Abstract:
An arc-resistant enclosure for electrical switchgear which includes solid front and back walls, a pair of solid side walls joined to the front and back walls, a ventilated roof joined to the side walls and the front and back walls, and a ventilated base joined to the side walls and the front and back walls. Internal partitions divide the space enclosed by the front, back, side, top and bottom walls into multiple compartments for housing different types of components. The ventilated base forms air-intake ports for admitting ambient air into a plurality of the compartments, and the ventilated roof forms air-exhaust ports for allowing air to be exhausted from the compartments. As air inside the enclosure is heated by the switchgear, the hot air rises through the switchgear compartments and is exhausted through the top air-exhaust ports, and replacement ambient air is drawn into the bottoms of the compartments through the air-intake ports.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to electrical switchgear enclosures and, more particularly, to arc-resistant electrical switchgear enclosures having ambient air ventilation systems. 
     BACKGROUND OF THE INVENTION 
     Switchgear enclosures are commonly employed in electrical power distribution systems for enclosing circuit breakers and other switching equipment associated with the distribution system. Typically, switchgear enclosures are comprised of a number of individual stacked or adjacent compartments, and receive electrical power from a power source and distribute is the electrical power through one or more feeder circuits to one or more loads. Switchgear enclosures typically include circuit protection device for interrupting electric power in a particular feeder circuit in response to hazardous current overloads in the circuit. A circuit protection device in electrical equipment can be a circuit breaker, fuse and switch combination, contactor and fuse combination or any other device intended to break or protect the load or secondary side of a circuit. 
     Switchgear is a general term covering switching and interrupting devices and their combination with associated control, instruments, metering, protective and regulating devices, and assemblies of these devices with associated interconnections, accessories, and supporting structures used primarily in connection with the generation, transmission, distribution, and conversion of electric power. Switchgear characteristics are described in ANSI/IEEE Standard No. C37.20.1, C37.20.2, C37.20.3-1999. However, the present invention can be used in many other types of electrical equipment where arc resistance is required. 
     The specified temperature limits applicable to switchgear assemblies are given in the above referenced standards. The rated continuous current of metal-enclosed (ME) switchgear is the maximum current that can be carried continuously by the primary circuit components, including buses and connections, without producing a temperature in excess of specified limits for any primary or secondary circuit component, any insulating medium, or any structural or enclosing member. The continuous current ratings of the main bus in ME switchgear are also defined by the above referenced standards. The short-time current ratings of the individual circuit-breaker compartments of ME switchgear are equal to the short-time ratings of the switching and protective devices used, or the short-time rating of the current transformers (see ANSI/IEEE C57.13-1993). 
     In addition to current overloads, switchgear enclosures may encounter other hazardous conditions known as arcing faults. Arcing faults occur when electric current “arcs,” flowing through ionized gas between conductors, such as between two ends of broken or damaged conductors, or between a conductor and ground in a switchgear enclosure. Arcing faults typically result from corroded, worn or aged wiring or insulation, loose connections and electrical stress caused by repeated overloading, lightning strikes, etc. Particularly in medium- to high-voltage power distribution systems, the ionized gases associated with arcing faults may be released at pressures and temperatures sufficient to damage the switchgear equipment and cause deadly harm to anyone in close proximity. 
     Presently, the most commonly employed method for enhancing the durability of switchgear enclosures in the event of arcing faults is to provide arc-resistant switchgear that meets switchgear standards, with a means for venting the gases from the compartment in which an arcing fault occurs. These compartments are designed to withstand the pressures and temperatures of the gases associated with an arcing fault and reduce the likelihood or extent of damage to the switchgear. This control of the explosion exhaust is what provides the increased safety to personnel working around the equipment. 
     Meeting the temperature limits in arc-resistant switchgear enclosures becomes more difficult as the current rating of the switchgear increases, and it becomes necessary to use air ventilation systems to maintain the required temperatures. For example, air intake and exhaust openings may be provided in the front and rear walls of a switchgear enclosure, along with automatic closure mechanisms to close such openings when an arcing fault occurs inside the enclosure. These closure mechanisms can add to the cost of switchgear enclosures, and can also introduce reliability issues in preventing an arc exhaust. 
