US2044133A - Circuit breaker - Google Patents

Description

June 16, 1936. STQLZ Er AL 2,044,133

CIRCUIT BREAKER Filed Oct. 14, 1932 ATTORNEY Patented June 16, 1936 CIRCUIT BREAKER.

Joseph Stolz, Sachsenhausen-in-Mark, and Josef Hanke and Gottlob Lux, Berlin-Charlottenburg, and Richard Logstadt, Berlin-Schoneberg, Germany, assignors to Westinghouse Electric and Manufacturing Company, a corporation of Pennsylvania Application October 14.

1932, Serial No. 637,764

In Germany October 17, 1931 19 Claims.

Our invention relates to electro-responsive apparatus suitable foruse as a relay or as a circuit breaker trip device, and is particularly applicable to circuit breaker trip devices having a latch biased into engagement with the circuit breaker operating mechanism and having a thermally responsive and a magnetically responsive element for releasing the latch, the vthermally responsive element releasing the latch and opening the breaker vafter a time delay upon the 'occurrence of la predetermined overload condition, and the magnetically responsive element releasing the latch and opening the breaker immediately upon the occurrence of a predetermined heavy overload condition. 'i

It is an object'of our invention to provide an improved device othis type that shall be more economical to manufacture, and that shall be more satisfactory and more reliable in operation than those devices which are already known to the art.

Another object of our invention is to provide an improved device of this type having a solenoid for attracting an armature, a spring for biasing the armature away from the solenoid, and a thermally responsive member for rendering the spring inoperative as a biasing member.-

A further object of our invention is to provide an improved supporting mechanism for the armature of a magnetically responsive trip device; the improved supporting mechanism to include a spring for biasing the armature away from its attracting solenoid and a thermally responsive element which is operable upon the occurrence of predetermined electrical conditions to permit movement of the armature without overcoming the force of the biasing spring.

A further object of our invention is to provide an improved temperature compensating device for the thermally responsive element of circuit breaker trip mechanisms, or like apparatus, which shall cause the operation thereoi` to be independent of changes in the temperature of the surrounding medium.

A still Vfurther object of-our invention is to provide an improved trip device for circuit breakers biased to the open position and held in the closed position by a releasable member which engag'es the trip device, our improved trip device to include a latch biased into engagement with the releasable memb-er and an electro-responsive element for rendering thebiasing force ineffective.

The principal field-for immediate application of our invention is in connection with trip devices for circuit breakers used in controlling lighting and distribution feeder circuits, and we shall hereinafter describe an embodiment of our invention as applied to such circuit breakers, without, however, in any way intending to restrict the scope of our invention except as indicated in the appended claims.

In this embodiment of our invention, we provide a manually operable mechanism for moving a switch member to the open or closed positions, a stationary contact member for cooperating with the movable switch member, an insulating base for mounting the various elements of the circuit breaker, an arc-extinguishing device affixed to thev base adjacent the path of movement of the movable switch member, a means for biasing the switch member to the open position, a releasable restraining means for holding the switch member in the closed position, and a trip device for engaging and releasably restraining the means holding the switch member in the closed position.

The features of our invention which we believe to be new are particularly pointed out in the appended claims, and for a full understanding of the principles of the invention, reference may be had to the accompanying drawing, in which Figure 1 is a. plan view of a single pole circuit breaker embodying the principal elements of our invention; the cover has been removed and part of the operating member has been cut away to more clearly show the structural features involved.

Fig. 2 is a sectional view on the line II-II of Fig. 1.

Fig. 3 is a schematic perspective view showing the principal features of our invention as applied to the trip device of the circuit breaker shown in Figs. 1 and 2.

Fig. 4 is a schematic perspective view showing one modification of our invention.

Fig. 5 is an elevational view showing a modified form of thermally responsive element for use with our invention, and

Fig. 6 is a plan view of the thermally responsive element shown in Fig. 5.

