US2696536A - Control arrangement for circuit breakers - Google Patents

Description

Dec. 7, 1954 T. R. COGGESHALL. El AL CONTROL ARRANGEMENT FOR CIRCUIT BREAKERS Filed Dec. 21, 1949 3 Sheets-Sheet 1 Inventors: Thellwell'RCo eshall, Nelson H.westervelt,

Their Attornqg.

1954 T. R. COGGESHALL ETAL 2,696,536

CONTROL ARRANGEMENT FOR CIRCUIT BREAKERS Filed Dec. 21, 1949 3 Sheets-Shea 2 Inventors: Thellwell R. Coggeshall, Nelson H. \A/estervelt.

bps W Their- Attorne g.

Dec. 7, 1954 T. R. COGGESHALL ETAL 2,696,536

CONTROL ARRANGEMENT F OR CIRCUIT BREAKERS 3 Sheets-Sheet 5 Filed Dec.

Inventor's: Thellwe H P. Cogge shall,

Nelson H. We ster'velt Their Attorne g.

United States Patent CONTROL ARRANGEMENT FOR CIRCUIT BREAKERS Thellwell R. Coggeshall, Bala Cynwyd, and Nelson H.

Westervelt, Drexel Hill, Pa., assignors to General Electric Company, a corporation of New York Application December 21, 1949, Serial No. 134,246

9 Claims. (Cl. 20089) This invention relates to control arrangements for circuit breakers and more particularly to an arrangement for controlling an electrically operated mechanism utilized to operate circuit breakers, particularly those wh ch are of the biased open type. The invention is an improvement upon the operating mechanism control scheme disclosed in Patent 1,723,103Walle, granted August 6, 1929, and assigned to the assignee of this invention and, in certain respects, is somewhat similar to the control arrangement disclosed in copending application Serial No. 68,363-Favre, filed December 30, 1948, now Patent No. 2,534,115, December 12, 1950, and assigned to the assignee of this invention.

An object of the invention is to provide an improved control arrangement for circuit breakers which deenergizes the circuit breaker closing means, such as a motor or solenoid, upon completion of the closing stroke; which also provides an earlier deenergization or control cutott whenever the circuit breaker trips free due to fault conditions existing during a closing or reclosing stroke of the breaker; and which further is readily adaptable to function as a latch checking device whereby power to reclose the breaker cannot be applied unless the trip latch is safely restored to its latching position as the proper sequel 'to an opening or a trip-free operation.

Another object of the invention is to provide an improved control arrangement for power operating means for circuit breakers wherein the controlled breaker is capable of trip-free operation and wherein provision is made for insuring that the power closing circuit cannot be reestablished until the trip latch for holding the breaker closed against its opening bias has reset in its safe latching posit-ion without any necessity for using a separate conventional so-called latch-checking switch for this purpose.

Still another object of the invention is to provide an electromagnetically operated control switch for use in a circuit breaker control arrangement which comprises a movable switch arm together with means for maintaining the switch arm in its momentarily closed position, and a rotatable pivotally mounted member disposed for engagement with an abutment member so that the movable element of the electromagnetic means is in driving relationship with the switch arm only during a switch closing operation and is divorced therefrom thereafter as during a switch opening operation.

In accordance with the invention, the energizing circuit for the electrically operated power means for closing a circuit breaker is controlled, i. e., established and then cut ofi, by switching means which, after just being closed electromagnetically, is caused to open positively at or near the end of the closing stroke of the breaker by mechanical means actuated by the electrically operated power means. An appropriately configured movable member disposed for movement in response to operation of the electrically operated closing means may be used for providing a variable cutoff characteristic for the closing control switch which is very desirable for circuit breaker trip-free operations and which member, in addition, serves as hold-open means for the switch thereby preventing a subsequent closing operation of the breaker if the mechanism trip latch failed to reset to its proper latching position at the termination of the preceding opening operation.

The invention will be better understood by reference to the accompanying drawing wherein Fig. 1 schemati- Patented Dec. 7, 1954 cally represents the invention with the parts in the positions which they occupy after the breaker is opened; Fig. 2 shows the parts in their respective positions during a circuit breaker closing operation; and Fig. 3 shows the parts in their respective latched-in positions after completion of a circuit breaker closing operation.