     SUMMARY 
     In one embodiment, an arc-resistant enclosure for electrical switchgear includes solid front and back walls, a pair of solid side walls joined to the front and back walls, a top wall joined to the side walls and the front and back walls, and a bottom wall joined to the side walls and the front and back walls. Internal partitions divide the space enclosed by the front, back, side, top and bottom walls into multiple compartments for receiving different types of switchgear. The bottom wall forms air-intake ports for admitting ambient air into a plurality of the compartments, and the top wall forms air-exhaust ports for allowing air to be exhausted from the compartments. An ambient air manifold below the bottom wall conducts ambient air to the air-intake ports in the bottom wall. As air inside the enclosure is heated by the electric paths within the switchgear, the hot air rises through the switchgear compartments and is exhausted through the top air-exhaust ports, and replacement ambient air is drawn into the bottom of the compartments through the air-intake ports. 
     In another embodiment, the compartments of the arc-resistant enclosure include a circuit protection device compartment, a cable compartment, and a main bus compartment between said circuit protection device compartment and cable compartments. An ambient air manifold conducts ambient air to the lower regions of all the compartments, and the top wall forms air-exhaust ports for allowing air to be exhausted from the compartments. In one implementation, a is portion of the cable compartment extends under the main bus compartment to be directly adjacent the circuit protection device compartment, and the ambient air manifold includes a pair of conduits located adjacent the front and back walls of the portion of the cable compartment extending under the main bus compartment to supply ambient air to the bottom of the main bus compartment. This arrangement permits the main bus compartment to be cooled with ambient air even though that compartment is located between, and isolated from, the other two compartments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages of the present disclosure will become apparent upon reading the following detailed description and upon reference to the drawings, in which: 
         FIG. 1  is a perspective view of an arc-resistant switchgear enclosure equipped with an ambient air ventilation system. 
         FIG. 2  is the perspective view similar to that of  FIG. 1  with the near side panel removed to reveal the internal structure. 
         FIG. 3  is an enlarged, partially exploded top perspective view of the ambient air intake manifold at the base of the arc-resistant switchgear enclosure of  FIGS. 1 and 2 . 
         FIG. 4  is a bottom perspective view of the ambient air intake manifold shown in  FIG. 3   
         FIG. 5  is a partially exploded top perspective view of a portion of the ambient air ventilation system in the switchgear enclosure of  FIGS. 1-4 , including the air intake manifold of  FIGS. 3 and 4 , a hood for attachment to an end portion of the top of the manifold, and a pair of vertical air conduits for conducting air to one of the compartments in the enclosure. 
         FIG. 6  is an enlarged perspective view of the components shown in  FIG. 5 , in their assembled positions. 
         FIG. 7  is a side elevation of the arc-resistant switchgear enclosure of  FIGS. 1-6 , with the addition of diagrammatic illustrations of the air flow through the ambient air ventilation system. 
         FIG. 8  is an enlarged section of a portion of one of the closure panels in the switchgear enclosure of  FIGS. 1-7 , in both its open and closed positions. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, is specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, 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 appended claims. 
     DETAILED DESCRIPTION 
     Turning now to the drawings and referring first to  FIGS. 1 and 2 , there is shown a switchgear enclosure  10  that includes vertical front and back walls  20  and  21 , a pair of vertical side walls  22  and  23  joined to the front and back walls, and top and bottom walls  24  and  25  joined to all the vertical walls  20 - 23 . Mounted on the front wall  20  are upper and lower hinged doors  26  and  27 , and the upper door  26  opens into a low-voltage compartment  28  that is isolated from the rest of the interior of the enclosure  10  by partitions  28   a  and  28   b  (see  FIG. 2 ). The lower door  27  opens into a circuit breaker compartment  31 , which is the first of three higher-voltage compartments  31 ,  32  and  33 . Compartment  32  is a main bus compartment, and compartment  33  is a cable compartment. 
