Abstract:
Arc resistant enclosures for dry-type transformers. More particularly, transformer enclosures having one or more arc-resistant features, including arc channels, arc fault dampers, and arc fault plenums, and methods for providing same.

Description:
FIELD OF INVENTION 
       [0001]    The present application is directed to arc resistant enclosures for dry-type transformers, and more particularly, to a transformer enclosure having one or more arc-resistant features, including arc channels, arc fault dampers, and arc fault plenums. The present application is also directed to methods for providing an arc resistant enclosure for dry-type transformers. 
       BACKGROUND 
       [0002]    Dry-type distribution and small power transformers are known in the art, and include a familiar core and winding configuration. It is common to house dry-type distribution transformers in metal enclosures for the purposes of protecting the components from the environment and limiting exposure of people to the equipment, among others. Arc flash events can occur in such electrical equipment during normal operation, system transients, or during maintenance. When an electric arc occurs within the enclosure, it results in a pronounced increase in the pressure and temperature of gas within the enclosure. This sudden increase in gas pressure and temperature poses a risk of hot gas escaping the enclosure in an uncontrolled manner, which in turn poses a severe risk to people in the vicinity. It is therefore desirable to minimize such risk. In particular, it is desirable to prevent or minimize hot arc gases escaping into the area surrounding the enclosure from the floor level to a height of 2 m (79 in.) from the floor level—ie., a standard measure approximating the area within which personnel of average height would occupy if such personnel were maintaining or operating the equipment. 
       SUMMARY 
       [0003]    Described herein are multiple embodiments of an arc resistant enclosure for dry-type transformer(s). In particular, in one embodiment, an arc resistant enclosure for housing dry type transformer(s) comprises base and roof structures secured to at least three walls forming an enclosed space. One of the walls is a front wall comprising a first and second corner piece, a first face frame proximate the first and second corner pieces defining a first access opening, and a first access panel arranged to cover the first access opening. At least one ventilation opening is cut into the either the roof or walls. The front wall contains at least one longitudinal seam covered by an arc channel, wherein the arc channel is attached in such a manner that, upon an arc event, arc gas is substantially prevented from escaping the enclosure through the covered longitudinal seam. In at least one embodiment, an arc fault plenum is attached to the at least one ventilation opening. 
         [0004]    In another embodiment, an arc resistant enclosure for dry-type transformer(s) comprises base and roof structures secured to at least three walls, forming an enclosed space. At least one of the walls contains at least one ventilation grating, and at least one ventilation opening is cut into either the roof or walls. An arc fault damper apparatus is affixed adjacent at least one of the ventilation gratings; providing, however, that an arc fault damper apparatus is affixed adjacent every ventilation grating that is located at or below a height of 79 inches from the floor level. Finally, each arc fault damper apparatus is configured to close upon an arc flash event, thereby substantially preventing the escape of arc flash gas through the at least one ventilation gratings. 
         [0005]    Methods for providing the aforementioned arc resistant enclosures are provided herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    In the accompanying drawings, structural embodiments are illustrated that, together with the detailed description provided below, describe exemplary embodiments of an arc resistant metal enclosures for dry-type transformers, or components thereof. One of ordinary skill in the art will appreciate that a component may be designed as multiple components or that multiple components may be designed as a single component. 
           [0007]    Further, in the accompanying drawings and description that follow, like parts are indicated throughout the drawings and written description with the same reference numerals, respectively. The figures are not drawn to scale and the proportions of certain parts have been exaggerated for convenience of illustration. 
           [0008]      FIG. 1A  is an isometric view of a prior art transformer enclosure housing a three-phase dry-type distribution transformer, with a sidewall removed. 
           [0009]      FIG. 1B  is an isometric view of an exemplary arc resistant dry-type transformer enclosure, with the arc plenums removed. 
           [0010]      FIG. 1C  is an exploded partial view of the enclosure of  FIG. 1B , showing the base structure, front wall, and first sidewall. 
           [0011]      FIG. 2A  is an isometric view of the inside surface of an exemplary arc channel, without end-cap pieces. 
           [0012]      FIG. 2B  is an elevational view along the longitudinal axis of the arc channel of  FIG. 2A , with an end-cap piece. 
