Note: Descriptions are shown in the official language in which they were submitted.
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1 41PR-6149
~ATC~ ASSEMBLY FOR
STATIC TRIP CIRCUIT BREAKERS
BACKGRQUND OF THE INVENTION
The present invention relates to circuit breaker
latch assemblies and particularly to an improved latch
assembly for static trip circuit breakers.
~n commonly assigned U.S. Patent No. 4,001,742
to Jencks et al dated January 4, 1977, there is
disclosed a static trip, three-pole industrial circuit
breaker to which the present invention has particular
but not necessarily limited application. The latch
assembly of this patent disclosure includes interacting
primary and secondary latch levers for sustaining the
cradle of the breakex operating mechanism in its 1atched
reset position against the bias of charged mechanism
springs. As long as this cradle reset position is
sustained, the breaker operating mechanism may be
articulated such as to motivate -the breaker movable
contacts to a stable closed circuit position. To
then open the breaker contacts, the secondary latch
lever is actuated, either manually or automatically
via a trip solenoid activated under the control of a
static trip unit in response to a sensed overcurrent
condition, to release the primary latch lever; the
latter then releasing the cradle from its reset
positionO While this two stage latch assembly
arrangement was generally efective, the tolerances
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imposed on the various latching surfaces were found to
be quite stringent and, unless these tolerances were
strictly observed, the requisite trip actuating force
to be exerted on the secondary latch lever by the trip
solenoid became unduly great.
It is accordingly an object of the present invention
to provide an improved latch assembly for static trip
circuit breakers.
An additional object is to provide a latch assem~ly
of the above character which accommodates less stringent
manufacturing tolerances.
A further object is to provide a latch assembly of
the above character which accommodates a relatively light
tri~ actuating force.
Yet another object is to provide a latch assembly of
the above character which is efficient in construction,
economical to manufacture, and reliable in operation.
Other objects of the invention will in part be
obvious and in part appear hereinafter.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is
provided an improved latch assembly for static trip indus-
trial circuit breakers. Basically, the subject latch
assembly is a three stage latch mechanism including a
primary latch lever, an intermediate latch lever and a
secondary latch lever, all interacting to latchingly
sustain the pivotally mounted cradle of a circuit breaker
operating mechanism in an untripped or reset position
against the bias of charged mechanism springs.
More specifically, the primary latch lever is
pivotally mounted adjacent its one end on a first pivot
pin and carries adjacent its other end a primary latch
pin adapted to latchingly engage a primary latch shoulder
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formed in the cradle pursuant to releaseably sustaining
the latter in its reset position. The primary latch
lever also carries, intermediate its ends, a second
pivot pin for pivotally mounting one end of the inter-
mediate latch lever. At the other end of the inter-
mediate latch lever there is formed an intermediate latch
shoulder and a secondary latch shoulder. The secondary
latch lever provides an intermediate latch pin which, in
the disclosed embodiment, is constituted by an extension
of a third pivot pin pivotally mounting the secondary
latch. This intermediate latch pin is adapted to engage
the intermediate latch shoulder of the intermediate latch
lever, while a secondary latch pin, carried by the second-
ary latch lever, is adapted to engage the secondary latch
lS shoulder of the intermediate latch lever.
The geometries of the various pivotally mounted
latch levers, latch shoulders and latch pins are such
that the rather large moment exerted on the primary latch
lever by the force of the breaker mechanism springs acting
through the cradle is ultimately counteracted by an ex-
ceptionally light force imposed at the engagement of the
secondary latch pin with the secondary latch shoulder.
This light counteracting force creates a correspond-
ingly light frictional force which can readily be over-
come by a relatively modest tripping force applied, forexample, by a trip solenoid to the secondary latch lever
in a manner to effect disengagement of the secondary
latch pin from the secondary latch shoulder. With this
disengagement, the intermediate latch pin and inter-
mediate latch shoulder also disengage to remove thepivotal restraint imposed on the primary latch lever by
the intermediate latch lever. The cradle is then freed
to push the primary latch pin out of its path of swinging
movement from its reset position to its tri~ped position
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under the urgence of the discharging mechanism springs.
Coincidentally, the discharging mechanism springs
abruptly propel the breaker movable contacts from their
closed circuit positions to their open circuit positions.
The invention accordingly comprises the features of
construction and arrangement of parts which will be
exemplified in the construction hereinafter set forth,
and the scope of the invention will be indicated in the
claims.
For a better understanding of the nature and objects
of the invention, reference should be had to the Eollowing
detailed description taken in conjunction with the accom
panying drawings in which:
DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a side elevational view of a circuit
breaker latch assembly constructed in accordance with the
present invention;
FIGURE 2 is a simplified side elevational view of
the latch assembly of FIGURE 1 illustrating the various
rotational moments imposed on the latch assembly parts
while releaseably sustaining the cradle of a spring-
powered breaker operating mechanism in its reset position;
FIGURE 3 is a simplified side elevational view of
the latch assembly of FIGURE 1 with the parts thereof
shown in their positions assumed in response to the
initiation of a breaker trip function; and
FIGURE 4 is a simplified side elevational view of
the latch assembly of FIGURE 1 with the parts thereof
shown in their positions assumed after the release of the
cradle from its reset position~
Corresponding reference numerals refer to like parts
throughout the several views of the drawings.
