Patent Publication Number: US-6218919-B1

Title: Circuit breaker latch mechanism with decreased trip time

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to circuit breaker assemblies having an improved latching system that substantially decreases mechanical trip time. The improved latching system can be utilized, but not limited to circuit breaker assemblies rated for residential and lower current industrial applications and for high ampere-rated circuit breaker assemblies. 
     Conventional circuit breaker assemblies utilize a thermal-magnetic trip unit to automatically sense overcurrent circuit conditions and to subsequently interrupt circuit current accordingly. It is the practice of the circuit protection industry to mount a magnet portion of the magnetic trip unit around a bimetal trip unit and to arrange an armature as part of the circuit breaker latching system. It is well appreciated in the electric circuit protection field that the latching surfaces within the circuit breakers latching system must be carefully machined and lubricated in order to ensure repeated latching and unlatching between the surfaces over long periods of continuous use. 
     The special machining that is required includes a time consuming polishing process or a special machining or shaving operation on the latch systems latch surfaces. The smooth low friction surfaces are required to minimize the amount of tripping force that must be applied to overcome the bias of the operating spring and the static friction on the contracting latch surfaces. The trip force is the amount of force that must be applied to the trip bar to overcome the latch spring bias and latch surface friction 
     In operation, a magnetic trip unit comprising an armature and a magnet is actuated upon the occurrence of an overcurrent condition. The actuation causes the armature, which is biased away from the magnet by a spring, to be rapidly driven towards the magnet so that a trip bar is activated. The thermal trip unit comprising a bimetal element senses overcurrent conditions by responding to the temperature rise on the bimetal element. When an overcurrent condition occurs over a period of time, the bimetal flexes and activates the trip bar. 
     Once activated, the trip bar sets in motion the activation and disengagement of a latching system comprising a primary latch, secondary latch, and a cradle. The trip bar, secured to the secondary latch, drives the secondary latch clockwise about a fixed point so that the secondary latch is moved out of contact with the primary latch. The primary latch in turn is positioned to prevent the rotation of the cradle. When the primary latch is released from the secondary latch, the cradle acts on the primary latch urging it to rotate clockwise about a fixed point. Once the primary latch is moved out of contact with the cradle, the cradle is released allowing it to rotate counterclockwise about a fixed point. As the cradle pivots the upper and lower links collapse under the biasing of an operating spring to draw a moveable contact arm containing a moveable contact to the open position. In the open position the moveable contact and a fixed contact are separated thereby terminating the circuit. 
     The primary latch and the secondary latch have a plurality of latching surfaces. The latching surfaces are defined as the surface of the latch that makes physical contact with any adjoining surface. The first latching surface of the secondary latch is positioned against the second latching surface of the primary latch. A first latching surface of the primary latch is positioned against the latching surface of the cradle. As previously described when the trip bar is actuated, it drives the secondary latch so that the secondary latch rotates about its pivot causing the first latching surface of the secondary latch to break contact with the second latching surface of the primary latch. Once this occurs, the first latching surface of the primary latch has a force bearing on it by the cradle at the cradle latching surface. If this force is great enough to overcome any resistant forces existing between the latching surfaces, the primary latch will rotate about its pivot point so that the first latching surface of the primary latch breaks contact with the latching surface of the cradle. Once released, the cradle rotates counterclockwise and set in motion a chain of events that trips the breaker. 
     Conventionally both the cradle and the primary latch are fabricated from a stamping operation followed by a shaving operation to flatten and smooth the latching surface of the cradle and the latching surfaces on the primary latch to maintain a low trip force between the cradle and the primary latch. To aid in the release of the latches there is a primary latching force provided by the operating spring. During use there is often a degradation of the latching surfaces due to wear and contaminates on the various latching surfaces. Even when the latching surfaces are prepared in an effort to minimize friction and the various springs provide a biasing force it is unpredictable if and when the latching system will be fully activated. If significant contaminates or excessive wear exists on the various latching surfaces, the latching system will not activate and result in a stalled situation between the cradle and the primary latch. In particular, once the primary latch is released by the secondary latch, the cradle through the latching surface of the cradle and supplied by provides a force on the primary latch at the first latching surface. This force must be great enough to overcome the friction forces acting between the first latching surface of the primary latch and the latching surface of the cradle. If contaminants or other sources cause the friction between these latching surfaces to become too large the first latching surface of the primary latch will not rotate and release the cradle so that the system is in a stalled situation. 
