispersion of conductive material is doped, or what may be referred to as positive temperature coefficient (PTC) materials. The transient or alternative current carrying path may include a series of splitter plates separated by air gaps and electrically in series with the variable resistance element. The transient current carrying path may thus present an essentially open circuit during normal operation of the device, and may comprise only mechanically static elements electrically in parallel with the normal current path through the interrupter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: 
     FIG. 1 is a perspective view of a circuit interrupter in accordance with the present technique for selectively interrupting an electrical current carrying path between a load and a source; 
     FIG. 2 is a sectional view through the assembly of FIG. 1, illustrating functional components of the assembly in a normal or biased position wherein a first current carrying path is established between the source and load; 
     FIG. 3 is a transverse sectional view through a portion of the device of FIG. 1, illustrating the position of a movable conductive element in the device adjacent to a stationary conductive element; 
     FIG. 4 is an enlarged detailed view of a portion of the device as shown in FIG. 2, including a variable resistance assembly for aiding in interrupting current through the device in accordance with certain aspects of the present technique; 
     FIG. 5 is a diagrammatical representation of certain functional components illustrated in the previous figures, showing a normal or first current carrying path through the device as well as a transient or alternative current carrying path through the variable-resistance structures; 
     FIG. 6 is a diagrammatical representation of the functional components shown in FIG. 5 during a first phase of interruption of the normal current carrying path through the device; 
     FIG. 7 is a diagrammatical representation of the functional components shown in FIG. 6 at a subsequent stage of interruption; 
     FIGS. 8 a ,  8   b ,  8   c ,  8   d  and  8   e  are schematic diagrams of equivalent circuits for the device in the stages of operation shown in FIGS. 5,  6  and  7 ; 
     FIG. 9 is a graphical representation of voltage and current traces during interruption of an exemplary conventional circuit interrupter; and 
     FIG. 10 is a graphical representation of exemplary voltage and current traces during interruption of a device in accordance with the present technique. 
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Turning now to the drawings, and referring first to FIG. 1, a modular circuit interrupter is represented and designated generally by the reference numeral  10 . The circuit interrupter is designed to be coupled to an incoming or source conductor  12  and to an outgoing or load conductor  14 , and to selectively complete and interrupt current carrying paths between the conductors. The interrupter module as illustrated in FIG. 1 generally includes an outer housing  16  and an inner housing  18  in which the functional components of the module are disposed as described in greater detail below. Outer housing  16  is covered by a cap  20 . 
     It should be noted that the circuit interrupter module  10 , shown in FIG. 1, is subject to various adaptations for incorporation into a wide variety of devices. For example, the interrupter module, and variants on the structure described below, may be incorporated into single phase or multi-phase interrupting devices such as circuit breakers, motor protectors, contactors, and so on. Accordingly, the module may be associated with a variety of triggering devices for initiating interruption, as well as with devices for preventing closure of the current carrying path following interruption. A range of such devices are well known in the art and may be adapted to function in cooperation with the module in accordance with the techniques described herein. Similarly, while in the embodiment described below a movable conductive element in the form of a spanner extends between a pair of stationary conductive elements or contacts, adaptations to the structure may include a movable element which contacts a single stationary element, or multiple movable elements which contact one another. 
     Returning to FIG. 1, also visible in this view is an interrupt initiator assembly, designated generally by the reference numeral  22 . As described below, in the illustrated embodiment the initiator assembly causes initial interruption of a normal or first current carrying path through the device under the influence of an electromagnetic field. On either side of the interrupter assembly a series of splitter plates  24  are positioned and separated from one another by air gaps  26 . Below each stack of splitter plates, a variable or controllable resistance assembly  28  is positioned for directing current through an alternative current carrying path during interruption of the normal current carrying path, and for aiding in rapidly causing complete interruption of current through the device. 
     FIG. 2 represents a longitudinal section through the device shown in FIG.  1 . As illustrated in FIG. 2, initiator assembly  22  is formed of a unitary core having a lower core portion  30  and an upper core portion  32 . Lower core portion  30  extends generally through the device, while upper core portion  32  includes a pair of upwardly-projecting elements or panels extending from the lower core portion  30 . These upwardly-projecting elements are best illustrated in FIG.  3 . In the illustrated embodiment, one of the conductors, such as conductor  14 , is wrapped around lower core portion  30  to form at least one turn  34  around the lower core portion, as illustrated in FIG.  2 . The turn or wrap around the core enhances an electromagnetic field generated during overload, overcurrent, and other interrupt-triggering events for initiating interruption. Lower and upper core portions  30  and  32  are preferably formed of a series of conductive plates  36  stacked and bound to one another to form a unitary structure. The individual plates in the core may be separated at desired locations by insulating members (not shown). 
