Abstract:
The present technique, applicable to low voltage, medium voltage, and high voltage MCCs and other power management systems, provides for substantially containing and directing an arcing fault and resultant ionized gases within a stab enclosure or housing disposed in the MCC. For example, the stab housing may have reduced stab-openings at the power bus interface to diminish the potential of an arc flash (and ionized gases) from reaching the power buses. Furthermore, phase-to-phase isolation barriers may be employed within the stab housing to reduce the potential of an arcing fault going phase-to-phase. Moreover, to reduce arc flash damage within the MCC, the walls and barriers, including walls around the stabs, within the housing may be configured to direct the arc on a preferred path to a desired location within the housing to extinguish the arc in less than 0.1 second or 6 cycles, or even less than 0.033 second or 2 cycles.

Description:
BACKGROUND  
       [0001]     The present technique relates generally to the field of power supply, such as that to motor control centers (MCCs). Specifically, the invention relates to techniques for connecting incoming power supply to certain types of electrical machinery, such as MCC&#39;s and components, for protecting such connections, and for containing and extinguishing arcing within such systems when faults do occur.  
         [0002]     Systems that distribute electrical power for residential, commercial, and industrial uses can be complex and widely divergent in design and operation. Electrical power generated at a power plant may be processed and distributed via substations, transformers, power lines, and so forth, prior to receipt by the end user. The user may receive the power over a wide range of voltages, depending on availability, intended use, and other factors. In large commercial and industrial operations, the power may be supplied as three phase ac power (e.g., 208 to 690 volt ac, and higher) from a main power line to a power management system. Power distribution and control equipment then conditions the power and applied it to loads, such as electric motors and other equipment. In one exemplary approach, collective assemblies of protective devices, control devices, switchgear, controllers, and so forth are located in enclosures, sometimes referred to as “motor control centers” or “MCCs”. Though the present technique is discussed in the context of MCCs, the technique may apply to power management systems in general, such as switchboards, switchgear, panelboards, pull boxes, junction boxes, cabinets, other electrical enclosures, and so forth.  
         [0003]     The MCC may manage both application of electrical power, as well as data communication, to the loads, such loads typically including various machines or motors. Within the MCC may be disposed a variety of components or devices used in the operation and control of the loads. Exemplary devices contained within the MCC are motor starters, overload relays, circuit breakers, and solid-state motor control devices, such as variable frequency drives, programmable logic controllers, and so forth. The MCC may also include relay panels, panel boards, feeder-tap elements, and the like. Some or all of the devices may be affixed within various “units” (or “buckets”) within the MCC. The MCC typically includes a steel enclosure built as a floor mounted assembly of one or more vertical sections containing the units or buckets. An MCC vertical section may stand alone as a complete MCC, or several vertical sections may be bolted and bused together. Exemplary vertical sections common in the art are 20 inches wide by 90 inches high.  
         [0004]     The MCC normally interfaces with (and contains) power buses and wiring that supply power to the units and components. For example, the MCC may house a horizontal common power bus that branches to vertical power buses at each MCC vertical section. The vertical power buses then extend the common power supply to the individual units or buckets. To protect the power buses from physical damage, both the horizontal and vertical buses may be housed in enclosures, held in place by bus bracing or brackets, bolted to molded supports, encased in molded supports, and so forth. Other large power distribution equipment and enclosures typically follow a somewhat similar construction, with bus bars routing power to locations of equipment within the enclosures.  
         [0005]     To electrically couple the MCC units or buckets to the vertical bus, and to simplify installation and removal, the units may be provided with self-aligning electrical connectors or metal stabs on the back of each unit. To make the power connection, the stabs, which may comprise spring-supported clamp devices, engage metal bars disposed on the vertical bus. For three phase power, three stabs per unit may accommodate three bus bars for the incoming power to give the phase terminals or terminations at the unit. An optional ground bus may also be used. Within the unit, three stab wires or power lead wires may route power from the stabs to a disconnecting device or component, typically through protective devices such as fuses and circuit breaker. It should be noted that though three phase ac power is discussed, the MCCs may also manage single phase ac power, as well as dc power (e.g., 24 volt dc power for sensors, actuators, and data communication). Moreover, the individual units or buckets may connect directly to the horizontal common bus by suitable wiring and connections.  