           [0013]      FIG. 3A  is an isometric, exploded, and enlarged view of the portion shown in the dashed line  2  of  FIG. 1B . 
           [0014]      FIG. 3B  is a sectional view of enclosure  100  along line  3 - 3 ′ in  FIG. 1B . 
           [0015]      FIG. 3C  is a sectional view of enclosure  100  along line  4 - 4 ′ in  FIG. 1B . 
           [0016]      FIG. 3D  is a sectional view of enclosure  100  along line  5 - 5 ′ in  FIG. 1B . 
           [0017]      FIG. 3E  is a sectional view of enclosure  100  along line  6 - 6 ′ in  FIG. 1B . 
           [0018]      FIG. 3F  is a sectional view of enclosure  100  along line  7 - 7 ′ in  FIG. 1B . 
           [0019]      FIGS. 4A and 4B  are isometric exploded views of an exemplary arc fault damper apparatus from the front and rear, respectively. 
           [0020]      FIG. 5A  is an isometric, enlarged view of the portion shown in the dashed line  8  of  FIG. 1B . 
           [0021]      FIG. 5B  is an isometric view from the rear of an exemplary arc damper apparatus in a closed configuration. 
           [0022]      FIG. 6A  is an isometric, enlarged view of the portion shown in the dashed line  8  of  FIG. 1B . 
           [0023]      FIG. 6B  is an isometric view from the rear of an exemplary arc damper apparatus in an open configuration. 
           [0024]      FIG. 7A  is an isometric view of an exemplary arc fault plenum, viewed from above and from the rear. 
           [0025]      FIG. 7B  is an isometric view of the arc fault plenum shown in  FIG. 7A , viewed from below and from the rear. 
           [0026]      FIG. 7C  is an isometric view of a flanged square piece used to build the arc fault plenum segment in  FIG. 7D . 
           [0027]      FIG. 7D  is an isometric view of an arc fault plenum segment used to build the arc fault plenum in  FIG. 7A . 
           [0028]      FIGS. 8A and 8B  are front and rear isometric views of the exemplary arc resistant dry-type transformer enclosure of  FIG. 1B , including the arc fault plenum in  FIG. 7A , as attached. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The enclosures and principles disclosed in this application are applicable to dry-type transformers of various sizes and ratings. Non-limiting examples of suitable dry-type transformers for use herein include power or distribution dry transformers with power ratings from 112.5 kVA to 25 MVA. Non-limiting examples of suitable commercially available dry transformers include vacuum cast coil, RESIBLOC® and open wound transformers offered by ABB, Inc. 
         [0030]      FIG. 1A  shows a typical three-phase dry-type distribution transformer  10  housed in enclosure  20 . For ease of reference, dry-type transformers will be referenced hereinafter simply as transformers. 
         [0031]    With reference to  FIGS. 1B and 1C , a transformer enclosure  100  according to one embodiment of the present invention is shown. Enclosure  100  includes a base structure  110 , walls  120 , and a roof structure  150 . The base structure may include means for supporting a transformer (not shown) within the enclosure, such as brackets  115 . The walls  120  are secured to the base structure  110 , typically at the bottom portion of the walls  120 . Walls  120  are preferably substantially perpendicular to the base structure  110 , e.g., at an angle of approximately 90°, such as between 80°-100°. As will be appreciated, in other embodiments, walls  120  and base structure  110  may form an angle substantially different from 90°, such as 30°, 45°, 60°, 120°, 135°, 150°, and any of various angles therebetween. Walls  120  are preferably secured around the perimeter of the base structure  110 . Alternatively, walls  120  are secured at any point of the base structure  110 . 
         [0032]    Although a rectangular enclosure is depicted in  FIGS. 1B and 1C , it will be understood that the enclosure walls  120  may form any of a number of geometric shapes, such polygonal, i.e., triangle, square, pentagon, etc., or can be circular, oval, elliptical, and the like. Moreover, any number of walls  120  may be employed. 
