Abstract:
A solenoid assembly for a circuit breaker having a clapper with an increased mass is provided. The mass, preferably at least one non-ferrous slug, increases the mass of the clapper so that the solenoid response time is slowed. That is, the added mass creates an inertial delay that slows the solenoid clapper as the clapper moves between the first position and the second position. Preferably, the delay is between 48 ms and 60 ms, and more preferably 50 ms. The clapper having a non-ferrous slug may also be incorporated into existing circuit breakers by, for example, coupling the non-ferrous slug to the preexisting clapper, or replacing the clapper with a clapper assembly having at least one non-ferrous slug thereon.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a medium voltage switchgear having a circuit breaker, the circuit breaker having an operating mechanism with an opening solenoid and, more specifically, to an opening solenoid having additional mass coupled to the solenoid clapper, the additional mass structured to cause an inertial delay to the operation of the solenoid clapper. 
         [0003]    2. Background Information 
         [0004]    A medium voltage switchgear, typically, comprises a switching mechanism housed in an enclosure. The switching mechanism, typically a circuit breaker, includes a plurality of separable contacts coupled to an operating mechanism having a common spring-operated closing and tripping device. The operating mechanism includes one or more opening springs which separate the contact and a pair of closing springs which close the contacts as well as charge the opening spring. The separable contacts are closed by releasing the energy stored in the closing springs through activation of a closing trigger mechanism. This can be done manually or remotely through a solenoid. The closing springs are charged manually by a lever arm through a ratchet coupling, or, more preferably, by a motor. An electronic trip circuit monitors the load currents and actuates an opening trigger mechanism through an opening solenoid if the current exceeds certain current-time characteristics. 
         [0005]    The opening solenoid includes an elongated clapper structured to move between a first position and a second position. The elongated clapper extends between the trip device and a trip lever in the operating mechanism. The clapper is, typically, made from steel. The trip lever is fixed to a D shaft that engages the operating mechanism trip latch. When the clapper is in the first position, the operating mechanism trip latch may engage the D shaft. When the trip latch is held by the D shaft, the separable contacts may be closed. Once the separable contacts are closed current may pass through the circuit breaker. If an external control device applies the appropriate voltage and current to the solenoid coil, or if the clapper is manually activated, the clapper moves to the second position. As the clapper moves into the second position, the clapper causes the trip lever to rotate which, in turn, causes the D shaft to rotate. As the D shaft rotates, the trip latch disengages from the D shaft and allows the operating mechanism to separate the contacts. 
         [0006]    The opening solenoid has a mass limit. That is, the opening solenoid is structured to move a mass, the clapper, and that mass has a maximum limit. The higher the mass limit, the greater the mass the opening solenoid is structured to move. Generally, an opening clapper has a mass that is between 5% and 20% of the mass limit of an opening solenoid. For example, a typical opening solenoid has a mass limit of about 1.89 kg and a typical opening clapper has a mass of about 0.2 kg. In this configuration, the response time, that is the time to move the clapper between the first and second position, is about 10 ms to 35 ms, and more typically 25 ms. When the mass of the opening clapper is reduced, the response time, i.e., the time required for the clapper to move between first and second positions, of the opening solenoid is decreased. That is, with a lighter opening clapper, the opening solenoid clapper moves between the first and second positions more rapidly. 
         [0007]    It is generally assumed that the response time of the opening solenoid should be as short as possible. That is, when the current exceeds certain current-time characteristics it is desirable to have the operating mechanism separate the contacts as quickly as possible to avoid, or minimize, damage to the circuit breaker and/or load side electrical components. To ensure that the operating mechanism responds rapidly, the opening solenoid must respond rapidly as well. However, it has also been determined that a typical over current, or “fault current,” situation includes a decaying direct current as well as an alternating current. That is, a current, either direct or alternating, has a wave form that may be expressed, generally, as a sine wave. A decaying direct current occurs just after the direct current wave form is at a peak. It has further been determined that, if the contacts are separated at, or near, the maximum wave peak, i.e., both the direct and alternating currents are at or near their peaks, the contacts may be damaged. Given that the fault current typically occurs when the direct current is at, or just past, a peak, separation of the contacts at the maximum wave peak could be avoided if the separation of the contacts was delayed until the direct current was off peak. 
