Abstract:
An auxiliary magnetic trip unit for a circuit breaker arranged on the load strap of an industrial-rated circuit breaker to interrupt circuit current upon occurrence of a high-level short circuit fault. The magnetic trip unit employs a magnet yoke, an armature, a trip lever for interacting with a latching mechanism of a circuit breaker operating mechanism, and a lever arranged to restrain the armature from moving toward the magnet yoke and to release the armature in response to a predetermined level of pressurized gas. Thus providing an auxiliary magnetic trip unit for use with circuit breakers for selective short circuit overcurrent protection in an electrical distribution system with circuit breakers connected in series.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to circuit breakers with a magnetic trip unit, and, more particularly, to circuit breakers with a pressure sensitive magnetic trip release mechanism. 
     Circuit breakers typically provide protection against the very high currents produced by short circuits. This type of protection is provided in many circuit breakers by a magnetic trip unit, which trips the circuit breaker&#39;s operating mechanism to open the circuit breaker&#39;s main current carrying contacts upon a short circuit condition. 
     Modern magnetic trip units include a magnet yoke (anvil) disposed about a current carrying strap, an armature (lever) pivotally disposed proximate the anvil, and a spring arranged to bias the armature away from the magnet yoke. Upon the occurrence of a short circuit condition, very high currents pass through the strap. The increased current causes an increase in the magnetic field about the magnet yoke. The magnetic field acts to rapidly draw the armature towards the magnet yoke, against the bias of the spring. As the armature moves towards the yoke, the end of the armature contacts a trip lever, which is mechanically linked to the circuit breaker operating mechanism. Movement of the trip lever trips the operating mechanism, causing the main current-carrying contacts to open and stop the flow of electrical current to a protected circuit. 
     In all circuit breakers, the separation of the breaker contacts due to a short circuit causes an electrical arc to form between the separating contacts. The arc causes the formation of relatively high-pressure gases as well as ionization of air molecules within the circuit breaker. These high-pressure gases can cause damage to the circuit breaker casing. The gases, therefore, must be vented from the circuit breaker enclosure. In addition, a phase-to-phase fault can occur if the arc gases from different phases are allowed to mix, and a phase-to-ground fault (e.g. single phase fault) can occur if the gases contact the grounded enclosure. To avoid a phase-to-phase or phase-to-ground fault, gases vented from different phases must be kept separate from each other and away from the grounded enclosure until the ionization has dissipated. 
     An exhaust port is conventionally employed to vent such gasses in a rotary contact circuit breaker; each phase (pole) employs two pairs of contacts, two contacts of which rotate about a common axis generally perpendicular to the current path from the line side to the load side of the circuit breaker. Each contact set in such an arrangement requires an exhaust port to expel gasses. One of the exhaust ports will be on the line side and one of the exhaust ports will be on the load side of the circuit breaker. In conventional units, the exhaust port on the line side is located proximate the top of the circuit beaker. Since gasses naturally flow in the direction of this port on the line side of the breaker, the port is effective. On the load side of the circuit breaker, the gasses formed consequent to a short circuit naturally migrates toward the lower corner of the breaker. Thus, an exhaust port is located at this corner providing there is sufficient room to exhaust gasses from this port. 
     An electrical distribution system may contain a series of circuit breakers, namely upstream breakers and downstream breakers. When circuit breakers are connected in series, it is desirable to ensure that a given fault caused by a short circuit condition will trip the circuit breaker closest to the fault. Such selectivity permits downstream breakers connected in series with an upstream breaker to trip without also tripping any upstream breakers. In this way, current to a room in a building can be shut off without shutting off current to the entire building. However, the upstream breaker must also be able to provide adequate protection for the circuit breaker when operating standalone in a non-selective application. If an upstream device trips at too low of a current threshold, there is no selectivity with any downstream breakers. If the upstream device trips at too high of a current threshold, there might not be adequate protection for the circuit breaker or its electrical system. Further, any tripping system must also ensure protection for the circuit breaker and the system in the event of a single-phase condition, e.g. only one phase becomes overloaded. In a multi-phase system, a single-phase condition exists when one pole experiences a fault thereby opening the contacts of that pole. The remaining poles do not experience the fault and therefore their respective contacts remain closed. A single-phase condition is not desirable in an application that uses a multi-phase component such as a three-phase motor. Therefore, it is desirable to provide a circuit breaker tripping system that will trip an upstream circuit breaker at a predefined short circuit fault level while ensuring protection of the circuit breaker and the electrical system should a single phase condition occur and, at the same time, avoiding unnecessary interruption of the performance of the circuit breaker. 
