Patent Publication Number: US-6703576-B1

Title: Arc chute with valve and electric power switch incorporating same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to apparatus relieving the pressure of arc gases that builds up in arc chutes of electric power switches during current interruption and to electric power switches incorporating such arc chutes. 
     2. Background Information 
     Electric power switches in which the contacts are exposed to air and that are designed to open circuits carrying appreciable current typically experience arcing as the contacts separate. These electric power switches, such as for instance, circuit breakers, commonly incorporate arc chutes to help extinguish the arc. Such arc chutes typically comprise a number of electrically conductive plates held in spaced relation around the separating contacts by an electrically insulative housing. The arc transfers to the arc plates where it is stretched and cooled until extinguished. A considerable volume of gas is generated by the arc. The pressure generated by this arc gas must be relieved to avoid catastrophic damage to the switch casing. However, this discharge of the arc gas must be controlled to avoid injury to persons nearby and to prevent phase to ground or phase to phase arcing in multiphase switches. Various arrangements for venting arc gases from electric power switches have been proposed. However, there is still room for improvement. 
     SUMMARY OF THE INVENTION 
     This invention is directed to an arc chute with a valve and to electric power switches incorporating such an arc chute. The arc chute comprises an arc chute chamber structure with a gas opening for releasing arc gases generated during contact separation. The valve comprises a flexible sheet member mounted at its two ends to the arc chamber structure to extend over and close the gas opening in the absence of arc gases. At least one of the two ends of flexible sheet member is free to slide along the arc chamber structure and bow the flexible sheet member between the two ends allowing arc gases to pass outward from the gas opening. The electric power switch incorporating this arc chute has a casing with a gas vent through which the arc gases released through the valve pass out of the casing. 
     More particularly, the invention is directed to an arc chute for an electric power switch that generates arc gases upon opening. The arc chute comprises an arc chamber structure having at least one gas opening for releasing arc gases and a valve. The valve comprises a flexible sheet member having two ends and side edges extending between the two ends, and mounting means mounting the flexible sheet member at the two ends to the arc chamber structure to extend over and close the at least one gas opening in the absence of arc gases. At least one of the two ends of the flexible sheet member is free to slide along the arc chamber structure and bow the flexible sheet member between the two ends allowing arc gases to pass from the gas opening out laterally between the side edges of the resilient member and the arc chamber structure. The mounting means can comprises a guide on the arc chamber structure slideably engaging the at least one end of the sheet member to allow it to slide along the arc chamber structure. The mounting means can further comprise at least one longitudinal slot through the one end of the flexible sheet member. In this case, the guide on the housing slideably engages the longitudinal slot to allow the one end of the flexible sheet member to slide. The mounting means can comprise a pair of laterally spaced longitudinal slots through the one end of flexible sheet member and a pair of guides on the arc chamber structure slideably engaging the pair of longitudinal slots. The second end of the flexible sheet member can be similarly mounted by the mounting means for sliding. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
     FIG. 1 is an isometric view with part peeled back of an arc chute in accordance with the invention. 
     FIG. 2 is a plan view of one of the arc plates which forms part of the arc chute of FIG.  1 . 
     FIG. 3 is a plan view of a flexible sheet member which forms part of the arc chute of FIG.  1 . 
     FIG. 4 is an isometric view of the arc chute in accordance with the invention illustrating release of low pressure gas. 
     FIG. 5 is an isometric view of the arc chute in accordance with the invention releasing high pressure gas. 
     FIG. 6 is a partial vertical sectional view through a circuit breaker incorporating the arc chute of the invention releasing high pressure gas. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring generally to FIGS. 1-5, an arc chute  1  in accordance with the invention comprises an arc chamber structure  3  and a valve  5 . The arc chamber structure  3  is constructed of a pair of electrically insulative sidewalls  7  forming between them an arc chamber  9 . A plurality of spaced apart arc plates  11  extend between the sidewalls  7 . As is shown in FIG. 2, the arc plates  11  have tangs  13  projecting from either side which extend through openings in the sidewalls  7 . These tangs  13  are pinged over to lock the side plates  7  and arc plates  11  together as shown in FIG.  1 . The exposed ends of the tangs are covered with an electrically insulative tape  15 , shown partially peeled back in FIG. 1 to expose tangs  13 . An arc runner  17  is similarly secured between the side plates by pairs of tangs  19 . The spaced apart sidewalls  7  form between them a gas opening  21  through which, as will be explained below, arc gases generated in the arc chamber  9  can escape. 