     Conductors  12  and  14  are electrically coupled to respective stationary conductors  38  and  40  on either side of the initiator assembly. A variety of connection structures may be employed, such as bonding, soldering, and so forth. Each stationary conductor includes an upper surface which forms an arc runner, indicated respectively by reference numerals  42  and  44  in FIG.  2 . Stationary contacts  46  and  48  are bonded to each stationary conductor  38  and  40 , respectively, adjacent to the arc runners. In the embodiment illustrated in the Figures, the stationary conductors, the arc runners, and the stationary contacts are therefore at the electrical potential of the respective conductor to which they are coupled. A movable conductive element or spanner  50  extends between the stationary conductors and carries a pair of movable contacts  52  and  54 . In a normal or biased position, the movable conductive spanner is urged into contact with the stationary conductors to bring the stationary and movable contacts into physical contact with one another and thereby to complete the normal or first current carrying path through the device. 
     Each stationary conductor  38  and  40  extends from the arc runner to form a lateral extension  56 . Each extension  56  is electrically coupled to a respective variable resistance assembly  28  to establish a portion of the alternative current carrying path through the device. In the illustrated embodiment, each variable resistance assembly includes a spacer  58 , a series of variable or controllable resistance elements  60 , a conductor block  62 , a biasing member  64 , and a conductive member  66 . The presently preferred structure and operation of these components of the assemblies will be described in greater detail below. In general, however, each assembly offers an alternative path for electrical current during interruption of the normal current carrying path, and permits rapid interruption of all current through the device by transition of resistance characteristics of the alternative path. Splitter plates  24 , separated by air gaps  26 , are positioned above conductive member  66 , and a conductive shunt plate  68  extends between the stacks of splitter plates. 
     Certain of the foregoing elements are illustrated in the transverse sectional view of FIG.  3 . As shown in FIG. 3, the plates  36  of the lower and upper core portions  30  and  32  form a generally H-shaped structure. An insulating liner  70  may extend between the upper core portions  32  and turns  34 , and the stationary and movable contacts, to protect the core and turns from the arc. Liner  70  may include an extension of an internal peripheral wall of inner housing  18  shown in FIG. 1. A biasing member, such as a compression spring  72 , is provided for urging the movable conductive spanner  50  into its normal or biased engaged position to complete the normal current carrying path. As mentioned above, in this orientation, movable and stationary contacts (see contacts  54  and  48  in FIG. 3) are physically joined to complete the normal current carrying path. In the illustrated embodiment lower core portion  30  also forms a trough  74  in which conductor  14  and at least one extension of turn  34  of the conductor are disposed. 
     The foregoing functional components of interrupter module  10  may be formed of any suitable material. For example, plates  36  of the core portions may be formed of a ferromagnetic material, such as steel. Stationary conductors  38  and  40  may be formed of a conductive material such as copper, and may be plated in desired locations. Similarly, movable conductive element  50  is made of an electrically conductive material such as copper. The stationary and movable contacts provided on the stationary and movable conductive elements are also made of a conductive material, preferably a material which provides some resistance to degradation during opening and closing of the device. For example, the contacts may be made of a durable material such as copper-tungsten alloy bonded to the respective conductive element. Finally, conductive members  66 , splitter plates  24  and shunt plate  68  may be made of any suitable electrically conductive material, such as steel. 
     The components of the variable resistance assemblies  28  are illustrated in greater detail in FIG.  4 . In the illustrated embodiment, each stationary conductor, such as stationary conductor  38 , includes a lower corner  76  formed between the arc runner (see FIG. 2) and the lateral extension  56 . The lateral extension is generally supported by the inner housing  16 . One or more variable resistance elements  60  are electrically coupled between each extension  56  and a respective conductive member  66 , through the intermediary of a conductor block  62 , if necessary. That is, where the spacing in the device requires electrical continuity to be assisted by such a conductive member, one is provided. Alternative configurations may be envisaged, however, where a conductor block  62  is not needed and electrical continuity between the stationary conductor and conductive member  66  is provided by the variable resistance elements alone. Moreover, in the illustrated embodiment, spacer  58 , which is made of a non-conductive material, is positioned within the lower corner  76  between the lateral extension and a side or end surface of the variable resistance elements. In general, such spacers may be positioned in the device to reduce free volumes  78 , or to change the geometry of such volumes, and thereby to limit or direct flow of gasses and plasma in the device during interruption. Again, where the geometry of the device sufficiently controls such gas or plasma flow, spacers of this type may be eliminated. 