         [0033]    Roof structure  150  is secured to the top of walls  120  and may comprise one or more generally flat, rigid panels. Roof structure  150  may contain one or more ventilation openings, or holes,  155  that permit ventilation of the interior of the enclosure. In one embodiment, roof structure  150  comprises three flanged and interlocked roof panels  150   a - c,  with each roof panel containing a ventilation opening  155   a - c  in the center thereof. As will be appreciated, although a flat, multi-paneled roof structure  150  is depicted in  FIGS. 1B and 1C , in other embodiments, roof structure  150  may be comprised of any suitable number of panels having any suitable geometric shape. For example, in one embodiment, roof structure  150  comprises a single flat, rigid panel containing a single ventilation opening. The roof structure and ventilation openings are described in more detail below, in the context of arc plenums. 
         [0034]    Enclosure  100  is fabricated using generally any material that is capable of providing the functional requirements of the user, including arc fault resistance. In one embodiment, enclosure  100  is fabricated using heavy gauge sheet steel; in other embodiments, enclosure  100  is fabricated using heavy gauge aluminum or stainless steel. The enclosure  100  may comply with National Electrical Manufacturers Association (NEMA) 250 Standards. 
         [0035]    With reference again to  FIGS. 1B and 1C , in the embodiment shown, rectangular enclosure  100  has a front wall  120   a,  a first sidewall  120   b,  a back wall  120   c  (not shown), and a second sidewall  120   d  (not shown). In this embodiment, the front and back walls are similarly configured, and the first and second sidewalls are similarly configured. As such, only front wall  120   a  and first sidewall  120   b  are referenced hereinafter. As may be appreciated, in other embodiments, the walls may be differently configured. 
         [0036]    In the embodiment shown, front wall  120   a  is comprised of a rigid face frame  125  that is itself comprised of two identical face frames  126  and  127  arranged in a coplanar and adjacent manner. Face frame  126  has first and second longitudinal edges bearing first and second longitudinal flanges  128 ,  129  that extend inwardly from and perpendicularly to the plane of face plate  126 . Likewise, second face frame  127  has first and second longitudinal edges bearing first and second longitudinal flanges  130 ,  131  that extend inwardly from and perpendicularly to the plane of face frame  127 . Longitudinal flanges  129 ,  130  are mechanically affixed, via bolts or otherwise, forming fourth longitudinal seam  170   d,  thereby providing rigid face frame  125 . As will be appreciated, rigid face frame  125  may also be comprised of a single face frame, thereby eliminating the need for longitudinal flanges  129 ,  130 . 
         [0037]    First and second face frames  126 ,  127  each contain first and second access openings  132   a,b  that define a majority of the surface area of the face frames and provide access to the interior of enclosure  100 . Access opening  132   a  is defined on its longitudinal sides by a pair of generally U-shaped channels  133   a,b,  that extend along the length of the access opening; likewise, access opening  132   b  is defined on its longitudinal sides by a pair of generally U-shaped channels  133   c,d,  that extend along the length of that access opening. The structure and function of channels  133  are detailed, below, in relation to  FIG. 3A . 
         [0038]    With continued reference to  FIGS. 1B and 1C , the front wall  120   a  is comprised of first and second corner pieces  134 ,  136 . Corner pieces  134 ,  136  are rigid, unitary panels that are curved or angled in a manner to form a first portion  134   a,    136   a,  and a second portion  134   b,    136   b.  The angle defined by first and second portions depends on the geometric shape of enclosure  100 . In the embodiment shown, the angle is 90°. First portion  134   a,    136   a  is generally co-planar with face plate  125  and forms part of front wall  120   a,  while second portion  134   b,    136   b  forms part of sidewalls  120   b,  d and are co-planar with the remaining components of those wall, described below. 
         [0039]    Corner piece  134  is adjacent first face frame  126 , and the longitudinal edge of corner piece  134  that is proximate face plate  126  bears a flange  135  that is directed inwardly and perpendicularly to the plane of front wall  120   a,  Likewise, corner piece  136  is adjacent second face frame  127 , and the longitudinal edge of corner piece  136  that is proximate face frame  127  bears a flange  137  that is directed inwardly and perpendicularly to the plane of front wall  120   a,  As assembled, flange  135  is mechanically affixed, by bolting or otherwise, to first flange  128  of face frame  126 , forming first longitudinal seam  170   a,  Likewise, as assembled, flange  137  of corner piece  136  is mechanically affixed to second flange  131  of face frame  127 , forming seventh longitudinal seam  170   g.    
