US3482069A

US3482069A - Voltage controlling shunting resistor with operating cam contour modifying means

Info

Publication number: US3482069A
Application number: US600926A
Authority: US
Inventors: Paul E Badey; John A Oppel; Ugo R Tognella
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1966-12-12
Filing date: 1966-12-12
Publication date: 1969-12-02
Anticipated expiration: 1986-12-02

Description

Dec. 2. 1969 P. E. BADEY ET Al. 3,482,069

VOLTAGE CONTROLLING SHUNTING RESISTOR WITH OPERATING CAM CONTOUR MODIFYING MEANS 3 Sheets-Sheet 1 Filed Dec. l2, 1966 /NvE/vro/es: PA UL E. BADEY Uso R. 75m/ELLA,

JoH/v A. OPPEL ATTORNEY Dec. 2. 1969 E. BADEY ET AL 3,482,069

VOLTAGE CoNTRoLLING sHUNTING REsIsToR WITH OPERATING CAM coNToUR MODIFYING MEANS 3 Sheets-Sheet 2 Filed Dec. l2, 1966 A) VIL afan HDPE AP 0 N Me WEAR MMWR A00 .D/.JG

gri/,71222.74 Wl ATTORNEY Dec. 2. 1969 P. E. BADEY ET AL 3,482,069

G SHUNTING RESISTOR WITH OPERATING CAM VOLTAGE CONTROLLIN CONTOUR MODIFYING MEANS 3 Sheets-Sheet 5 Filed Dec. l2, 1966 .y MP5 oAPN W50@ MEAT. N/ NDM U y JU ATTORNEY United States Patent O U.S. Cl. 200-148 4 Claims ABSTRACT OF THE DISCLOSURE Discloses a high voltage circuit breaker in which a resistor is connected across the main contacts of the breaker just prior to their engaging during a closing operation. The resistor remains so connected until an intermediate point in a subsequent opening operation. The resistor is inserted and removed in this manner by means of a resistor switch controlled by a cam coupled to the movable main contact of the breaker. Means is provided for modifying the effective contour of the cam during breakeropening as compared to its contour during closing. Such contour-modification is accomplished by extending a dwell portion of the cam during breaker-opening to delay initiation of resistor switch-opening.

This invention relates to an electric circuit breaker of the type that is provided with a voltage controlling resistor shunting its main contacts and resistor switch means for interrupting the current flowing through the resistor after the main contacts have been opened.

For controlling the voltage developed across the main contacts of a high voltage circuit breaker during a circuit interrupting operation, it has been customary to shunt these contacts with a resistor through which current is transferred -when the main contacts are opened to interrupt the circuit. For interrupting the current through the resistor, there is customarily provided a resistor switch connected in series with the resistor and in shunt with the main contacts.

In order for the resistor consistently to carry out its intended function during the interrupting process, it is important that the resistor switch always be opened at the expiration of a definite predetermined time interval following operation of the main contacts. An arrangement for obtaining this coordination between the main contacts and the resistor switch contacts is shown and claimed in U.S. lPatent 3,211,868-Barken and Oppel, assigned to the assignee of the present invention.

In certain circuit breaker applications, it is highly desirable that, during closing, the resistor shunting the main contacts be inserted into the power circuit at a precisely controlled instant prior to closing of the main contacts. This type of operation, which is referred to as resistor preinsertion, materially reduces the ma-gnitude ofl the voltage trainsients developed by circuit breaker closing. The circuit breaker disclosed in the aforementioned. Barkan and Oppel patent inserts the resistor at about the same time as the main contacts are closed and is not capable of preinserting the resistor.

An object of the present invention is to provide a circuit breaker which can preinsert its shunting resistor d-ul-v ing a closing operation and can open the circuit through the resistor during an opening operation at a precisely controlled time following separation of the main contacts of the circuit breaker. K

Another object is to control the shunting resistor in the manner set forth in the preceding object by meansof a simple and compact linkage interconnecting the main contacts and the resistor switch contacts.

