US2571933A - Air circuit breaker - Google Patents

Description

Oct. 16, 1951 P. OLSSON ET AL 2,571,933

AIR CIRCUIT BREAKER Filed July 30, 1948 3 Sheets-Sheet 1 Fig. I.

WITNESSES: INVENTORS ATTORNEY 1951 P. OLSSON ET AL 2,571,933

AIR CIRCUIT BREAKER Filed July 30, 1948 5 Sheets-Sheet 2 Fig.2.

WITNESSES: INVENTORS Russell E.Frink BY and Paul Olsson. W, M {M ATTO RN EY 1951 P. OLSSON ET AL AIR CIRCUIT BREAKER 3 Sheets-Sheet 3 Filed July 50, 1948 Fig 5.

INVENTORS Russeil E. Frink 0nd Poul-0lsson.

WITNESSES:

ATTORNEY Patented Oct. 16, 1951 AIR CIRCUIT BREAKER Paul Olsson and Russell E. Frink, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application July 30, 1948, Serial N 0. 41,645

Claims. 1

This invention relates to circuit breakers and more particularly to contact structures of air circuit breakers.

Conventional air circuit breakers are usually provided with one or more auxiliary sets of contacts which are designed to by-pass a large portion of the current for a short interval of time during circuit interruptions for the purpose of minimizing damage to the main contacts due to burning or pitting. The sets of contacts usually separate in sequence during a circuit opening operation with the main contacts separating first, the auxiliary contacts second and the arcing contacts last to draw the are. In the usual contact arrangement wherein the moving contacts of the respective pairs of coacting main, auxiliary and arcing contacts are carried by a pivoted contact arm, the main and auxiliary contacts being closer to the pivot of the contact carrying arm,

are separated a lesser distance than the arcing contacts at the time of interruption. Under certain conditions a switching surge is produced of sufficient magnitude to cause the arc to restrike and since, at this time, the main contacts have separated a lesser distance than the arcing contacts, the tendency is for the arc to restrike across the main contacts.

Should a restrike of the arc occur across the main contacts, considerable burning and pitting of contacts may result. The interrupter also may not clear the circuit on such a restrike if the arc fails to transfer to the auxiliary and arcing contacts and continues to play outside the arc extinguishing structure.

It is an object of the invention to improve the circuit interrupting ability of circuit breakers of the foregoing type.

A more specific object of the invention is to provide a circu't breaker embodying an improved contact structure which obviates the above-mentioned electrical and mechanical difiiculties.

Another object of the invention is to provide a circuit breaker embodying an improved contact structure in which increased main contact separation is provided at the time of arc extinction.

The novel features that are considered characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to structure and operation, together with additional aspects and advantages thereof will be best understood from the following detailed descript'on of several embodiments thereof when read in conjunction with the accompanying drawings.

In said drawings:

Figure 1 is a side elevational View, partly in section, of a circuit breaker embodying the principles of the invention.

Fig. 2 is an enlarged side elevational view, partly in section, showing the improved contact structure in the closed contact position.

Fig. 3 is an enlarged elevational view, partly in section, showing the improved contact structure with the parts in the position they assume at the instance the arcing contacts separate.

Fig. 4 is an elevational view, partly in section, showing a modification of the improved contact structure.

Fig. 5 is a detail view of the latch mechanism.

Referring to Fig. l of the drawings, the circuit breaker is of the roll out type and is mounted on a truck comprising a pair of side plates ll (only one being shown) which are rigidly joined by cross members l3, l5, l1 and 19 to form a rigid frame for supporting the circuit breaker. The truck is mounted on wheels 20 to facilitate rolling the circuit breaker into and out of a cubicle in order to connect and disconnect the breaker in an electrical circuit.

The circuit breaker is of the multi-pole type (only one pole being shown) and comprises a plurality of sets of separable contact means indcated generally at 2|, a common operating mechanism 23, a trip device 25 and a closing means 21.

