Molded case circuit breaker having movable contact finger releasably locked to an operating mechanism

A subframe is coaxially pivotally mounted with a movable frame of the operating mechanism of a molded case circuit breaker and is locked to the frame by a bell crank pawl which is pivotally mounted on the subframe. A leaf spring assembly rigidly attached to the subframe overlies the movable contact finger to engage the same in the circuit ON condition of the circuit breaker to provide desired contact force. The bell crank pawl is released upon an incrementally small amount of initial movement of the movable contact arm in response to blow-open magnetic repulsion forces to release the movable contact finger for pivotal opening movement relative to the movable frame of the circuit breaker operating mechanism.

BACKGROUND OF THE INVENTION 
This invention relates to molded case circuit breakers operable in response 
to fault currents to open the contact and interrupt a circuit in which the 
circuit breaker is connected. More particularly, this invention relates to 
circuit breakers of the aforementioned type which are operable upon high 
fault currents to limit the peak let-through current, commonly known as 
current limiting circuit breakers. In current limiting circuit breakers, 
the movable contact must be capable of moving relative to the operating 
mechanism such that the magnetic repulsion forces created by high fault 
currents in the contact area are able to blow the movable contact away 
from the stationary contact to effect separation independently of 
operation of the operating mechanism. To permit the relative movement, the 
movable contact finger is customarily connected to the operating mechanism 
by a resilient spring connection. The resiliency of such connection 
renders it difficult to achieve proper contact pressure during the normal 
operated ON position or engaged position of the circuit breaker contacts. 
SUMMARY OF THE INVENTION 
This invention provides a molded case circuit breaker having a pivoted 
movable contact finger which is made electrically conductive with a first 
conductor mounted in the case of the circuit breaker. An operating 
mechanism connects a collapsible toggle linkage to the movable contact 
finger by the releasable locking means of this invention. The operating 
mechanism includes a movable frame pivotally mounted within the breaker 
case substantially coaxially with the movable contact finger. A subframe 
is also pivotally mounted coaxially with the frame and movable contact 
finger and extends along the movable contact finger, having a portion 
overlying the contact finger. A bell crank shaped pawl is pivotally 
mounted on the subframe. One arm of the bell crank shaped pawl has a 
portion overlying the movable contact finger in proximity to the overlying 
portion of the subframe. The other arm of the bell crank shaped pawl 
carries a projection which engages a notch in the movable frame. Biasing 
means are provided between the subframe and the pawl to bias the 
projection of the pawl into engagement with the notch of the movable 
frame, thereby locking the subframe to the movable frame. A leaf spring 
assembly is rigidly affixed to the subframe to overlie the movable contact 
finger, engaging the finger near its distal end to apply force on the 
movable contact finger to achieve the desired contact pressure in the 
engaged position of the circuit breaker contacts. The pivot of the movable 
contact finger includes an elongated opening in the finger to afford 
limited movement of the finger away from the stationary contact upon 
creation of repelling magnetic forces as a result of high fault currents. 
These magnetic forces also counteract the contact pressure force and cause 
limited movement of the movable contact finger against the leaf springs. 
Such movement of the contact finger away from the stationary contact 
causes the finger to engage the overlying portion of the bell crank pawl, 
driving the projection out of the notch and releasing the contact finger 
and subframe from the movable frame of the circuit breaker operating 
mechanism, permitting movement of the finger relative to that frame such 
that the finger may pivot to a contact open position. 