         [0008]    There is, therefore, a need for an operating mechanism structured to delay the separation of the contacts until the direct current was off peak. 
         [0009]    There is a further need for an opening solenoid that is structured to delay the movement of the clapper thereby delaying the release of the operating mechanism trip latch. 
         [0010]    There is a further need for an opening solenoid that is structured to delay the movement of the clapper that may be incorporated into existing circuit breakers. 
       SUMMARY OF THE INVENTION 
       [0011]    These needs, and others, are met by the device disclosed herein provides for a solenoid assembly with a clapper having mass coupled thereto. The mass, preferably at least one non-ferrous slug, increases the mass of the clapper so that the solenoid response time is slowed. That is, the added mass creates an inertial delay that slows the clapper as the clapper moves between the first position and the second position. Preferably, the delay is between 48 ms and 60 ms, and more preferably 50 ms. The clapper having a non-ferrous slug may also be incorporated into existing circuit breakers by, for example, coupling the non-ferrous slug to the preexisting clapper, or replacing the clapper with a clapper assembly having at least one non-ferrous slug thereon. 
         [0012]    In an alternate embodiment, the clapper may simply be constructed to have a greater mass. For example, the clapper may be thicker, wider, or made from a material of greater density than 7.85 kg/dm 3 . Such a change in size or material is not simply a design choice. That is, generally it is desirable to have smaller, lighter components on a circuit breaker. Smaller, lighter components typically provide a better response time. Smaller components further allow the circuit breaker to occupy a smaller space. Conversely, larger components and/or heavier materials are typically used to improve the strength of a component. The clapper, however, is not subjected to an excessive stress and, as such, the strength of the clapper is not typically a design issue. Therefore, an improvement to a clapper generally involves making a clapper lighter, smaller, or both. The improvement disclosed herein, however, relates to increasing the mass of the opening clapper. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    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: 
           [0014]      FIG. 1  is a side elevational view with some parts cut away with a typical medium voltage circuit breaker in accordance with the invention shown in the disconnected position. 
           [0015]      FIG. 2  is a front elevational view of a typical circuit breaker as seen in  FIG. 1  with the cover removed. 
           [0016]      FIG. 3  is a partial side view of a circuit breaker. 
           [0017]      FIG. 4  is a detailed side view of a portion of the charging mechanism. 
           [0018]      FIG. 5  is an isometric view of the charging mechanism shown in  FIG. 4 . 
           [0019]      FIG. 6   a  is a sectional view taken along the line  6 - 6  in  FIG. 2  shown with the breaker in the open position and the closing springs discharged.  FIG. 6   b  is similar to  FIG. 6   a  but showing the breaker closed with the closing springs charged. 
           [0020]      FIG. 7   a  is a sectional view taken along the line  7 - 7  in  FIG. 2  showing the breaker open and the closing spring discharged. 
           [0021]      FIG. 7   b  is similar to  FIG. 7   a  but showing the breaker in the open position and the closing springs charged. 
           [0022]      FIG. 7   c  is similar to  FIGS. 7   a  and  7   b  but showing the breaker closed and the closing springs discharged. 
           [0023]      FIG. 8  is a side view of an opening solenoid assembly including a clapper with additional mass. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    As shown in  FIGS. 1 and 2 , a switch gear apparatus  10  includes a cabinet or enclosure  13  for enclosing a circuit breaker  15 . The exemplary circuit breaker  15  is, preferably, a draw-out three-phase vacuum circuit interrupter having controls on a front panel  17  for manually operating the circuit breaker  15 . The circuit breaker  15  has wheels  19  which engage rails  21  for inserting the circuit breaker  15  into and removing the circuit breaker  15  from the enclosure  13 . The enclosure  13  includes at least one line terminal  27  and at least one load terminal  29 . The circuit breaker  15  includes at least one line terminal  23  and at least one load terminal  25 . Typically, the switch gear apparatus  10  has three circuit breaker line and load terminals  23 ,  25  and three corresponding enclosure line and load terminals  27 ,  29 . The circuit breaker line and load terminals  23 ,  25  are positioned to engage, and be electrically coupled to, the enclosure line and load terminals  27 ,  29 . Movement of the circuit breaker  15  along the rails  21  also effects connection and disconnection of circuit breaker line and load terminals  23 ,  25  with the enclosure line and load terminals  27 ,  29 . While a medium voltage vacuum interrupter is shown for the circuit breaker  15 , the invention is also applicable for use with air circuit breakers. 
