Abstract:
A scalable earthing switch that incorporates a torsion spring to effect rapid closure of the switch. The torsion spring is supported coaxially about a rotatable shaft on which contact blades are mounted resulting in compact design. The blade contacts are separated axially along the length of the shaft by one or more spacers. By using difference size spacers the distance between adjacent blade contacts can be changed and, thus, the earthing switch can be easily scaled for different applications. A latching (detent) mechanism is provided for latching the switch in an open position.

Description:
BACKGROUND 
       [0001]    The present exemplary embodiment relates to electrical switching mechanisms. It finds particular application in conjunction with medium voltage earthing switches, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications. 
         [0002]    It is common to provide protection to technicians servicing an electrical component enclosure through the provision of an earthing switch. A typical earthing switch includes one or more blade contacts mounted on a rotatable shaft. An actuating mechanism rotates the shaft to move the blade contacts between an open position and a closed position in contact with a grounding electrode. The earthing switch is typically installed between a distribution bus and a circuit breaker connecting the distribution bus to a main line. The earthing switch, when closed, grounds the distribution bus. 
         [0003]    Prior to earthing the line or bus terminals, it is typical to disconnect the upstream source of electrical power. In certain situations, however, the circuit may inadvertently be live during grounding. In other situations, the upstream source of electrical power may be inadvertently reenergized before performing closing of the switch. In still other situations, there could be back feed of electricity to the distribution bus such as, for example, in the case of a spinning electric motor producing current that back feeds to the distribution bus. Thus, even when the circuit breaker connecting the distribution bus to the main bus is open, current may exist in the distribution bus. In each of the foregoing situations, a properly operating earthing switch can protect technicians and equipment from harm. 
         [0004]    Arcing can occur when an earthing switch is closed on a fault. The arcing, in turn, can cause melting of the contact material which can result in welding of the contacts. If the contacts are not opened while the metal is still fluid, a rough surface is produced. The voltage concentrations caused by the spikes on the now rough surface result in an even earlier striking of the arc the next time and can lead to permanent welding of the contacts. 
         [0005]    To minimize arcing, many conventional earthing switches include coil springs configured to rapidly close the switch when actuated. Such coil springs are often supported adjacent to the rotatable shaft and operatively coupled to the shaft by a crank arm or other mechanism. When the switch is actuated to close, the spring is configured to act on the crank arm to rapidly rotate the shaft and thereby quickly close the switch. 
         [0006]    Current earthing switch designs relying on coil springs are generally bulky since the coil springs and associated mechanisms are supported adjacent the rotating shaft and blade contacts. Further, such prior art earthing switches are not easily scalable to various applications, since most often the blade contacts are welded or otherwise permanently secured to the rotatable shaft. Thus, separate shaft/blade assemblies typically need to be manufactured for different applications. 
       BRIEF DESCRIPTION 
       [0007]    The present disclosure provides a scalable earthing switch that incorporates a torsion spring to effect rapid closure of the switch. The torsion spring is supported coaxially about a rotatable shaft on which contact blades are mounted resulting in a more compact design. The blade contacts are separated axially along the length of the shaft by one or more spacers. By using difference size spacers the distance between adjacent blade contacts can be changed and, thus, the earthing switch can be easily scaled for different applications. A latching (detent) mechanism is provided for latching the switch in an open position. 
         [0008]    In accordance with one aspect, an earthing switch for a connecting a power source to ground comprises an actuating mechanism, a rotatable shaft adapted to be rotated by the actuating mechanism, at least one moveable contact secured to the rotatable shaft for movement therewith between an open position and a closed position, a torsion spring for biasing the at least one moveable contact towards the closed position, and a detent mechanism for latching the at least one moveable contact in the open position. 
         [0009]    The switch can further include a plurality of moveable contacts secured to the rotatable shaft for movement therewith, the moveable contacts being axially spaced apart along the shaft by at least one spacer. The at least one spacer can be coaxially received over the rotatable shaft, and may be conductive. The at least one moveable contact can include a pair of spaced apart blades adapted to receive a stab therebetween when in the closed position. The at least one moveable contact can include a non-circular bore adapted to be received on a non-circular section of the shaft for fixing the contact for rotation therewith. The actuating mechanism can include a rotary actuating mechanism for rotating the shaft. 
         [0010]    The earthing switch can further comprise a mounting bracket, wherein the rotatable shaft is supported on the mounting bracket for rotation, and wherein a coil of the torsion spring is received coaxially over the rotatable shaft, a first end of the torsion spring being engaged with said mounted bracket, and a second end of the torsion spring being operatively connected to the movable contact, whereby rotation of the rotatable shaft in a first direction is opposed by the torsion spring while rotation of the rotatable shaft in the second direction is assisted by the torsion spring. 