While the drawings show the invention as applied to a solenoid operated mechanism, the invention is equally applicable to other types of operating mechanism, such, for example, as motor driven mechanisms such as that disclosed in the above mentioned Walle patent.

Referring now to Fig. l, the numeral 1 designates the operating rod for the controlled circuit breaker and the numeral 2 designates the main lever or output crank from the operating mechanism. Crank 2 is biased by the tension spring 3 for counterclockwise movement about the fixed pivot 4. Spring 3 biases the breaker contacts toward the open position so that a clockwise driving movement of lever 2 about the pivot 4 is necessary to close the breaker contacts against this spring bias. Closing force is applied to lever 2 through the operating mechanism by means of the main solenoid 5 and its armature 6. When solenoid 5 is energized, the upward force exerted by armature 6 is transmitted to lever 2 through the trip-free or collapsible linkage system generally indicated at 7. The linkage 7 cooperates with the trip latch 8 which is rotatable about the pivot 9. The latch 8 is biased into engagement with the stop 10 by means of the torsional spring 11 and, as will be observed from Figs. 2 and 3, this latch constitutes a shiftable prop for the link-age against which breaker opening forces react during and after a breaker closing operation. Latch 8 is rotatable from its holding position by means of the armature 12 acting against tension spring 14 whenever the tripping solenoid 13 is energized in any suitable manner as from some abnormal current responsive relay or the like. The energization and cutoff of the circuit of solenoid 5 are accomplished by the closing and reopening respectively of the contacts 15 and 16 of the control device of this invention, contact 16 being a fixed contact while contact 15 is moved into and out of contact by two distinct actuating means respectively as will become apparent as the description proceeds. Thus, when contact 15 'is moved into engagement with contact 16, a circuit is completed through solenoid 5 and thereafter upward movement of armature 6 closes the breaker by exerting a force thereon through the trip-free linkage 7, lever 2 and rod 1 in a manner very similar to that described in U. S. Patent 1,827,626 to Thumim which is assigned to the same assignee as this present application.

The construction of the operating mechanism for the movable contact 15 of the control device will now be described. The contact 15 is mounted on the switch arm 17 which in turn is pivotally mounted on the fixed pin 19. Switch arm 17 is maintained in its open position by biasing means in the form of a permanent magnet 20 which cooperates with an armature 21 mounted on switch arm 17. Also, when operated to close, the switch arm 17 and its contact 15 are then maintained in the closed position by means of another permanent holding magnet 22 which cooperates with an armature 23 mounted on the switch arm. It will be understood that the magnets 20 and 22 constitute an equivalent for the conventional overcenter spring arrangement whereby a movable contact continues in the position to which last moved until shifted therefrom to another position. Magnet 22 is the stronger, for it must furnish adequate contact pressure for the closed contacts, while magnet 20 need be only sufficient to hold the contacts safely parted in the open position.

The motive means for imparting closing movement to contact 15 and switch arm 17 comprises the electromagnet 24 and its cooperating armature 25 which transmits its pull through an intermediate force-transmitting crank 33. Armature 25 is biased toward the left against stop means 27 by tension spring 26. Energization of coil 24 results from the closing of any suitable control means, such as the manually operable switch 28. As is well known, automatically operable switch means arranged for rapid reclosing could be utilized instead of manually operable switch 28. Switch arm 17 is provided with an outwardly projecting flange portion 29, the edges of which are adapted for engagement with the projecting ends 30 and 31 of a spring member 32 coiled about the shaft 19. The purpose of spring 32 and projection 29 Will become apparent as the description proceeds. In order to relate operably the operating movement of armature 25 with the switch arm 17 during switch closing operation, an intermediary member comprising the actuating crank 33 is pivotally supported on the fixed fulcrum pin 19 and is provided with an outwardly pro ecting flange portion 34 which, like flange portion 29, is normally in engagement with the ends 30 and 31 of spring 32. Thus, due to the arrangement of spring 32 and its projecting ends 30 and 31 with respect to flange 29 of the switch arm 17 and the flange 34 of actuating crank 33, a limited resilient relationship exists between actuating crank 33 and switch arm 17 which may become a positive drive as soon as the flange 34 encounters the closely spaced flange 29 as a result of initial rotation of the crank 33. By this means, movement in the clockwise direction, for example of actuating crank 33 about the fixed pin 19, results in downward pressure on the end 31 of spring 32 exerted by flange portion 34 which causes end 36 of spring 32 to exert a downward force on flange portion 29 on switch arm 17, thereby to gradually build up a torsional force in the clockwise direction on switch arm 17 which eventually overcomes the holding force of magnet 20 so that the control device contact will move to the closed poistion after this predetermined relative lost motion between the actuating crank 33 and switch arm 17. If desired, suitable notches could be placed in the ends of flanged portions 29 and 34 to hold the ends 30 and 31 of spring 32 in position.