The invention and its advantages will become more apparent in the following 
description of the preferred embodiment when read in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, the molded case circuit breaker 2 in which the movable 
contact finger lock of this invention is embodied comprises a molded 
insulating base 4 and a molded insulating cover 6 shown fragmentarily in 
dot-dash phantom line. A preformed rigid conductor 8 is mounted to base 4 
as seen in the lower left-hand portion of FIG. 1. The conductor 8 extends 
through a separate removable trip unit assembly 10 to the outside of the 
breaker for attachment of a wire connecting lug thereto. A stationary 
contact 12 is mounted to the base 4 as seen in the lower right-hand 
portion of FIG. 1. Contact 12 is a turn-back contact commonly employed in 
current limiting circuit breakers to provide a current path of substantial 
length parallel to the current path in the movable contact finger and 
directed in an opposite direction to the current path in the movable 
contact finger. Although a vertically oriented U-shaped stationary contact 
has been shown, many other configurations of current limiting stationary 
contacts are known and are equally applicable to the circuit breaker 
embodying this invention. Stationary contact 12 extends to an exterior end 
of base 4 for attachment of a wire connecting lug. A contact element 12a 
of high quality circuit making and breaking material is affixed to the 
stationary contact 12 at one end thereof. 
A movable contact finger assembly 14 is pivotally mounted in the base 4 of 
circuit breaker 2 on a pin 16 which is positioned and fixed within the 
base 4. Referring to FIGS. 5, 6 and 7, it can be seen that movable contact 
assembly 14 comprises a pair of movable contact fingers 18 and 20, each of 
which comprise a pair of conductive finger members 18a, 18b and 20a, 20b, 
respectively. The members 18a, 18b and 20a, 20b are welded together at one 
end. The members are shaped for parallel spacing over the central portion 
of their length and to diverge to a greater spacing at the left-hand ends 
as viewed in FIG. 7. The left-hand ends of each of the members have 
aligned openings 18c, 20c, respectively, the openings being elongated 
vertically. The welded ends of the movable contact fingers 18 and 20 have 
contact elements 18d and 20d affixed to the lower surfaces thereof for 
engagement with a common stationary contact 12a. As seen in FIGS. 1, 2 and 
6, conductor 8 has a pair of spaced projections 8a and 8b oriented 
vertically and having slots 8c open to the top thereof. The left-hand ends 
of contact finger elements 18a and 18b are disposed on opposite sides of 
the projection 8a. Similarly, the left-hand ends of movable contact finger 
elements 20a and 20b are disposed on opposite sides of projection 8b as 
seen in FIG. 6. A lateral thrust washer 22 is disposed between movable 
contact finger elements 18b and 20b at the center of the assembly, 
positioned by a spacer 24. Similar lateral thrust washers 26 are 
positioned at the outside surface of contact elements 18a and 20a, 
positioned by discs 28. Washers 22 and 26 urge the inner surface of 
contact fingers 18 and 20 against the respective opposite surfaces of 
projections 8a and 8b to form a wiping conductive pivot joint. 
Alternately, a flexible braided conductor could be connected between 
conductor 8 and the movable contact finger assembly 14 to make the 
assembly 14 electrically conductive. 
Enlarged openings 60b at the ends of a U-shaped subframe 60 are disposed 
over discs 28, the discs providing a pivot surface for the openings 60b of 
the subframe 60 which will be described in greater detail hereinafter. A 
U-shaped movable frame 30 is disposed over the contact assembly as seen in 
FIG. 6 with the outer legs 30a being disposed adjacent discs 28. Pin 16 is 
inserted through aligned holes in the spaced legs 30a of frame 30, in 
discs 28, the spacer 24, the elongated openings 18c and 20c in the movable 
contact fingers and in the open slots 8c of projections 8a and 8b of 
conductor 8. The projecting ends of pin 16 are fixed within molded base 4. 
Movable contact finger assembly 14, subframe 60 and movable frame 30 are 
all permitted to pivot coaxially about the axis of pin 16. 