The contact means consists generally of stationary contact blocks 29 and 3| mounted respectively on upper and lower terminal members 33 and 35 which, in turn, are rigidly supported on a back plate 31. The back plate 3'! is rigidly supported by angular brackets 39 (only one being shown) rigidly secured to the side plates ll of the frame. A switch arm 4| pivotally mounted on a stud 43 on the lower terminal member 35 carries a bridging contact member 45 for cooperating with the stationary contacts 29 and 3| to open and close the circuit. The switch arm 4| also carries a mcvable arcing contact 41 which cooperates with a stat onary arcing contact 49 mounted on the upper terminal member 33 for drawing the are upon opening movement of the switch arm M.

The detailed construction of the improved con tact structure will be more fully described hereinafter.

The specific structures of the operating mechanism, trip mechanism and the closng means as shown, are fully disclosed in cocending application Serial No. 716, 04, filed December 13,

1946, now Patent No. 2,515,994, July 18, 1950, by Joseph D. Findley, Alvin W. Ogg and Fritz E. Florschutz and assigned to the assignee of the instant invention, for which reason only a brief description of these mechanisms is given herein.

There is a switch arm II for each pole of the breaker and each of the switch arms is operatively connected by means of an insulating operating rod 5I to the operating mechanism 23 for simultaneous operation. The upper ends of operating rods 5i are pivotally connected by means of pivot pins 53 to the corresponding switch arms II and the lower ends of the operating rods are connected by suitable means to an angular cross bar 59 which is common to all of the poles of the breaker. The cross bar 5-9 is pivotally connected at its ends by means of pivot pins 55 (only one being shown) to an operating lever Ill. The operating lever GI is pivotally supported on a shaft 61 mounted in the side members of a generally U-shaped frame 59 which is supported on the cross members I3, I5 and I1, the frame being shown partly broken away more clearly to show the operating mechanism.

In addition to the operating lever BI, the operating mechanism consists of a closing lever 15 also mounted on the shaft 5? and connected by means of a link BI to the movable armature 85 of the closing solenoid 21. The operating lever BI and the closing lever 15 are connected by means of an under set thrust transmitting toggle comprising toggle links 8'! and 89 pivotally connected together by means of a knee pivot pin SI. The toggle link 31 is pivotally connected to the operating lever BI and the toggle link 89 is pivotally connected to the closing lever 15.

The closing lever is held in closed position by means of a spring biased latch 95 pivoted on a rod 91 supported in the side members of the frame 69. The latch 95 is biased into engagement with a roller iIII mounted on the closing lever 15 and normally holds the latter in the position shown.

The toggle 81-89 is normally held in a slightly underset position by means of a latch member I53 rotatively mounted on the shaft 51 and connected by means of a link I115 to the knee of the toggle, the link I35 being connected to the latch I03 by means of a pivot pin I51 and to the to gle 8185 by the knee pin 9 I The latch member I03 is normally releasably restrained in latching position by the latching and tripping mechanism shown in Fig. 5. The latching and tripping mechanism comprises a trip member I09 pivoted on a pin III and carrying a latch roller H5 which is biased by means of a spring I23 into engagement with a shoulder on the latch member I 53 to restrain this member and the toggle 81-89 in thrust transmitting position.

The latch I69 is of the slip-01f type and a trigger latch I21 is provided to releasably restrain the latch member I09 and the latch roller H9 in latching position. The trigger latch I21 is pivoted on a pin I29 and is provided with a latching projection I3I which normally engages a latching projection I33 on the trip member I09. A spring I35 biases the latch member I21 to latching position.

The latch member I21 may be operated either manually or automatically by means'of a tripping electromagnet I31 (Fig. 1) to efiect release of the operating mechanism and opening of the circuit breaker. The electromagnet I31 consists 4 of a yoke I39 secured to a plate I4I mounted on the left-hand end of the U-shaped frame 69, an energizing coil I43, a movable armature (not shown) and a trip rod I45 attached to the movable armature.