41PR-6149
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DETAILED DESCRIPTION
Referring first to FIGURE 1, the latch assembly of
the present invention, generally indicated at 10, is
illustrated in its application latchingly sustaining a
cradle 12 of a spring-powered circuit breaker operating
in the untripped or reset position shown. For a detailed
description of an applicable breaker operating mechanism,
reference may be had to the above-noted U.S. patent No.
4,001,742 dated January 4, 1977. ~atch assembly 10 includes
a primary latch lever 14 which is pivotally mounted
adjacent its upper end by a pin 16 in turn transversely
mounted between a pair of paralle, spaced side plates
illustrated in phantom at 18. In practice, primary
latch lever 14 may be U-shaped in transverse cross-
section such as to provide a pair of identical lever
arms 14a joined by a bight portion 14b.
Carried by the lower end portion of the primary
latch lever is a transverse primary latch pin 20 for
engaging a primary latch shoulder 12a formed in the edge
of the cradle tail end portion. The primary latch lever
also carries, at a location intermediate its ends, a
pivot pin 22 for pivotally mounting one end of an inter-
mediate latch lever 24. At its other end, the inter-
mediate latch lever is formed to provide an intermediate
latch shoulder 24a and a separate secondary latch
shoulder 24b. In the cradle latching condition of latch
assembly 10, intermediate latch shoulder 24a engages an
intermediate latch pin 26 which preferably is constituted
by an extension of a sha~t mounted transversely between
side plates 18 and serving to pivotally mount a secondary
latch lever 28. Mounted adjacent the lower end of
secondary latch lever is a secondary latch pin 30 in
position to engage secondary latch shbulder 24b of the
intermediate latch lever while the latch assembly is in
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its loaded or cradle latching condition. The secondary
latch lever is also preferably U-shaped having a pair
of lever arms 28a and 28b integrally joined at their
lower ends by a transverse bight portion 28c having a
longi-tudinal extension to serve as a trip actuating arm
28d. Completing the description of the latch assembly
parts seen in FIGURE 1, a double acting torsion spring 32
is carried on intermediate latch pin 26 with one end 32a
thereof engaging secondary latch pin 30 to bias secondary
latch lever 28 to its illustrated, unactuated, clockwise-
most position determined by the abutment of the secondary
latch pin against the edges of enlarged openings 18a in
side plates 18 through which the ends of the secondary
latch pin project. The other end 32b of this spring acts
to bias primary latch lever 14 in the counterclockwise
direction such as to resiliently urge primary latch pin
20 into intercepting relation with primary latch shoulder
12a when cradle 12 is swung in the counterclockwise
direction about its pivotal mounting shaft 12b from its
tripped position to its illustrated reset position
incident with a breaker resetting function. Finally, a
torsion spring 34 is carried on pivot pin 22 and acts
between pivot pin 16 and intermediate latch lever 24 to
assist in controlling the position of the intermediate
latch lever during latch assembly reset, as will be
detailed below.
Having described the latch assembly parts in con-
nection with FIGURE 1, reference is now had to FIGURE 2
for an analysis of the forces involved in sustaining the
for an analysis of the forces involved in sustaining the
cradle in its reset position. As can be observed from
the disclosure of the above-noted patent, the charged
breaker operating mechanism springs exert a torque on
the cradle attempting to swing it away Erom its reset
position in the cl~ckwise direction about its pivotal
mounting shaft 12b, as seen in FIGURE 2. With primary
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latch pin 20 engaging primary latch shoulder 12a to sus-
tain the cradle in its reset position, the eharged
mechanism springs exert, by virtue of the bias angle
imparts to the primary latch shoulder, a clockwise
moment on primary latch lever 14. This moment, exerted
about pivot pin 1~, is composed of a force Fl having a
line of force 38 and a relatively short arm 40. Since
the angular relationship of intermediate latch lever 24
with the primary latch lever is effectively fixed by the
engagements of its latch shoulders 24a and 24b with pins
26 and 30, the clockwise moment on the primary latch
lever associated with force Fl is transposed to a clock-
wise moment on the primary latch lever about pin 16
composed of a force F2 having a line of force 41 and a
moment arm 42. Note that moment arm 42 is considerably
longer than moment arm 40, and thus the magnitude of
force F2 is proportionately less than the magnitude of
force Fl.
Force F2 is opposed by an equal and oppositely
directed force F3 whieh exerts a counterclockwise moment
on intermediate latch lever 24 about pin 22 having a
moment arm 44. It is seen that moment arm 44 is con-
siderably shorter than moment arm 42, and thus the
counterclockwise moment on intermediate latch lever 24
is proportionately less than/ and yet effectivel~ counter-
balances, the clockwise moment exerted on the primary
latch lever by the charged breaker mechanism springs.
