Note: Descriptions are shown in the official language in which they were submitted.
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MOLDED CASE CIRCUIT BREAKER
-
WITH REDUCED CONTACT MOUNTS
Background of the Invention
Molded case circuit breakers for industrial
application are described within U.S. Patent 3,155,803
issued November 3, 1964, entitled "Electric Circuit
Breaker with Toggle Positioning Means" in the name of
Keith W. Klein and David B. Powell. This patent
describes a toggle operation mechanism which consists
of a pair of upper and lower links joined together by
a toggle pin. To set the alignment of the upper links
when the circuit breaker contacts are closed, the
cradle is provided with a stopping surface at the
forward end of the cradle to receive the toggle pin
when the operating handle is moved to the ON position.
The angle defined between the upper and lower links is
maintained slightly less than 180 to allow manual
opening of the contacts without undue resistiny force.
The advent of rapid operating mechanisms
within industrial si~e molded case circuit breakers
allows the circuit to be interrupted in a time during
the current waveform before the current amplitude has
reached a maximum value. The following U.S. Patent
numbers describe various operating mechanisms and
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contact arm configurations which accomplish the rapid
contact separation without requiring a re-design of
the other breaker components. U~S. Patent Number
4,480,2~2 issued October 30, 1984 entitled "Variable
Torque Contact Arm for ~lectric Circuit Breakers";
U.S. Patent Number ~,482,877 issued November 13, 1984
entitled "Electric Circuit ~reakers Having Fast Short
Circuit Response"; and U.S. Patent Number 4,553,119
issued November 12, 1985 entitled "Electric Circuit
Breaker Having Reduced Arc Energy" all in the names of
R. Castonguay and C. Jencks.
To facilitate such rapid contact separation,
an S-shaped contact arm configuration is generally
required. It was discovered during excessive high
current test conditions, that the separating contacts
would arc at the approximate location at which the
contacts would mate upon closing. This could cause
localized contact heating upon repeated opening and
closing of the contacts under excessive current test
conditions eventually causing the silver contact
material to melt and fuse the contacts.
It was further discovered that the same
contact arm configuration allowed the contacts to
separate to an approximate 90 angle measured between
the upper and lower links resulting in a large
separation distance between the contacts when the
contacts are manually opened to set the operating
mechanism. When the contacts were summarily closed,
the force required to overcome the large separation
distance caused the movable contact to strike the
fixed contact with a substantial velocity causing the
movable contact to bounce away from the fixed
contact. This "contact bounce" between energized
breaker contacts could cause localized arcing between
the contacts increasing contact wear as well as
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providing contact weld as described earlier.
The purpose of this invention is to provide
means for setting the contact separation distance upon
manual opening of the contacts to thereby
substantially reduce the stress on the mechanism and
the occurrence of contact bounce and contact welding
upon manual closing of the contacts.
Summary of the Invention
A molded case circuit breaker operating
mechanism cradle is provided with a stopping surface
for the operating mechanism toggle pin during manual
opening and closing of the breaker contacts. The
stopping surface behaves as a cam to direct the toggle
pin as a cam-follower. The stopping surfaces
accurately set the angle of the upper and lower links
joined by the toggle pin to facilitate manual opening
and closing of the contacts with substantially reduced
contact bounce.
Brief Description of the Drawings
Figure 1 is a side view of a molded case
circuit breaker with part of the casing removed to
expose the lower portion of the contact operating
mechanism;
Figure 2 is a side view of a portion of the
operating mechanism used within the breaXer depicted
in Figure l; and
Figure 3 is a side view of a portion of the
operating mechanism according to the invention.
Description of the Preferred Embodiment
A molded case industrial breaker 10 is shown
in Figure 1 wherein a cover 11 is arranged on a
case 12 manufactured from a thermoset plastic
material. An ON-OFF handle 13 wherein the OFF
position 13' is shown in phantom is mounted to the top
of the cover and an upper link 14 having an upper
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pin 15 is pivotally connected to a lower link 18
having a lower pin 20 by means of a toggle pin 17.
The arrangement of the upper and lower links 14, 18
comprise the toggle mechanism generally described at 3.
An operating spring 16 fixedly attached to a portion
of the operating handle apron 30 as shown in Figure 2
by means of a support 7 operatively connects to the
toggle operating mechanism 3 by connection to the
toggle pin 17 at its opposite end. As described in
the aforementioned patent to K. W. Klein et al., the
movement of the operating handle tLanspOseS the
movable contact arm 19 from the closed positicn
indicated wherein the movable contact 21 touches the
fixed contact 5 to the open position identified as 19'
with the movable contact indicated as 21'. The
movable contact arm 19 is supported on a contact
carrier 25 and secured thereto by means of pivot
pin 22 and support pin 20 to allow the movable contact
arm to rotate about the contact carrier pivot 24 when
the handle is moved from the ON to the OFF position.