     Electrical continuity between extensions  56  and conductive members  60  is further enhanced by biasing member  64 . A variety of such biasing members may be envisaged. In the illustrated embodiment, however, the biasing member consists of a roll pin positioned between a lower face of lateral extension  56  and a trough formed in the inner housing. The biasing member forces the extension upwardly, thereby insuring good electrical connection between the extension, the variable resistance elements, and conductive member  66 . 
     In the illustrated embodiment, a group of three variable resistance elements is disposed on either side of the initiator assembly. The variable resistance elements are electrically coupled to one another in series, and the groups of elements form a portion of the transient or alternative current carrying path through the device as discussed below. Depending upon the desired resistance in each of these assemblies, more or fewer such elements may be employed. Moreover, various types of elements  60  may be used for implementing the present technique. In the illustrated embodiment, each element  60  comprises a conductive polymer such as polyethylene doped with a dispersion of carbon black. Such materials are commercially available in various forms, such as from Raychem of Menlo Park, Calif., under the designation PolySwitch. In the illustrated embodiment, each of the series of three such elements has a thickness of approximately 1 mm. and contact surface dimensions of approximately 8 mm.×8 mm. In addition, to provide good termination and electrical continuity between the series of elements  60 , each element body  80  may be covered on its respective faces  82  by a conductive terminal layer  84 . Terminal layer  84  may be formed of any of a variety of materials, such as copper. Moreover, such terminal layers may be bonded to the faces of the element body by any suitable process, such as by electroplating. 
     While the conductive polymer material mentioned above is presently preferred, other suitable materials may be employed in the variable resistance structures in accordance with the present technique. Such materials may include metallic and ceramic materials, such as BaTiO 3  ceramics and so forth. In general, variable resistance elements such as elements  60  change their resistance or resistive state during operation from a relatively low resistance level to a relatively high resistance level. Commercially available materials, for example, change state in a relatively narrow band of operating temperatures, and are thus sometimes referred to as positive temperature coefficient (PTC) resistors. By way of example, such materials may increase their resistivity from on the order of 10 mΩcm at room temperature to on the order of 10 MΩcm at 120°-130° C. In the illustrated embodiment, for example, each element transitions during interruption of the device from a resistance of approximately less than 1 mΩ to a resistance of approximately 100 mΩ. 
     The voltage provided by these elements during fault interruption is a function of time that also depends on external circuit parameters which may vary. For example, under a typical 480 volt AC, 5 kA available conditions with 70% power factor, each element generates a back-EMF that rises smoothly from zero to approximately 12 volts at 1.5 ms after fault initiation and holds relatively constant thereafter until the fault current is terminated. As discussed more fully below, in the present technique, the elements do not pass current during normal operation, that is, as current is passed through a normal current carrying path in the device. Thus, during normal operation the elements do not offer voltage drop with normal load currents. 
     FIGS. 5,  6  and  7  illustrate current carrying paths through the device described above, both prior to and during interruption. As illustrated diagramatically in FIG. 5, a normal or first current carrying path through the device, represented generally by reference numeral  86 , includes segments A, B and C. Segment A includes conductor  12  extending up to and partially through stationary conductor  38 . Similarly, section B includes conductor  14  and a portion of stationary conductor  40 . It should be noted that the turn around the interrupt initiator assembly described above is not illustrated in FIGS. 5,  6  and  7  for the sake of simplicity. Section C of the normal current carrying path  86  is established by the stationary conductors  38  and  40 , by movable conductive spanner  50 , and the stationary and movable contacts disposed therebetween. Thus, during normal operation, current may flow freely between the source and load. The normal current carrying path is maintained by biasing of the movable conductive spanner against the stationary conductors. 
     A transient or alternative current carrying path is defined through the variable resistance assemblies described above. As illustrated in FIG. 5, this transient current carrying path, designated generally by the reference numeral  88 , includes section A described above, as well as a section D extending through the extension  56  of stationary conductor  38 , the variable resistance elements  60  associated therewith, the conductor block  62 , if provided, and conductive member  66 . The transient current carrying path then extends through the series of air gaps and splitter plates, and therefrom through shunt plate  68 . Moreover, the transient current carrying path also is defined by section B described above, through conductor  14 , and through extension  56  of stationary conductor  40 , as well as through the variable resistance elements, conductor block and conductive member  66  associated therewith, as indicated by the letter E in FIG.  5 . Thus, the alternative or transient current carrying path through the device extends between the source and load conductors, through the variable resistance assemblies, the splitter plates, air gaps, and shunt plate, these various components being electrically connected in series. It should be noted, however, that during normal operation, the resistance offered by the transient current carrying path, particularly by the air gaps between the splitter plates, forms an open circuit preventing current flow through the transient current carrying path, and forcing all current through the device to be channeled via the normal current carrying path  86 . 