     Conventional circuit breakers have a size limitation imposed upon them in order to fit into panel boards of residential, office and light industrial applications. While the outer dimensions of the circuit breaker are fixed, short circuit current magnitudes available from electrical utilities have increased, requiring circuit breaker designers to seek new and improved operating and trip mechanisms which limit the energy let-through. To do this, one must minimize the current and/or the time from the onset of overload to arc extinction. One way to accomplish this is to provide an extremely fast acting circuit breaker capable of early contact separation upon detection of an overload. 
     SUMMARY OF THE PRESENT INVENTION 
     It is therefore desirable to provide a molded case circuit breaker capable of exceedingly fast tripping action effective in limiting to acceptable levels let-through energy incident with a high fault current interruption. This is accomplished by utilizing an improved latching system employed to immediately release the primary latch once the secondary latch is disengaged by the actuation of the trip bar. Once the primary latch is set free it subsequently releases the cradle so that the breaker mechanism is tripped by the movement of the link system comprising an upper link, a lower link and the operating spring thereby allowing the moveable contact and the fixed contact to separate thereby terminating the circuit. This immediate release of the primary latch, upon the secondary latch disengagement, achieves contact separation in significantly shorter time than when reliance for the release of the cradle is solely dependent upon the cradle forces and minimal friction between the cradle surface and the primary latch surface. 
     The improved latching system comprises the primary latch, the secondary latch and the trip bar. The improved latching system is designed to function so that upon activation of the trip bar and the disengagement of the secondary latch, the primary latch, being in direct physical contact with the trip bar/secondary latch configuration is immediately released. The primary latch and the secondary latch are shaped and positioned so that once the trip bar is activated, an extension on the secondary latch acts directly on an extension on the primary latch. Therefore the secondary latch drives the primary latch clockwise about its pivot point to positively release the cradle. The timing is such that as soon as the secondary latch clears the primary latch the primary latch is also freed. The timing of the release of the cradle is immediately after the release of the primary latch from the secondary latch. 
     Because the trip bar/secondary latch configuration is in direct physical contact with the primary latch the mechanical trip time is decreased thereby limiting the energy let-through to an acceptable value. Additionally, the release of the cradle is no longer only dependent on the cradle forces and the finishing of the latching surfaces to reduce friction to effectuate tripping of the breaker. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings wherein like elements are numbered alike in the several Figures: 
     FIG. 1 is a side view of a single contact arm molded case circuit breaker shown with the contacts closed according to the prior art; 
     FIG. 2 is a side view of a trip bar according to the prior art; 
     FIG. 3 is a side view of the secondary latch according to the prior art; 
     FIG. 4 is a side view of the trip bar assembled to the secondary latch according to the prior art; 
     FIG. 5 is a side view of a single contact arm molded case circuit breaker with an improved latching system according to the present invention; 
     FIG. 6 is a side view of a second embodiment of a single contact arm molded case circuit breaker with an improved latching system according to the present invention; 
     FIG. 7 is a side view of the improved latching system according to the present invention; 
     FIG. 8 is a perspective view of a self actuating primary latch according to the present invention; and 
     FIG. 9 is a side view of a secondary latch according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a conventional circuit breaker assembly of the prior art, which is generally indicated at  10 . It is to be appreciated that this invention deals with one, two, three, or four-pole circuit breakers formed with one or multiple adjacent compartments for housing multiple pole units, a common operating mechanism is provided to simultaneously actuate the interrupter of each pole. For ease of illustration the Figures will show only one pole. FIG. 1 shows a circuit breaker used for lower circuit interruption applications. Although not shown, the invention can also be used in many different types of circuit breaker assemblies. When activated, the invention operates in the same manner regardless of which circuit breaker assembly in which it is mounted. Therefore, when describing the prior art, FIG. 1 will be referenced however it is to be appreciated that the improved latching system  92  can be utilized in any type circuit breaker assembly. 
     The circuit breaker assembly  10  includes an insulative housing  12  shown with one side of the circuit breaker removed. At one end of the housing  12  exists a line strap  14  and a line terminal screw  16 . Permanently affixed to the line strap  14  is a fixed contact  18 . When the circuit breaker assembly  10  is in an on mode the fixed contact  18  makes electrical contact with a moveable contact  20  which is permanently affixed to a first end  22  of a moveable operating arm  24 . At the opposite end of the housing  12  exists a load lug  26  that connects with a bimetal  28  by means of a load strap  30 . A braided conductor  32  electrically connects the bimetal  28  to the moveable operating arm  24 . 