An operating mechanism of well known type is provided for the circuit 
breaker 2. The operating mechanism comprises an operating handle 32 
projecting through an opening in cover 6, the handle comprising an 
insulating molded cap sitting on top of a U-shaped bracket which is 
pivoted within the case at 34. A latching mechanism 36 comprising an arm 
38 pivotally mounted in the case 4 at 40 is held by a latch 42 of the trip 
unit 10. A collapsible toggle linkage is connected between arm 38 and 
movable frame 30, a lower end of the lower link 44 being pivotally 
connected to the frame 30 at 46. The upper end of upper link 48 is 
pivotally connected to arm 38 at 50. The inner ends of links 44 and 48 are 
joined by a knee pin 52. Drive springs 54 connect between the knee pin 52 
and the bight portion of the handle 32 to provide a resilient driving 
connection between the handle and the toggle linkage. Movement of the 
handle 32 to the right-hand position shown in FIG. 1 moves the line of 
action of drive springs 54 across the pivotal connection 50 of upper 
toggle link 48 with arm 38, straightening or extending the collapsible 
toggle linkage to rotate frame 30 clockwise about pin 16 and cause movable 
contact finger assembly 14 to engage with stationary contact 12. In a 
known manner, movement of operating handle 32 to a left-hand position 
moves the line of action of drive springs 54 to the left of the pivotal 
connection 50, effecting collapse of the toggle linkage and movement of 
the movable contact finger assembly 14 out of engagement with stationary 
contact 12. This manual OFF position is well known and has not been 
illustrated in the drawings. Also in a well known manner, release of latch 
42 by trip unit 10 in response to overcurrent conditions effects collapse 
of the toggle linkage and movement of the movable contact finger out of 
engagement with stationary contact 12 by permitting the arm 38 to pivot 
about the pivot point 40, thereby carrying the pivotal connection 50 and 
the upper end of toggle link 48 to the right of the line of action of the 
drive springs 54 to effect collapse of the toggle linkage. This trip 
position is also well known and has not been illustrated in the drawings. 
In a multi-pole circuit breaker, the operating mechanism is provided in 
only one pole and additional poles are linked thereto by a cross bar 56 
which extends transversely across the additional poles of the circuit 
breaker and is clamped to each of the movable frames 30 by a clamp 58. In 
this manner, all of the poles operate simultaneously. 
In current limiting circuit breakers, the movable contact finger assembly 
14 must be capable of moving independently of the frame 30 of the 
operating mechanism to permit blow-open of the contacts in response to 
high fault currents. However, the movable contact finger must also be 
connected in some manner to the movable frame 30 in order to be operated 
by the operating mechanism and by the trip unit. This is commonly 
accomplished by tension springs which join the movable finger to the 
movable frame. However, known constructions have not always provided 
satisfactory contact pressure between the movable and stationary contacts 
in the engaged position. This invention provides a releasable lock between 
the movable contact assembly and the movable frame which under normal 
operating conditions provides significant contact force by virtue of a 
leaf spring assembly bearing upon the movable contact finger. 
Referring to FIGS. 1, 2 and 7, an inverted U-shaped subframe 60 (mentioned 
earlier) has side legs 60a having large circular openings 60b at the 
distal ends thereof which are disposed over the discs 28 at the inner 
surfaces of legs 30a of movable frame 30. A common leg 60c extends between 
the side legs 60a at the right-hand end thereof and is provided with a 
pair of holes 60d. A multi-leaf leaf spring assembly 62 is attached to the 
under surface of common leg 60c together with a reverse leaf spring 64 and 
a spring keeper 66 by rivets 68. Leaf spring assembly 62 extends outwardly 
of subframe 60 to the right to overlie the welded ends of movable contact 
fingers 18 and 20. 