As fully described within the referenced patents, a
contact spring 23 is arranged around the movable
contact arm pivot 22. Electrical connection with the
movable contact arm is made by means of braided
conductor 4. Electrical connection with the fixed
contact 5 is made through the contact strap 26
supported upon the insulated base 8. The movable
braided conductor 4 is fixedly attached to the circuit
breaker load terminal (not shown~ at the opposite end
and is provided with a length in excess of the
separation distance between the line terminal and the
movable contact arm to allow sufficient flex upon
movement of the movable contact arm without breaking
the braid connection at either end. The braid is
further provided with a diameter in excess of that
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necessary to meet the steady state current operating
conditions in order to provide sufficient mass to
prevent the braid from undergoing a whipping movement
when the movable contact is moved to the open position
either manually or upon operation of the circuit
breaker trip unit.
The latch assembly consists of a secondary
latch 32 pivoted about a secondary latch pin 33 and a
primary latch 34 which contains a latch surface 35 for
removably retaining a portion of the cradle 2~. It is
noted that the handle 13 is in its OFF position such
that the operating spring 16 is in an unstressed
condition and the angle A' defined between the upper
and lower links 14, 18 is equal to or less than 90.
The separation distance between the movable contact 21
and the fixed contact 5 is defined by L. The cradle
operating membér 28 is shown to contact a handle
pin 31 on the handle apron 30 when the circuit breaker
is moved to its OFF position and the cradle rotates in
a counterclockwise direction about the cradle
pivot 29. In the counterclockwise rotation of the
cradle, the toggle pin 17 has moved from a stop 37
formed on the cradle as described within the Klein et
al patent and has moved within the toggle pin
clearance area generally described at 36 to a position
of rest within the clearance area. The large contact
separation distance L generates the high movable
contact arm velocity described earlier, and
necessitates the increased length and cross sectional
area of the braided conductor 4.
The improved operating mechanism arrangement
according to the instant invention is shown in
Figure 3 to consist of a similar handle 13 shown in
the OFF position with the handle pin 31 in contact
with a portion of the cradle 28 and with the cradle
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rotated in a counterclockwise direction about the
cradle pivot 29. It is noted that the angle A"
defined between the upper and lower links 14, 18 is
now less than 180 ~ut larger than that shown at A'
for the operating mechanism depicted earlier in
Figure 2. The contact separation distance L' is
substantially less than the contact separation
distance L shown in Figure 2. This is brought about
by the movement of the toggle pin 17 from the 37 stop
formed on the surface of the cradle 28 to set the
angle of the upper and lower links 14, 18, when the
contacts are closed, through a new clearance area 36'
formed on the cradle 28 to a second stop position 38
formed on the cradle opposite that of the first
stop 37. This prevents the movable contact arm 19
from any further counterclockwise rotation and thereby
sets both linkage angle A" as well as the contact
separation distance L'. Since the braided conductor 4
no longer has to travel as great a distance its length
can be decreased by approximately 15~ without
adversely affecting either the electrical or
mechanical properties. The reduced counterclockwise
movement of the movable contact arm 19 also allows a
reduction in the diameter of the braided conductor
which accounts to an overall decrease of roughly 30
in the weight of the braided conductor. Besides
providing increased contact operating life by avoiding
contact bounce, the careful control of the contact
separation distance also provides a substantial
savings in the copper material used to form the
braided conductor. Additionally, a substantial
savings in fabrication cost of the braided conductor
is provided by reducing the flex exerted on the
braided conductor and allowing the use of a #36 AWG
size wire as opposed to a #44 AWG size wire in the
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braid construction. The reduced energy in the "off"
operation because of the reduced shock, or bounce,
also allows use of lower trip bar return spring force
and lower primary latch engagement without nuisance
trip. An additional feature resulting from the
careful control over the contact separation distance
when the breaker is in the OFF position, is the use of
a smaller area for the movable and fi~ed
contacts 21, 5. This allows for more silver to be
employed within both of the contacts to cause a
further reduction in the contact operating
temperatures because of the good electrical and
thermal conduction properties of silver. When such
contacts are operated within the operating mechanism
shown in Figure 3 the toggle pin 17 moves between the
cradle stop 37 with the contacts in the closed
position along the surface provided by the clearance
area 36' to the stop 38 provided on the cradle surface
for governing the contact separation distance L' when
th~ handle is in the OFF position. This is similar to
the operation of a cam within a cam-follower whereas
the cam is defined by the location of the two
stops 37, 38 provided on the clearance area 36' on the
surface of cradle 28. The location of stops 37, 38
accurately define the contact separation distance L'
and the angle A" defined between the upper and lower
links 14, 18, as described earlier.