Referring to the drawing, the base I is of moulded insulating material and has mounted thereon the terminal contacts 3 and 5, the trip device 'I, the circuit breaker operating mechanism 9 which has associated therewith the switch member II, the arc extinguisher I3, and the main stationary contact I5. The cover I1 is likewise of moulded insulated material and is fastened to the base I by means of two screws I9 which engage suitable threaded openings 2| in the base I. The upper portion 25 of the operating member 23,

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which is of moulded insulating material, has a projection 21 which extends through a suitable opening 29 in the cover I1 to permit manual operation of the circuit breaker mechanism. The various parts of the trip device 1 are mounted on suitable moulded projections extending upwardly from the base I, through the agency of screw fastenings, as will be described in some detail later.

The electrical circuit through the breaker is very direct; beginning with the terminal contact 5, the current iiows successively through the conducting strip 3|, the stationary contact I5, the moving contact 33, the resilient conducting member 35, the flexible shunt 31, the energizing coil 39 for the magnetically responsive element of the trip device, the conducting strip 4I which connects one end of the coil 39 with the heater 43 for the thermally responsive trip element, the heater 43, the conducting strip 45, and thence to the terminal contact 3.

The switch member I I has a channel shaped frame 41 which is pivoted tothe U-shaped frame B3 of the circuit breaker operating mechanism 9 through the agency of the pivot pin 49. The resilient conducting member 35 is preferably constructed of spring steel, and as previously pointed out, the moving contact 33 is rigidly affixed to one end thereof by means of the screw 53. The other end of the conducting member 35 and one end of the ilexible shunt 31 are fastened to the central portion of the channel shaped switch member frame 41 by means of the rivets 55. Round spacing washers 56 are provided to position the switch member frame 41 between the sides of the U.

We prefer to use arc-resisting material for the main contacts I5 and 33, the former being composed of finely divided silver and graphite compressed into a conglomerate mass, and the latter being composed of a silver-molybdenum alloy.

The arc extinguisher I3 is of the spaced-plate type in which a plurality of slotted magnetic plates, each insulated from the other and having a slot therein, are provided.. 'I he extinguisher is positioned adjacent the arc paththe slots being of substantially the same outline as the moving contact-and upon the establishment of an arc, due to the altering of the magnetic circuit adjacent to the arc path, force the arc into the spaces between the plates where it is quickly cooled and extinguished.

In the structure which we prefer to use, the magnetic plates 51, having slots 59 therein, are assembled between two end plates 6| of insulating material, projecting lugs (not shown) being provided in the magnetic plates 51 for engaging the end plates 6|.

The operating mechanism 9 comprises, in general, a U-shaped basel 63, a pair of toggle links 65 and 61 for actuating the frame 41 of the switch member II, a releasable carrier member 1| for restraining the toggle links 65 and 61 in an operative position, the operating member 23, and the springs 13 for connecting the operating member 23 to the knee of the toggle links. The U-shaped base 63 is fastened to the insulating base I of the circuit breaker proper by means cftwo screws 15 `which engage suitable threaded openings therein. The pivot pin 49 which provides a pivot point for the switch member I I extends through aligned Aopenings in the U-shaped base 63. Other holes are provided for the reception of the pivot pins 11-one on either side of the U--which provide pivot points for the bifurcated portion 19 of the operating member 23.

'I'he lower end (with respect to the base I) of the toggle link 61 is pivoted to the frame `41 of the switch member II by means of the pivot pin 8 I. The upper end of the toggle link 61 is pivoted to one end of the second toggle link 65 by means of the knee .pivot pin 83. One of the operating springs 13 engages each end of the knee pivot pin 83 and thus serves to operatively connect the knee of the toggle with the operating member 23. The carrier 1 I, which is pivoted about the pin 85, provides a releasable restraining means for holding .the toggle links 65 and 61 in an operative positionthe upper end (Fig. 2) of the toggle link 65 being pivotally fastened to the carrier 1I through the agency of the pin 81. 'Ihe movement of the carrier 1I .in a clockwise direction (Fig. 2) is limited by the projection 89 which extends inwardly from one side of the U-shaped base 63. The limits of movement of the biiurcated portion 19 of the operating member 23 are defined by the off-set projections 9| and 93 which extend upwardly from the U-shaped base 63.