     SUMMARY OF THE INVENTION 
     In an exemplary embodiment of the present invention, an auxiliary magnetic trip unit is arranged on the load strap of an industrial-rated circuit breaker to interrupt circuit current upon occurrence of a high-level short circuit fault. The separation of the contacts upon a short circuit overcurrent condition creates pressurized gas that is vented from the circuit breaker. The magnetic trip unit employs a U-shaped magnet (magnet yoke) disposed about the load-side contact strap, an armature, a trip lever for interacting with a circuit breaker operating mechanism latch, and a lever arranged to restrain the armature from moving toward the magnet yoke and to release the armature in response to a predetermined level of pressurized gas. Thus providing an auxiliary magnetic trip unit for use with circuit breakers for selective short circuit overcurrent protection in an electrical distribution system with circuit breakers connected in series. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a circuit breaker cassette assembly of the type employing a rotary contact operating mechanism; 
     FIG. 2 is an isometric view of the magnet assembly; 
     FIG. 3 is a perspective view of the circuit breaker assembly of FIG. 1; 
     FIG. 4 is an isometric projection of the vent housing; 
     FIG. 5 is a side perspective view of the vent structure; and 
     FIG. 6 is an illustration of the pressure sensitive magnetic trip release mechanism. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a circuit breaker rotary contact assembly  10  is shown in an electrically insulative cassette half piece  2 . Electrically insulative cassette half piece  2  is attached to a similar cassette half piece (not shown) to form a cassette. Opposing line-side and load-side contact straps  11 ,  12  are adapted for connection with an associated electrical distribution system and a protected electric circuit, respectively. Fixed contacts  24 ,  26  connect with the line-side and load-side contact straps,  11 ,  12  respectively, while the moveable contacts  23 ,  25  are attached to ends of a rotary contact arm  22  for making movable connection with the associated fixed contacts  24 ,  26  to allow electrical current to flow from the line-side contact strap  11  to the load-side contact strap  12 . 
     The rotor  19  in the circuit breaker rotary contact assembly  10  is intermediate the line-side contact strap  11  and load-side contact strap  12  and associated arc chutes  13 ,  14 . The arc chutes  13 ,  14  are similar to that described within U.S. Pat. No. 4,375,021 entitled RAPID ELECTRIC ARC EXTINGUISHING ASSEMBLY IN CIRCUIT BREAKER DEVICES SUCH AS ELECTRIC CIRCUIT BREAKERS. The moveable contact arm  22  is arranged between two halves of circular rotor  19 . The moveable contact arm  22  includes first and second moveable contacts  25 ,  23  that are arranged opposite first and second fixed contacts  26 ,  24 . The moveable contact arm  22  moves in unison with the rotor  19  that, in turn, connects with the circuit breaker operating mechanism (not shown) by means of an elongated pin (not shown) and linkage assembly (not shown) to move the movable contacts  23 ,  25  between the CLOSED position, depicted in dashed lines, and the OPEN position depicted in solid lines in FIG.  1 . Upon a short circuit overcurrent condition, the contact pairs  23 ,  24 ,  25 ,  26  are separated. When the contact pairs  23 ,  24 ,  25 ,  26  are separated, electrical arcing occurs between the contact pairs  23 ,  24 ,  25 ,  26 . These arcs are cooled and quenched within arc chutes  13 ,  14 , thus preventing damage to the circuit breaker  10 . 