         [0011]    The detent mechanism can include at least one pawl adapted to engage a surface of a hub associated with the actuating mechanism for latching the switch in an open position. The at least one pawl can be pivotally mounted to a housing of the actuating mechanism for movement between a radially outer position and a radially inner position relative to the hub whereat the pawl is received in a recess in the hub thereby latching the switch open. A cam member can be provided for radially displacing the at least one pawl from its radially inner position, and the hub and cam can be mounted coaxially on an input shaft of the actuating mechanism whereby rotation of the input shaft from a position corresponding to a latched position of the switch towards a position corresponding to a closed position of the switch causes the cam to radially outwardly displace the at least one pawl from the recess and allow the switch to close. 
         [0012]    In accordance with another aspect, a modular earthing switch assembly comprises a support member, a rotatable shaft having a non-circular cross-section supported for rotation on said support member, a moveable contact mountable on the rotatable shaft in a plurality of positions, the moveable contact having a bore with a non-circular cross-section for telescoping over the non-circular cross-section of the rotatable shaft thereby fixing the movable contact for rotation with the rotatable shaft, and at least one spacer received coaxially on the rotatable shaft and located adjacent the moveable contact, the at least one spacer axially locating the moveable contact along the rotatable shaft. 
         [0013]    The switch can further include a torsion spring for biasing the movable contact towards a closed position. A mounting bracket can be provided, wherein the rotatable shaft is supported on the mounting bracket for rotation, and wherein a coil of the torsion spring is received coaxially over the rotatable shaft, a first end of the torsion spring being engaged with said mounted bracket, and a second end of the torsion spring being operatively connected to the movable contact, whereby rotation of the rotatable shaft in a first direction is opposed by the torsion spring while rotation of the rotatable shaft in the second direction is assisted by the torsion spring. The at least one moveable contact can include a pair of spaced apart blades adapted to receive a stab therebetween when in the closed position. 
         [0014]    The switch can also include an actuating mechanism for rotating the shaft to effect movement of the at least one movable member between an open position and a closed position. A detent mechanism can be provided including at least one pawl adapted to engage a surface of a hub associated with the actuating mechanism for latching the switch in an open position. The at least one pawl can be pivotally mounted to a housing of the actuating mechanism for movement between a radially outer position and a radially inner position relative to the hub whereat the pawl is received in a recess in the hub for latching the switch open. A cam member can be provided for radially displacing the at least one pawl from its radially inner position, and the hub and cam can be mounted coaxially on an input shaft of the actuating mechanism whereby rotation of the input shaft from a position corresponding to a latched position of the switch towards a position corresponding to a closed position of the switch causes the cam to radially outwardly displace the at least one pawl from the recess and allow the switch to close. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a perspective view of an exemplary earthing switch in accordance with the disclosure; 
           [0016]      FIG. 2  is an exploded view of the exemplary earthing switch of  FIG. 1 ; 
           [0017]      FIG. 3  is an enlarged view of the exemplary earthing switch of  FIG. 1  showing details of the torsion spring; 
           [0018]      FIG. 4  is an side elevational view of the exemplary earthing switch showing the torsion spring and set screw for adjusting torsion spring tension; 
           [0019]      FIG. 5  is a perspective view of a latching mechanism of the exemplary earthing switch in a first position; 
           [0020]      FIG. 6  is a front elevational view of the earthing switch in the position shown in  FIG. 5 ; 
           [0021]      FIG. 7  is a perspective view of the exemplary earthing switch in a second position; 
           [0022]      FIG. 8  is a front elevational view of the earthing switch in the position shown in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    With reference to  FIG. 1 , an exemplary earthing switch  10  in accordance with the disclosure is illustrated. The earthing switch  10  generally includes a rotatable actuating shaft  14  on which a plurality of blade contacts  18  are mounted for rotation therewith between an open position and a closed position wherein said contacts  18  engage respective line/load stabs. An actuating mechanism, including an input shaft  22  and gearbox  26 , is connected to the actuating shaft  14  for moving the blade contacts  18  between the open and closed positions. Unlike prior art earthing switches that utilize coil-over springs, the earthing switch  10  utilizes a torsion spring  30  arranged coaxially with the actuating shaft  14  for biasing the blade contacts  18  towards the closed position. This results in a compact design that can be easily scaled for various applications. All of the components are supported on a mounting bracket  34  that can be mounted to a desired surface, such as within an electrical cabinet or the like. 