The closing solenoid 21 comprises a fixed core member I41 and an energizing coil I49. The solenoid 21 is supported between the frame 59 and an end plate I5I secured thereto by means of bolts I53 and spacers I55.

Upon the occurrence of an abnormal circuit condition, such for instance as an overload current in the circuit controlled by the breaker, the tripping electromagnet I31 is energized by well known means such as a fault relay. Upon energization of the tripping magnet the movable armature (not shown) thrusts the trip rod I45 upwardly to engage and operate the trip member I21 to thereby effect release of the latch member I59 which, in turn, releases the latch member I03. Upon release of the latch I03, the toggle 81-83 .immediately collapses permitting clockwise rotation of the operating lever 5| to thereby cause opening of the breaker contact means.

The toggle 81-89 is resetto its underset position and the latch member I03 is reset and relatched automatically during the opening operation. This is effected by means of a pair of springs I69 (only one being shown) tensioned between the movable armature B5 and the closing lever 15 at a point near the shaft 51. During the collapse of the toggle 8189, a roller I15 carried on the toggle link 89 engages av projection I19 on the latch 95 and moves the latch downward to unlatching position to release the closing lever 15. Thereupon, the springs I69 draw the movable armature B5 of the closing solenoid toward the left, and acting through the link 8| rotates the closing lever 15 in a clockwise direction, thereby extending the toggle 8189 to its underset position. As the toggle is extended it acts through the link I05 to rotate the latch member I 53 clockwise to its latching position at which time the latching mechanism is reset and relatched to restrain the toggle 8189 in thrust transmitting position. Thereafter the breaker is closed by energization of the closing solenoid 21, from any suitable source. When energized. the solenoid 21 attracts its armature and, through the link SI, rotates the closing lever 15 in a counter-clockwise direction about the shaft 51. Since, at this time, the toggle 8189 is held in thrust transmitting condition, the movement of the closing lever 15 is transmitted through the toggle to actuate the operating lever 6| and the rods 5| to close the contact means ZI. As the closing lever reaches its closed position the latch re-engages the roller IElI to restrain the breaker mechanism in the closed circuit position.

Referring now to Fig. 2 of the drawings, the switch arm 4I comprises a casting having a generally channel-shaped recess I1I in which the main bridging contact 45 is disposed. The bridging contact 45 is pivotally mounted on a pin I13 supported in the side walls of the channel-shaped recess I1I. The bridging contact 45 is provided with a slightly elongated opening I15 through which the pin I13 extends. The bridging contact 45 is provided with contact elements I11 and I82 which are adapted to cooperate respectively with contact elements I8I and I83 on the contact blocks 29 and 3I. Contact pressure between the contacts I11-I8I and I82-I83 in the closed position of the switch arm 4| is provided by means of two sets of springs I85 and I31 disposed on opposite sides of the pivot pin I13 and compressed between the bottom ofthe recess Ill and the bridging contact 45. The springs I85 and I81 surround guide studs I89 and |9| respectively mounted on the bottom wall of the recess I1I. It may be noted here that the springs I81 are somewhat stronger than springs I85.

A projection I93 which is integral with the switch arm 4| extends singularly-upwardly therefrom toward the stationary arcing contact. A contact block I95 is securely mounted by means of bolts I91 and I99 to the forward side of the projection. The contact block I95 carries the movable arcing contact 41 and also a movable intermediate contact 2IiI. The movable intermediate contact 2|i| cooperates with a stationary intermediate contact 203, which, together with the stationary arcing contact '49, is mounted on a contact platform 205. The contact platform 205 is mounted on the upper stationary contact block 29 for limited movement relative thereto by means of a pair of spaced plates 29! disposed one on each side of the contact block 29 and rigidly secured thereto by bolts 299.