         [0025]    The circuit breaker  15  has a front low voltage section  31  adjacent to the front panel  17  and a rear high voltage section  33  containing a vacuum interrupter  35  for each phase. The low and high voltage sections  31 ,  33  are electrically insulated from each other by upper and lower insulators  37 ,  39 . Within each vacuum interrupter  35 , a pair of separable contacts  40  including a stationary contact  41  and a moveable contact  43  are provided. The contacts  40  are operated between the open position (shown) and a closed position by a linkage  45  which includes a bell crank  47  (shown schematically) pivoted at pivot point  49  and an insulated push rod  51  extending into the low voltage section  31 . 
         [0026]    An operating mechanism  53  for opening and closing the separable contacts  40  through the linkage  45  is contained in the low voltage section  31 . This operating mechanism  53  has a number of driven parts  54  which include a pole shaft  55  which is rotatably journaled in sidewalls  57 ,  59  of a housing  61  ( FIG. 2 ). A pole arm  63  ( FIG. 1 ) for each phase projects laterally from the pole shaft  55  and is pivotally connected to the associated push rod  51  so that rotation of the pole shaft  55  simultaneously opens or closes the separable contacts  40  of each pole. The pole shaft  55  is rotated counter-clockwise as viewed in  FIG. 1  to open the contacts  40  by an opening spring  65  in the form of a helical tension spring connected at one end to an upper portion of the housing  61  of the low voltage section  31  and at the other end to a lever arm  67  mounted on the pole shaft  55 . 
         [0027]    The operating mechanism  53  also includes a pair of helical tension closing springs  69 ,  71  each of which is connected at its upper end to the housing  61  and at its lower end through a spring link  73 ,  75  to an eccentric pivot  77 ,  79  on a spring crank  81 ,  83 , respectively. The spring cranks  81 ,  83  are mounted on opposite ends of a crank shaft  85  rotatably supported between a pair of spaced supports  87 ,  89 . Fixed on the crank shaft  85  between the supports  87 ,  89  is a closing cam  91  which includes a notch  93  in the peripheral cam surface thereof (see  FIGS. 7   a - c ). 
         [0028]    The crank shaft  85  is rotated to extend or charge the two closing springs  69 ,  71  by a charging mechanism  200 . As shown in  FIG. 2 , the charging mechanism  200  includes a motor  202 , preferably electric, having a motor shaft  204 , and a drive gear eccentric  206 . As further shown in  FIGS. 3-5 , the charging mechanism  200  further includes at least one charge pawl  208 , at least one hold pawl  209  a ratchet wheel  210 , at least one charging plate  211 , at least one drive lever  213 , a motor control cam  212 , and a motor control switch  218  having a switch lever  220 . The switch lever  220  is, preferably, a rectangular beam  221  having a diameter thickness between about 0.031 and 0.062 inch, and more preferably about 0.040 inch. The motor shaft  204  extends in a direction generally parallel to the crank shaft  85 . The drive eccentric  206  is coupled to the motor shaft  204 . The ratchet wheel  210  is fixedly mounted to freely rotate about the crank shaft  85  within rotational boundaries set by an integral detent  223  and at least one charging plate  211 . The at least one charging plate  211  is fixedly mounted to the crank shaft  85 . The charge pawl  208  is coupled to at least one drive lever  213 , which in turn freely rotates about the crank shaft  85 . The drive gear eccentric  206  is structured to operatively engage the ratchet wheel  210  through at least one drive lever  213  so that, when the motor  202  is energized, the crank shaft  85  is rotated counterclockwise as shown by the arrows in  FIGS. 7   a - c . That is, rotation of the motor shaft  204  is transferred to the crank shaft  85  via the linking of the drive gear eccentric  206 , the charge pawl  208 , at least one drive lever  213 , and the ratchet wheel  210 . Reverse rotation of the crank shaft  85  is substantially limited by the at least one hold pawl  209  which is coupled to the housing  61  and also structured to engage the ratchet wheel  210 . 