         [0006]     A problem in the operation of MCCs and other power management systems, such as switchboards and panelboards, is the occurrence of arcing (also called an arc, arc fault, arcing fault, arc flash, arcing flash, etc.) which may be thought of as an electrical conduction or short circuit through gas or air. Initiation of an arc fault may be caused by a momentary or loose connection, build-up of foreign matter such as dust or dirt mixed with moisture, insulation failure, or a short-circuit (e.g., a foreign object establishing an unwanted connection between phases or from a phase to ground) which causes the arc to be drawn, and so forth. Once initiated, arcing faults may proceed in a substantially continuous manner. On the other hand, arcing faults may be intermittent failures between phases or phase-to-ground, and may be discontinuous currents that alternately strike, extinguish, and strike again.  
         [0007]     In either case, the result is an intense thermal event (e.g., temperatures up to 35,000° F.) causing melting and vaporization of metals. An arcing fault is an extremely rapid chain of events releasing tremendous energy in a fraction of a second, and is known for quick propagation. Once the arcing begins, heat is generated and ionized gases are produced that provide a medium by which the arcing fault can propagate. An arc may travel along one stab wire and jump to other stab wires, melting and/or vaporizing the stab wires. As a result, more ionized gas and arcing may be created, engulfing all three phases and possibly reaching the power buses. A phase-to-ground or phase-to-phase arcing fault can quickly escalate into a three-phase arcing fault due to the extensive cloud of conductive metal vapor which can surround the power leads and terminals. If not contained, the arc may propagate throughout the entire MCC, especially if the arc reaches the power buses. Arcing faults can cause damage to equipment and facilities, and drive up costs due to lost production.  
         [0008]     It has been well documented that incident energy of an arcing fault is directly proportional to the time the fault persists. As the arcing fault flows for 6, 12, or 30 cycles or more, for example, the incident energy and force of the arc fault increases dramatically. Thus, circuit breakers, for example, on the line side operating with typical time delays (e.g., greater than 6 cycles) may be problematic with arcing faults. In general, it is desirable that the arcing fault be extinguished in a short time, such as within 6 cycles, and in certain applications, in less than 2 cycles. Testing has shown that if the arc (e.g., for 65,000 amps available current at 480 volts) does not extinguish quickly (e.g., in less than 0.1 seconds or six cycles), it can cause extensive damage. Moreover, although the amount of energy released in an arc flash may be greater for higher voltage installations, such as those found in petrochemical and other industrial plants, the sheer volume of lower voltage equipment in commercial and industrial facilities means that such installations account for a great number of arc flash incidents. Thus, there has been interest in arc flash protection for medium and low voltage MCCs, in addition to interest for protection of high voltage systems. Finally, as known by those skilled in the art, there are several industry and regulatory standards around the world that govern arc flash prevention.  
         [0009]     Arc characteristics and incident energy levels have many variables, such as system voltage, arc current, arc duration, arc electrode spacing, and so forth. In recent years, significant progress has been made in understanding arcing faults. For example, analytical tools have been developed to better assess arcing faults. As a result, it has been found that current-limiting devices, low impedance circuit components such as low impedance transformers, reduce the occurrence of arcing faults and/or the arc energy. However, such advances have proved deficient in mitigating arcing fault incidents.  
         [0010]     There is a need, therefore, for improved stab housing and enclosure designs that reduce the potential of arcing faults going phase-to-phase and reaching the power buses. Similarly, there is a need for a technique that efficiently and quickly extinguishes arcing faults to reduce damage to the MCC and other power management systems.  