     In the breaker compartment  31 , a circuit breaker (not shown) is plugged into a circuit breaker stab assembly  34  that includes a pair of circuit breaker sockets  34   a  and  34   b  which are connected to line-side bus bars  36  located in the main bus compartment  32  and load-side bus bars  37  located in the cable compartment  33 . The bus bars  36  are connected to a power-input line from a utility grid, and the bus bars  37  are connected to a load to be furnished with power. It will be noted that a portion of the cable compartment  33  extends under the main bus compartment  32  to be directly adjacent the circuit breaker compartment  31 , for connecting the bus bars  37  to the lower circuit breaker socket  34   b , through a vertical partition  38 . The main bus compartment  32  is located between the breaker compartment  31  and the cable compartment  33 , above the extended portion of the cable compartment  33 . A vertical partition  39  and a horizontal partition  40  separate compartments  32  and  33 . A pair of doors  41   a  and  41   b  is mounted on the exterior surface of the back wall  21  to permit access to the cable compartment  33 . 
     As the bus bars  36 ,  37  and the circuit breaker stab assembly  34  increase in temperature during operation, the air within the compartments  31 - 33  also increases in temperature. To allow the heated air to naturally rise and exit the compartments  31 - 33 , and thereby remove heat from those compartments, air exhaust ports are provided at or near the tops of the compartments  31 - 33 . As the heated air rises and exits the enclosure  10 , the exhausted hot air is replaced by cooler is ambient air that enters each compartment through air-intake openings at or near the bottoms of the compartments  31 - 33 . The replacement air is heated by the switchgear as it rises through the compartments  31 - 33 , thereby continuously removing heat from, and thus cooling, the switchgear. 
     In the illustrative embodiment, the three compartments  31 - 33  have respective air exhaust ports  42 ,  43  and  44  in their top walls, as can be seen in  FIGS. 1 and 2 . Each of the exhaust ports  42 ,  43  and  44  is formed by an array of cutouts in the metal panels that form the top walls of the respective compartments  31 - 33 , and which collectively form the top wall  24  of the enclosure  10 . This configuration of the air exhaust ports prevents any material larger than the size of the individual cutout openings from entering the enclosure  10 . Any particulate material that might enter the enclosure through the small cutout openings is captured in removable trays  45  mounted in the respective compartments  31 - 33 , above the switchgear in those compartments. 
     Replacement air enters the enclosure  10  from a hollow base  50  beneath the bottom wall  25  of the enclosure. In the illustrative embodiment, the hollow base  50  is coextensive with the width and depth of the switchgear enclosure  10 . Ambient air enters the hollow base  50  through multiple air-intake openings  51  formed in the front, back and both end walls of the hollow base  50 . Each of the air intake openings is formed by an array of cutouts in the metal panels that form the short vertical walls of the hollow base  50 , to prevent debris from entering the hollow base  50 . The interior of the hollow base  50  functions as a manifold that distributes the ambient air to air-intake ports  52 ,  53  and  54  formed in the bottom wall  25  of the enclosure  10 , where the ambient air is drawn upwardly into the enclosure  10 . The bottom wall  25  of the enclosure  10  also serves as the top wall of the hollow base  50 . 
     As in the case of the exhaust ports  42 - 44 , each of the air-intake ports  52 - 54  is formed by an array of cutouts in the metal panels that form the bottom wall  25  of the enclosure  10 . Associated with each of the ports  52 - 54  is a movable closure panel which will be discussed below. 
     The two air-intake ports  52  and  53  are aligned with the breaker compartment  31  so that air can be drawn into this compartment at a relatively high rate, because the single hottest region within the enclosure  10  is typically in the space around the breaker stabs  34   a  and  34   b  in the breaker compartment  31 . As the relatively cool ambient air flows upwardly from the intake ports  52  and  53  through the breaker compartment  31 , the breaker stabs  34   a  and  34   b  are cooled by the transfer of heat to the passing air stream, and then the heated air continues to rise and is ultimately exhausted from the enclosure via the large exhaust port  42  in the top wall of the breaker is compartment  31 . 
