Abstract:
An arc containment device is presented. The arc containment device includes a shock shield further having a multiple apertures for escape of gas, the shock shield configured to surround an arc source. The device further comprises an inner enclosure having a multiple openings generally aligned with the multiple apertures, the inner enclosure configured to provide an electrical insulation base for the arc source. An outer enclosure disposed is provided around the inner enclosure, the outer enclosure configured to direct the gas to the environment outside the device.

Description:
BACKGROUND 
     The invention relates generally to techniques for mitigating the effects of arcs, and more particularly to arc containment. 
     An arc flash may be defined as a condition associated with the release of energy caused by an electric arc. This release of energy is in the form of light and heat, often causing a pressure or shock wave. Arc flashes occur when the insulation between two conductors (often only air) can no longer withstand the voltage between them, resulting in an insulation breakdown. The energy produced by an arc flash event is a function of the voltage between the conductors, current flow during the event, and the duration of the event. To reduce or mitigate the deleterious effects of these events, design engineers have options such as grounding practices and current limiting fuses to reduce system voltage or fault currents. However, under certain conditions reducing arc fault clearing time is another approach to reducing the let-through energy resulting from the arc fault. 
     When arc flashes are contained, high energy levels released can involve very high pressure waves, on the order of tens to hundreds of bar, the transient and ultimate pressures of which depend upon the magnitude of short circuit current, and the volume and nature of a container. Consequently, the cost of the container increases exponentially with the magnitude of current. Shock waves are generated due to instantaneous heating of the gas or vaporized components around the arc. Pressures created by the shock wave may also be quite high, on the order of hundreds of bar, and are a function of the current magnitude and distance of the container wall from the arc. The shock waves occur during initial stages of arc formation. The ultimate pressure resulting from the expanding gas builds inside the container, and is generally a function of such factors as the duration of the event, the magnitude of the short circuit current and the volume of the containment chamber. 
     Therefore, there is a need for an arc containment approach designed to withstand both shock waves and high pressures with minimized size and cost. 
     BRIEF DESCRIPTION 
     According to an embodiment of the invention, an arc containment device is presented. The arc containment device includes a shock shield further having a plurality of apertures for escape of gas, the shock shield configured to surround an arc source. The device further comprises an inner enclosure having a plurality of openings generally aligned with the plurality of apertures, the inner enclosure configured to provide an electrical insulation base for the arc source. An outer enclosure is provided around the inner enclosure, the outer enclosure configured to direct the gas to the environment outside the device. 
     According to another embodiment, a method of manufacturing an arc containment device is presented. The method includes disposing a shock shield within an inner enclosure, the shock shield comprising a plurality of apertures generally aligned with openings in the inner enclosure. Further the method includes disposing an outer enclosure around the inner enclosure, the outer enclosure configured to provide a passageway for a gas between the inner enclosure and the outer enclosure. Further the method includes fixing an arc source on an electrical insulation base within the shock shield. 
     According to another embodiment, a method of containing an arc within an arc containment device is presented. The method includes containing a shock wave originating from an arc source by a shock shield, venting of gas via a plurality of apertures of the shock shield and a plurality of openings on an inner enclosure surrounding the shock shield and channeling the gas via the passageway between the inner enclosure and an outer enclosure. 
     According to another embodiment, an arc containment device is presented. The device includes a shock shield surrounding an arc source, the shock shield configured to contain a shock wave and an enclosure surrounding the shock shield, the enclosure configured to provide an electrical insulation base for the arc source. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a schematic representation of an electrical system including an arc containment device; 
         FIG. 2  is a diagrammatic representation of an arc containment device; 
         FIG. 3  is an exploded view of an arc containment device illustrating certain exemplary component parts and an arc source; 
         FIG. 4  is a partial sectional view of the arc containment device of  FIG. 3 ; 
         FIG. 5  is a cross sectional view of the arc containment device illustrating vents for channeling gas; and 
         FIG. 6  is a cross sectional view of a non-vented arc containment device according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an electrical power system is illustrated and designated generally by the reference numeral  10 . In the illustrated embodiment, the electrical power system  10  includes a power source  12  configured to deliver power to a load  18  via a circuit breaker  14 . In an exemplary embodiment, the power source  12  is configured to deliver alternating current or AC power to the common bus  16 . The electrical power system  10  illustrated herein includes a three phase configuration. In another embodiment, the electrical power system  10  may include a single phase configuration. The power source  12  and the load  18  are further coupled via a common bus  16  to an arc electrode system  20  (arc source). An example of the arc electrode system  20  includes but not limited to an arc crow bar device. The arc electrode system  20  is enclosed within an arc containment device  22 . 