       BRIEF DESCRIPTION  
       [0011]     The present technique is designed to respond to such needs. The technique, applicable to low voltage, medium voltage, and high voltage power management systems, provides for substantially containing and directing an arcing fault and resultant ionized gases within a stab enclosure or housing disposed, for example, in an MCC. The stab housing may have reduced stab-openings at the power bus interface to diminish the potential of an arc flash (and ionized gases) from reaching the power buses. Furthermore, the stab housing may employ phase-to-phase isolation barriers to reduce the potential of an arcing fault propagating from one phase to another. Moreover, to reduce arc flash damage within the MCC, the stab housing barriers (including walls around the stabs) may be configured to direct the arc on a preferred path to a desired location within the housing to extinguish the arc in less than 0.1 second or 6 cycles, or even in less than 0.033 second or 2 cycles. For example, the arc may be allowed to progress along a stab wire into the stab housing where the arc and gases are contained by the walls and barriers. To extinguish the arc, the stab housing walls and barriers may direct the arc to a reduced (neck) area of a stab to sever the stab wires (power leads) to interrupt the current and thus extinguish the arc before significant damage occurs in the MCC or other power management system.  
         [0012]     In one embodiment, a power stab housing has a plurality of barriers configured to direct an electrical arc to a desired location within the power stab housing to extinguish the electrical arc. The barriers in the stab housing may include at least one of a partition, a wall, and a substantially conical section. The desired location may include a reduced region within the housing, and/or a neck of a power stab which couples an external power source to a power lead wire. The barriers may be configured to direct the arc to sever the power lead wire.  
         [0013]     In another embodiment, a power management system has an enclosure having at least one unit containing a component for managing a load. At least one electrical connector electrically couples an external power supply to at least one power lead wire which routes power to the component. A connector housing substantially enclosing the at least one electrical connector, wherein the housing comprises a plurality of barriers configured to direct an arcing flash on a preferred path.  
         [0014]     In yet another embodiment, an electrical power supply system has a plurality of electrical connectors which couple a multi-phase power supply to a power component. A plurality of barriers configured to substantially isolate the electrical connectors from one another, wherein the plurality of barriers are configured to direct an arc on a preferred path to interrupt the arc.  
         [0015]     The technique provides a method of managing an arc fault within an electrical power supply system, including supplying main power to the electrical power supply system, allowing an arc fault current to propagate through a stab wire, isolating a stab and at least a portion of the stab wire within a stab housing assembly, and causing arc fault current interruption within the stab assembly housing. The technique also provides another method for interrupting an arc fault, including substantially surrounding electrical connectors with an enclosure, wherein the electrical connectors receive power from a multi-phase power bus, and directing arcing to a desired location within the enclosure. 
     
    
     DRAWINGS  
       [0016]     The foregoing and other advantages and features of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:  
         [0017]      FIG. 1  is a perspective diagrammatical representation of a motor control center, in accordance with one embodiment of the present technique;  
         [0018]      FIG. 2  is a perspective diagrammatical representation of the unit of  FIG. 1 , in accordance with one embodiment of the present technique;  
         [0019]      FIG. 3  is a top diagrammatical representation of a stab and vertical bus bar assembly, in accordance with one embodiment of the present technique;  
         [0020]      FIG. 4  is a top diagrammatical representation of the MCC unit engaging the vertical bus bars, in accordance with one embodiment of the present technique;  
         [0021]      FIG. 5  is a perspective view of a stab with a neck component for receiving a crimp ring, in accordance with one embodiment of the present technique;  
         [0022]      FIG. 6  is a perspective view of a stab with an integral crimp ring, in accordance with one embodiment of the present technique;  
         [0023]      FIG. 7  is a perspective view of the inside of the rear piece of a stab housing assembly, in accordance with one embodiment of the present technique;  
         [0024]      FIG. 8  is a perspective view of the outside of the rear piece of a stab housing assembly, in accordance with one embodiment of the present technique;  
         [0025]      FIG. 9  is a perspective view of the inside of the front piece of the stab housing assembly, in accordance with one embodiment of the present technique;  
         [0026]      FIG. 10  is a perspective view of the outside of the front piece of the stab housing assembly, in accordance with one embodiment of the present technique;  
         [0027]      FIG. 11  is a perspective view of a stab with a neck component for receiving a crimp ring, and showing the point at which an arc is extinguished, in accordance with one embodiment of the present technique; and  
         [0028]      FIG. 12  is a block diagram of a method for extinguishing an arc flash within an MCC, in accordance with one embodiment of the present technique. 