3,482,069 Patented Dec. 2, 1969 ICC Still another object is to provide a simple interconnecting linkage that comprises a cam movable in one direction to effect closing of the resistor switch and in a reverse direction to effect openin-g of the resistor switch.

Still another object is to provide an interconnecting linkage of a type that obviates the need for the holdclosed latch used for holding the resistor switch closed in the aforesaid Barkan and Oppel patent.

In .carrying out our invention in one form, Iwe interconnect a movable one of the main contacts and a movable one of the resistor switch conta-cts by means of a linkage comprising a cam coupled to the movable main contact. This cam is movebale between first and second positions in response to movement of the main contact through a closing stroke and is returnable in a reverse direction to said first position in response to movement of said main contact through an opening stroke. A follower coupled to the movable resistor switch contact and actuated by the cam imparts closing motion to the movable resistor switch contact in response to movement of the cam between said first and second positions. The speed of said closing motion is determined by the effective contour of said cam during said closing stroke. Spring means biases the movable resistor switch contact toward open position and operates in response to return movement of the cam from its second to its first position to impart opening motion to the movable resistor switch contact at a starting ypoint determined by the effective contour of said cam during said opening stroke. Means is provided for modifying the effective contour of the cam during said opening stroke as compared to its effective contour during said closing stroke.

For a better understanding of the invention, reference may be had to the following description taken in con .junction with the accompanying drawings, wherein:

FIG. 1 is a side-elevational view, partially in section, showing a circuit breaker embodying one form of our invention. The circuit breaker is depicted in the closed position.

FIG. 2 is a sectional view taken along the line 2 2 of FIG. l.

FIG. 3 is a view similar to that of FIG. 2 except showing the parts of FIG. 2 in the circuit breaker-open position.

FIG. 4 is an enlarged sectional view of a portion of the apparatus of FIGS. l-3 at an intermediate point during a circuit breaker closing operation.

Referring now to FIG. l, the circuit breaker shown therein is of the general type disclosed and claimed in the aforesaid Barkan and Oppel Patent No. 3,211,868 and in Patent No. 2,783,338-Beatty, both assigned to the assignee of the present invention. This circuit breaker comprises an enclosed interrupting chamber 11 defined, in part, by a metallic casing 12 which is filled with pressurized arc-extinguishing gas.

A pair of elongated conductive studs 15 and 20 project into the casing 12 from diametrically-opposed points, and each of these studs carries a suitable stationary contact assembly 16 at its radially-inner end. Cooperating with each stationary contact assembly is a movable contact 28 pivotally mounted upon a stationary pivot 29. These pivots 29 are supported upon stationary brackets 31 which are integral with one end of a stationary operating cylinder 32. Suitable means (not shown) are provided for transferring current between the movable contacts 28 and the brackets 31, so that the brackets 31 together with the cylinder 32 form a conductive path electrically interconnecting the two movable contacts 28.

The cylinder 32, at its left hand end, is suitably supported from a generally cylindrical housing 33, which, in turn, is suitably secured at its left hand end to the metallic casing 12.

When each pair of

main contacts

16 and 28 are sepa- Irated, an arc is drawn therebetween, and this arc is subjected to a gas blast action which helps to extinguish it. The gas blast is produced in a conventional manner and ows through nozzles 38 and the hollow housing 33 to the surrounding atmosphere. A normally-closed blast valve (not shown) within the housing 33 is used for controlling this gas blast. When the blast valve is opened, the gas-blast is initiated and continues until the blast valve is closed.

For operating the blast valve and the movable contacts 28, a combined operating mechanism preferably of the fluid-actuated type yshown in the aforementioned Beatty patent is provided within the

cylinders

32 and 33. The details of this operating mechanism form no part of the present invention and, hence, such details are not shown in the present application. An adequate understanding of the present invention may be had if it is understood that the operating mechanism acts during an opening operation to drive a crosshead shown at 59 in FIG. 1 to the right and also acts to open the blast valve. The crosshead 59 is coupled to the contacts 28 (by means soon to be described) and, hence, such movement to the right of crosshead 59 serves to drive the main contacts 28 open. At a predetermined instant after the main contacts 28 have been opened, the then-open blast valve is driven closed by the operating mechanism, as is described in detail in the Beatty patent. This prevents further loss of gas from the interrupting chamber 11.