     A magnet assembly  40  is attached to the load end of the circuit breaker  10  by positioning a magnet yoke  30  on a top surface  12 B of load-side contact strap  12 . The cassette enclosure insulates the top portion of magnet assembly  40 . An insulator  45  envelops the underside and sides of the magnet yoke  30  thereby preventing the magnet yoke  30  and the load-side contact strap  12  from making contact. Further, the insulator  45  is attached to the top surface  12 B of the load-side contact strap  12  by two molded pins (not shown). The molded pins extend outward from the underside surface of the insulator  45  and extend through corresponding openings (not shown) in the load-side contact strap  12 . The magnet yoke  30  is thus positioned proximate to the load-side contact strap  12 . 
     A latch mechanism (latch)  46  is mounted such that it pivots on an axis positioned in the circuit breaker operating mechanism (not shown). A trip lever  28  has a first end  42  located proximate to the latch  46  and a second end  44  positioned near magnet assembly  40 . Upon a high-level short circuit condition, armature  38  is attracted to the magnet yoke  30  due to the magnetic field created around the magnet yoke  30 . This attraction causes the armature  38  to make contact with second end  44  of trip lever  28 . Trip lever  28  then rotates in a counterclockwise direction causing the first end  42  of the trip lever  28  to make contact with latch  46 . Latch  46  activates the circuit breaker operating mechanism (not shown) that causes the moveable contacts  23 ,  25  to separate from the fixed contacts  24 ,  26 . In other words, movement of latch  46  by trip lever  28  causes the circuit breaker to trip. The construction and operation of the circuit breaker operating mechanism is known in the art. 
     Trip lever  28  is pivotally mounted to an external face of the cassette half-piece (not shown) opposite cassette half-piece  2 . The trip lever  28  includes a first molded pin  50  extending radially outward from the trip lever along axis  51  and inserted through an opening (not shown) in the cassette half-piece (not shown) opposite cassette half-piece  2 . Also, the trip lever  28  rotates about the first molded pin  50 . It should be noted that if trip lever  28  is to be used with a second pole of the circuit breaker, then the trip lever  28  also includes a second molded pin  50  extending radially outward from trip lever  28 , opposite to the first molded pin. The second molded pin  50  is inserted through a corresponding opening in an outer cassette half piece for that respective pole. 
     Referring to FIG. 2, the magnet assembly  40  is shown in more detail. Magnet yoke  30  includes a first side arm  32  and a second side arm  34  containing an armature slot  36 . An armature  38  is positioned onto the magnet yoke  30  by insertion of a pivot arm  43 , shaped on one end of the armature  38 , within the armature slot  36 . An actuator arm  41 , shaped on the opposite end of the armature  38  extends beyond the sidearm  32 . Armature arm  41  has a top surface  82 . Actuator arm  41  extends through the cassette half-piece (not shown), and is located proximate the second end  44  of trip lever  28  (FIG.  1 ). 
     Referring to FIG. 3, a vent structure  70  is shown assembled to the outer surface of cassette half piece  2  for a three-phase system. Vent structure  70  is connected to the cassette half-piece  2  by means of a connector member  73 . Vent structure  70  includes a first side  120  and a second side  122 . First side  120  includes a depressed, bifurcated path  76 . 
     A trip lever  28  is shown positioned between two cassettes. Second end  44  of trip lever  28  includes an outwardly extending fin  48 . Actuator arm  41  of the first pole  130  is proximate to second end  44 ; an actuator arm ( 41 ) of the second pole  132  is also proximate to second end  44 . Fin  48  separates actuator arm  41  of the first pole  130  and actuator arm (not shown) of the second pole  132 . Consequently, in a three-phase system, an actuator arm (not shown) of a third pole  134  would operate a second trip lever  28 . Latch  46  operates in conjunction with each trip lever  28 . In this way, during a short circuit condition in any phase (pole) of the electrical distribution system, the respective trip lever  28  will activate the latch  46  causing all phases in the circuit breaker to open. This avoids a single-phase condition where the contacts of only one phase of a multi-phase system would open while the contacts  25 ,  26  of the remaining poles remain closed. A magnet block lever (lever)  56  and spring  72  for the first pole  130  are shown and will be discussed in reference to FIG.  5 . 