     The moveable operating arm  24  is pivotally connected at a second end  34  intermediate to a pivot  35  and pivotally connected by a pivot  37  at a distance from the second end  34  to a first end  36  of a lower link  38 . A second end  40  of the lower link  38  is pivotally connected to a first end  42  of an upper link  44 , which in turn is pivotally connected at a second end  46  to a cradle  48 . The cradle  48  is used to mechanically interact with a latching system  68  and a trip unit assembly  50  with the moveable operating arm  24 . An on-off handle  52  operatively connects with the moveable operating arm  24  by means of a handle yoke  54 , a mechanism spring  56  and the upper and lower links  44 ,  38 . The handle yoke  54  connects the mechanism spring  56  with the upper and lower links  44 ,  38  through an operating springs support pin  58 . 
     Useful in detecting short circuit conditions is a magnetic trip unit  60  comprising an armature  62  and a magnet  64 . When the circuit breaker assembly  10  is subjected to short circuit conditions, a magnetic attraction is immediately generated between the armature  62  and the magnet  64 . Subsequently, the armature  62  is drawn in the direction of the magnet  64  which strikes a trip bar  66  thereby setting into motion the activation of a latching system  68 . Additionally, useful in detecting overcurrent conditions is a thermal trip unit  70  that reacts to temperature rise on the bimetal element  28  causing the bimetal  28  to flex and strike the trip bar  66  which in turn activates the latching system  68 . 
     The latching system  68  comprises a primary latch  72 , a secondary latch  74  and the trip bar  66 . When the circuit breaker assembly  10  is in the “ON” mode, the fixed and moveable contacts  18 ,  20  are closed so that electrical continuity is retained throughout the assembly  10  allowing the current to flow. 
     A cradle latching surface  76  exists at the end of the cradle  48  located opposite the cradle  48  connection with the upper link  44 . When the circuit breaker assembly  10  is in the “ON” mode the latching system  68  is set. Setting the latching system  68  includes positioning the cradle latch surface  76  under a first primary latching surface  78  so that the first primary latching surface  78  prevents the cradle  48  from rotating counterclockwise about its pivot point. A second primary latching surface  80  is positioned against a first secondary latching surface  82  so that the secondary latch  74  is in the path of the primary latch  72  preventing the primary latch  72  from rotating clockwise about its pivot point. Referring to FIGS. 2-4, FIG. 2 showing the trip bar  66 , FIG. 3 showing the secondary latch  74  and FIG. 4 showing the trip bar  66  assembled in the secondary latch  74 . The trip bar  66  comprises a projection  84 , a leg  86  and a crosspiece  87  wherein the trip bar crosspiece  87  fits in a slot  89  on the secondary latch  74 . A secondary latch pivot pin  88  allows the trip bar projection  84  and the trip bar leg  86  to rotate clockwise upon contact with the bimetal  28  or the armature  62 . The secondary latch further comprises a leg  91  which snappingly engages a lip  93  on the trip bar  66  so that when activated, the two rotate together. 
     In operation, when the magnetic trip unit  60  is subjected to tripping conditions. A magnetic attraction is immediately generated between the armature  62  and the magnet  64  drawing the armature  62  in the direction of the magnet  64  thereby striking the projection  84  of the trip bar  66 . When dealing with lower level overload conditions, the bimetal  28  flexes and strikes the leg  86  of the trip bar  66 . Once the projection  84  or leg  86  of the trip bar  66  is contacted the trip bar  66  rotates clockwise. When this occurs the secondary latch  74  is also rotated clockwise so that the secondary latching surface  82  is moved from the path of the primary latch  72 . Acting under tension from the mechanical spring  56  biasing the cradle  48  to rotate in a counterclockwise direction about its pivot point, the biasing force pulls at the cradle  48  so that the cradle latching surface  76  pushes up on the first primary latching surface  78 . When the force exerted by the cradle  48  acting on the primary latch  72  overcomes the friction force between the two latching surfaces, it drives the primary latch  72  in a clockwise direction thereby freeing the cradle latching surface  76 . Once the cradle latching surface  76  is freed, the cradle  48  rotates counterclockwise thereby collapsing the upper link  44  and the lower link  38  so that the moveable operating arm  24  can move to the open position. This separates the moveable contact  20  and the fixed contact  18  so that the current flow is terminated. 
     In order to improve the circuit breaker assembly mechanical trip time and eliminate a potential latch and cradle stall condition an improved latching system  92  in accordance with an exemplary embodiment of the present invention will be described in detail. Referring to FIGS. 5 and 6, FIG. 5 showing the exemplary embodiment of the present invention and FIG. 6 showing a second embodiment of the present invention, when like components are used reference numbers remain the same. Conventional trip systems as described above depend on the cradle forces alone to apply the appropriate forces required to rotate the primary latch  72  thereby releasing the cradle latching surface  76  from contact with the first primary latching surface  78 . In these conventional systems, the mechanical trip time is slow and results in excess energy let-through. The improved latching system  92  depicted in FIGS. 5 and 6 limits energy let-through to acceptable levels by decreasing the mechanical trip time. 