         [0040]    Front wall  120   a  may also comprise one or more rigid access panels  140 . In the embodiment shown, front wall  120   a  comprises first and second rigid access panels  140   a,  b that are configured and arranged to cover access openings  132  of face frame  125 . Access panels  140  are mechanically affixed to face frame  125  by any suitable means. In one embodiment, access panels  140  are configured such that each longitudinal side is flanged in a manner to mate with U-shaped channels  133  of face frame  126 ,  127 , and are bolted along their length to face frame  125  in the manner described below. 
         [0041]    Front wall  120   a  may also comprise one or more ventilation gratings  180  that allow gas to pass into and out of the interior of the enclosure. In the embodiment shown, access panels  140  each contain two ventilation gratings  180 . In other non-limiting embodiments, the one or more ventilation gratings are located in one or more different locations, such as sidewalls  120   b,d,  and/or back wall  120   c.    
         [0042]    Sidewall  120   b  comprises one or more rigid sidewall plates  145 . In the embodiment shown, sidewall  120   b  comprises two identical sidewall plates separated by, and affixed to, an elongated sidewall support piece  146 . Additionally, sidewall  120   b  comprises second portion  136   b  of corner piece  136 , as well as an analogous second portion of counterpart corner piece  138 . 
       Arc Channels 
       [0043]    With continued reference to  FIGS. 1B and 1C , arc channels  160  according to one embodiment of the present invention are shown. In general, arc channels  160  are elongated flat metal pieces having first and second ends  161 ,  162  that are positioned on the enclosure at a first point proximate the floor and a second point greater than 2 m (79 in.) above the floor level, respectively. Arc channels  160  are affixed to the outer surface of walls  120  at any longitudinal seam or portion of any longitudinal seam, as that term is defined herein, that is located anywhere from the floor level to 2 m (79 in.) from the floor level. The terms “seam” and “joint” are used interchangeably herein and refer to any longitudinal seam in the outer surface of walls  120  caused by the abutment or overlap of two adjacent wall panels, frames, or support pieces, that are likely to release expanding gas resulting from an arc fault event, and that are thereby likely to cause harm directly to any adjacent bystander or indirectly by igniting adjacent flammable material. 
         [0044]    Arc channels  160  act to contain rapidly expanding gases resulting from an arc fault event inside the enclosure, or to direct expanding gases to a point that will not be likely to cause harm (e.g., to a point higher than 79 in. above floor level). Referring to  FIGS. 2A and 2B , in one embodiment, arc channels  160  have a central flat elongated portion  163  and two side portions  164 . Side portions  164  are formed by angling each side twice at approximately 90°, creating a turned-up portion  164   a  and a flanged portion  164   b  that is approximately parallel to the central portion  163 . Preferably, both ends  161  and  162  of arc channel  160  are substantially closed or capped by, for example, welding a small flat metal end-cap piece  165  to either end such that the cross-sectional area between each turned-up portion  164   a  is substantially covered, as shown in  FIG. 2B . Each arc channel  160  is attached to the outer surface of the enclosure walls  120  such that the flanged portions  164   b  abut the outer surface, thereby creating an enclosed space (not shown) between the outer surface of the enclosure walls  120  and the inner surface of the flat elongated portion  163 . 
         [0045]      FIG. 3A  is an exploded and enlarged view of the portion shown in the dashed line  2  in  FIG. 1B , showing an upper section of arc channel  160   b,  a portion of access panel  140   a,  and a portion of face frames  126 ,  127  that are mechanically affixed by longitudinal flanges  129 ,  130  and that contain U-shaped channels  133   b  and  133   c,  respectively. As assembled, access panel  140   a  is brought into contact with face frame  126  such that its first longitudinal side, which is flanged perpendicularly to its surface, is seated in first U-shaped channel  133   a  (not shown) of face frame  126  and its second longitudinal side, which is also flanged perpendicularly to its surface, is seated in second U-shaped channel  133   b.  Likewise, although not shown in  FIG. 3A , access panel  140   b  is brought into contact with face frame  127  such that its first longitudinal side, which is flanged perpendicularly to its surface, is seated in first U-shaped channel  133   c  of face frame  127  and its second longitudinal side, which is flanged perpendicularly to its surface, is seated in second U-shaped channel  133   d.    