     Referring to FIGS. 3 and 4, where FIG. 4 shows a vent housing  110  including a first half  104  and a second half  106 . External to first half  104  is a depressed, bifurcated path  76 . Vent structure  70  is assembled with the vent housing  110  by joining the first half  104  of the vent housing  110  with the first side  120  of vent structure  70 . Upon assembly, bifurcated path  76  of the vent housing  110  mates with bifurcated path  76  of the vent structure to form an enclosed load gas passage  76 . Upon assembly of the vent structure  70  with the vent housing  110 , an inlet  94  and an outlet  98  are also formed. Arc gases, upon exiting a cassette, enter the inlet  94  and are released into the load gas passage  76 . The arc gases finally exit the circuit breaker through outlet  98 . 
     Vent housing  110  houses a commercially available current transformer (not shown) for providing power to electronic components within the circuit breaker, as is known in the art. An opening  100  is formed by first and second sides  104 ,  106  of vent housing  110 . Opening  100  permits through passage of a load-side strap extender (not shown) for connection with the load-side contact strap  12  (FIG.  1 ). 
     The vent housing  110  and the vent structure  70  are similar to the type described in U.S. patent application Ser. No. 09/225,988 entitled CIRCUIT BREAKER VENTING ARRANGEMENT, filed Jan. 5, 1999, which is incorporated herein by reference. 
     Referring to FIG. 5, first side  120  of vent structure  70  is shown in more detail. Vent structure  70  includes an opening  62  in connector member  73 . A chamber  64  is formed within the vent structure  70  when the connector member  73  is attached to the cassette half piece  2  (FIG.  3 ). Opening  62  is in fluid communication with the chamber  64  and the load gas passage  76 . Thus, opening  62  is a passageway for arc gases to enter the chamber  64  from the load gas passage  76 . Chamber  64  has an exterior wall  80  that is proximate to the connector member  73 . Chamber  64  also includes an opening  60  in exterior wall  80 . Each cassette in a multi-pole circuit breaker includes a separate chamber  64 . 
     It should be noted that in order to accommodate multi-phases within a circuit breaker, vent structure  70  is preferably located on each side of vent housing  110 . Therefore, if vent structure  70  is employed between two vent housings  110 , the above-discussed features will be located on both sides of vent structure  70 . If vent structure  70  is employed on the last vent housing  110  of a multi-pole circuit breaker, the above-discussed features will be located on only one side of vent structure  70 . 
     Referring to FIG. 6, a pressure sensitive magnetic trip release mechanism  59  (magnetic trip unit) is shown. Magnet block lever  56  includes a first arm  54  and a second arm  58 . The magnet block lever  56  rotates about a pivot  52  located proximate to the first arm  54 . Pivot  52  is located on the exterior of cassette half-piece (not shown) which mates with cassette half-piece  2  (FIG.  3 ). The first arm  54  is positioned over the top surface  82  of the actuator arm  41  thereby preventing movement of the actuator arm  41  towards the magnet yoke  30 . The second arm  58  extends through opening  60  of the vent structure  70  and into chamber  64 . A link  68  is located within chamber  64 . Link  68  is pivotally mounted at one end to a pin  66 . At the opposite end, link  68  slidable contacts second arm  58 . Spring  72  has a moveable end attached to lever  56  and a fixed end attached externally to the cassette half-piece (not shown) which mates with cassette half-piece  2  (FIG.  3 ). Spring  72  biases the first arm  54  of the magnet block lever  56  over the top surface  82  of the actuator arm  41 . 
     Although the pressure sensitive magnetic trip release mechanism  59  is shown in FIG. 6 for a single pole  130 , it is understood that a separate pressure sensitive trip lever mechanism including a magnet yoke  30 , actuator arm  41 , magnet block lever  56 , and link  68  can be arranged for each pole in a circuit breaker housing having a plurality of poles  132 ,  134 . 