Theplatform 205 is provided with laterally extending studs 2|| (only one being shown) disposed adjacent the lower end thereof and extending into elongated slots 2I3' in the plates 201 for supporting the platform for limited movement. Ears 2|5 (only one being shown) extend laterally from the sides of the platform 205 into openings 2|1 in-the plates 201.

The platform 205 is biased outwardly away from the contact block 29 by means of a coil spring 2|9 (Fig. 3) disposed adjacent the upper end of the platform and a coil spring 22I lo cated near the lower end of the platform 295. The. springs 2I9 and '22I are compressed between the contact block 29 and the contact platform 205 and the spring 2| 9 is provided with a guide stud 223 secured to the contact block 29.

The contact platform 205 is electrically connected to the upper contact block 29 by means of a flexible shunt conductor 225 more clearly shown in Fig. 3, and a flexible shunt conductor 221 electrically connects the switch arm 4| to the lower contact block 3|. In the closed circuit position of the breaker the circuit extends from the upper terminal 33, contact block 29, contacts I8I-|11, the bridging contact member 45, contacts I82-I83, and the contact block 3| to the lower terminal 35. Another circuit extends through the arcing contacts and intermediate contacts as follows: Upper contact block 29, flexible shunt 225 (Fig. 3), contact platform 295, arcing contacts 4941 and the intermediate contacts 203-20|, contact block I95, switch arm 4|, and the flexible shunt 221 to the lower contact block 3|.

When the trip device I31 (Fig. l) is energized to effect collapse of the toggle 81-89, the operating lever 6| starts to rotate clockwise permitting downward movement of the rod and movement of the switch arm 4| about its pivot 43 in opening direction. As the switch arm 4| moves in opening direction the contacts I11--- I82 under the pressure of springs I85-I81 remain in contact respectively with the stationary contacts |8|-|83 until the pin I13 engages the left end wall of the slot I16. At thistime the bridging contact member 45 will start to move in opening direction with the switch arm, but since the springs I81 are stronger than the springs I85, the contact I82 will be maintained in engagement with the stationary contact I83 and the bridging-contact member 45 will be rotated counterclockwise about its pivot I13 until the upper end thereof engages the end of the spring guide stud I89. The counterclockwise movement of the bridging contact relative to the switch arm will effect higher speed separation of the main contacts |11|8| than would be attained if the springs I85-I81 were of equal strength. Thereafter, the bridging member 45 will move to the open position with the switch arm 4| without further movement relative thereto.

The contact platform 235, under the influence of the springs 2|9 and 22| (Fig. 3) will follow the moving arcing and bridging contacts 41-2Il| respectively until the studs 2|I engage the left end walls of the slots 2|3 in the plates 201. Thereafter the platform 205 will turn counterclockwise about the studs 2|! until the ears 2| 5 engage the left walls of the openings 2|1. The intermediate contacts 20I2Ii3 separate upon engagement of the studs 2|I with the ends of the slots 2| 3 and the arcing contacts 41-49 will separate when the ears 215 are arrested by the walls of the openings 2|1.

Figure 3 illustrates the improved contact structure at the point, during an opening operation, when the arcing contacts 41-49 are about to separate. Continued movement of the switch arm 4| in opening direction will effect separation of the arcing contacts 4149 and drawing of the arc therebetween. The are is drawn into an arc extinguishing structure indicated generally at 229 (Fig. 1) where it is quickly extinguished. The are extinguishing structure 229 may be of any suitable type, but is preferably of the type fully disclosed in Patent No. 2,442,- 199, issued May 25, 1948, to R. C. Dickinson and R. Frink and assigned to the assignee of the present invention.

In the type of circuit breaker illustrated the arc will usually be extinguished when the arcing contacts 4149 have separated approximately two inches. Under certain conditions when the circuit constants give alow damping factor, and the current is of sufficient magnitude, a switching surge may be produced which will cause the arc to restrike across a gap of this dimension. If, at this time, the main contacts I11|8I which are outside the arc chute 229 are closer together than the arcing contacts, the arc will restrike across the main contacts |11|8| and will not g back into the arc chute and damage will result.