     The third air-entry port  54  opens into the portion of the L-shaped cable compartment  33  that extends under the main bus compartment  32 . A portion of the ambient air drawn through the port  54  is channeled to the main bus compartment  32  by a pair of vertical air conduits  60  and  61  extending upwardly from opposite ends of the port  54  to conduct ambient air from the port  54  into the bottom of the main bus compartment  32 . The lower ends of the two conduits  60  and  61  are connected to a shallow hood  63  that fits over, and is fastened to, the right-hand portion of the bottom wall  25 , including the air-intake port  54 . The hood  63  forms an array of cutout openings  64  to allow air from the port  54  to enter the cable compartment  33 , and a pair of rectangular openings  65   a  and  65   b  that allow air rising from the port  54  to enter the two conduits  60  and  61 , respectively. In  FIG. 5 , lines  31   a ,  32   a  and  33   a  illustrate the air flow paths from the air-intake ports  52 - 54  to the respective compartments  31 - 33 . In certain applications, one of the conduits  60  and  61  might be sufficient to maintain the temperature of the main bus compartment below the applicable limit. 
     Flanges  66  and  67  on the lower ends of the respective conduits  60  and  61  are fastened to the top of the hood  63  so that the only paths from the cable compartment  33  into the main bus compartment require 180-degree vertical turns in the constricted space between the openings  64  and  65   a,b . The two air streams then pass upwardly through the conduits  60  and  61  and are discharged into the lower region of the main bus compartment  32 . A pair of air deflectors  68  and  69  guide the air from the respective conduits  60  and  61  into the central region of the compartment  32 , where the most heat is generated because that is where the circuit breaker stabs  34   a  and  34   b  are connected to the bus bars  37 . 
     When an arcing fault occurs within one of the compartments  31 - 33  in the enclosure  10 , the temperature and pressure in that compartment can increase rapidly, and the materials involved in or exposed to the arc produce hot decomposition products, either gaseous or particulate, which should be discharged outside the enclosure. In the illustrative enclosure, such gaseous and particulate materials can only be discharged through the air-exhaust ports in the top wall  24  because the air-intake ports  52 - 54  are equipped with closure devices that automatically close the air-intake ports in response to the sudden pressure increase that occurs inside the enclosure when an arcing fault occurs. Specifically, the air-intake ports  52 - 54  are equipped with respective closure panels  72 ,  73  and  74  that are held open by biasing springs during normal pressure conditions. When the pressure within the enclosure  10  increases due to the occurrence of an arcing fault, that pressure overcomes the spring biasing forces and presses the panels  72 - 74  down against the upper surface of the bottom wall  25 , thereby closing the air-intake ports  52 - 54 . This type of closure device is described in U.S. Pat. No. 5,767,440, which is assigned to the assignee of the present invention. 
     As can be seen in  FIG. 8 , spring biasing forces are applied to each of the closure panels  72 - 74  by multiple compression springs  75  captured on cap screws  76  by nuts  77  and washers  78 . The cap screws  76  extend through the panels  72 - 74  so that each of the panels  72 - 74  can slide vertically on a set of six of the screws  76 . The compression springs  75  normally hold the panels  72 - 74  in their raised (open) positions where they are vertically spaced from the bottom wall  25  so that ambient air can freely pass through the air-intake ports  52 - 54 . When an arcing fault occurs, the resulting sudden pressure increase inside the enclosure  10  forces the panels  72 - 74  downwardly, overcoming the upward biasing force of the compression springs  75  and moving the panels  72 - 74  down into firm engagement with the bottom wall  25 . This closes the air-intake ports  52 - 54  so that there is no risk to personnel or equipment that might be positioned near the exterior openings in the hollow base of the enclosure. 
     The gases cannot be vented through the front, back or end walls because those walls are all completely closed. Thus, the gases can exit only through the top wall  24 , which is not accessible to personnel, or is equipped with an exhaust plenum that receives gases exhausted through the top wall  24  and conducts these gases to a safe exhaust region where personnel are prohibited. 
     The ambient air ventilation system provided by this invention has been found to be highly effective in cooling switchgear. Tests have demonstrated that the illustrative ventilation system is capable of reducing the temperature within the switchgear enclosure by more than 20° C., or more than 70° F., effectively cooling switchgear capable of handling currents as high as 2000 to 3000 amperes. This temperature reduction can be achieved without the use of any air vents in any of the vertical walls of the enclosure above the hollow base, thereby avoiding the need for movable closure devices on any of the vertical walls of the enclosure. 
     While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations will be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.