     An arc flash detection system  24  is configured to detect an arc flash event  36  within the electrical power system  10  and further includes an electrical signal monitoring system  26 , arc flash decision system  28  and a sensor  30 . The electrical signal monitoring system  26  is configured to monitor current variations in the electrical power system that may arise due to the arc flash event. In an example, the electrical signal monitoring system  26  includes a current transformer. Furthermore, the arc flash decision system  28  is configured to receive electrical parameters  32  from the electrical signal monitoring system  26  and parameters  34  from the sensor  30 . As used herein, the term ‘parameters’ refers to parameters such as, for example, optical light, thermal radiation, acoustic, pressure, or radio frequency signal originating from an arc flash  36 . Accordingly, in such an embodiment, the non-electrical sensor includes an optical sensor. Based on the parameters  32  and  34 , the arc flash decision system  28  generates an arc fault signal  38  in an event of the arc flash event  36 . The arc fault signal  38  further triggers the arc electrode system  20 . As will be appreciated by those skilled in the art, the arc electrode system  20  helps mitigate effects of the arc flash event. 
     The arc electrode system  20  is configured to create an arcing fault that creates a second arc flash  40  within the arc containment device  22 . The arc flash  40  emits a substantial amount of energy in the form of intense light, sound, pressure waves and shock waves. It further causes vaporization of electrodes resulting in high pressure. (Such arcing fault facilitates diverting energy away from the arc flash  36 ). It may be noted that the arc electrode system  20 , by virtue of its functionality, includes an enclosure or arc containment device  22  robust enough to contain shock waves and high pressure resulting from arc flash  40 . The construction and functionality of the arc containment device  22  is discussed in detail below. 
     In one embodiment of the invention, the arc containment device may be a vented arc containment device as described in  FIGS. 2 ,  3 ,  4  and  5 . In another embodiment of the invention, the arc containment device may be a non-vent arc containment device ( FIG. 6 ). Typically, the non-vent arc containment devices occupy more volume. For example, in a 600 volt system, for a 65 kA/5 cycles arc flash energy, the non-vent arc containment device may occupy about 0.1 meter cube in volume, while the vented arc containment device may occupy about less than 0.01 meter cube in volume, for same arc flash energy level. However, it may be noted that appropriate arc containment device (vented or non-vented) may be used depending on requirement of location of installation. 
       FIG. 2  illustrates an exemplary arc containment device  42  implemented according to an aspect of the present technique. It may be noted that the arc containment device  42  may be implemented as the arc containment device  22  for the arc electrode system  20 , as referenced in  FIG. 1 . In the illustrated embodiment the arc containment device  42  includes an outer enclosure  44 . The outer enclosure may be made of any suitable material, such as metal, non-conducting material, composites and so forth. Ribs  46  are provided around the outer enclosure surface to improve its mechanical strength (particularly its ability to resist high internal pressures resulting from arc flash within the device). Vents  48  and  50  are provided at bottom sides of the outer enclosure  44 . However, it may be noted that in the illustrated exemplary embodiment, a single such vent extends around substantially the entire lower periphery of the outer enclosure. The outer enclosure is fixed to a support assembly  52 . The support assembly  52  includes an electrical insulation base (not visible in  FIG. 2 ) that will be positioned within the enclosure when the device is assembled as shown. 
       FIG. 3  illustrates an exploded view of the exemplary arc containment device  42  of  FIG. 2 . According to the illustrated embodiment, arc containment device  42  comprises various components such as the outer enclosure  44 , an inner enclosure  58 , a shock shield  62  and support assembly  52  as depicted in  FIG. 3 . In a particular embodiment, the shock shield includes an electrically conducting material or electrically non-conducting material. In one embodiment of the invention, the inner enclosure includes an electrically conducting material or an electrically non-conducting material 
     In a presently contemplated embodiment, the outer enclosure  44  is fastened on to the inner enclosure  58  via bolts (not shown) running through holes such as indicated by reference numeral  60 . The bolts are received through generally aligned holes in the outer enclosure  44 , the inner enclosure  58  and the support assembly  52 . The components are thus properly located and solidly held together to resist shock waves and high pressures resulting from arc flash events within the arc containment device. The outer enclosure is disposed around the inner enclosure  58 . The shock shield  62  is disposed within the inner enclosure  58 . In a presently contemplated embodiment, the shock shield  62  comprises corrugations  66  around its periphery. Corrugations  66  help in absorbing the shock waves by way of diffusion and flexing. As will be appreciated by those skilled in the art, by using a shock shield  62 , the volumetric construction of the arc containment device  42  may be substantially reduced, as compared to a device without a shock shield to absorb similar magnitudes of shock waves and high pressure. On the top surface of the shock shield  62 , apertures  64  are provided that are generally aligned with the openings  100  on the inner enclosure  58  for escape of gas that results from heating by the arc flash  40  as referenced in  FIG. 1 . The outer enclosure and the inner enclosure are fastened on to the support assembly  52 . The support assembly  52  includes hole  68  aligned with the holes  60  to accommodate fasteners. Electrodes  70 ,  72  and  74  are mounted onto the support assembly  52  forming an arc source. Electrical contact rods (not shown) are provided that extend through the support assembly to facilitate connection of the electrodes to the power source (e.g., to the power bus). The support assembly  52  may be made of any suitable electrically insulating material and composites to provide an electrical insulation base  76  for the electrodes. 
