Abstract:
A pair of spring mounted masses have sufficient inertia to remain stationary in response to a shock force applied to the housing of a trip assembly in a circuit breaker to block movement of the trip bar and thereby prevent a nuisance trip.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to circuit breakers, and particularly to a circuit breaker with an inertia device which prevents tripping of the circuit breaker by shock loading. 
     2. Background Information 
     Multipole circuit breakers commonly have a trip unit that monitors each pole for currents exceeding certain current/time characteristics. The response of the trip unit to an overcurrent in any pole is coupled to a single spring powered operating mechanism through a trip bar mounted for rotation about its longitudinal axis. A latch arm on the trip bar unlatches the spring powered operating mechanism when rotated by the trip unit. The latch is designed such that a relatively light force applied to the trip bar is adequate to release the relatively strong force stored in the spring powered operating mechanism. Such an assembly can be susceptible to “shockout” or unintentional tripping of the circuit breaker in response to mechanical shock. The resultant nuisance trips are unacceptable in many applications. 
     There is a need therefore for an improved circuit breaker that is not unduly susceptible to shock loads. 
     SUMMARY OF THE INVENTION 
     This need and others are satisfied by the invention which is directed to a circuit breaker incorporating one or more inertia devices that restrict movement of trip bar in response to shock loading but do not interfere with normal operation of the trip bar in unlatching the operating mechanism. More specifically, the circuit breaker includes a casing, separable contacts, and an operating mechanism incorporating a latch and that opens the separable contacts when the latch is unlatched. The circuit breaker further includes a trip assembly comprising a trip bar unlatching the latch when moved to a trip position and a trip device moving the trip bar to the trip position in response to selected conditions of current flowing through the separable contacts. An inertia device prevents movement of the trip bar to the trip position in response to a mechanical shock. The trip bar comprises an elongated member mounted for rotation about a longitudinal axis. The inertia device comprises at least one mass and a compliant mount mounting the mass adjacent the trip bar to engage the trip bar in response to the mechanical shock and prevent unlatching of the latch. The compliant mount is a spring mount which comprises a mounting support, a mounting rod carrying the at least one mass and slideable relative to the mounting support and a spring coupling the mass to the mounting support. The spring can be a helical compression spring mounted coaxially on the mounting rod. The trip assembly can include a trip housing in which the elongated member is rotatably mounted and to which the mounting support is mounted. 
     A pair of the masses can be provided, each mounted by a compliant mount on opposite sides of the trip bar to counteract shock loading in opposite directions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 is a schematic diagram of a circuit breaker incorporating the invention. 
     FIG. 2 is a simplified elevation view of a trip unit of the circuit breaker of FIG. 1 illustrating the invention. 
     FIG. 3 is a sectional view taken along the line  3 — 3  in FIG.  2 . 
     FIG. 4 is a sectional view taken along the line  4 — 4  in FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the schematic diagram of FIG. 1, the circuit breaker  1  includes a casing  3  in which is mounted a pair of separable contacts  5 . The separable contacts  5  are opened and closed by a spring powered operating mechanism  7 . The contacts can be manually opened and closed by a handle  9  on the operating mechanism. The spring powered operating mechanism  7  is well known in the art and typically includes a latch  11  which, when actuated, automatically opens the separable contacts  5 . 
     The latch  11  is actuated by a trip assembly  13  which responds to selected overcurrent conditions such as a persistent overcurrent condition or a short circuit condition. The trip assembly  13  includes a trip bar  15  that is mounted for rotation about a longitudinal axis  17 . The trip bar is typically molded of an insulative resin and has a latch arm  19  projecting laterally. A metal latch plate supported by the latch arm engages the latch  11  and prevents it from being unlatched by movement in the direction of the arrow  23 . 