         [0046]    With continued reference to  FIG. 3A , one exemplary arc channel bolting arrangement is shown. In this embodiment, both the arc channel  160   b  and access panel  140   a  are bolted to face frame  126  using two alternating sets of bolts. A first set of bolts  300  passes through arc channel  160   b  along a line proximate one longitudinal edge of arc channel  160   b.  Thereafter, the bolts  300  pass through access panel  140   a  and screw into a securing means, e.g., a tinnerman nut (not shown), in face frame  126 . A second set of bolts  310  originate inside arc channel  160   b  (i.e., the head of bolts  310  lie within the enclosed space between flat elongated portion  163  of arc channel  160  and the access panel  140 ), pass through access panel  140   a,  and screw into a securing means, e.g., a tinnerman nut (not shown), in face frame  126 . Likewise, this bolting arrangement is utilized along a line proximate the opposite longitudinal edge of arc channel  160   b  to affix arc channel  160   b  and access panel  140   b  to face frame  127 . In this manner, arc channel  160   b  covers the third, fourth, and fifth longitudinal seams  170   c,d,e,  described n more detail below. 
         [0047]      FIG. 3B  is a sectional view of enclosure  100  along line  3 - 3 ′ in  FIG. 1B , showing an assembled cross-section of a portion of front wall  120   a,  and specifically portions of corner piece  134   a,  arc channel  160   a,  face frame  126 , and access panel  140   a,  As shown, first longitudinal seam  170   a  is formed by the abutting flange portions  135 ,  128  of corner piece  134  and face frame  126 , respectively. Also, second longitudinal seam  170   b  is formed by the overlapping portion of access panel  140   a  and face frame  126 . A first set of bolts  320  is proximate a first longitudinal edge of arc channel  160   a,  bolting it to corner piece  134 ; second and third alternating sets of bolts  300 ,  310 , bolts a second longitudinal edge of arc channel  160   b  to access panel  140   a  and face frame  126 . In this manner, arc channel  160   a  covers first and second seams  170   a,b.    
         [0048]      FIG. 3C  is a sectional view of enclosure  100  along line  4 - 4 ′ in  FIG. 1B , showing an assembled cross-section of a portion of front wall  120   a,  and specifically portions of arc channel  160   b,  access panels  140 , and face frames  126 ,  127 . As shown, third and fifth longitudinal seams  170   c,e  are formed by overlapping portions of access panel  140  and face frames  126 ,  127 , respectively, as described above. Similarly, fourth longitudinal seam  170   c  is formed by the abutting flange portions  129 ,  130  of face frames  126  and  127 , respectively, as described above. Also as shown, arc channel  160   b  is bolted to access panel  140   a  and face frame  126 , and access panel  140   b  and face frame  127 , using bolts  300 ,  310 , as described above, thereby covering third, fourth, and fifth longitudinal seams  170   c,d,e.    
         [0049]      FIG. 3D  is a sectional view of enclosure  100  along line  5 - 5 ′ in  FIG. 1B , showing an assembled cross section of a portion of front wall  120   a  and sidewall  120   b,  and specifically portions of access panel  140   b,  face frame  127 , arc channel  160   c,  corner piece  136 , arc channel  160   d,  and sidewall panel  145   a.  As shown, sixth seam  170   f  is formed by the overlapping portion of access panel  140   b  and face frame  127 . Seventh seam  170   g  is formed by the abutting flange portions  131 ,  137  of face frame  127  and corner piece  136   a,  respectively. First and second alternating sets of bolts  300 ,  310  are proximate a first longitudinal edge of arc channel  160   c,  bolting it to access panel  140   b  and face frame  127 , in the manner described above. Also, a third set of bolts  320  is proximate a second longitudinal edge of arc channel  160   c,  bolting it to corner piece  136   a.  In this manner, arc channel  160   c  covers sixth and seventh seams  170   f,g.    