As previously pointed out, the trip device 1 includes a magnetically responsive element 95 and a thermally responsive element 91. A latch 99 which is formed integral with the movabley armature I 0| of the magnetically responsive element 95 is provided for releasably engaging the end 98 of the carrier 1I.l The core member |03 of the magnetically responsive element 95 is held in place by two screws |05 which pass through suitable openings in the moulded member |01 extending upwardly (Fig. 2) from the base I. .The core |03 has a bifurcated portion |09 which provides a pivot point for the combined latch and armature |0I, the. pivot pin IIO being provided for engaging the armature |0I. The single-turn energizing coil 39 is aflixed to the base I by means of the screw III which also serves to electrically connect one end of the coil 39 with the conducting strip 4|; the other end of the coil 39 is conductively fastened to one end of the flexible shunt 31.

As shown particularly in Figs. 2 and 3, the armature I0| supports, at its upper end, tworelatively movable members II,3 and ||5 which are pivotally fastened to each other by means of the pivot pin II1. Themember I3 has an upwardly projecting portion I I9 and the member I I5 has an upwardly projecting portion I2I. A biasing spring |23 engages each of the portions II9 and `|2I and thereby yieldably connects the two members II3 and ||5.v 'I'he member ||5 has a portion |25 which extends outwardly from the pivot pin I|1 and is adapted to engage a stop |21 fastened to the member II3. 'Ihe engagement of this stop I 21 and the projection |25 dene the limit of relative movement of the members II3 and IIE in the direction toward which they are biased by the spring |23.

One end of the spring |23 is provided with a threaded portion |29 which engages the thumb nut I3| to provide a means for regulating the biasing force exerted by the spring |23 on the two members I|3 and II5. A rod member |33 extends upwardly from the threaded portion |29 of the spring |23 and provides a convenient indicating means forshowing the amount of stress to which the spring is subjected, a suitable opening and indicator scale |31 being vprovided in the cover I1.

'I'he thermally responsive element 91 includes a heater 43 of resistance material which is adapted to be connected in the electrical circuit through the agency of the screw fastenings |39. 'A bimetallic member I4I is thermally conductively joined to the heater 43 by means of two rivets |43. The unfastened end of the bimetallic member |4| has a cutout' portion |45 therein for engaging the movable p ortion |41 of the armature supporting latch |49. 'I'he bimetal heater unit is completely removable and thereby provides a convenient means for changing the rating of the trip device.

The armature latching means |49 comprises a Acrank-shaped frame member |5| having a cylindrical opening |50 (Fig. 3) therein which provides a support and a guide for the movable portion |41. The central portion |53 of the movable member |41 is enlarged and threaded.

This threaded enlargement provides a stop for limiting the movement of the movable member |41 toward the latched position. The upper end (Fig. 1) of the movable member |41 engages a guide opening |55 in an extension of the base and is supported thereby. A spring |51 is positioned between that portion of the base adjacent the guide opening |55 and the threaded nut |59 disposed on the enlarged threaded portion |53 -of the movable member |49. This spring |51 biases, the movable member |49 into the latched position. The lower end (Fig. 1) of the frame member |5| is supported by a bimetallic supporting member |6I; by making the supporting member |6| of bimetallic material it is possible to secure a thermally responsive element which is not affected by changes in temperature in the surrounding medium. The movement of the support member exactly compensates for the movement of the electrically heated bimetallic trip member due to changes in room temperature. The spring |51 andthe adjustable nut |59 provide a convenient means for controlling the time relay. between the occurrence of an overload and the tripping point of the thermally responsive element.

The end |63 of the movable member |41 extends through the cylindrical guide opening |50 in the frame member |5| and is adapted to releasably engage the end |65of the member ||5 which is supported on the movable armature. The member I|5 is biased to a position beneath the end portion |63 of the movable latch member |41 by the spring |61.