     
    
     DETAILED DESCRIPTION  
       [0029]     Beginning with  FIG. 1 , an exemplary motor control center (MCC)  10  formed of a large metal enclosure includes a stab housing  12  that isolates electrical connectors, such as metal power stabs  14 . The stabs  14  are configured to engage the vertical power bus (bars)  18  through openings in a vertical bus cover  16  at the back wall  20  of the MCC  10 . In this example, the vertical power bus  18  receives power from a horizontal power bus which in turn receives power, such as 208 to 690 volt ac power, from an external power source  22 . Thus, in this embodiment, the three vertical power bus bars  18  deliver three phase ac power to the three stabs  14  at the bucket or unit  24 . To form an electrical connection or termination, the stabs  14  engage the bars  18  as the unit  24  is slid into its respective cavity where the unit  24  resides during normal operation.  
         [0030]     Power leads  26  electrically couple to the stabs  14  and deliver power to components  28 , such components  28  including fuses, circuit breakers, motor starters, variable frequency drives, and the like. It should be noted that the various components  28  within the units  24  may require power other than 3 phase ac power. For example, some components  28  may operate on 120 volt single phase ac power. Still other components  28 , such as with those that manage data communication and control signals, may operate on 24 volt dc power. To facilitate operation of the components  28 , a control or secondary power may be split from the main power or be transformed to a more accessible secondary power level. The MCC  10  may house a collection of removable units  24  having various components  28 , and an access panel or door  30  may cover the front of the units  24 . As discussed below, the units  24  may employ stab housings  12  designed to contain and interrupt arcing faults within the MCC  10 .  
         [0031]     As indicated, in an arc flash or arcing fault, a substantial electric current may pass through air (and resultant ionized gas), generating an enormous amount of concentrated radiant energy. Such energy may thrust outward creating pressure waves, a high intensity flash, and extremely high temperatures. The arcing fault may melt or vaporize metal components, wires, and terminations or terminals, and if not contained, may propagate throughout the entire MCC  10 , especially if the arc reaches the power buses. Accordingly, as discussed below, the stab housing  12  may be mounted in the rear area of a unit  24  to enclose the stabs  14  (and engaged bars  16 ) to reduce arc flashing between phases, to prevent arc flashes from reaching the power buses, and to extinguish arc flashes in a relatively controlled and timely manner.  
         [0032]      FIG. 2  is a perspective view of a diagrammatical representation of the MCC unit  24  of  FIG. 1  and illustrates the placement of the stab housing  12 . In general, the stab housing  12  may reside fully inside or outside of the unit  24 , or may straddle the rear wall  34  of the unit  24 . In this example, the stab housing  12  mounts to the unit rear wall  34 . Indeed, the illustrated embodiment depicts the stab housing as mounted to the inside surface of the unit rear wall  34  inside the unit  24 . The outside surface of the unit rear wall  34  interfaces with the inside surface of the MCC vertical bus cover  16 . The slots or openings  36  of the stab housing  12  receive the stabs  14  from within the unit  24 . The indentations  38  of the stab housing  12  receive protrusions  17  (not illustrated) of the vertical bus cover  16  to provide stability of the interface between the stab housing  12  and vertical bus cover  16 . Furthermore, if the stab housing  12  is to be mounted on the interior of the unit  24 , then openings may be formed in the rear wall  34  to facilitate the interface with the vertical bus. Again, as illustrated in  FIG. 1 , the vertical bus  18  supplies power via stabs  14  to the unit  24  and components  28 .  
         [0033]      FIG. 3  is a diagrammatical representation of a top view of a terminal  42  which may reside partially or fully with the stab housing  12  and is the electrical coupling or connection of the power stab  14  to the vertical power bus bar  18 . The outer surface of the bar  18  engages the inner surface of the stab  14  to make the electrical connection. The stab  14  is typically made of steel or copper, while the bar  16  is typically constructed of copper. Other suitable materials may, of course, be employed for these components. The stab  14  may be spring supported such that the engagement of the stab  14  and bar  18  is tight enough to provide for adequate electrical contact.  