         [0050]    With continued reference to  FIG. 3D , eighth longitudinal seam  170   h  is formed by the overlapping portion of sidewall panel  145   a  and corner piece  136   b.  A first set of bolts  330  is proximate a first longitudinal edge of arc channel  160   d,  bolting it to corner piece  136   b.  Second and third alternating sets of bolts  340 ,  350  are proximate a second longitudinal edge of arc channel  160   d,  bolting it to sidewall panel  145   a  and to corner piece  136   b.  In this manner, arc channel  160   d  covers eighth seam  170   h.    
         [0051]      FIG. 3E  is a sectional view of enclosure  100  along line  6 - 6 ′ in  FIG. 1B , showing an assembled cross section of a portion of sidewall  120   b,  and specifically portions of sidewall panels  145   a,b,  sidewall support piece  146 , and arc channel  160   e.  As shown, ninth and tenth seams  170   i,j,  are formed by the overlapping portions of sidewall panels  145   a,b,  and sidewall support piece  146 , respectively. First and second alternating sets of bolts  340 ,  350  are proximate to both longitudinal edges of arc channel  160   e,  bolting it to sidewall panels  145  and to sidewall support piece  146 . In this manner, arc channel  160   e  covers ninth and tenth seams  170   i,j.    
         [0052]      FIG. 3F  is a sectional view of enclosure  100  along line  7 - 7 ′ in  FIG. 1B , showing an assembled cross section of a portion of sidewall  120   b,  and specifically portions of sidewall panel  145   b,  arc channel  160   f,  and corner piece  138 . As shown, eleventh seam  170   k  is formed by the overlapping portion of sidewall panel  145   b  and corner piece  138 . First and second alternating sets of bolts  340 ,  350  are proximate a first longitudinal edge of arc channel  160   f,  bolting it to sidewall panel  145   b  and to corner piece  138 . A third set of bolts  330  is proximate a second longitudinal edge of arc channel  160   f,  bolting it to corner piece  138 . In this manner, arc channel  160   f  covers eleventh seam  170   k.    
         [0053]    Arc channels  160   a - f,  described above, cover longitudinal seams  170   a - k,  thereby preventing or minimizing the escape of hot gas resulting from an arc flash event in the area surrounding enclosure  100  below a height of 2 m (79 in.). In this way, any personnel in the vicinity are protected from exposure to such hot gases, as well as any flammable materials. As may be appreciated, the arc channels described herein are merely one embodiment of the present invention, and different configurations, geometries, and attachment means for other arc channel embodiments are contemplated herein that may still perform the functions describe above. Likewise, different seam geometries and arrangements may be present in different enclosure embodiments, depending on the particular enclosure embodiment. 
       Arc Fault Damper Apparatus 
       [0054]    Embodiments of the present invention may also include one or more arc fault damper apparatus. In general, an arc fault damper apparatus is a damper device that is located and coupled with ventilation gratings described above. According to the invention described herein, any ventilation grating that is present in an arc resistant transformer enclosure at a location that is at or below a height of 2 m (79 in.) from the floor level must have an arc fault damper apparatus coupled therewith. 
         [0055]    With reference to  FIGS. 4A and 4B , an arc fault damper apparatus  400  according to one embodiment of the present invention is shown. Damper apparatus  400  includes a damper plate  405  that is made from any material suitable to prevent hot arc gases from escaping an enclosure and that is shaped so as to completely cover the ventilation grating that it is associated with. In one embodiment, damper plate  405  is made of steel and is rectangular, with an area greater than the area covered by ventilation grating  180 . A damper handle  410  is attached to the front surface (shown in  FIG. 4A ) of damper plate  405  and is arranged to protrude through a suitable opening in ventilation grating  180 . 
         [0056]    In one embodiment, the top edge of damper plate  405  bears a flange  415  that extends in a direction toward the rear surface (shown in  FIG. 4B ) of damper plate  405 , and that is perpendicular to the surface plane. Also, both side edges of damper plate  405  bear first and second side flanges  420   a,b  that extend in a direction toward the rear surface of damper plate  405  and that are perpendicular to the surface plane. Side flanges  420  each include throughhole  441  and bolt channel  451 , described in more detail below. 