According to the present invention, this is obviated by giving the main contacts I11-|8| a greater separation than the arcing contacts 41-49 at the time of arc extinction so that any restriking of the arc will occur between the arcing contacts within the arc extinguisher structure 229.

As the switch arm 4| continues its opening travel from the Figure 3 position, the bridging member 45 rotates counterclockwise about the pin I13 relative to the switch arm 4! due to the fact that the springs I81 are stronger than the springs I85. The counterclockwise rotation of the bridging member 45 is arrested by the upper end thereof striking the spring guide or stop I89. In this position, and at the time the arc is extinguished, the contacts I11-|8I are separated a greater distance than the arcing contacts 4149, hence, any restrikin of the arc will occur across the lesser gap between the arcing contacts within the arc extinguisher. By the time the restriking arc has been extinguished the contacts will have separated far enough to prevent a, second restriking of the arc. Continued movement of the switch arm 4| in opening direction after the bridging member 45 engages the stop I89 causes separation of the contacts |82|83. Thereafter the bridgin member 45 moves to the fully open position with the switch arm 5| without further rotative movement relative thereto. The fully open position of the movable contact structur is indicated generally at |95 in Figure 2.

When the closing solenoid 21 (Fig. 1) is energized it acts through the operating mechanism in the previously described manner to thrust the rods 5| upwardly and rotates the switch arms 4| clockwise from the fully open position to close the contacts. The sequence of engagement of the contacts is the reverse of the opening thereof. The main contacts |82|83 engaging first, then, in order, the arcing contacts 41-49, the intermediate contacts 20|203 and finally the main contacts |11l 8 5.

According to the modification of the invention illustrated in Figure l, a bridging member 23l, similar to the bridging member 45 (Fig. 3) but omitting the elongated slot, is pivoted on the pin I13 for rotative movement only relative to the switch arm 4|. The springs I85 (Fig. 3) are omitted and the bridging member 23| is biased in a counterclockwise direction about the pin H3 by the springs N31. The bridging member 23| is provided with contact members I11 and I82 which are the same as those in the Fig. 3 modification. The contact member I82 cooperates with the stationary contact I33 on the lower contact block 3| and the contact member I11 cooperates with a stationary contact member 233 -mounted for limited movement on an upper contact block 235, simi ar to the upper contact block 29 (Fig. 3), mounted on the upper terminal 33. The contact member 233 is supported by means of a spring clip 231 secured to the contact block 235 by means of a screw 239, and is biased into engagement with the contact member I11 of the bridging member 23! by means of a spring 24| compressed between a projection on the contact block 235 and the contact member 233. The reciprocal sliding movement of the contact member 233 is limited by a. projection 243 thereon engaging a slot 245 in the contact block 235. The construction and operation of intermediate and arcing contact of the Fig. 4 modification are the same as those shown in Fig. 3, and, like parts thereof have been given the same reference characters.

The sequence of contact separation is the same for the Fig. 4 mod fication of the invention as that of the modification shown in Fig. 3, that is, the contacts 233 separate first followed by separation of the intermediate contacts 2|l|2!l3, the arcing contact 41-49 and finally the contacts |82-|83 separate. When the switch arm 4| starts its counterclockwise rotation in opening direction, the springs I81 maintain the contact I82 in engagement with the contact I83 and cause the bridging member 23| to rotate counterclockwise about the pin I13, the contact member 233 following under the influence of the spring 24| until its movement is arrested by the projection 243 engaging the end wall of the slot 245. As the switch arm 4| continues its counterclockwise travel the bridging member 23| continues its counterclockwise rotation relative thereto until the upper end thereof strikes the stop member I89. Thereafter the bridgingmember 23| will move with the switch arm 4| without further movement relative thereto. The main contacts |1'|233 at the time the arc is extinguished will have separated a greater distance than the arcing contacts 41-49, hence, any restriking of the arc, due to a switching or other surge, will occur across the arcing contacts within the arc extinguisher.