         [0024]    With additional reference to  FIG. 2 , the details of the exemplary earthing switch  10  will be described. The mounting bracket  34  includes a base plate  36 , a gear box end plate  38  secured to the base plate  36 , and a shaft end plate  38  also secured to the base plate  36 . The mounting bracket  34  includes a plurality of holes for securing the same to a desired surface using one or more suitable fasteners. The gear box  26  is secured to the base plate  36  and end plate  38  via a plurality of bolts  44 . A first end of the actuating shaft  14  is received through an opening  46  in the gear box  26  and supported therein for rotation. A second end of the actuating shaft  14  is supported for rotation by a bearing  48  secured to the shaft end plate  38  by bolts  50 . 
         [0025]    The actuating shaft  14  includes a non-circular portion  54  thereof on which the plurality of blade contacts  18  are mounted. In the illustrated embodiment, the non-circular portion  54  of the actuating shaft  14  has a hexagonal cross-section, but other non-circular shapes could be used. Each blade contact  18  comprises a pair of individual blades  56 , each having an opening  58  in an end thereof having a cross-sectional shape corresponding to the cross-sectional shape of the non-circular portion  54  of the actuating shaft  14 . When received on the non-circular portion  54 , each blade  56  is fixed for rotation with the actuating shaft  14 . 
         [0026]    The axially outer blade contacts  18  are mounted to the actuating shaft  14  with a ground spacer  60  disposed between each respective blade  56  at its point of attachment to the actuating shaft  14 . Like each blade  56 , each ground spacer  60  is keyed to the actuating shaft for rotation therewith. To this end, each ground spacer  60  has a central bore  62  having a cross-sectional shape that corresponds to the non-circular portion  54  of the actuating shaft. As will be described in more detail below, each ground spacer  60  also includes first and second radially extending ears  64  having stop surfaces  66  for limiting the extent of rotation of the actuating shaft  14 . The stop surfaces  66  make contact with the baseplate  36  when the actuating shaft  14  is rotated a predetermined amount in either direction. Accordingly, the ground spacers  60  act as limiters to prevent over-rotation of the shaft  14 . 
         [0027]    Each ground spacer  60  further includes a bore  68  provided for connecting each ground spacer  60  to a grounding strap (not shown). The middle blade contact  18  has a spacer  69  between respective blades  56 . The spacer  69  is not a ground spacer (e.g., it does not have a tab for connection to a ground strap), although a ground spacer could be utilized in that position as well if desired. 
         [0028]    A pair of tubular spacers  70  are provided for locating and/or spacing the blade contacts  18  axially along the actuating shaft  14 . The tubular spacers  70  also support the torsion spring  30  and, as such, can have an outer circumference that is closer in size to an inner circumference of the torsion spring  30  than is the outer circumference of the actuating shaft  14 . Together, the actuating shaft and blade contact assembly including ground spacers  60 , spacer  69 , and spacers  70 , define a conductive ground path from the blade contacts  18  to ground. 
         [0029]    Opposite tails  74  of the dual coil torsion spring  30  are received in spring holes  76  that secure the spring  30  to respective blade contacts  18 . With reference to  FIGS. 3 and 4 , a central portion  78  of the spring  30  between respective coils includes tab  79 . Tab  79  is a generally u-shape extension of the spring  30  that is configured to engage a set screw  80  mounted to the bracket  30  to thereby restrict rotation of the tab  79  relative to the bracket. Set screw  80  can be adjusted to adjust the tension (preload) of the torsion spring  30 . For example, the set screw can be unscrewed from the position shown in  FIGS. 3 and 4  thereby displacing the tab  79  upward and increasing the spring preload. In contrast, if the set screw is screwed in further from the position shown, the preload of the spring will be reduced. 
         [0030]    All of the components mounted on the actuating shaft  14  are secured thereon between hex nut portion  81  at a first end of the shaft  14 , and a hex nut  82  and washer  83  secured to the opposite end of the shaft  14 . As will be appreciated, the actuating shaft and blade contact assembly can be configured using components of differing sizes to produce a switch having a desired size and/or rating. For example, the spacing between the individual blades  56  of the blade contacts  18  can be changed by utilizing ground spacers  60  having a desired thickness. Also, the orientation of the blade contacts  18  can be changed by locating each blade in a desired angular position on the non-circular portion  54  of the actuating shaft  14 . Further, the spacing between each respective blade contact  18  can be altered by using spacers  70  of a desired length. In some cases, a given actuating shaft  14  can be used to support a plurality of configurations of the blade contacts  18 , etc., thereon. In other instances, an actuating shaft having a longer or shorter axial length may be provided instead of the illustrated actuating shaft  14  to accommodate larger or smaller contact assemblies. 