An inverted U-shaped bell crank pawl 70 is pivotally mounted between the 
legs 60a of subframe 60 by a pair of outwardly directed rivets 72 
extending through aligned holes in side legs 70a of pawl 70 and 60e in 
side legs 60a of subframe 60. The common leg 70b of pawl 70 joins the side 
legs 70a to the right of the axis for pivoting formed by rivets 72, 
thereby providing a first lever arm of the bell crank pawl. Side legs 70a 
extend downwardly from the rivets 72 and have holes 70c located at the 
distal ends of the side legs 70a, thereby providing the second leg of the 
bell crank pawl The common arm 70b extends across the top side of the 
movable contact finger assembly 14 below and spaced from the common arm 
60c of subframe 60. A pin 74 is inserted through the aligned holes 70c of 
pawl 70 to lie just below the bottom surface of movable contact finger 
assembly 14. Pin 74 is maintained assembled to the pawl 70 by C-clips (not 
shown) or other suitable fastening means received in annular grooves in 
the pin 74. Pin 74 projects beyond the outer surfaces of side legs 70a and 
beyond the outer surfaces of side legs 30a of movable frame 30. The outer 
edges of legs 30a of movable frame 30 are provided with a notch 30b for 
receiving the projecting ends of pin 74. Leaf spring 64 bears downwardly 
as oriented in the drawings upon the upper surface of common leg 70b of 
bell crank pawl 70 to bias the pawl clockwise about the pivot 72' biasing 
pin 74 into engagement with the outer edges of legs 30a of movable frame 
30. Particularly, spring 64 biases bell crank pawl 70 and its pin 74 into 
engagement in the notches 30b to positively lock subframe 60 to movable 
frame 30. When so locked and the contact operating mechanism is moved to 
cause engagement of movable contact finger assembly 14 with stationary 
contact 12, leaf spring assembly 62 bears firmly against the upper side of 
movable contact assembly 14 and biases it into engagement with the 
stationary contact 12, thereby establishing the desired contact pressure 
between the contact elements. When the operating mechanism is moved to the 
opposite position to cause the contact to move out of engagement, pin 74 
engages the under side of movable contact assembly 14 and rotates it 
counterclockwise about pin 16 to effect opening under the influence of the 
toggle linkage. 
The contact structure is arranged to be current limiting, i.e. the 
stationary contact 12 is arranged to have a significant current path 
extending parallel to the current path of the movable contact finger 
assembly 14, but in opposite directions. High currents in these paths 
generate repelling electromagnetic forces which drive the movable contact 
finger assembly 14 away from the stationary contact 12. When such forces 
are present, the left-hand end of the movable contact finger assembly 14 
at the pivot 16 can move upwardly within the limits of the elongation of 
the openings 18c and 20c. Also, the right-hand end of the movable contact 
finger assembly 14 can move upwardly a limited amount, deflecting the leaf 
spring assembly 62 slightly. The common leg 70b of bell crank pawl 70 is 
disposed very close to the upper surface of movable contact finger 
assembly 14 and is immediately engaged by that surface after a small 
increment of upward movement of the movable contact finger. Such 
engagement and continued upward movement of the movable contact finger 
drive bell crank pawl 70 counterclockwise about the pivot 72, moving pin 
74 out of notches 30b to release the subframe 60 and movable contact 
finger assembly 14 from the movable frame 30 and the circuit breaker 
operating mechanism as shown in FIG. 3. When so unlocked, the movable 
contact finger assembly 14 is free to pivot about pin 16 by virtue of the 
repelling electromagnetic forces to the contact open position as shown in 
FIG. 4, while the movable frame 30 and operating mechanism remain 
stationary in the original ON position until the fault current causes the 
trip mechanism 10 to trip the breaker open. The movable contact mechanism 
and its lock can be reset by moving the operating mechanism manually to 
the OFF position whereupon the movable frame 30 is pivoted 
counterclockwise about pin 16 to bring the notches 30b into alignment with 
pin 74, causing it and the bell crank pawl to again reengage with the 
frame 30. 
The foregoing describes a preferred embodiment and best mode contemplated 
of carrying out the invention, providing a lock for positively securing a 
subframe to a movable frame of a molded case circuit breaker movable 
contact finger assembly and providing stiff leaf springs on the subframe 
which bear against the movable contact finger in the ON position of the 
circuit breaker to provide the desired contact force. The lock is readily 
released upon the occurrence of magnetic blow-open forces caused by high 
fault currents to permit the movable contact finger assembly to pivot open 
independently and relative to the circuit breaker operating mechanism 
which may remain briefly in its ON position until the trip mechanism 
trips. Although a single preferred embodiment has been described, it is to 
be understood that the invention is susceptible of various modifications 
without departing from the scope of the appended claims.