         [0034]      FIG. 4  is a top view of a diagrammatical representation of the MCC unit  24  and the terminals  42  of the stabs  14  engaged with the vertical bus bars  18 . In the illustrated embodiment, a molded rear piece  46  mates with a molded front piece  48  to form the stab housing  12 . The molded pieces  46  and  48  may be constructed, for example, of a glass-filled polyester thermoset. It should be noted that because of the scale of the view in  FIG. 4 , the stab openings  36  and indentations  38  are not delineated. However, the protrusions  17  and openings  19  of the bus cover  16  are depicted. Again, in this example, the stab housing  12  is mounted at the unit rear wall  34  outside the unit  24 . However, as mentioned, the stab housing may be mounted partially or fully inside of the unit  24 . In either case, the power leads  26  which supply three phase ac power to the component  28  may be coupled to the stabs  14 . The power leads  26  then exit the housing  12  and are routed to the component  28 . For reference, the front wall of the unit  24  is denoted by reference numeral  50 .  
         [0035]      FIGS. 5 and 6  better illustrate details of exemplary stabs  14 A and  14 B.  FIG. 5  is a perspective view of an exemplary stab  14 A having a neck component  52 A for receiving a crimp ring that couples the stab  14 A to a power lead  26 .  FIG. 6  is a perspective view of an exemplary stab  14 B having an integral crimp ring  52 B that couples the stab  14 B to a power lead  26 . Whether a separate crimp ring or integral crimp ring  52 B is employed, the power leads  26  couple to the stabs  14  at or near the neck  56  of the stab  14 A and  14 B. The engagement surface  54  of the stabs interfaces with the vertical bus bar  18 . Wire springs  58  provide for support and flexibility to the stabs  14 A and  14 B to facilitate receipt of the bus bars  18  and a relatively tight electrical connection between the stabs  14  and bus bars  18 . It should be noted that the illustrated stabs  14 A and  14 B are given as examples only, and the configuration, shape, and features of the stabs  14 A and  14 B may vary depending on the application and other factors. Further, electrical connectors other than stabs may be employed and may benefit from the present technique.  
         [0036]      FIGS. 7 and 8  are perspective views, respectively, of the inside and the outside of an exemplary rear piece  46  of the stab housing assembly  12 . In this embodiment, the outside surface  60  of the rear piece  46  interfaces with the inside surface of the MCC vertical bus cover  16  (see  FIG. 1 ) and the vertical bus bars  18 . Phase partitions  62  and conical sections  64  and  66  separate the three power phase stabs  14  and portions of the individual power leads  26  within the housing  12 . The stabs  14  are further enclosed by stab walls  68  which may operate with the phase partitions  62  and conical sections  64  and  66  to separate the power phases and reduce arcing and propagation of arcing between the power phases. To enhance phase-to-phase separation, the conical sections  64  and  66  may nestle inside components, such as other conical sections or cavities, on the front piece  48  ( FIGS. 9 and 10 ) that mates with the rear piece  46 .  
         [0037]     The walls  68 , partitions  62 , and conical sections  64  and  66  may also reduce the potential of an arc flash or fault from reaching the vertical bus  18 , horizontal bus, other units  24 , and so forth. For example, the ionized gases typically generated during an arc flash may be substantially contained to reduced propagation of the arc. Moreover, the stab walls  68 , phase partitions  62 , and/or conical sections  64  and  66  may facilitate extinguishment of an arc flash by directing the arc flash to a region, such as to the reduced area at the neck  56  of a stab  14 , where the arc may sever (melt and even vaporize) the leads  26  to interrupt power supply or current to the unit  24  and the arc. The geometry and shapes of the walls  68 , partitions  62 , and conical sections  64  and  66  may be configured to interrupt or extinguish the arc relatively quickly, such as in less than 0.1 second to avoid significant damage to the MCC. As will appreciated by those skilled in the art, such timing of the extinguishment and related lack of damage to the MCC may be validated by subjecting the stab housing assembly  12  to typical arc flash testing conducted in the industry.  