Armature 25 is operatively related momentarily to the actuating crank 33 by means of an abutment member 37 mounted on armature 25 which is engageable with a pivotally mounted pawl member 38 pivoted to actuating crank 33 by a pin 39. Pivotally mounted pawl 38 is biased in the clockwise direction against crank 33 through its projection 43 by torsional reset spring 40, one end of which engages the pin 41 supported in crank 33 and the other end of which engages a projecting portion 42 forming an integral part of the pawl 38.

When the circuit is completed through coil 24, as by an operator closing the switch 28, armature 25 moves toward the right against the bias of its return spring 26 so that when its abutment member 37 engages the tip of the pawl 38, a clockwise moment is applied to the actuating crank 33 which first produces clockwise movement of the lower spring arm 31 while the upper arm 30 applies an increasing downward pressure upon the flange 29. After predetermined rotation of crank 33, its flange 34 impacts the switch arms flange 29 to thereby positively break the bond between the hold-open magnet 2t) and the switcharm armature 21, so that the contacts 15 and 16 thus close the energizing circuit for coil 5 of the breaker operating mechanism and a breaker closing stroke is instantly initiated.

For dependable and foolproof operation, a circuit breaker control device must rely upon manual means for initiating an operating sequence only, and thereafter the subsequent portions of the operating cycle must proceed automatically and inevitably in such a Way that, irrespective of whether the operator continues holding the switch 28 closed or instantly releases it, he has no power over the further operation of closed contacts 15 and 16. For this reason, it is necessary that the armature now be rendered incapable of maintaining the control device contacts closed and this condition is fulfilled in the device of our invention by providing an appropriate overtravel for the armature 25 during which the abutment member 37 rides over the driving tip of the pawl 38. This overtravel position of the armature 25 is shown clearly in Fig. 2; and in arriving at this position, it will be understood that the abutment 37 rode over the tip of pawl 38 during a corresponding overtravel of the clockwise rotation of crank 33 with respect to the switch arm 17 which had been arrested by the hold-close magnet 22, this overtravel being available by virtue of the aforementioned resilient connection between the crank and lever constituted by the torsion spring 32. Accordingly, if the operator maintains the switch 28 closed, the armature and its abutment 37 continue attracted to the right beyond the switch arm 17 and the contacts 15 and 16 will be broken at an appropriate later instant by the action of the breaker operating mechanism, as will be described. On the other hand, should the operator instantly release switch 28 to deenergize coil 24, the armature 25 can return harmlessly against its stop 27 through the pull of its return spring 26 without affecting the closed switch arm 17; for in this return movement, the returning abutment 37 will merely have rotated the pawl 38 counterclockwise with respect to the switch actuating crank 33 because the pawl spring 49 is very light compared to the torsion spring 32 which constitutes the resilient coupling medium between the crank 33 and the switch arm 17. In other words, the pawl 38 really constitutes a resilient catch which permits but a one-way transitory drive between the actuating armature 25 and the driven crank 33 of the control device switch arm 17.

Thus, once the control device contact 15 has been closed electromagnetically by the operator, the only way these contacts can be reopened to cut off the breaker closing coil 5 is by causing counterclockwise rotation of the crank 33 by means other than, and independent of, the above described closing means, and the manner in which this opening movement is effected automatically will be made evident as the description proceeds.