The means for coupling the crosshead 59 to the main contacts 28 comprises two sets of connecting links 60, each pivotally connected at 61 and 62 to the crosshead 59 and movable contacts 28, respectively.

In the position of FIG. 1, the movable contacts 28 are biased into closed position by means of overcenter compression springs 64 acting to the left on crosshead 59. Each of these springs 64 has one end pivotally supported at `65 on a projecting portion of one of the stationary brackets 31. At their inner ends, the springs 64 are pivotally supported on the crosshead 59. These overcenter springs 64 tend to urge the contacts 28 closed while the crosshead 59 is to the left of a reference line connecting the pivots 65. But when the crosshead is moved to the right beyond this reference line (as occurs during a contact-opening operation), the overcenter springs thereupon tend to urge the contacts in a contact-opening direction. This action coupled with that of the operating mechanism acts to hold the contacts 28 in a fully-open position until the operating mechanism is subsequently operated to close the contacts 28.

Shunting the upper pair of

main contacts

16, 28 is a resistor 70, shown schematically as a resistor element Wound about an insulating core 70a carried by a conductive tube 71. The conductive tube 71 is supported from the conductive stud 20 by means of conductive webs 72 electrically interconnecting the tube 71 and the stationary contact assembly 16. The lower terminal of the resistor 70 is connected to the tube 71, whereas the upper terminal is locally insulated from the tube 71 and is connected by means of a conductor 74 to an electrode 73 of an auxiliary or resistor switch 75.

The resistor switch 75 comprises, in addition to the electrode 73, a second electrode 76 spaced from electrode 73 and coacting with electrode 73 to form an interrupting gap between the two electrodes. The electrode 73 is supported on the central housing 33 by an insulator 77 which is capable of electrically isolating the electrode 73 from housing 33 when the resistor switch is open. The electrode 76 is also supported on the housing 33 but is electrically connected to the housing. Electrically bridging the two

stationary electrodes

73 and 76 is a movable electrode 78, which in its closed position of FIG. 1, butts against the two stationary electrodes. Thus, it will be seen that the resistor 70 is connected in shunt with the upper contacts 16. 25 by means of a. circuit which extends ,4 through the

parts

72, 71, 70, 74, 73, 78, 76 and 33. As will be apparent from FIG. 1, the lower

main contacts

16, 28 are shunted by a similar circuit. Since the parts forming this lower shunting circuit are substantially identical to those forming the upper circuit, corresponding lower parts have 'been assigned corresponding reference numerals followed by the suffix a.

Referring more particularly to the resistor switch 75, it will be noted from FIG. 1 that each of the

movable electrodes

78 and 78a has an elongated pin 80 integral therewith and projecting therefrom to the right. Each of these pins is freely received in an opening formed in a reciprocable cross-beam 81 of insulating material. A pair of compression springs 82 and 82a, each of which is disposed between the cross-beam 81 and one of the

movable electrodes

78 or 78a, urge the movable electrodes into firm circuit-closing engagement with the stationary electrodes. While the switch 75 is in its normal closed position of FIG. l, the cross-beam 81 is restrained from moving to the right as will soon appear more clearly. When, however, the cross-beam 81 is driven to the right during a switch-opening operation, it eventually impacts against suitable abutments 83 adjustably secured to the outer ends of the pins and 80a, and, thus, drives the

movable electrodes

78 and 78a to the right into an open position spaced from the stationary electrodes. To aid in guiding the cross beam 81 during such opening movement, a suitable guide pin 84 is shown secured to a stationary portion of the switch 75 and projecting through a guide opening formed in the crossbeam. Additional guidance is provided by a metal tube 89 xed to the cross-beam 81 and slidably mounted in bearings 87 on a stationary part 88 of the circuit breaker, as may be seen in FIGS. 2 and 3.