     The valve  5  is formed by a flexible sheet member  23  having two ends  25  and side edges  27 . The flexible sheet member  23  in the exemplary arc chute  1  is made of fish paper. Other suitable flexible materials that are electrically insulative and resistant to the hot gases could also be utilized. The flexible sheet member  23  is mounted over the gas opening  21  by mounts  29 . These mounts  29  permit the ends  25  and  27  of the flexible sheet member to slide along the arc chute chamber structure  3  in response to high pressure in the arc chamber  9  causing the flexible sheet member to bow in the middle. The mounts  29  include pairs  31  of elongated slots  33  at the two ends  25  of the flexible sheet member  23 . The mounts  29  further include guides projecting from the arc chamber structure  9 . In the exemplary embodiment of the invention these guides are in the form of extensions  35  on two of the arc plates  11 . These pairs of extensions  35  extend through the elongated slots  33 . Retainers  37  are pressed onto the extensions  35  to retain the flexible sheet member while allowing the ends to slide. Alternatively, just one end of the flexible sheet member  23  could be mounted for sliding while the other end is fixed; however, having both ends free to slide increases the valve action. 
     The flexible sheet member  23  can also be provided with an auxiliary valve  39  formed by slits in the flexible sheet member  23 . The exemplary flexible sheet member  23  is provided with a pair of slits  41  and  43  which intersect intermediate their end points to form four flaps  45 . 
     FIG. 1 shows the flexible sheet member  23  in the relaxed position flat against the edges of the sidewalls  7  to close the gas opening  21 . As gas is generated in the arc chamber  9  during current interruption and the pressure begins to build, the flaps  45  of the auxiliary valve  39  open as shown in FIG. 4 to allow the low pressure gas to escape. As the gas pressure in the arc chamber  9  builds, the pressure exerted against the flexible sheet member  23  causes it to bow, pulling the ends  25  toward each other. As shown in FIG. 5, this allows the higher pressure gas to escape laterally outward between the side edges  27  of the flexible member  23  and the edges of the sidewalls  7  of the arc chamber structure  3 . In the exemplary embodiment of the arc chute, the flaps  45  of the auxiliary valve  39  open first before the flexible sheet member  23  begins to bow to any substantial degree. 
     The arc chute  1  of the invention is used in electric power switches such as the circuit breaker  47  shown in FIG.  6 . This circuit breaker  47  has an electrically insulative casing  49 . Only the base of the casing  49  is shown; however, as is well known, the bottom portion has a mating cover. Housed within the casing are separable contacts  51  including a fixed contact  53  and movable contact  55 . The fixed contact is secured to a line conductor  57  which extends through an end wall  59  of the casing  49  into a terminal recess  61  where it is connected to an external conductor (not shown). The movable contact  55  is carried by a pivoted movable contact arm  63 , which is rotated to open and closed the separable contacts by an operating mechanism indicated generally at  65 . As the contacts  51  separate an arc is drawn between the fixed contact  53  and the movable contact  55 . This arc must be extinguished in order to interrupt the current. As the movable contact arm  63  fully opens, the arc is driven outward by electromagnetic forces along the arc runner  17  and into the arc chamber  9  where it is stretched and divided by the arc plates  11 , shown in FIGS. 1,  4  and  5 , until it is extinguished. During this process, a large quantity of arc gas can be generated. The pressure generated by this gas must be relieved to avoid damage to the circuit breaker. The circuit breaker casing  49  is provided with a gas vent  67  which extends vertically behind the arc chute  1 . For lower pressure gases, the flaps  45  of the auxiliary valve  39  open, as shown in FIG. 4, to permit gases to pass from the arc chamber  9  into the gas vent  67 . For higher gas pressures, such as would be generated by a bolted short in the protected circuit, the center of the flexible sheet member  23  bows out as shown in FIG. 6, allowing the higher pressure gases to escape more easily into the gas vent  67 . A barrier  69 , which could be made, for instance, from fish paper, prevents the discharge arc gas from entering the terminal recess  61 . 
     While the arc chute  1  of the invention has been shown as applied to a circuit breaker  47 , it will be appreciated that it could also be used in other electric power switches in which arc gases are generated in order to provide controlled relief of the gas pressure. 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.