         [0057]    One or more hinges are attached to the damper plate in order to rotatably attach the damper plate to the inside surface of enclosure  100 . In one embodiment, elongated hinge  425  is attached to top flange  415 . 
         [0058]    Arc fault damper apparatus  400  includes one or more brackets. In one embodiment, first and second brackets  430   a,  b include a flanged portion that is substantially co-planar with the surface of damper plate  405  and a main portion that extends rearwardly from the flanged portion and that is substantially perpendicular to the flanged portion. The main portion comprises at least one wheel bearing channel  431  having a notch  432 , and at least one cutout portion  433 , all of which are described in more detail below. 
         [0059]    With continued reference to  FIGS. 4A and 4B , the upper portion of bracket  430  is rotatably attached to damper plate  405  by bolt  435  in the following manner. Bolt  435  extends through angled outer spring retainer  440 , cutout portion  433 , throughhole  441 , and thereafter through inner spring retainer  442 , and is secured by locking washer  491  and nut  436 , which is threadably attached. Flat washers  490  are included at appropriate positions, as shown. Cutout portion  433  is configured to provide a suitable throughhole for bolt  435  at the top portion of bracket  430 . In the embodiment shown, cutout portion  433  includes a relatively larger upper portion to accommodate bolt  435 , a relatively narrow necking portion that has a width less than the diameter of bolt  435  (thereby preventing bolt  435  from moving past it), and a relatively large lower portion that serves to reduce the weight of the bracket  430  and to allow increased ventilation when damper plate  405  is open. 
         [0060]    The lower portion of bracket  430  is slidably attached to damper plate  405  by bolt  445  in the following manner. Bolt  445  extends through angled outer spring retainer  450 , bearing wheel  455 , bearing channel  431 , bolt channel  451 , and thereafter through inner spring retainer  452 , and is secured by locking washer  492  and nut  446 , which is threadably attached. Flat washers  490  are included at appropriate positions, as shown. 
         [0061]    Outer springs  460  are attached at a first end to outer spring retainer  440 , and at a second end to outer spring retainer  450 . Similarly, inner springs  465  are attached at a first end to inner spring retainer  442  and at a second end to inner spring retainer  452 . Bearing wheels  455  are situated in bearing channel  431 . 
         [0062]    The operation of arc fault damper apparatus  400  is described with additional reference to  FIGS. 5A ,  5 B,  6 A and  6 B.  FIG. 5B  is a rear view of arc fault damper apparatus  400 , as assembled and in a closed position. Damper apparatus  400  is aligned with a ventilation grating  180 , described above and shown in  FIG. 5A . Side brackets  430  and hinge  425  are attached to access panel  140   a  of enclosure  100 , as by bolting or the like, such that damper  405  completely covers grating  180 . An exemplary bolting pattern is shown in  FIGS. 5A and 5B , and comprises the aligned bolt holes  500   a,  b of side brackets  430   a,  b with bolt holes  505   a,b  of access panel  140   a,  and the aligned bolt holes  500   c  of hinge  425  with bolt holes  505   c  of access panel  140   a.    
         [0063]    Arc fault damper apparatus  400  is configured such that it is in a normally closed position, as shown in  FIGS. 5A and 5B . In the embodiment shown, the normally closed configuration is accomplished through the use of outer and inner springs  460 ,  465 , combined with bearing wheels  455  and an angled bearing channel  431 . In particular, a first end of outer and inner springs  460 ,  465  is attached to outer and inner spring retainers  440 ,  442 , respectively, which in turn are rotatably affixed in position by bolts  435 . A second end of springs  460 ,  465  is attached to outer and inner spring retainers  450 ,  452 , respectively, which in turn are rotatably mounted on bearing wheels  455 . Bearing wheels  455 , which are mounted in angled bearing channel  431 , allow springs  460 ,  465  to transfer a contraction force into a lateral force that effectively pulls damper  405  to a closed position. As may be appreciated, other arrangements may be configured to result in a normally closed damper  405 , and are encompassed herein. In one non-limiting example, bearing wheels  455  are replaced by steel pins that are capable of sliding in bearing channels  431 . In another non-limiting example, torsion springs are utilized at bolts  435  in lieu of the components discussed above. 