The operation of our invention may best be described in `conjunction with Figs. 1 and 2 which show the circuit breaker in the closed position. To open the breaker lmanually the operating member 23 is moved counter-clockwise (Fig. 2) about its pivot pins 11. This movement of the operating handle causes the overcenter springs 13 to exert a component of force on the knee pivot pin 83 tending to move that pin to the left (Fig. 2). And when this cornponent becomes suiiiciently large, the toggle links 65 and 51 collapse and thereby open the circuit breaker contacts with a snap action. The upper movement of the switch member it is limited by the engagement of the resilient conducting member 35 with the projection 89 on the frame 63. l

To close the circuit breaker manually the operating handle 23 is moved clockwise about its pivot point until the component of force exerted by the over-center springs 13 is sufficiently large to set the knee oi the toggle in motion toward the right (Fig. 2). The movement of the toggle during both the opening and the closing operation proceeds at a progressively increasing rate because the effective component of force increases as the knee .of the toggle moves toward the closed or opened position. Thus the contacts are perforce opened and closed with a snap action.

Upon the occurrence of a heavy overload or short circuit condition in the controlled circuit the attractive force exerted on the armature |0| by the energized core |03 becomes sufliciently large to move the armature against the force of the biasing spring |23, the member ||3 moving with respect to the member |5 by pivoting about the pivot pin ||1. 'I'his movement of the armature 0| causes the latch 99 to release the end 98 of the carrier member 1|. The'carrier 1| is biased about its pivot pin 85 in a clockwise direction (Fig. 2) by the over center springs 13, and begins to move in that direction as soon as it is released by the latch 99. The clockwise movement of the carrier 1| moves the upper pivot pin 81 to the right (Fig. 2) of the center line of the toggle; the over center springs 13 then cause the toggle to collapse and open the cireuitbreaker contacts in exactly the same manner as for manual operation.

Upon 'the occurrence of a moderate overload condition which does not energize the core |03 suiliciently to overcome the force of the biasing spring |23 and move the armature |0| to release the latch 99, the bimetallic member |4| begins` to fiex in a direction toward the threaded nut |59. If the overload continues for a sufficient length of time, the end |1| of the bimetaiiic member |4| engages the nut |59 and moves it together with the attached movable member |41 away from the frame member |5| against the biasing spring |51. After a short time, the end portion |63 of the member |41 disengages the end |65 of the movable member ||5. The biasing spring |23 is then rendered inoperative as a biasing means for the armature |0 I-the spring |61 is relatively weak and exerts but a small force-and the attractive force of the energized core |03 is suiiicient to move the armature and the two relatively movable members ||3 and ||5 supported thereon about the pivot pin ||0 thereby causing the latch 99 to disengage the carrier 1|. This disengagement of the latch 99 and the accompanying release of the carrier 1| permits the breaker contacts to open as described in the previous paragraph.

After the opening of the circuit breaker contacts by either the magnetically responsive trip element or the thermally responsive trip element 91, it is necessary to restore the breaker mechanism to. an operative position. To do this the operating handle 23 is moved counter-clockwise (Fig. 2) about its pivot 11; the projection |13 on the operating member 23 then engages the portion |15 of the carrier 1i, and continued movement of the operating member causes the carrier tomove clockwise about its pivot 85. When the operating member 23 has nearly reached the off position the portion 98 of the carrier 'li reengages the latch 99--the armature |0| being forced to move toward the core |03 in order to clear `the latch 99.

The trip device 1 is entirely automatically resettable. It the breaker has been opened by the magnetically responsive trip element 95, the armature ||l| is immediately moved back to the position shown in Fig. 2 by the biasing spring |23. If the breaker has been opened by the thermally responsive trip element 91, the armature |0i and the two relatively movable members H3 and H5 are restored to the position shown in Fig. 2 by the secondary biasing spring |61. It is important to note in this connection that the lower edge of the portion |65 of the member H5 never moves above the upper edge of the end |63 of the movable member |41. rIhus the member H5 will not be blocked against downward movement following the opening of the breaker contacts and the accompanying removal of the force holding the movable member lill out of engagement with the member H5.