         [0029]    The motor control cam  212  is also fixedly coupled to the crank shaft  85 . The motor control switch  218  is coupled to the housing  61  adjacent to the motor control cam  212 . The motor control switch lever  220  extends toward and engages the cam surface of the motor control cam  212 . The motor control switch  218  is electrically coupled to the motor  202  and provides a control signal thereto. That is, the motor control switch  218  is structured to selectively actuate the motor  202  in response to the position of the switch lever  220 . The switch lever  220  is structured to engage the motor control cam  212  and move in response to the changing diameter of the motor control cam  212 . The motor control cam  212  includes a first, reduced diameter portion  230 , and a second, wide diameter portion  232 . The switch lever notch  214  is located at one boundary between the first, reduced diameter portion  230 , and the second, wide diameter portion  232 . The motor control switch  218  is structured to provide an actuation signal to the motor  202  when the motor control switch lever  220  engages the second, wide diameter portion  232  of the motor control cam  212 . When the motor control switch lever  220  engages the first, reduced diameter portion  230  of the motor control cam  212  the motor  202  is not actuated. 
         [0030]    Alternatively, as is known, the crank shaft  85  can be manually rotated to charge the closing springs  69 ,  71  by a charging lever (not shown) which engages the charging mechanism  200 . The closing springs  69 ,  71  are retained in the charged condition and released by a first, closing spring release  99  (see  FIGS. 6   a  and b) which includes a closing spring release latch  101  pivotally connected on a shaft  103 . This closing spring release latch  101  has a latch surface  105  which is engaged by a latch roller  107  supported between a pair of roller support arms  109  fixed to the crank shaft  85 . 
         [0031]    With the circuit breaker  15  open and the closing springs  69 ,  71  discharged as shown in  FIG. 6a , operation of the charging mechanism  200  causes the crank shaft  85  to rotate in a counterclockwise direction as shown by the arrow. This causes the eccentric pivots  77 ,  79  to move downward thereby extending the closing springs  69 ,  71 . Just after the eccentric pivots  77 ,  79  carry the lines of action of the closing springs  69 ,  71  through the center of the crank shaft  85 , the closing latch roller  107  engages the latch surface  105  on the closing spring release latch  101 . The tendency of the closing springs  69 ,  71  to continue the rotation in the clockwise direction is blocked by the engagement of an extension  111  on the release latch  101  with a fixed pin  113 . 
         [0032]    The release latch  101  is operated by a release lever  115  pivotally connected at one end to an arm  117  on the pole shaft  55 . The other end of the release lever  115  rests on a close clapper  119 . The close clapper  119 , in turn, is pivotally supported on a bracket  121  which also supports a close solenoid  123 . Rotation of the close clapper  119  counterclockwise in  FIG. 6   a  about a pivot axis  125 , either manually by pressing on the lower end of the close clapper  119 , or automatically by energization of the close solenoid  123 , causes clockwise rotation of the release lever  115 . The release lever  115  engages a projection  128  on the close spring release latch  101  which is rotated clockwise until the close latch roller  107  slips off of the latch surface  105 . This permits the closing springs  69 ,  71  to rapidly rotate the crank shaft  85 . This results in rotation of the pole shaft  55  to close the separable contacts  40  of the circuit breaker  15 . The force generated by two closings springs  69 ,  71  is required as they not only operate the mechanism  53  to close the separable contacts  40 , but they also charge the opening spring  65 . With the circuit breaker  15  closed as shown in  FIG. 6   b , the release lever  115  is lowered so that if the closing springs  69 ,  71  are recharged (as shown), the release lever  115  will not engage the closing spring release latch  101  and thus the closing springs  69 ,  71  cannot be discharged. The closing springs  69 ,  71  maintain the circuit breaker  15  ready for a recharge should the circuit breaker  15  trip open. 