         [0064]    In normal operation, an operator sets damper  405  to an open position (as shown in  FIGS. 6A and 6B ) by pushing damper handle  410  until bearing wheel  455  is seated in notch  432 . Once seated, damper  405  will remain open and thereby allow ventilation of enclosure  100  to occur. Upon an arc event, however, the concussive force of a the rapidly expanding gases unseats bearing wheel  455 , causing the damper  405  to move to a closed position, thereby preventing substantial escape of the heated arc gases from enclosure  100  through the ventilation gratings  180 . 
       Arc Fault Plenum 
       [0065]    Embodiments of the present invention may also include one or more arc fault plenums. In general, an arc fault plenum is an enclosure apparatus that channels expanding arc fault gases out of the arc resistant transformer enclosure to a location where they may be safely discharged. 
         [0066]    Referring to  FIGS. 7A-7D , arc fault plenums  700  according to one embodiment of the present invention are shown. In general, arc fault plenum  700  may be constructed of any material suitable for containing arc fault gases. In one non-limiting embodiment, arc fault plenum is constructed of light gauge sheet metal. 
         [0067]    Arc fault plenum  700  may be constructed in segments  710 , of any suitable shape or length, that are mechanically attached as by bolting or the like. In one non-limiting embodiment, each segment  710  is cubic and comprised of identical square pieces  720  that are flanged at each side in a direction perpendicular to its surface. Each segment  710  is formed by attaching a first flange of side square pieces  730  to a first surface (ie., the surface that does not intersect a flange) of top square piece  740  proximate two of its opposing edges. Similarly, a second flange (ie., opposite the first flange) of side square pieces  730  are attached to the first surface of bottom square piece  750  proximate two of its opposing edges. Each segment  710  is thereafter attached via flanges to another segment  710  to arrive at arc fault plenum  700 , with the proviso that a bottom square piece  750  is not attached to one or more consecutive segments  710 , so as to provide an open space  760 . 
         [0068]    Referring to  FIGS. 8A and 8B , arc resistant enclosure  100  is shown having arc fault plenum  700  attached. In the embodiment shown, arc fault plenum  700  is bolted to roof structure  150  such that open spaces  760  are aligned with ventilation opening  155 , shown in  FIGS. 1B and 1C , and such that expanding arc fault gases may exit the interior of enclosure  100  via ventilation opening  155  and arc fault plenum  700 . In one non-limiting embodiment, arc fault plenum  700  is connected to a duct system that terminates in a safe location outside of the electrical room and/or building housing enclosure  100 . 
         [0069]    As may be appreciated, other arc fault plenum and ventilation opening configurations are within the scope of the present invention. For example in one non-limiting embodiment, roof structure  150  comprises a single panel with a single ventilation opening, to which a single arc fault plenum is attached. In other non-limiting embodiments, ventilation openings  155  are provided in one or more enclosure wall  120 , at a point above 2 m (79 in.) from the floor, and one or more arc fault plenums are attached thereto. 
         [0070]    To the extent that the term “includes” or “including” is used in the specification or the claims, it is intended to be inclusive in a manner similar to the term “comprising” as that term is interpreted when employed as a transitional word in a claim. Furthermore, to the extent that the term “or” is employed (e.g., A or B) it is intended to mean “A or B or both.” When the applicants intend to indicate “only A or B but not both” then the term “only A or B but not both” will be employed. Thus, use of the term “or” herein is the inclusive, and not the exclusive use. See, Bryan A. Garner, A Dictionary of Modern Legal Usage 624 (2d. Ed. 1995). Also, to the extent that the terms “in” or “into” are used in the specification or the claims, it is intended to additionally mean “on” or “onto.” Furthermore, to the extent the term “connect” is used in the specification or claims, it is intended to mean not only “directly connected to,” but also “indirectly connected to” such as connected through another component or components. 
         [0071]    While the present application illustrates various embodiments, and while these embodiments have been described in some detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention, in its broader aspects, is not limited to the specific details, the representative embodiments, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the applicant&#39;s general inventive concept.