     The trip assembly  13  also includes a trip unit  25 . The exemplary trip unit  25  utilizes a bimetal  27  which is fixed at one end to a load conductor  29 . The free end of the bimetal  27  is connected by a flexible shunt  31  to the separable contacts  5  to form the main current path through the circuit breaker  1  which extends from a line conductor  33 , through the separable contacts  5 , the flexible shunt  31 , the bimetal  27  and the load conductor  29 . A persistent overcurrent heats the bimetal  27  causing it to bend counterclockwise as viewed in FIG.  1  and shown by the arrow  35 . Thus, the bimetal  27  engages a trip arm  37  on the trip bar  15  and rotates the trip bar clockwise as shown by the arrow  39 . The clockwise rotation of the trip bar  15  causes the latch plate  21  to disengage from the latch  11  to unlatch the operating mechanism and therefore trip the separable contacts  5  open. 
     FIG. 2 shows the trip bar  15  as an elongated member rotatably mounted at its ends and in a molded trip assembly housing  41 . The exemplary trip assembly  13  is for a three-pole circuit breaker, and therefore, the trip bar  15  has three trip arms,  37 A,  37 B and  37 C, one for each pole. Corresponding bimetals (not shown) would be provided for each pole. The trip unit  25  can also include magnetic trip devices (not shown) which respond to very high instantaneous overcurrents, such as would be caused by a short circuit, as is well known. In addition, an electronic trip unit can be provided, for instance, to provide ground fault or arc fault protection. In such case, a solenoid (not shown) would engage an arm (not shown) on the trip bar to also rotate the trip bar to unlatch the latch of the operating mechanism, as is well known. Alternatively, the trip unit could be an all-electronic trip unit in which a solenoid engages a paddle on the trip bar to release the latch. 
     Regardless of the particular type of trip unit, shock loading, particularly in the vertical direction, can unintendedly release the latch  11 . In order to prevent this “shockout” of the circuit breaker, an inertia device  43  is provided. The inertia device  43  includes a pair of masses  45  and  47 , each supported by a compliant mount  49  and  51 , respectively. Each compliant mount  49  and  51  is a spring mount that includes a mount support  53 ,  55 , which can be integrally molded into the trip assembly housing  41 , and a mounting rod  57 ,  59  carrying the respective masses  45  and  47  and slideable relative to the mount supports  53 ,  55 . The compliant mounts further include helical compression springs  61 ,  63 , concentrically mounted on the mounting rods  55 ,  57 . The springs  61 , 63  are retained on the mounting rods  57 ,  59  by washers  65 ,  67  and retaining nuts  69 ,  71  threaded onto the ends of the mounting rods so that the springs  61 ,  63  bear against the washers  65 ,  67  and the mount supports  53 ,  55 . 
     The masses  45 ,  47  are supported by the mount supports  53 ,  55  so that they are adjacent lateral projections  73  and  75  on the trip bar  15 . With the trip unit housing  41  oriented vertically as shown in FIG. 2, the mass  45  is supported above the projection  73  by the spring  61  while the mass  47  rests on the mount support  55 . 
     If the trip assembly housing  41  is subjected to a shock force in the upward direction as indicated by the arrow  77  in FIG. 3, the impulse lifts the housing, and with it the trip bar  15  in the same upward direction. However, the mass  45  having more inertia remains substantially relatively fixed in space relative to the remainder of the trip assembly so that the lateral projection  73  is restrained by the mass  45  and does not unlatch the latch to trip the circuit breaker. As the mount support  53  is carried by the housing  41  in the upward direction slightly, the spring  61  compresses proportionally and then relaxes as the housing recovers from the shock. 
     On the other hand, if the shock generates an impulse force  79  in the downward direction as shown in FIG. 4 by the arrow  79 , the mass  47  resists the impulse and remains fixed to block movement of the projection  75 , and therefore prevent unlatching of the trip latch. Again, the mount support  55  moves downward with the casing  41  resulting in compression of the spring  63 . 
     Under normal operating conditions when the trip bar is actuated and rotated in a direction of the arrow  81 , the projections  73  and  75  rotate away from the masses  45  and  47 , and therefore, the latter do not interfere with normal operation of the trip unit. 
     The masses  45  and  47  are selected to balance the mass of the trip bar, and thus, can be of unequal size depending upon the configuration of the trip bar. 
     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 the invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.