Opening and closing of the resistor switch 75 are pre cisely coordinated with opening and closing, respectively, of the

main contacts

16, 28 by a linkage 85 which is connected between movable main contact 28 and movable resistor switch contact 78. More specifically, this linkage isconnected at one end to the crosshead 59 that ac tuates the movable main contacts 28 and at its other end to the cross beam 81 that actuates the movable

resistor switch contacts

78, 78a.

Thislinkage 85 comprises a cam 86 mounted for linear movement and coupled to the main contact crosshead 59 bya connecting lever 90. The connecting lever 90 has one end mounted on a suitable pivot 91 (FIG. 2) and its opposite end pivotally connected to cam 86 through a link 92. Link 92 is pivotally connected at its opposite ends to cam 86 and lever 90. The pivot 91 on which connecting lever is mounted is carried by a guide link 93 pivotally mounted on a stationary pivot 94. 'I'he crosshead 59 is pivotally connected to the lever 90 through a pivot 95.

During a circuit breakeropening operation, the crosshead `5,9 is moved to the right from its position of FIG. 2 into, its position of FIG. 3. Such movement forces lever 90 clockwise about pivot 91 to drive cam 86 to the right into its position of FIG. 3. During a circuit breaker-closing operation, the crosshead 59 returns from its position of FIG. 3 to its position of FIG. 2, acting 'through lever 90 to drive cam 86 leftward from its position of FIG. 3 to its position of FIG. 2. The cam is guided during such movement by a plurality of spaced guide rollers 98 and 99, which force the cam to follow a linear path."

For controlling the resistor switch in accordance with this motion of cam 86, a follower 100 mounted on a bell crank 102 is provided. This follower 100 is preferably a roller rotatably mounted on a supporting pin 103 fixed to bell crank 102. lBell crank 102 is pivotally mounted on a stationary pivot 104 and has one arm pivotally connected through a link 106 to the cross beam 8l of the resistor switch 75. Link 106 is pivotally conw nected at its opposite ends to crank 102 and cross beam 81, respectively.

The cross beam 81 of resistor switch 75 is biased toward open position by means of a resistor-switch opening spring 110 and the wipe springs 82 and 82a. Opening spring 110 is a compression spring bearing at its left hand end on a stationary cylinder 111 and at its right hand end on a hub portion of the cross beam 81. This opening spring 110- and the wipe springs 82, 82a bias crank -2 clockwise about its pivot 104, thereby urging follower 100 upwardly against cam 86. When cam 86 is moved to the left from its position of FIG. 3 toward its position of FIG. 2 during a circuit breaker closing operation, a steep ramp surface 112 on cam 86 first engages follower 100, thereby pivoting the follower bell crank 102 counterclockwise about its pivot 104 to carry cross beam 81 to the left to produce resistor-switch closing. When the cam 86 has moved through only a portion of its total closing stroke, the cross beam 81 has moved sufficiently to the left to drive the movable resistor switch contact 78 into engagement with

stationary contacts

73 and 76. Shortly thereafter, a less steep portion 112a of the ramp 112 passes into engagement with follower 100 to continue driving the cross beam 81 to the left but at a much reduced speed. This additional leftward movement of cross beam 81 compresses wipe springs 82, 82a on the resistor switch contacts. Leftward motion of cross beam 81 is terminated when a zeroslope, or dwell, portion 112b of cam 86 passes into engagement with follower 100. Cam 86 continues moving a slight additional distance to the left but imparts no further leftward motion to cross beam 81 since the follower 100 is then on the dwell portion 112b. The ramp surfaces 112 and 112a constitute what is referred to hereinafter as a motion-producing portion of the cam. It will be noted that the dwell surface 112b extends substantially parallel to the linear path of motion followed by cam 86, and the ramp surfaces 112 and 112a extend transversely of said linear path.