The modification of our invention shown in Fig. 4 is essentially similar to the construction shown in Figs. 1, 2 and 3. The bimetallic element |71 corresponding to the element |4| has been modified to serve as a support for the movable member |19 which corresponds to the member |41, and a bimetallic compensating portion |8| has been substituted for the member ISI. It will be seen that this modified construction is somewhat simpler than the previously described structure but the functioning is substantially the same.

The modified thermally responsive element |83 which is shown in Figs. 5 and 6 is intended for use in place of the heater 43 and the attached bimetallic member |4l, the holes |85 being adapted to engage the screw fastenings |39. This modified form of bimetal is an improvcment over the two part structure previously described,

- exactly as described before.

in that the bimetallic memberitself is used as a heater. Both types of thermally responsive elements are removable and interchangeable and permit the ready adjustment of the trip d evice to any desired ratings. v

Should-'an attempt be made to close the breaker against an overload` or against ashort circuit condition the latch 99 will be released instantaneously or after a predetermined time delay, depending upon the magnitude of the overload',

The releasable carrier 1| will likewise be released and will cause the toggle mechanism to break so as to open the circuit independent of the position of the operating handle. Therefore, it is not possible to hold the breaker closed against a continued overload or against a short circuit condition.

It will thus be seen that we have disclosed an improved electro-responsive device which includes a'thermally responsive element, a magnetically responsive element, a spring for biasing the armature of the magnetically responsive element away from the attracting means, and a means for rendering the spring ineffective as a biasing means. In addition we have disclosed a means yfor compensating for changes in the temperature of the medium surrounding the thermally responsive element, and we have shown how. this improved mechanism can be applied to a quick-make and quick-break circuit interrupter which is. biased to the open position and is held in the closed position by a releasable carrier.

While in accordance with the patent statutes, we have given the foregoing details of a practical embodiment of our invention, it isto be understood that many of these are merely illustrative and that variations of their precise form will be desirable in some applications. We desire therefore that the language of the accompanying claims be accorded the broadest reasonable construction and that our invention be limited only by what is explicitly stated in the claims and by the prior art.

We claim as our invention:

1. In electrical apparatus, an armature, magnetic means for attracting said armature, means biasing said armature away from said attracting means, electro-responsive means operable to permit said attracting means to move said armature without overcoming more than a fraction of the force normally exerted by said biasing means, and means for preventing the movement of said armature in response to the attraction of said magnetic means from subjecting said electro-responsive means to any substantial mechanical stress.

2. In electricalapparatus, a'. movable armature, magnetic means for attracting said armature, means biasing said armature away from said attracting means, electro-responsive means, including a thermally responsive member, for causing a reduction in the biasing force normally exerted by said biasing'means upon` the occurrence of predetermined electrical conditions, and a thermally responsive compensating means for making the operation of said thermally responsive member independent of variations in the temperature of the surrounding medium.

3. In electrical apparatus, a movable armature, magnetic means for attracting said armature including a .core member of magnetic material and a coil for energizing said core member, spring means biasing said armature away from said attracting means, electro-responsive means, including a thermally responsive member, for causing a reduction in the biasing force normally exerted by said spring biasing means upon the occurrence of predetermined electrical conditions, and a, thermally responsive compensating means having an element of bimetallic thermally responsive material for making the op- 3 eration of said electro-responsive means independent of variations in the temperature of the surrounding medium.

4. In a trip device for a'circuit interrupter, a magnetically responsive element for actuating the tripping mechanism immediately upon the occurrence of predetermined heavy overload conditions in the controlled circuit, and a thermally responsive element for actuating the tripping mechanism after a time delay upon the occurrence of predetermined moderate overload condi- 2tions in the controlled circuit, said magnetically responsive element including a core member, an i energizing Winding therefor, and an armature 1 yieldably held away from said core member by means including two interconnected relatively movable members and a biasing spring which biases said members against relative movement in one direction, one end of one of said members being retained in position by releasable means actuable by said -thermaily responsive means to release said two interconnected members thereby permitting said armature to be attracted to said member without stressing said spring.