         [0033]    As shown in  FIGS. 7   a - 7   c , the operating mechanism  53  also includes a coupling mechanism  127  for coupling the crank shaft  85  to the pole shaft  55 . This coupling mechanism  127  includes a pair of parallel main links  129  each pivotally connected at one end to the pole shaft  55  through a crank arm  131  and rotatably supporting a main link roller  133  between their free ends. This main link roller  133  engages the peripheral surface of the closing cam  91  which, as the crank shaft  85  rotates, pushes on the main links  129  to rotate the pole shaft  55  through the eccentricity in the closing cam  91  surface. Opening spring release mechanism  135  includes a banana link  137  pivoted at one end on a common axis  125  with the main link roller  133  and at the other end to one end of a hatchet  139 . The hatchet  139  is mounted on a fixed pivot pin  141  and has a free curved end  143  forming a latch edge  145 . Opening spring release mechanism  135  also includes a trip lever  147  fixed to a rotatable trip lever “D” shaft  149 . The trip lever  147  is coupled to an opening solenoid assembly  300 . The opening solenoid assembly  300  includes an opening solenoid  302  and an opening clapper assembly  304 . The opening solenoid  302 , preferably, has a mass limit of between about 1.5 kilograms and 2.0 kilograms, and more preferably about 1.89 kilograms. The opening clapper assembly  304  has an elongated body  305  with a first upper end  306  and a second, lower end  308 . The trip lever  147  rests on the opening clapper upper end  306 . The opening solenoid assembly  300  also includes a bracket  310  which is coupled to the housing  61 . The opening clapper assembly  304  is pivotally coupled to the bracket  310  at the opening clapper pivot point  312 . The opening clapper pivot point  312  is disposed on a medial location on the opening clapper body  307  (described below). The opening solenoid  302  is also coupled to, or mounted on, the bracket  310 . A trip latch reset spring  159  connected to this bracket  310 , biases the hatchet  139  clockwise as shown in  FIG. 7   a  to the hatchet  139  position as shown in  FIGS. 7   b  and  7   c  wherein the latch edge  145  is engaged by the D shaft  149 . 
         [0034]      FIGS. 7   a - c  illustrate the coupling of the crank shaft  85  to the pole shaft  55  to close the circuit breaker  15  and tripping of the opening spring release mechanism  135  to open the circuit breaker  15 .  FIG. 7   a  illustrates the position of the parts with the circuit breaker  15  open and the closing springs  69 ,  71  discharged. As can be seen, the push rod  51  is retracted so that the separable contacts  40  are open. The sequence is initiated by operation of the charging mechanism  200  to rotate the crank shaft  85  in the counterclockwise direction to charge the closing springs  69 ,  71  in the manner described above. The trip latch reset spring  159  biases the main link roller  133  against the peripheral caming surface of the closing cam  91  until it falls into the notch  93  with the closing springs  69 ,  71  latched in the charged condition. This permits the trip latch reset spring  159  to rotate the hatchet  139  clockwise to the latched position in which the latch edge  145  is engaged by the D shaft  149  as shown in  FIG. 7b . When the closing spring release  99  is actuated, the closing springs  69 ,  71  rapidly rotate the crank shaft  85  in the manner described above with reference to  FIGS. 6   a  and  6   b . The increasing effective diameter of the closing cam  91  produced by the eccentricity of the cam  91  surface, pushes the main links  129  downward and to the position shown in  FIG. 7   c . This rotates the pole shaft  55  in the counterclockwise direction to drive the push rod  51  to the left to close the separable contacts  40  while, as can be seen in  FIG. 7   c , the hatchet  139  remains engaged by the D shaft  149 . 
         [0035]    The circuit breaker  15  is opened manually by pressing on the lower end of the opening clapper assembly  304 . In addition, the circuit breaker  15  can be opened automatically by actuation of the opening solenoid  302  which rotates the opening clapper assembly  304  clockwise. The opening solenoid  302  is energized by an electronic trip unit in response to current which exceeds predetermined current/time characteristics. Alternatively, the opening solenoid  302  can be energized from a remote source to open the circuit breaker  15 . In any case, rotation of the opening clapper assembly  304  in the clockwise direction rotates the open trip lever  147  and with it the D shaft  149 . The force generated by the charged opening spring  65  through the main links  129  and banana link  137  rotates the hatchet  139  counterclockwise past the D shaft  149 . This allows the opening spring  65  to rotate the pole shaft  55  to withdraw the push rods  51  and open the separable contacts  40  as the main link roller  133  rolls along the outer surface of the closing cam  91  to the position shown in  FIG. 7   a.    
         [0036]    The opening solenoid  302  includes a housing  320 , a coil  322  disposed within the opening solenoid housing  320 , and an opening clapper assembly  304 . The opening clapper assembly  304  has a body assembly  305  with an elongated body  307 . The opening clapper assembly  304  is structured to move between a first, extended position and a second, retracted position. The time it takes the opening clapper assembly  304  to move between the first, extended position and the second, retracted position is the response time. As noted above, the response time for the opening solenoid  302  is increased, that is, the speed and/or acceleration of the opening solenoid assembly  300  is slowed, by increasing the mass of the opening clapper assembly  304 . 