         [0038]     Finally, a variety of fastening elements may connect the rear piece  46  to the front piece  48  ( FIGS. 9 and 10 ) of the stab housing assembly  12 . In this example, screws are inserted in screw holes  70  to couple the two housing pieces  46  and  48 . Screw holes  72  receive screws for mounting the housing assembly  12  to the MCC unit  24 .  
         [0039]      FIGS. 9 and 10  are perspective views of the inside and outside, respectively, of an exemplary front piece  48  of the stab housing assembly  12 . The outside surface  74  illustrated in  FIG. 10  of the front piece  48  is the front of the stab housing  12  facing into the MCC unit  24 . Screw holes  76  receive screws for attaching the housing pieces  46  and  48  to one another. Screw holes  78  receive screws for mounting the assembly  12  to the back of the MCC unit  24 . Conical sections  80  surround the screw holes  76  on the inside of stab housing  12  and nest with the matching conical sections  64  on the rear piece  46  ( FIGS. 7 and 8 ) to advance phase separation within the stab housing  12 . Similarly, conical section  82  may nest with the conical section  66  of the rear piece  46 . The opening  84  provide an exit region for the power leads  26  from the stab housing assembly  12  into the MCC unit  24  in route to the component  28 . A variety of structural components, such as structural members  86 , may be formed on the rear and front pieces  46  and  48  to provide support. Another example are the walls or indentations  85  which help hold and support the stab housing  12  in place.  
         [0040]      FIG. 11  illustrates a perspective view of stab  14 A with neck component  52 A configured to receive a crimp ring  90  used to couple the power lead wires  26  to the stab  14 A. The crimp ring  90  is depicted in the open position prior to crimping to facilitate view of where the power lead wires  26  may sever and where an arc flash may thus be interrupted. Arcing or an arc flash inside the stab housing assembly  12  may be directed towards a reduced area of the stab  14 A (and  14 B), such as at the neck  56  and crimp ring  90 . The directed flow of the arc flash is depicted by arrow  92 . To accomplish extinguishment of the arc flash, the heat and other forces generated by the arc are directed to the neck  56  and crimp ring  90  at extinguishment region  94 , a desired location, to break (e.g., vaporize or melt) the leads  26 . Severing of the leads  26  discontinues the power supply or current and extinguishes the arc.  
         [0041]      FIG. 12  is a block diagram of an exemplary method  100  for containing and extinguishing arc flashes within an MCC  10 . Initially, the stabs  14 , which may be coupled to vertical bus bars  18 , are enclosed in a molded stab housing  12 , as referenced in block  102 . The stab housing  12  be formed of one or more pieces and may be constructed of a glass-filled polyester thermoset, for example. Supports and other structural members may be provided for strength and mechanical integrity. Further, the stabs  14  may be substantially isolated from the vertical and horizontal power buses, as referenced in block  104 . In other words, the stab openings  36  which provide pathways for engagement of the stabs  14  to the bars  18  may be significantly reduced in size to contain the ionized gases and to reduce the potential of arc propagation to the power buses and throughout the MCC. Furthermore, the phases within the stab housing  12  may be partitioned from one another to reduce the spread of ionized gas and arcing between the phases (e.g., phase lead wires  26  and stabs  14 ), as referenced in blocks  106  and  108 . Also, the partitions may be configured to direct the arc toward a desired location within the housing (block  110 ). In fact, the arc may be allowed to progress along a stab wire or lead wire into the stab housing  12  where the arc and resultant gas are contained by phase partitions and other barriers. At the desired location within the housing, the arc may be interrupted or extinguished, as depicted in block  112 . An exemplary desired location is a reduced area at the neck of a stab  14  where the ionized gas, heat, and arc are directed, resulting in severing of the power lead wire  26  at the stab  14  neck and thus interruption of the arc. Further, the arc life may be reduced to avoid damage to the MCC (block  114 ). The directing and interruption of the arc may be take place within 0.1 second or 6 cycles, for example. Indeed, the configuration of barriers, walls, and partition, and so forth, may take advantage of the rapid propagation of an arc fault to quickly direct and interrupt the arc.  
         [0042]     While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, 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.