5. In a trip device for a circuit interrupter, a movable armature, magnetic means for attracting said armature, means for holding said armature away from said attracting means including two interconnected relatively movable members, a spring for opposing relative movement of said members, and a means which includes a thermally responsive element for releasably engaging said holding means, said thermally responsive element being operable to permit said armature to be attracted to said attracting means without stressing said spring upon the occurrence of predetermined electrical conditions, and said releasable engaging means including means for preventing the. movement of said armature in response to the attraction of said magnetic means from subjecting said electro-responsive means to any substantial mechanical stress.

6. In a circuitJ interrupter, a switch member movable to open and closed positions, a movable operating'member, a spring operatively connecting the operating member to the switch member, a releasable restraining means that is movable to an inoperative position for moving the switch member to open position, and a trip device for releasably engaging said restraining means, said trip device including a latch means for engaging Vsaid restraining means, a movable armature of magnetic material for actuating said'latch, magnetic means, including a core member of magnetic material and an energizing coil for said core, for attracting said armature to release said latch, adjustable spring means for supplying a force for biasing said armature away from said attracting means, and electro-responsive means including a member of bimetallic material for reducing the biasing force normally exerted by said adjustable spring biasing means upon the occurrence o f predetermined electrical conditions in the circuit controlled by said interrupter, said attracting means and said electro-responsive means being energized by the current flowing through said interrupter.

'7. In a circuit interrupter, a switch member movable to open and closed positions, a movable operating member, a spring operatively connecting the operating member to the switch member, a releas'able restraining means that is movable to an inoperative position for moving the switch member to open position, and a trip device for releasably engaging said restraining means, said trip'device including a latch means for engaging said restraining means, a movable armature for actuating said latch, magnetic means for attracting said armature, means biasing said armature away from said attracting means, electro-responsive means, including a thermally responsive member, for reducing the force normally exerted by said biasing means upon the occurrence of predetermined. electrical conditions to facilitate the operation of said trip device, and a thermally responsive compensating means for making the operation 'ci said thermally responsive member independent of variations in the temperature of the medium surrounding said interrupter.

8. In electrical apparatus, an electro-responsive means movable to actuate said apparatus, spring means normally biasing said electro-responsive means against movement to actuate said apparatus, a second electro-responsive means actuable to permit said rst electro-responsive means to move to actuate said apparatus without having to overcome the biasing force normally exerted by said spring biasing means tending to prevent such movement, and means disposed intermediatesaid two electro-responsive means for preventing said last mentioned electro-responsive means irom being subjected to any substantial mechanical stress during the operation of said rst mentioned electro-responsive means.

9. In electrical apparatus, an armature, magnetic means for attracting said armature, means which normally prevents said magnetic means from moving said armature except when the attracting force exerted by said magnetic means reaches a predetermined large magnitude, electro-responsive means, including a bimetallic element, operable upon the occurrence of predetermined conditions to permit said attracting means to move said armature without overcoming more than a fraction of the force ,normally exerted by bimetallic element, operable upon the occurrence oi predetermined conditions to permit said magnetic means toI attract said armature Without overcoming more than a fraction of the force normally exerted by said biasing means, and means for preventing the movement of said armature in response to the attraction of said magnetic means vfrom subjecting said bimetallic element to any substantial mechanical stress.

l1. In electrical apparatus, an armature, magnetic means for attracting said armature, means biasing said armature away from said attracting means, an electro-responsive means, including a bimetallic element, operable upon the occurrence of predetermined conditions to permit said magnetic means to attract said armature without overcoming more than a portion of the force normally exerted by said biasing means, and means for preventing the movement of said armature in response to the attraction of said magnetic means from subjecting said bimetallic element to any substantial mechanical stress, said means for preventing movement of said armature from stressing said bimetallic element comprising a slidably supported member positioned intermediate said bimetallic element and said armature and adapted to be moved in response to movement of said bimetallic element.