By virtue of the engagement of secondary latch shoulder
24b with secondary latch pin 30, the counterclockwise
moment on the intermediate latch lever by force F3 is
transposed to a counterclockwise moment-thereon exerted
about pin 22 composed of a force F4 having a line of
force 46 and a moment arm 48. Since moment arm 48 is
seen to be considerably longer than moment arm 44, force
F4 is proportionately smaller than force F3. Thus, the
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mechanical advantage built into latch assembly 10 enables
the very light force F4 to effectively oppose the large
force Fl and thus sustain cradle 12 in its reset position.
By the same token, this light secondary latch force F4
is indicative of a low frictional resistance to dis-
engaging secondary latch pin 30 from secondary latch
shoulder 24b pursuant to initiating a trip function.
Thus, an exceptionally low trip force such as indicated
at F5 applied to actuating arm 28d along line of force
49 can exert sufficient counterclockwise moment on
secondary latch lever 28, particularly considering the
length of moment arm 50, to swing the secondary latch pin
rightward out from under the secondary latch shoulder.
It will be noted that, while the trip force F5 exerted
on the secondary latch lever is opposed by spring 32
(FIGURE 1), it is aided by the counterclockwise ~oment
exerted on the secondary latch lever by force F4.
From FIGURE 3, it is seen that when secondary lever
28 is pivoted in the counterclockwise direction by trip
force F5 developed by a trip solenoid (not shown),
secondary latch pin 30 is swung rightward out from under
secondary latch shoulder 24b. The counterclockwise moment
on intermediate latch lever 24 by force F4 (FIGURE 2) then
becomes unopposed, and the left end of the intermediate
latch lever drops downward, disengaging its intermediate
latch shoulder 24a from intermediate latch pin 26. The
clockwise moment on primary ].atch lever 14 previously
exerted by force F2 is transposed to the now unrestrained
clockwise moment of force Fl(FIGURE 2) exerted on the
primary latch lever by the charged breaker mechanism
springs. The cradle is thus free to pivot the primary
latch lever in the clockwise direction, thereby disengaging
primary latch pin 20 from primary latch shoulder 12a~ The
cradle thus swings to its clockwisemost tripped position
of FIGURE 4 as the mechanism springs discharge. In the
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process, intermediate latch lever 24 is propelled left-
ward between latch pins 26 and 30; adequate clearance
being afforded by relieving the intermediate latch lever
edges leading rightwardly away from latch shoulders 24a,
24b, as indicated at 24c in FIGURE 4. These edge reliefs
terminate in stop shoulders 24d serving to limit the
extent of leftward movement of the intermediate latch
lever motivated by cradle 12. In its tripped position,
the cradle provides a trailing edge 12c disposed to engage
a pin 14a carried by the primary latch lever, as seen in
FIGURE 4, and thus maintain, against the contrary bias of
latch reset spring 32 (FIGURE 1), primary latch pin 20 in
non-interfering relation with the cradle as it is being
returned to its reset position.
When the cradle is returned to its reset position,
its trailing edge 12c is in position to pick up primary
latch pin 20 before clearing pin 14a. Thus, when the
cradle is returned counterclockwise beyond its reset posi-
tion and primary latch shoulder 12a swings below the
primary latch pin, reset spring 32 pivots the primary
latch lever in the counterclockwise direction to swing
the primary latch pin into position to intercept primary
latch shoulder 12a when the applied breaker charging and
cradle resetting force is removed. Coincidentally, spring
32 returns secondary latch lever 28 to its clockwisemost
position of FIGURE l; the trip solenoid having previously
been reset to remove trip force F5.
To cooperate with torsion spring 34, seen in FIGURE
1 to be exerting`a clockwise bias on the intermediate
latch lever, in properly positioning int~rmediate latch
shoulder 24a relative to intermediate latch pin 26 during
latch reset, a positioning shoulder 24e is formed in the
intermediate latch lever immediately below the inter-
mediate latch shoulder. Thus, for the brief inter~al
. 35 during -the breaker resetting operation from the time the
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primary latch pin swings into intercepting relation with
the primary latch shoulder to the time the latch assembly
assumes the burden of restraining the cradle in its reset
position, the clockwise bias of spring 34 on the inter-
mediate latch lever brings positioning shoulder 24e tobear against intermediate latch pin 26. The intermedia~e
latch lever is thus correctly positioned to ensure that
its intermediate latch shoulder 24a reliably engages the
intermediate latch pin when the latch assembly is loaded
up by the charged breaker mechanism springs.
It will thus be seen that the objects set forth
above, among those made apparent in the preceding
description, are efficiently attained and, since certain
changes may be made in the above construction without
departing from the scope of the invention, it is intended
that all matter contained in the above description or
shown in accompanying drawings shall be interpreted
as illustrative and not in a limiting sense.