Once the contacts 15 and 16 are closed so as to energize the circuit breaker operating solenoid 5, its armature 6 moves upwardly to drive the actuating tripfree toggle linkage 7. Linkage 7 comprises a force-transmitting roller 44 which is rotatable about a pin 45 which constitutes the knee of the toggle comprising the thrust links 46 and 47. The left-hand end of link 46 is pivoted to a guide link 48 through the pin 49 while a tripping roller 50 is mounted rotatably about the pin 49. Guide link 48 is pivoted to a fixed fulcrum pin 51 at its lefthand end and a resetting spring 52 exerts a clockwise turning moment on the link 48 so that in normal circumstances the tripping roller 50 lies in the position shown between a fixed stop 54 and the arcuate tip of the trip latch 3 so that the upward thrust of armature 6 is translated through the link 47 and lever 2 into a downward pull on the rod 1 to the breaker-closed position. in order to maintain the breaker in the closed position, a prop 55 is pivotally supported about a fixed pin 56 and is biased in the counterclockwise direction to its holding position by means of the torsional reset spring 57. Accordingly, during a breaker closing operation the prop is brushed aside as in Fig. 2, but it instantly returns to engage the pin 45 and prop the breaker closed at the end of a full operating stroke as shown in Fig. 3.

If now the trip coil 13 is energized, i. e., with breaker closed, trip latch 8 (which now constitutes the abutment for receiving the end thrust of the toggle linkage through the tripping roller 59 due to the breaker opening spring 3) will be rotated in the clockwise direction about its fixed shaft 9, thus allowing the guide link 48 to rotate counterclockwise as the linkage 7 collapses to the left so that the breaker opens at high speed.

In order to deenergize the breaker solenoid 5 at or near the end of a closing stroke of the operating mechanism, it is desirable to provide an appropriate mechanical llnkage between the mechanism and the control device whereby the latter is caused to reopen its momentarily closed contact 15. To this end, there is provided a suitable bell crank mounted adjacent the breaker operating mechanism for actuation thereby through the agency of the toggle knee pin 45 which is extended outwardly a suitable amount for this purpose. This bell crank 58 is mounted for rocking about a fixed pivot 59 and it carries a connecting member 60 pivotally connected to the short arm thereof by a pin 61. For permitting the already described closing movement of the control device, a lost motion connection with the control device actuating crank 33 is provided, as depicted in Fig. l, in the form of a slot 62 formed in the lower end of connecting member 60 about the driving pin 63. It will be observed that this slot becomes substantially closed, as shown in Fig. 2, coincident with the closing of contacts 15 and 16.

At this point it will be understood that clockwise movement of the bell crank 58 produces a downward motion to the link 61) whereby counterclockwise rotation is transmitted to the control device crank 33 and its switch arm 17 so that the movable contact 15 separates from the fixed contact 16 to effect the cutoff point of the closing coil 5. Accordingly, if, in conventional manner, it were desired only to cut oif the closing stroke at a constant predetermined pointnear or at the end of the stroke, it would be necessary to provide only the long horizontal portion 64a. of control arm 64 of the bell crank for engagement thereof by some suitable member moving in unison with the mechanism armature 6. In such a case, however, no provision is made for achieving an earlier cutoff point in the event of trip-free closing operations wherein the latch 8 is tripped during the latter part of the breaker closing stroke.

As another feature of the invention then, a variable cutoff arrangement, for minimizing closing slam, is achieved by actuating the bell crank 58 rather in accordance with the toggle action by means of the toggle knee pin 45 and by extending the bell crank control arm 64 downwardly in an arcuate tailpiece 65 which renders the bell crank 58 capable of cutting 01f the closing circuit at any point in the closing stroke at which a trip-free response occurs. This is because the tailpiece 65, although lying closly adjacent the upward arcuate closing sweep of the knee pin 45 so as not to be affected thereby except at the end of a normal full-closing stroke, nevertheless lies directly in the path of this pins instant displacement to the left which results whenever the latch 8 is tripped, as from a protective relay, during the concluding part of a breaker closing stroke. For example, let it be assumed that the breaker has been closed in on a short circuit so that the latch 8 is tripped from its holding position at a point short of the complete stroke, i. e., at the instant depicted in Fig. 2. At such a moment, the toggle link 46 will move generally bodily over to the left (following the guide link 48) with the result that the pin 45, after just having ascended along the arcuate path a, will now move to the left along the approximate path b until the toggle links become generally straightened out (the circuit breaker opening in the meantime), after which the pin 45 will descend along the path 0 as the links reset back to the normal breakeropen position illustrated in Fig. 1. Obviously, in following its lateral path 1;, the pin 45 will have imparted the necessary clockwise motion to the bell crank 58 so as to instantly open the control device contacts 15 and 16 at an earlier point in the stroke than would have occurred had there been no trip-free response and the breaker had closed all the way in on a normal power line. Thus, by shrouding the pin 45 by the arcuate extension 65 of the control arm 64, the closing slam of the mechanism is minimized by arranging its power cutoff to take place either precisely at the end of a normal closing operation, or selectively and automatically at any point during the closing operation at which the trip latch 8 might prematurely respond to fault conditions on the line.