For decelerating cross beam 81 at the end of its resistor-switch closing stroke, a dashpot 120 is provided. This dashpot 120 comprises a dashpot piston 122 which is slidably mounted in a stationary dashpot cylinder 111. When the dashpot piston 122 is driven to the left from its position of FIG. 3, its motion is opposed by fluid trapped in the cylinder space to the left of the piston. The rate at which the piston 122 can move to the left is determined by a metering orifice 123 through which the dashpot fluid is expelled as the dashpot piston moves to the left. The dashpot piston 122 comprises a projecting portion 124 which is adapted to engage a buffer rod 125 xed to crossbeam 81. A reset spring 126 biases dashpot piston 122 to the right toward its position of FIG. 3, where it engages a stop 127. Large vent openings 129 at the right hand end of cylinder 111 permit free movement of the piston 122 to the right to allow for resetting thereof when the resistor switch opens.

When the circuit breaker is open, as shown in FIG. 3, there is a space present between buffer rod 125 and dashpot piston 124 which permits cross beam 81 to move a predetermined distance to the left before the dashpot 120 becomes eective. The dashpot becomes effective when the leftward moving buffer rod 125 strikes dashpot piston portion 124. When this occurs continued movement of the buffer rod 125 to the left drives dashpot piston 122 to the left against the opposition of uid in the cylinder space to the left of the dashpot piston, forcing the dashpot fluid at a controlled rate through metering orifice 123. Motion of the resistor switch cross beam 81 is thus smoothly terminated at the end of its closing stroke. The parts of the circuit breaker are then in the fully-closed position depicted in FIG. 2.

This dashpotting action at the end of the closing stroke reduces closing shock that could cause harmful contact bounce and also reduces the tendency for cross-beam 81 to oscillate when its closing motion is terminated.

As pointed out hereinabove, in certain circuit breaker applications it is highly desirable that the resistor switch contacts engage well ahead of the instant at which the main contacts engage. Our cam-type linkage 85 readily lends itself to such resistor preinsertion inasmuch as the steep ramp portion 112 of the cam is shaped in such a manner that it quickly drives the resistor switch crossbeam 81 into a position that produces resistor-switch contactengagement Well ahead of the point at which the main contacts engage.

During a circuit breaker opening operation, the resistor switch must be opened at a precisely controlled instant, e.g., 22 milliseconds, after the main contacts part. For controlling this instant of resistor-switch contact-part, we utilizeI the same cam 86 that is used for closing; but we change the effective contour during a closing operation. More specifically, We provide the cam with a contourmodifying member in the form of a slider 130, visible in FIGS. 2, 3 and 4, but best shown in FIG. 4. The illustrated slider 130 preferably is a bar member that is guided for linear motion in a guide slot 132 in the cam 86. Suitable guide pins 134, fixed to cam 86, project through an elongated slot 135 in the slider to retain it in its guide slot 132 and to assist in its guidance. A reset spring 136 urges the slider toward the left, as shown in FIG. 4.

During a resistor-switch closing operation, such as described hereinabove, the ramp portions 112 and 112a of the cam freely engage the follower without any significant interference from slider 130. As the follower moves along the ramp portions 112 and 112a, it simply brushes aside the slider, forcing it to the right against its low force reset spring 136, thus remaining in rm engagement with the ramp portions 112, 112g despite the presence of the slider. When the follower 100 passes on to the dwell portion 112b of the cam, it exerts no further force on the slider, and the slider therefore becomes free to return to the left into its position of FIGS. 2 and 4. The surface of dwell portion 112b is located slightly Ibelow the lowermost surface of slider 130, and thus the follower 100 is held out of engagement with the slider when on the dwell portion. This allows the slider to reset without any opposition from the follower 100 when the follower is on the dwell portion.