The switch arm 4| (Fig. 4) is operated by the mechanism shown in Figure 1 in the previously described manner.

While the invention has been disclosed in accordance with the provisions of the patent statutes, it is to be understood that various changes in the structural details and arrangement of parts thereof may be made without departing from some of the essential features of the invention.

We claim as our invention:

1. A circuit interrupter comprising a pivotally mounted switch arm, means for actuating said switch arm to open and closed positions, a main bridging contact member pivotally mounted on said switch arm for limited movement relative thereto, two spaced stationary contacts engageable by said bridging contact members on actuation of said switch arm to closed position, separable arcing contact means actuated by said switch arm for drawing an arm, and spring means disposed between said bridging contact member and said switch arm operable upon openin movement of said switch arm to rotate said main bridging contact member in opening direction relative to said switch arm whereby at least one end of the main bridging contact member is separated from its corresponding stationary contact a greater distance than the arcing contacts when the arcing contact means are separated a distance most favorable for arc extinction.

2. A circuit interrupter comprising a pivotally mounted switch arm, means for actuating said switch arm between an open and a closed position, a mean bridging contact member, a pivot mounting said bridging contact member on said switch arm for limited movement relative thereto, spaced main stationary contacts engageable by said bridging contact upon actuation of said switch arm to the closed position, separable arcing contact means operable by said switch arm upon opening movement of said switch arm for drawing an arc, spaced spring means disposed on opposite sides of said pivot and compressed between said switch arm and said bridging contact member, one of said spring means being stronger than the other and operative upon opening movement of said switch arm to rotate said main bridging contact member in opening direction relative to said switch arm whereby said bridging contact member and one of said main contacts are separated a greater distance than said arcing contacts at the time the arcing contacts have separated a distance most favorable for arc extinction.

3. A circuit interrupter comprising a pivotally mounted switch arm, means for actuating said switch arm between an open and a closed position, a main bridging contact member pivotally mounted on said switch arm for limited movement relative thereto, two spaced stationary contacts engageable by said bridging contact member on actuation of said switch arm to the closed position, one of said stationary contacts being mounted for limited movement, separable arcing contact means actuated by said switch arm duringmovement therepf in opening direction to draw an arc, means for extinguishingsaid are following separation of said arcing contacts, and spring means compressed between said switch arm and said bridging contact member operable during an opening movement to rotate said main bridging memberin opening direction relative to said switch arm whereby said bridging'contact member and said one stationary contact are separated a greater distance than said arcing contacts at the time the arcing contact means is in a position most favorable for arc extinction.

4. A circuit interrupter comprising spaced stationary contacts, a main switch arm pivoted on a fixed pivot and movable between open and closed position, a main bridging contact member pivoted on said switch arm and engageable with said stationary contacts when said switch arm is moved to closed position, arcin contacts adiacent one end of said bridging contact member for establishing an arc during movement of said switch arm to open position, and resilient means disposed between said main switch arm and said bridging contact for moving said bridging member in opening direction relative to said switch arm during opening movement of said switch arm to thereby provide a greater contact separation between said one end of said main bridging contact member and its correspondin stationary contact than between said arcing contacts at a predetermined time during the opening movement of said switch arm when said arcing contacts separate to a position most favorable for are extinction.

5. A circuit interrupter comprising spaced stationary contacts, a switch member movable between open and closed positions, a main bridging contact rotatably mounted on said switch member and engageable with said stationary contacts when said switch member is moved to the closed position, and spring means between said switch member and said main bridging contact for rotating said main bridging contact in opening direction relative to said switch member whereby the separation between one end of said bridging contact and its corresponding stationary contact is increased at a predetermined time during opening movement of said switch memher.