     Referring now to FIGS. 6 and 7, interruption of current flow through the device is illustrated in subsequent phases. From the normal or biased position of FIG. 5, interruption is initiated as shown in FIG. 6 by repulsion of the conductive spanner  50  from the stationary conductors. In the illustrated embodiment, this repulsion results from a strong electromagnetic field generated by the initiator assembly. Other types of interruption initiation may, of course, be provided. As the conductive spanner  50  is moved from its normal or biased position, as indicated by arrow  90  in FIG. 6, arcs  92  form between the movable and stationary contacts of the spanner and stationary conductors. These arcs migrate from the contacts outwardly along the arc runners and contact conductive members  66  of each variable resistance assembly. At this initial phase of interruption, variable resistance elements  60  are placed electrically in parallel with a respective arc  92  and, following sufficient movement of the conductive spanner, offer a lower resistance to current flow between a respective stationary conductor and conductive member  66 . Current flow then transitions from the arc path through the variable resistance assemblies, extinguishing the arc at the location illustrated in FIG. 6, and directing current through the transient or alternative current carrying path. As illustrated in FIG. 7, further movement of the conductive spanner may then proceed with complete interruption of the normal current carrying path, and current flow only through the transient current carrying path. 
     The interruption sequence described above is illustrated schematically in FIGS. 8 a - 8   e  through equivalent circuit diagrams. As shown first in FIG. 8 a , with conductive spanner  50  in its biased position, the normal current carrying path is establish between conductors  12  and  14 . The variable resistance assemblies, represented by variable resistors  96  in FIG. 8 a , in combination with air gaps between conductive members  66  and splitter plates  24 , represented by resistors  98  in the Figure, offer sufficient resistance to current flow to establish an open circuit through the transient current carrying path. 
     Upon initial interruption of the normal current carrying path, arcs established between the movable and stationary conductive elements define resistances  100   a  between the stationary conductors and spanner  50  as shown in FIG. 8 b . At this stage of operation, resistors  96  defined by the variable resistance assemblies, remain at their relatively low resistivity levels. Subsequently, a shown in FIG. 8 c , expanding arcs established between the stationary conductors  38  and  40 , and spanner  50 , extend to contact conductive members  66 , to establish equivalent resistances  100   b  and  100   c  on each side of the device. It will be noted that equivalent resistances  100   b  established by the arcs are electrically in parallel with variable resistors  96 . When the resistance offered by these assemblies, balanced with the resistance offered by the expanding and migrating arcs, favors transfer of current flow through the transient current carrying path, the transient current carrying path begins conducting all current through the device, extinguishing the arcs at the initial locations and resulting in heating of the variable resistance assemblies. Thus, in a subsequent phase of interruption, illustrated schematically in FIG. 8 d , all current flows through the transient current carrying path. During this intermediate stage of interruption, the transient current carrying path extends through the variable resistors  96 , through arcs  100   c  and through spanner  50 . As the spanner is displaced further in its movement, as indicated by arrow  90 , interruption is eventually completed, terminating all current flow through the device, as indicated in FIG. 8 e.    
     With heating during these progressive phases of interruption, the variable resistance assemblies transition to their higher resistivity level. In the illustrated embodiment, for example, each variable resistance assembly provides, in the subsequent phase of interruption, a voltage drop of approximately 75 volts. Each air gap between the splitter plates, indicated at reference numeral  98  in FIGS. 8 a ,- 8   e , provides an additional 17 volts of back-EMF. A total back-EMF is provided in an exemplary structure, therefore, of approximately 900 volts, of which approximately 150 volts is provided by the variable resistance elements. It is believed that in the current structure, certain of the upper splitter plates and shunt plate  68  may contribute little additional back-EMF for interruption of current through the device. However, it is currently contemplated that one or more variable resistors comprising one or more layers of material, such as that defining assemblies  28 , may be added at upper levels in the transient current-carrying path to provide additional assistance in establishing back-EMF and interrupting current flow. 
     It has been found that the present technique offers superior circuit interruption, reducing times required for driving current to a zero level, and thereby substantially reducing let-through energy. Moreover, it has been found that the technique is particularly useful for high voltage (e.g. 480 volts) single phase applications. FIGS. 9 and 10 illustrate a contrast between the performance of conventional circuit interrupters and performance of the exemplary structure described above. 