     As shown in FIG. 7, the improved latching system  92  comprises a quick release primary latch  94 , an interactive secondary latch  96  and the trip bar  66 . Although the interactive secondary latch  96  and the trip bar  66  are described as two separate elements, the secondary latch  96  and the trip bar  66 , could have their features combined into one interactive secondary latch/trip bar element  140 . FIG. 8 details the quick release primary latch  94  and FIG. 9 shows the interactive secondary latch  96 . The quick release primary latch  94  comprising a top cross bar  100  having a primary latch extension  102  extending generally perpendicular to the top cross bar  100  at approximately the midpoint of the top cross bar  100 . The primary latch extension  102  being of sufficient length so that a bottom surface  104  of the extension  102  becomes a first primary latching surface  106  capable of interfacing with the cradle latch surface  76  to prevent the cradle  48  from counterclockwise rotation. 
     Referring to FIG. 8, extending at an angle from the top cross bar  100  in the same direction as the primary latch extension  102  on either side of the primary latch extension  102  are two primary legs  108 . Extending generally perpendicular to the two primary legs  108  away from the primary latch extension  102  are two primary arms  110 . The two primary arms  110  each having a generally oblong opening  112  through which a primary latch pivot pin  114  passes. At a distal end  116  of at least one of the primary arms  110 , a cam element  124  extends. The formation of the cam element  124  as shown in FIG. 8 is illustrative and is not meant to be limiting. 
     The trip bar  66 , as shown in FIG. 7, comprises the trip bar projection  84  and the trip bar leg  86 . When the trip bar  66  is assembled to the interactive secondary latch  96 , the trip bar  66  can freely rotate. Shown in FIG. 9, the interactive secondary latch  96  further comprises a step  130  and a leg  132 . Wherein the leg  132  securely snaps into the lip  93  on the trip bar  66  such that when the trip bar  66  is activated by movement of the armature  62  or the bimetal  28 , the interactive secondary latch  96  pivots clockwise with the trip bar  66 . The step  130  is designed to make physical contact with the cam element  124  upon the release of the interactive secondary latch  96 . 
     As shown in FIG. 5, the improved latching system  92  is set in the manner previously described, a second primary latching surface  134  is positioned against a first secondary latching surface  136  so that the quick release primary latch  94  is prevented from rotating clockwise about its pivot point. When the trip bar  66  is activated, it drives the interactive secondary latch  96  clockwise so that the second primary latching surface  134  and the first secondary latching surface  136  are moved out of contact with each other thereby releasing the quick release primary latch  94 . At this point in a conventional system, the activated latching system  68  would depend on the cradle forces to drive the primary latch  72  clockwise so that the first primary latching surface  78  moves thereby releasing the cradle latching surface  76 . 
     In the improved latching system  92 , instantaneously upon the interactive secondary latch  96  clearing the quick release primary latch  94 , the step  130  makes physical contact with the cam element  124 . This results in the immediate rotation of the quick release primary latch  94  thereby moving the first primary latching surface  106  out of contact with the cradle latching surface  76 . Once the cradle latching surface  76  is freed, the cradle  48  rotates counterclockwise thereby collapsing the upper link  44  and the lower link  38  so that the moveable operating arm  24  can move to the open position. This separates the moveable contact  20  and the fixed contact  18  so that the current flow is terminated. 
     The cam element  124 , located on the quick release primary latch  94 , and the step  130 , located on the interactive secondary latch  96 , are designed so that the moment the first secondary latching surface  136  clears the second primary latching surface  134 , the step  130  makes physical contact with the, cam element  124 . 
     As shown in FIG. 6 a second embodiment of the present invention relies on a linkage mechanism  138  positioned between and physically connecting the trip bar  66  and the quick release primary latch  94 . The linkage mechanism  138  is utilized to drive the quick release primary latch  94  clockwise about its pivot point as the trip bar  66  is activated. This insures positive tripping and the elimination of any possibility of a stalled situation. 
     It will be understood that a person skilled in the art may make modifications to the preferred embodiment shown herein within the scope and intent of the claims. While the present invention has been described as carried out in a specific embodiment thereof, it is not intended to be limited thereby but is intended to cover the invention broadly within the scope and spirit of the claims.