6. A circuit interrupter comprising spaced stationary contacts, a switch member movable between open and closed positions, a main bridging contact rotatably mounted on said switch member and engageable with said stationary contacts when said switch member is moved to the closed position, and spring means compressed between said switch member and said main bridging contact for rotating said main bridging contact in opening direction relative to said switch member whereby the separation between one end of said bridging contact and its corresponding stationary contact is increased at a predetermined time during opening movement of said switch member, and stop means for limiting the opening movement of said bridging contact relative to said switch member.

7. A circuit interrupter comprising spaced stationary contacts, a switch member movable between open and closed positions, a main bridging contact pivotally mounted on said switch member for limited rotative movement relative thereto, said bridging contact engaging said stationary contacts in the closed position of said switch member, and a plurality of springs disposed between said switch member and said bridging contact, certain of said springs being stronger than certain others of said springs to cause rotation of said main bridging contact in opening direction relative to said switch member whereby the separation between one end of said bridging contact and its corresponding stationary contact is increased at a predetermined time during opening movement of said switch member.

8. A circuit interrupter comprising spaced stationary contacts, a switch member movable between open and closed positions, a main bridging contact pivotally mounted on said switch member for limited movement relative thereto, said bridging contact engaging said stationary contacts in the closed position of the switch member, and a plurality of springs disposed between said switch member and said bridging contact to provide contact pressure in the closed position, certain of said springs being stronger than certain others of said springs to effect rotation of said main bridging contact in opening direction relative to said switch member whereby the extent of separation between one end of said bridging contact and its corresponding stationary contact is increased at a predetermined time during opening movement of said switch member.

9. A circuit interrupter comprising a pivotally mounted switch arm, means for actuating said switch arm between open and closed positions, a main bridging contact member, a pivot mounting said bridging contact member on said switch arm for limited movement relative thereto, spaced stationary contacts engageable by said bridging contact upon actuation of said switch arm to closed position, separable arcing contact means adjacent one end of said bridging contact member operable by said switch arm upon opening movement thereof for drawing an arc, spaced spring means disposed on opposite sides of said pivot and compressed between said switch arm and said bridging contact member to provide contact pressure in the closed position of said switch arm, one of said spring means being stronger than the other and operative upon opening movement of said switch arm to rotate said main bridging contact member in opening direction relative to said switch arm whereby said one end of said bridging contact member is separated a greater distance from the associated stationary contact than said arcing contacts at the time the arcing contacts have separated a distance most favorable to arc extinction.

10. A circuit interrupter comprising a pivotally mounted switch arm, means for actuating said switch arm to open and closed positions, a main bridging contact member pivotally mounted on said switch arm for limited movement relative thereto, spaced stationary contacts cooperating with said main bridging contact member in the closed position of said switch arm, a stationary arcing contact, a movable arcing contact rigidly mounted on said switch arm and cooperating with said stationary arcing contact during an opening operation of said switch arm to draw an arc, means for extinguishing said arc, and spring means disposed between said main bridging contact member and said switch arm operable upon opening movement of said switch arm to rotate said main bridging contact member in opening direction relative to said switch arm whereby one end of the main bridging contact member is separated from its corresponding stationary con- 11 12 tact a greater distance than the arcing contacts 4 when the arcing contacts have separated a dis- UNITED STATES PATENTS tance most favorable for are extinction. Number Name Dat 633,772 Wright et a1 Sept. 26, 1899 PAUL OLSSON- 6 687,065 Read Nov. 19, 1901 RUSSELL FRINK- 1,935,428 Atwood Nov. 14, 1933 2,227,160 Seaman Dec. 31, 1940 REFERENCES CITED 2,241,810 Dickinson May 13, 1941 The following references are of record in the 2,280,616 Baskerville Apr. 21, 1942 file of this patent: I0 7

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