         [0031]    As noted, a first end of the actuating shaft  14  is received in the gear box  26  and supported therein for rotation. In this regard, a miter gear  84  is keyed to the end of the actuating shaft via a key  86  received in a keyway of the miter gear  84 . In the illustrated embodiment, the miter gear is secured on the end of the actuating shaft  14  via a e-type circlip  90 , but could be secured to the shaft  14  in any suitable manner. 
         [0032]    Miter gear  84  is engaged with a corresponding miter gear  92  that is secured to an end of the input shaft  22  and supported for rotation on a bearing  94  that is secured to the base plate  36 . As will be appreciated, rotation of the input shaft  22  results in rotation of the actuating shaft  14  and corresponding movement of the blade contacts  18 , for example, between their open and closed positions. In order to maintain the switch in an open position against the bias of the torsion spring  30 , miter gear  92  includes a contoured hub  93  that is part of a latching mechanism  96  designed to hold the switch in the open position. 
         [0033]    The latching mechanism  96  (also referred to as a detent mechanism) includes a pair of roller pawls  98  adapted to engage and follow respective outer hub surfaces  99  of the contoured hub  93  in a manner that restricts rotation of the gear  92  from a position associated with the contacts  18  being in their open position. In other words, the latching mechanism  96  operates to latch the switch in the open position against the force applied by the torsion spring  30 . Once dislodged from the open position, the latching mechanism  96  allows the torsion spring  30  to rotate the switch contacts  18  unimpeded to the closed position. 
         [0034]    Referring now to  FIGS. 5 and 6 , the latching mechanism  96  is shown in an unlatched position with the blade contacts  18  being in a closed or partially open position (e.g., not open). The outer hub surfaces  99  of the hub  93  extend from the gear box  26 , with the miter gear  92  itself generally enclosed within the gear box  26 . Each roller pawl  98  is pivotally mounted to the gear box  26  by a bolt  100 , and is biased against the hub  93  via a pawl torsion spring  101  ( FIG. 2 ). Rollers  102  of each roller pawl  98  engage respective hub surfaces  99  of the hub  93  at diametrically opposed positions. 
         [0035]    As will be appreciated, the hub surfaces  99  are discontinuous and also diametrically opposed. Each hub surface  99  extends approximately ¼ of the circumference of the hub  93 . In between the hub surfaces  99  are a pair of diametrically opposed recesses  106  in which the respective roller pawls  98  are adapted to reside when the switch is locked in the open position. 
         [0036]    With reference to  FIGS. 7 and 8 , it will be understood that the pawl torsion springs  101  (only shown in  FIG. 2 ) bias the pawls  98  against the hub surfaces  99  such that, when input shaft  22  is rotated and the pawls  99  become aligned with the recesses  106 , the pawls  98  will pivot radially inwardly into the recesses  106  and secure the switch in the open position against the bias of the torsion spring  30 . Once in the position of  FIGS. 7 and 8 , the rollers  102  engage end surfaces  110  of the hub  93  and restrict rotation of the hub  93  and by extension the input shaft  24  and actuating shaft  14 . In this position, the pawls  98  are in an “over-center” position with respect to their point of attachment to the housing  26  such that as the torsion spring  30  acts upon the actuating shaft  14  and thereby the hub  93 , the pawls are further driven radially inwardly thereby preventing rotation of the hub  93  and latching the switch open. 
         [0037]    To release the latching mechanism  96 , a cam  112  is provided on the input shaft  22  and mounted for rotation therewith. Cam  112  has a pair of diametrically opposed cam lobes  116  adapted to urge the pawls  98  radially outwardly when the input shaft  22  is rotated from the position shown in  FIGS. 7 and 8  (e.g., the switch open and latched position) towards a switch closed position (e.g., as shown in  FIGS. 5 and 6 ). The cam lobes  116  are positioned radially about the input shaft  22  in a position such that they immediately engage and urge radially outwardly a surface of the pawls  98 , for example the rollers  102 , when the input shaft  22  is rotated from the open and latched position. As the shaft  22  is rotated, the cam lobes  116  radially displace the pawls  98  until the rollers  102  clear the end surfaces  110  at which point the pawls  98  no longer restrict rotation of the hub  93 , and by extension the input shaft  24  and actuating shaft  14 . Accordingly, the torsion spring  30  then can act to rapidly transition the switch to a closed position. 
         [0038]    As will now be appreciated, the latching mechanism  96  enables the switch to be maintained in the open position against the force of the torsion spring  30  and then to quickly become unlatched and allow the full force of the torsion spring  30  to act upon the actuating shaft  14  to close the switch. This results in a rapid closure to avoid or minimize arcing issues that can sometimes occur when closing the switch against a fault. 
         [0039]    The exemplary embodiment has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiment be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.