     Referring to FIGS. 1,  2 ,  3 ,  4 ,  5  and  6 , a circuit breaker with a pressure sensitive magnetic trip release mechanism  59  operates as follows. Under high-level short circuit faults, the contact arm  22  is opened due to the magnetic forces at the stationary and moveable contacts  24 ,  26 ,  23 ,  25 . As the contact arm  22  is opened and the moveable contacts  23 ,  25  are separated from the stationary contacts  24 ,  26 , a plasma arc is formed between the stationary and moveable contacts  24 ,  26 ,  23 ,  25 . This arc generates arc gases of relatively high pressure that exit the arc chute  14  and enter into load gas passage  76  from inlet  94 . The pressurized gas enters the chamber  64  via opening  62 . The increased high level of current being carried through load-side contact strap  12  also induces a magnetic field around the magnet yoke  30 . 
     To the extent that when a specific current is exceeded, the magnetic force generated by the magnet yoke  30  is sufficient to attract the armature  38 . However, due to the positioning of the magnet block lever  56 , the actuator arm  41  is not permitted to move toward the magnet yoke  30 . 
     Generally, the level of pressure created in the chamber  64  is proportional to the level of the short circuit fault. Therefore, once the pressure inside the chamber  64  reaches a predetermined level that is consistent with the desired short circuit overcurrent level for which a trip of the circuit breaker is desired, link  68  rotates counter-clockwise about pin  66  in response to the increased pressure within chamber  64 . The movement of link  68  causes the magnet block lever  56  to rotate counter-clockwise about pivot  52 . Thus, first arm  54  is no longer positioned over the top surface  82  of the actuator arm  41 . Once actuator arm  41  is released, the armature  38  is permitted to move vertically upward toward the magnet yoke  30 . The armature  38  moves in response to the magnetic field around the magnet yoke  30  caused by the overcurrent condition. The actuator arm  41  then makes contact with second end  44  of the trip lever  28 . The trip lever  28  rotates clockwise about pin  50  thereby unlatching the latch  46  causing all phases of the circuit breaker to trip in response to the short circuit condition. 
     The pressure sensitive magnetic trip release mechanism  59  can be arranged for use in a circuit breaker having a plurality of cassettes. Each pole or phase or the circuit breaker utilizes a pressure sensitive magnetic trip release mechanism  59  which interacts with the corresponding chamber  64  of the corresponding side of the vent structure  70 . When a high level short circuit occurs, the most loaded pole will trip due to the pressure increase in chamber  64 . Therefore, since each pole employs a pressure sensitive magnetic trip release mechanism  59 , a trip of one pole causes all poles of the circuit breaker to open. Thus, a single-phase condition is prevented. 
     Further, when circuit breakers are in series, for example, an upstream circuit breaker in series with a downstream circuit breaker, the pressure sensitive magnetic trip release mechanism  59  permits selectivity between two circuit breakers of different ratings having the same short circuit current flowing through them. Selectivity ensures that the circuit breaker closest to the fault will trip. Under low overcurrent conditions, it is desirable to selectively not permit an upstream circuit breaker to trip thereby permitting the downstream breaker to trip. Selectivity is also needed when a fault in the electrical distribution system occurs closest to a downstream circuit breaker. For example, if a larger magnet yoke  30  cannot be utilized in an upstream circuit breaker to prevent saturation at too low of an overcurrent, then the movement of the armature  38  must be prevented until a predetermined high-level short circuit occurs. At such a predetermined high level short circuit condition, the movement of the armature must be released so that the selected circuit breaker can trip. 
     Since the level of pressure in the chamber  64  is proportional to the fault current, the sensitivity of the pressure sensitive magnetic trip release mechanism  59  in each cassette can be adjusted independently to any desired level. This adjustment can be achieved by changing the size or location of the opening  62 , the size or shape of the magnet block lever  56 , or by changing the force generated by the spring  72 . In this case, the pressure sensitivity of the trip blocking mechanism utilized in an upstream circuit breaker is set at a lower level than downstream breakers thereby preventing the upstream breaker from tripping under lower current short circuit conditions in the electrical distribution system. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.