     As shown in FIG. 9, where circuit interruption begins at a time to, a back-EMF voltage trace  102  in a conventional device rises sharply, as does a trace of current  104  through a splitter plate and shunt bar arrangement. The back-EMF voltage reaches a peak  106 , then declines and oscillates as shown at reference numeral  108 . In exemplary tests of a single phase device, with a 480 volt source, an available current of approximately 8,000 Amps, and a power factor of approximately 60%, a clearing time (t 0  to t f ) of approximately 3.8 ms was obtained. A peak back-EMF was realized at a level of approximately 913 volts. Let-through energy, represented generally at reference numeral  112  in FIG. 9 was approximately 10.7×10 4  A 2 s. 
     As illustrated in FIG. 10, a back-ENF voltage trace  114  for an interrupter of the type described above exhibits a similar rise following initiation of interruption at time t 0 , while a trace of current  116  rises significantly more slowly than in the conventional case. Moreover, the voltage trace reaches an initial level  118 , followed by a further rise to a higher sustained peak, as indicated at reference numeral  120 , before falling off with the decline of current to a zero level at time t f , as indicated at reference numeral  122 . In exemplary tests, with similar conditions to those set forth above, a clearing time of approximately 2.72 ms was obtained, with a peak back-EMF of 1010 volts. Let-through energy, represented generally at reference numeral  124 , was approximately 7.60×10 3  A 2 s. 
     In addition to establishing a transient or alternative current carrying path for rapidly interrupting current through the device as described above, the present technique serves to reduce or eliminate arc retrogression during interruption. As will be appreciated by those skilled in the art, arc retrogression is a common and problematic failure mode in circuit breakers and other circuit interrupters, particularly under high voltage, single-phase conditions. In this failure mode, parasitic arcs external to the splitter plate stack provide parallel paths to arcs within the splitter plate stacks. Arc retrogression is believed to be caused by residual ionization resulting from prior arcing, and from strong electric fields due to high back-EMF concentrations. When new arcs are initiated, back-EMF drops precipitously and older arcs in the splitter plate stack are extinguished as volt current transfers to the new lower voltage, lower resistance arc. The new arc then folds into the splitter plate stack, increasing its back-EMF until the retrogression threshold is reached again and the process is repeated, giving rise to a characteristic high frequency voltage oscillation. As a result of such oscillations, the average back-EMF through the successive retrogression cycles is lower than it would be without such cycles, prolonging the process of driving the current to a zero level, and permitting additional let-through energy. 
     Through the present technique, such retrogression is significantly reduced or eliminated. In particular, the use of the variable or controlled resistance material in the transient current carrying path, provides additional back-EMF, removing some of the load from the splitter plate stack which can then operate below the retrogression threshold and circumvent the retrogression-related voltage oscillations. The use of the material adjacent to the core in the preferred embodiment also redistributes the back-EMF within the device, shifting an additional portion of the back-EMF to a location adjacent the core where magnetic field density is greater and aids in opposing retrogression by raising its threshold. 
     As noted above, additional variable resistance material may be provided at elevated levels in the transient current carrying path. Such additional structures are believed to enable further reduction in the occurrence of retrogression. In particular, prior to transition of the materials to an elevated resistance level, they provide a short circuit or lower resistance path, preventing the retrogression effects. Upon heating and transition to a higher resistance level, such structures would provide additional sources of back-EMF to assist in driving the fault current to a zero level. It is also noted that because a time delay is inherent in conversion of the additional structures from one resistance level to another by heating, such delays would permit residual ionization (associated with arc commutation to the splitter plates adjacent to such variable resistance structures) to decay somewhat before the electric field subsequently appears. As the level of residual ionization decreases, the electric field or voltage per unit length required to initiate retrogression increases. Thus, the delay in transition of the material to a higher resistance level permits a higher back-EMF to be eventually applied to more rapidly bring the fault current to a zero level without initiating unstable arc retrogression. 
     While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown and described herein by way of example only. It should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the following appended claims. For example, those skilled in the art will readily recognize that the foregoing innovations may be incorporated into various forms of switching devices and circuit interrupters. Similarly, certain of the present teachings may be used in single-phase devices as well as multi-phase devices, and in devices having different numbers of poles, and various arrangements for initiating circuit interruption. Moreover, the present technique may be equally well employed in interrupters having a single movable contact element or multiple movable elements. As mentioned above, the variable resistance elements and assemblies may be placed in different locations of the transient current carrying path described, including in locations above the stationary conductors, such as adjacent to or in place of the shunt bar, for example.