         [0037]    Preferably, the opening clapper assembly  304  and/or the clapper body assembly  305  has a mass that is between about 20% and 30% of the mass limit of the opening solenoid  302 , and more preferably, about 25% of the mass limit of the opening solenoid  302 . Thus, for an opening solenoid  302  having the mass limit described above, the opening clapper assembly  304  has a mass that is between about 0.38 kilograms and 0.56 kilograms, and more preferably, about 0.48 kilograms. To achieve the increase in mass, the clapper body assembly  305  may be manufactured with the same dimensions/shape as a prior art opening clapper, but be made from a ferrous material that is more dense than what was used in the prior art. Alternatively, the clapper body assembly  305  may be made from the same material and have the same general shape as a prior art clapper, but have an increased dimension, for example, the clapper body  307  may be thicker. 
         [0038]    However, as shown in  FIG. 8 , in the preferred embodiment the opening solenoid assembly  300  utilizes an clapper body  307  made from the same material and in the same general size/shape as in the prior art, but the clapper body assembly  305  includes additional mass added thereto in the form of one or more slugs  332 . The clapper body assembly  305 , preferably, has two slugs  332 A,  332 B disposed on opposite sides of the opening clapper pivot point  312 . That is, for an opening solenoid  302  having the mass limit described above, the clapper body  307  has a mass of about 0.2 kilograms and the at least one slug  332 , preferably, has about the same mass which is between 0.24 kilograms and 0.32 kilograms, and more preferably, 0.28 kilograms. If there are two slugs  332 A,  332 B, the slugs  332 A,  332 B, preferably have about the same mass which totals about 0.28 kilograms. That is, a single slug  332 A has a mass of about 0.14 kilograms. Further, the two slugs  332 A,  332 B are also, preferably, disposed about the same distance from the opening clapper pivot point  312 . In this configuration, the balance of the clapper body assembly  305  is comparable to the prior art clapper. As such, the opening solenoid  302  may be the same, or similar, to prior art opening solenoids. By maintaining the same, or substantially similar, operating characteristics, other than the total mass of the clapper body assembly  305 , the operation of the opening solenoid assembly  300  is substantially similar, and the components are substantially similar, to the prior art opening solenoid assembly. While this is the preferred embodiment, the invention contemplates other configurations. 
         [0039]    That is, by locating the one or more slugs  332  at various distances from the opening clapper pivot point  312  and/or by having one or more slugs  332  on one side of the opening clapper pivot point  312  with a greater mass than the one or more slugs  332  on the opposite side of the opening clapper pivot point  312 , the operating characteristics of the opening solenoid  302  may be controlled as desired. 
         [0040]    Preferably, the one or more slugs  332  are coupled to the clapper body  307  by a bolted connection such as, but not limited to, a nut and bolt, or, a threaded rod and nut as described below. Other coupling devices include, but are not limited to, brazing the slugs  332  to the clapper body  307 , use of other mechanical fasteners, use of adhesives, snap-fit slugs  332  structured to clip onto the opening clapper assembly  304 , or welding. A mechanical fastener  340  such as a threaded rod  342  may, for example, be incorporated into the slug  332 . In this embodiment, the opening clapper assembly  304  would have one or more openings  344 . The opening clapper openings  344  could be threaded so that the slug  332  may be coupled thereto without an additional element, or, the opening clapper openings  344  may be smooth and a nut  346  may be provided. A snap-fit slug  332  may be structured to be movably coupled to the clapper body  307 . That is, the slug  332  may be structured to slide longitudinally on the clapper body  307 . 
         [0041]    With an opening solenoid  302  having a mass limit of about 1.89 kilograms, and an opening clapper assembly  304  having a mass that is about 25% of the opening solenoid  302  mass limit, the response time of the opening solenoid  302  is between 48 ms and 60 ms, and more preferably 50 ms. This response time is delayed relative to the prior art opening solenoids and, as such, the opening solenoid assembly  300  delays the separation of the separable contacts  40  until the direct current wave form of a fault current is off peak. 
         [0042]    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 invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.