A circuit-breaker opening operation is performed by driving the crosshead 59 to the right from its position of FIG. 2 into its position of FIG. 3. Such motion of the crosshead 59 opens the

main contacts

16, 28 of the circuit breaker and also causes the cam 86 to move to the right from its position of FIG. 2 to that of FIG. 3. Cam 86 must move a predetermined distance to the right before the resistor switch is permitted to open; and, by this time, the

main contacts

16, 28, have already parted. So long as the cam follower 100 is held in its position of FIG. 2, the resistor-switch opening spring is prevented from driving the cross head 81 of the resistor switch through its opening travel. But when follower 100 is released, opening spring 110, assisted by springs 82, 82a, rapidly drives cross beam 81 and the connected resistor switch contacts 75, 75a into their open positions. Cam 86 holds follower 100 in its position of FIG. 2 during initial movement of the cam to the right because during this time the follower rides on the dwell portion 112b of the cam and then upon the lower surface of slider 130. But when the cam 86 has been moved sufficiently to the right to carry the lower surface of the slider out of engagement with follower 100; the follower is, in effect, released. The resistor switch opening spring 110 then becomes effective to pivot the follower crank 102 clockwise about pivot 104 driving the resistor switch cross beam 81 through an opening stroke. The cam 86 is moving at a relatively high speed when the follower is released. The follower, on the other hand, begins moving at a relatively low speed due to inertia, and thus the follower does not engage the ramp 112 during a switch-opening operation.

It will be noted that no significant opening travel of the resistor switch cross beam 81 was permitted until slider 130 passed beyond follower 100. The lower surface of slider 130, in effect, formed an extension of the dwell portion 112b which delayed initiation of resistor-switch opening motion while the cam 86 was being moved through additional travel.

The presence of slider 130 permits us to introduce the required delay in resistor-switch opening without compromising on the cam contour used for closing, Even though we use the same cam 86 for controlling both closing and opening speeds and use a cam contour which approaches the ideal shape for closing speed control, we are still able to introduce the required delay on opening. Had the cam contour during closing conformed to that present during opening, then an unsatisfactory closing contour would be present. In this respect, the cam contour during opening is the contour defined by ramp 112 and the surface of the fully-extended slider 130, as viewed in FIG. 4. In effect, the steep slope of the ramp would extend all the way down to the dwell portion at the bottom surface of slider 130. This cam shape, if used during closing, would cause the closing speed just prior to the end of the closing stroke to be so high that objectionable oscillations would occur at the end of a closing stroke. These oscillations could interfere with opening the resistor switch at the desired instant on those opening operations that immediately follow closing. The closing dashpot 120 helps to damp out these oscillations but not sufficiently to permit use of such a closing cam contour. In the illustrated linkage, the presence of the gradually sloping ramp portion 112a assures that the closing speed will be more gradually reduced at the end of the closing stroke. This enables the closing dashpot 120 to act with more effectiveness to damp out any oscillations tending to occur at that point. The buffer rod 125 engages the dashpot piston rod 124 only after the follower 100' has passed on to the gradually sloping ramp surface 112a.

In the resistor switch of the aforementioned Barkan and Oppel patent a hold-closed latch is used to hold the resistor switch cross beam 81 in its fully closed position when the circuit breaker is closed. By using the cam-type linkage 85 described hereinabove, we can eliminate this latch. In this respect, the dwell portion 112b of cam 86 acts to hold the resistor switch cross beam 81 in its closed position of FIG. 2 by blocking clockwise opening motion of the follower crank 102 when the cam 86 is so positioned. This holding action, combined with that performed by slider 103 during the early stages of an opening operation, obviates the need for a hold-closed latch.

It should be apparent that our resistor switch control linkage 85 is of a simple and compact design. This simplicity and compactness have enabled us to incorporate this linkage in a commercial circuit breaker of the design shown in the aforementioned Barkan and Oppel patent without substantially relocating or redesigning any of its major components- By incorporating this linkage into such a circuit breaker, we make the circuit breaker capable of resistor preinsertion, thus reducing the voltage transients produced by a closing operation. A factor'that contributes to the simplicity and compactness of our linkage 85 is that the same cam 86 is used for both opening and closing control of the resistor switch, and this cam is of a simple reciprocating type.