l2. In a circuit interrupter, separable contact means for opening and for closing the electrical circuit through said interrupter, actuating means for moving said contact means from one position to the other, and an electro-responsive tripping means operable upon the occurrence of predetermined condition to cause said actuating means to move said contact means to the open circuit position, said tripping means including a movable armature, a solenoid means for attracting said armature, means which normally prevents said armature from being attracted toward said solenoid means except upon the occurrence of a predetermined large overload, electro-responsive means operable upon the occurrence of predetermined small overloads whichpersist for a substantial period of time to allow said solenoid means to attract said armature without overcoming more than a small portion of the force which normally prevents said armature from being at-' tracted toward said'solenoid means, and means intermediate said electro-responsive means and said armature which permits said armature to be moved by said solenoid means without subjecting said electro-responsive means to any substantial mechanical stress.

armature, means which normally prevents said 75 armature from being attracted toward said solenoid except upon the occurrence of predetermined large current magnitude overloads, and a current-carrying electro-responsive means, including a bimetallic element and'a member disposed intermediate said bimetallic element and said armature, operable upon the occurrence of predetermined small current magnitude overloads, which persist for a substantial period of time, to allow said solenoid means to attract said armature without overcoming more than a portion of the force which normally prevents said armature from being attracted toward said solenoid means, said intermediate member allowing said armature to be moved independently of said bimetallic element.

14. In a circuit interrupter, separable contact means for opening and for closing the electrical circuit through said interrupter, actuating means for moving said contact means from one position to the other, and an electro-responsive tripping means'operable upon the occurrence of predetermined conditions to cause said actuating means to move said contact means to the open circuit position, said tripping means including an armature which is movable to actuate said tripping means, magnetic means for attracting said armature, means biasing said armature away from said attracting means, a bimetallic element operable upon the occurrence of predetermined conditions to permit said magnetic means to attract said armature without overcoming more than a portion of the force normally exerted by said biasing means, and means for preventing the movement of said armature in response to the attraction of said magnetic means from subjecting said bimetallic element to any substantial mechanical stress.

15. In electrical apparatus, an armature, magnetic means for attracting said armature, means biasing said armature away from said attracting means, electro-responsive means operable to permit said attracting means to move said armature without overcoming more than a fraction of the force normally exerted by said biasing means, and means disposed intermediate lsaid electroresponsive means and said armature forA preventing the movement of said armature in response to the attraction of said magnetic means from subjecting said electro-responsive means to any substantial mechanical stress.

16. In electrical apparatus, an armature, magnetic means for attracting said armature, means biasing said armature away from said attracting means, a latch movable from a normal latched position to an unlatched position to permit said attracting means to move said armature without overcoming more than a fraction of the force normally exerted by said'biasing means, and an electro-responsive means operable to move saidv latch from said latched to said unlatched position.

17. In electrical apparatus, an armature, magnetic means for attracting said armature, spring means biasing said armature away from said attracting means, electro-responsive means, which includes a movable member of bimetallic material, operable upon the occurrence of predetermined conditions to permit said attracting means to move said armature without overcoming more than a fraction of the force normally exerted by said spring biasing means, and means disposed intermediate said electro-responsive means and said armature for preventing the movement of said armature in response to the attraction of said magnetic means from subjecting said mv' able'bimetallic element to any substantial mechanical stress.

18. In electrical apparatus, an armature, magnetic means for attracting said armature, means biasing said armature away from said attracting means, electro-responsive means operable to permit said attracting means to move said armae ture without overcoming more than a fraction of the force normally exerted by said biasing means, and a thermally responsive compensating means for making the operation of said thermally responsive means independent of variations in the temperature of the surrounding medium.

19. In electrical apparatus, an armature, magnetic means for attracting said armature, means biasing said armature away from said attracting means, an electro-responsive means including a bimetallic element operable upon the occurrence of predetermined conditions to permit said magnetic means to attract said armature without overcoming more than a fraction of the force normally exerted by said biasing means, means for preventing the movement of said armature in response to the attraction of said magnetic means fromsubjecting said bimetallic element to any substantial mechanical stress, and thermally responsive compensating means for making the operation of said electro-responsive means independent of variations in temperature of the surrounding medium.

JOSEPH STOLZ. JOSEF HANKE. GOTTLOB LUX. RICHARD IDGSTDT.

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