It will be observed that the downward arcuate portion 65 of the bell crank 58 extends down much lower than would be required for any conceivable premature or trip-free operation and, in accordance with the invention, this further extension serves another useful function, viz., that of providing latch-checking insurance which heretofore has been provided only by the use of an additional switch arranged in the closing coil energizing circuit. By way of explanation, and referring to Fig. 3, it will be understood that the knee pin 45 will follow a closing are A and, for normal tripping from the fully closed position by the latch 8, will follow generally the return path C as determined by the leftward collapse of the toggle links 46 and 47 followed by their return to the normal mechanism-open position of Fig. 1 under the influence of the resetting spring 52 for the guide link 48. It further will be observed that the crank actuating knee pin 45 follows the same return path after tripping, irrespective of whether it has been a trip-free closing operation or a normal tripping operation; in other Words, the trip-free return path c of Fig. 2 is the same as the lower portion of the curved path C of Fig. 3. It follows, then, that the resetting of the pin 45 to its normal mechanism-open position D is a good indication that the latch 8 has properly reset above the trip roller 50, it being well understood that it would be futile for the armature 6 to attempt closing or rapidly reclosing the mechanism without the latch 8 being back in its normal position to counteract the toggle forces imposed by the breaker opening spring 3. Because the lower end of the arcuate extension 65 constitutes interference means which prevents the restoration of bell crank 58 under the influence of its resetting spring 66 until the pin 45 returns to position D, it will be apparent that not until this safe moment can the crank 33 of the control device move to reclose the contacts 15 and 16 for reenergizing the 6 breaker closing coil 5. Thus, by making it impossible for the control device contacts 15 and 16 to reclose until the operating mechanism has reset, the contacts 15 and 16 are arranged to function further in the additional latch-checking capacity which formerly was provided by a special and separate switch. The dotted outlines of the bell crank 58 and the toggle knee pin 45 in Fig. 3 illustrate several positions which the crank must assume as the knee pin 45 follows its close-open path A-C.

From the foregoing explanation, it will be appreciated that the rod or link 60 constitutes mechanical means operated in accordance with the breaker operating mechanism for automatically controlling the opening, and preventing the reclosing, of the control device contacts 15 and 16, these contacts already having been closed by separate means controlled manually by an operator or by other suitable means, such as a reclosing relay.

Furthermore, it will be appreciated that this closing control of the invention is nonpumping, for it is impossible for an operator to cause reclosure of the control device contacts until he first releases the manual switch 28. Coincident with this ensuing return to the left of armature 25, it will be understood that the abutment 37 simply brushes by the pawl 38 (which is freely rotatable against its light spring 40 in this direction) so that the abutment is once again in condition for picking up its driving relationship with the pawl 38 in response to the next energization of the control device coil 24.

Those skilled in the art will appreciate that, in carrying out the invention, the specific control device or switch described here need not necessarily be employed in conjunction with the above described mechanical control means associated with the breaker operating mechanism. For example, any control switch capable of being closed by one means having self-disconnecting properties and reopened by means independent from the closing means may be employed with equally beneficial results.

While there have been shown and described particular embodiments of the invention, it is not desired that the invention be limited thereto and it is intended in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

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

1. An arrangement for controlling the energization of an electroresponsive device comprising a switch arm movable between one position and another position, a crank, yieldable means for operably relating said crank and said switch arm, movable motive means, pivotally mounted means movable with said crank and rotatable in one direction from a normal position but not in the other direction from said normal position, said pivotally mounted means being biased toward its normal position, abutment means movable with said motive means and engageable with said pivotally mounted means upon actuation of said motive means for operably relating said motive means and said crank to cause movement of said switch arm from said one position to the other position, and means operatively connected between said switch arm and said electroresponsive device for restoring said switch arm to said one position in response to a predetermined operation of the electroresponsive device.

2. A control arrangement for an electric circuit breaker comprising electroresponsive means for imparting closing movement to the breaker, a switch arm movable from one position to another position to control said electroresponsive means, a force-transmitting linkage mechanism movable from one position toward another position by said electroresponsive means for closing the breaker, latching means engageable by said linkage and operable to cause said linkage to collapse to open the breaker, and means including a movable control arm disposed for engagement by said linkage upon collapse thereof for causing said switch arm to return to said one position.