While we have shown and described a particular ernbodiment of our invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from our invention in its 'broader aspects; and we, therefore, intended in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In an electric circuit breaker of the type comprising separable main contacts, a resistor shunting said main contacts, separable resistor switch contacts connected in series with said resistor and in shunt with said main contacts, and means for producing resistor preinsertion during circuit breaker closing'comprising a linkage connected between a movable one of said main contacts and a movable one of said resistor switch contacts, said linkage comprising:

(a) a cam coupled to said movable main contact, said cam being movable between first and second positions in response to movement of said main contact through a closing stroke and returnable in a reverse direction to said first position in response to movement of said main contact through an opening stroke,

(b) means comprising a follower coupled to said movable resistor switch contact and actuated by said cam for imparting closing motion to said movable resisytor switch contact in response to movement of said cam between said first and second positions, the speed of said closing motion being determined by the effective contour of said cam during said closing stroke,

(c) said cam having a motion-producing portion and a dwell portion sequentially engaged by said follower during a circuit breaker-closing operation,

(d) said follower engaging said motion-producing portion until said movable resistor switch contact substantially reaches the end of its closing travel and thereafter engaging said dwell portion, said dwell portion thereafter acting to hold said movable resistor switch contact closed while said circuit breaker is closed,

(e) spring means for biasing said movable resistor switch contact toward open position and operable in response to return movement of said cam from said second to said first position to impart opening movement to said resistor switch contact at a starting point determined by the effective contour of said cam during said opening stroke,

(f) and means for modifying the effective contour of said cam during said opening stroke as compared to its effective `contour during said closing stroke comprising means for extending said dwell portion during a circuit breaker-opening operation across a part of said motion-producing portion, thereby delaying initiation of resistor switch-opening during circuit breaker-opening.

2. The circuit breaker of claim 1 in which:

(a) said dwell portion of said cam comprises a main dwell surface that is engaged by said follower dur ing circuit breaker-closing,

(b) said means for extending said dwell portion cornprises a contour-modifying member mounted on said cam for movement relative to said cam and having an auxiliary dwell surface which is substantially a continuation of said main dwell surface during circuit breaker-opening,

(c) means is provided for causing said follower to engage said auxiliary dwell surface after leaving said main dwell surface and before returning toward said switch-open position during a circuit breaker-opening operation, and

(d) means is provided for rendering said contourmodifying member ineffective to control movement `of said follower during movement of said cam from said first to said second position incident to circuit breaker closing.

3. The circuit breaker of claim 2 in which:

(a) spring means is provided for biasing said contourmodifying member toward an effective position where its auxiliary dwell surface is substantially a continuation of said main dwell surface,

(b) said spring means is operable during switch-opening to hold said contour-modifying member in said effective position,

(c) said contour-modifying member is so mounted that during switch-closing said follower brushes aside said contour-modifying member against said spring means 9 10 and is thus essentially unaffected in its closing motion 2,739,206 3/ 1956 Florschutz et al. 20G-148.6 by said contour-modifying member. 2,902,570 9/ 1959 Roxburgh et al ZOO-148 `4'. The circuit breaker of claim 3 in which said 4cam 3,211,868 10/ 1965 Barkan et al. 20051482 X is mounted for motion in a linear path between said first 3,275,778 9/ 1966 Morioka 20G-148 and second positionsz said motion-producing surface is a 5 FOREIGN PATENTS ramp surface extending transversely of said llnear path, and said main dwell surface extends substantially paral- 1,181,297 11/1964 Germany- 1 lt d l' th.

e o sal mear pa ROBERT K. SCHAEFER, Primary Examiner References Cited 10 R. A. VANDERHYE, Assistant Examiner UNITED STATES PATENTS US C1 XR 2,294,824 9/1942 Baker at a1 20d-148.6

2,581,822 1/1952 Thommen ai a1 20o-148 200-144. 153; 74-567