       FIG. 4  is a cross sectional assembled view of the exemplary arc containment device  42 . As mentioned above, the construction of the arc containment device  42  is made rigid to withstand high pressure and shock waves from arc flash events. The inner enclosure  58  is housed on an electrical insulation base  76 . It may be noted that the electrical insulation base  76  is part of the support assembly  52  as referenced in  FIG. 3 . A shock shield  62  is disposed around the electrodes. The shock shield  62  is configured to absorb shock waves generated in the event of an arc flash by way of the corrugations  66  on the surface on the shock shield  62 . The inner enclosure  58  is disposed around the shock shield  62 . Apertures  64  are provided on the shock shield  62  and openings on the inner enclosure  58  are provided for passage of gas. The outer enclosure  44  is disposed around the inner enclosure  58  to facilitate a passageway  80  between the inner enclosure  58  and the outer enclosure  44  for escape of gas. A plasma gun  82  is placed at the center of electrodes  70 ,  72  and  74  that are fixed to the electrical insulation base  76 . In one embodiment, the plasma gun  82  injects plasma as an arc mitigation technique, to create an arcing fault in response to the arc fault signal  38 , as referenced in  FIG. 1 . The electrodes are connected to the external circuitry via electrical contacts  84  and  86  and a third electrical contact (not shown). The outer enclosure  44  and the inner enclosure  58  are fastened to the electrical insulation base  76  via fasteners  88  and  90 . De-ionizing plates  92  are disposed in the passageway  80  to de-ionize the gas prior to expulsion from the arc containment device  42 . 
       FIG. 5  is a partial sectional view of the arc containment device  42 . The construction of the device  42  includes an outer enclosure  44  disposed around an inner enclosure  58  to provide a passageway  80  between the inner enclosure  58  and the outer enclosure  44 . An ablative layer  96  is disposed on the inner surface of the outer enclosure  44 . A second ablative layer  98  is disposed on the outer surface of the inner enclosure  58 . In an exemplary embodiment, the ablative layer comprises an ablative polymer such as but not limited to Delrin, Teflon or Polypropylene. Various methods of disposing the ablative layers  96  and  98  such as spraying, fixing a sheet, and so forth may be incorporated. The passageway  80  has vents  48  and  50  at the bottom to expel gas out of the device  42 . The ablative layers  96  and  98  absorb heat generated by gas in the event of arc flash  40 , as referenced in  FIG. 1 , in the passageway  80  via ablation. A shock shield  62  is disposed within the inner enclosure  58 . The electrodes  70 ,  72  and  74  are housed on an electrical insulation base  76 . The shock shield has apertures  64  aligned to the openings  100  on the inner enclosure  58 . Two such apertures  64  and openings  100  are shown here by way of example. Many such apertures  64  and respective openings  100  may be disposed respectively on the shock shield  62  and the inner enclosure  58 . As will be appreciated by one skilled in the art, the apertures  64  and openings  100  are aligned for passage of gas. De-ionizing plates  92  are disposed adjacent to the apertures  64 . 
       FIG. 6  illustrates a perspective view of a non-vent arc containment device  106 . The device  106  includes an enclosure  108 , a shock shield  110 , an electrical insulation base  112  and electrodes  70 ,  72  and  74 . In the illustrated embodiment, the electrodes forming an arc source are enclosed within non-vent arc containment device  106 . In the event of an arc flash  40  as referenced in  FIG. 1 , the shock shield  110  is configured to absorb shock waves released by the arc flash. The shock shield  110  includes corrugation around its surface that provides flexing during absorption of shock waves. It may be noted that corrugation provides diffusion of the shock wave by way of providing more surface area of exposure to the shock wave. The enclosure  108  is disposed around the shock shield  110  and fixed on to the electrical insulation base  112 . The electrical insulation base  112  provides support for the electrodes  70 ,  72  and  74 . 
     Advantageously, such arc containment devices reduce high pressure within the device enabling lower operating pressure. Also the device diffuses shock waves thereby facilitating compact construction. Hence, simplified construction design and compact size of the arc containment device are achieved in accordance with the disclosed techniques. 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.