3. A control arrangement for an electric circuit breaker comprising electroresponsive means for imparting closing movement to the breaker, a switch arm movable from one position to another position to control said electroresponsive means, a force-transmitting linkage mechanism movable from one position by said electroresponsive means, latching means engageable by said linka e and v le to enable said linkage to collapse and means including a movable control arm having a portion thereof disposed for engagement by said linkage 7 upon collapse thereof and another portion thereof disposed for engagement by said linkage upon movement thereof by said electroresponsi've means for causing said Switch arm to return to said one position. 7

4. A control arrangement for an electric circuit breaker comprising electroresponsive means for imparting closing movement to the breaker, a switch arm movable from one position to another position to control said electroresponsive means, a force-transmitting linkage mechanism movable from one position toward another position by said elect'roresponsive means for closing the breaker, latching means engageable by said linkage and operable to cause said linkage to collapse thereby to open the breaker, and means including a control arm movable from one position to another position Whenever said linkage collapses for preventing return of said switch arm from said one position to said another position until said linkage is reset to said one position.

5. An arrangement for controlling the energization of an electroresponsive device comprising a switch arm movable between one position and another position, movable motive means, pivotally mounted means operably related with said switch arm and rotatable in one direction from a normal position, abutment means movable with said motive means and engageable with said pivotally mounted means upon actuation of said motive means for operably relating said motive means and said switch arm to cause movement thereof from said one position to said another position, a control arm movable from one position to another position in response to actuation of said device, and means for causing movement of said switch arm from said another position back to said one position in response to movement of said control arm to said another position.

6. An arrangement for controlling a biased open electric circuit breaker comprising electroresponsive means for imparting closing movement to the breaker, a switch arm movable from one position to another position for energizing said electroresponsive means, motive means, pivotally mounted means operably related with said switch arm, abutment means movable with said motive means and engageable with said pivotally mounted means upon actuation of said motive means to cause movement of said switch arm to said another position for energizing said electroresponsive means, a linkage mechanism normally disposed for engagement by said electroresponsive means and movable thereby to impart closing movement to the breaker, a control arm disposed for engagement by said linkage upon movement thereof by said electroresponsive means, and means inte'rrela'ting' said control arm and said switch arm to cause said switch arm to move back from said another position upon actuation of said control arm by said linkage.

7. An arrangement for controlling a biased open electric circuit breaker comprising electroresponsive means for imparting closing movement to the breaker, a switch arm movable from one position to another position for actuating said electroresponsive means, motive means, pivotally mounted means operably related with said switch arm, abutment means movable with said motive means and engageable withv said pivotally mounted means upon actuation of said motive means to cause movement of said switch arm to said another position for actuating said electroresponsive means, a linkage mechanism normally disposed for engagement by said electrore'sponsive means and movable thereby to close the breaker, said linkage being collapsible to cause the breaker to open, a control arm disposed for engagement by said linkage and movable thereby when said linkage collapses, and means "controlled by said control arm for causing movement of said switch arm back to said one position.

8. An arrangement for controlling the energization of an elect'roresponsive trip-free operating mechanism having a collapsible force-transmitting structure comprising a control device including a switch arm movable between a first and a second position, said switch arm when in said second position effecting energization of said electroresponsive mechanism, actuating means for driving said switch arm from said first to said second position, and means independent of said actuating means and operated from said force-transmitting structure in response to collapse of said force-transmitting structure for returning said switch arm back to said first position.

9. In combination, an electroresponsive trip-free operating mechanism comprising a latch-restrained collapsible force-transmitting structure, an arrangement for controlling said e'lectrorespons'ive mechanism compris ing a control device including a switch member movable from a first to a second position to effect energization of said electrorespons'ive mechanism, actuating means for moving said switch member from said first to said second position, a control linkage operatively coupled to said switch member and actuable to return said switch memher from said second to said first position, said control linkage having a position for blocking switch member movement from said first to said second position, and interference means coupled between said force-transmitting structure and said control linkage and operable to maintain said control linkage in said blocking position whenever said force-transmitting structure is collapsed whereby to prevent energization of said mechanism whenever said structure is collapsed.

References Cited in the file of this patent UNITED STATES PATENTS

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