US3534305A - Current limiting circuit breaker - Google Patents

Description

Oct. 13, 1970 Filed Nov. 21, 1968 A. sTRoBL-:L 3,534,305

CURRENT LIMITING CIRCUIT BREAKER 6 Sheets-Sheet 2 Ir-E. 1; /d

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CURRENT LIMITING CIRCUIT BREAKER Filed Nov. 21, 1968 6 Sheets-Sheet 3 n Oct. 13, 1970 A. STROBEL.

CURRENT LIMITING CIRCUIT BREAKER Filed NOV. 2l, 1968 6 Sheets-Sheet 4.

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2%/ Il" y \\\\\\\\\\\\\\\\\\\\J /z &\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ "ya Il; 25 C Patented Oct. 13, y1970 k3,534,305 CURRENT LIMITING CIRCUIT'BREAKER Albert Strobel, Cherry Hill, NJ., assigner to I-T-E Imperial Corporation, Philadelphia, Pa., a corporation of Delaware l Filed Nov. 21, 1968, Ser. No. 777,704

Int. Cl. H01h 77/10 U.S. Cl. 335-16 10 Claims ABSTRACT F THE DISCLOSURE My invention relates to circuit breaker devices of the type generally referred to as a current limiting circuit breaker.

Current limiting circuit breakers of the type which the instant invention generally relates are described in Edmunds U.S. Pat. No. 3,012,118; Bodenschatz U.S. Pat. Nos. 3,127,488, and 3,164,700, all assigned to the l-T-E Circuit Breaker Company. Such circuit breakers include one or more pairs of cooperating contracts, which upon the occurrence of a relatively minor overload or short circuit current condition are moved to their disengaged condition by a trip unit mechanism. The trip unit mechanism typically includes a thermally or magnetically actuated fault responsive means, which upon the occurrence of a predetermined fault current condition serves to release a latch. The release of the latch thereupon permits movement of the spring biased circuit breaker operating mechanism to the automatic trip condition. The advent of increased available source magnitudes has subjected circuit breakers to the possible occurrence of very severe short circuit conditions. In order to safely interrupt the load current under such severe short circuit conditions without requiring a circuit breaker of unduly high interruption rating, the current limiting circuit breaker includes provision to eiect rapid separation of the contacts and a substantial limitation of the current flow through `the circuit breaker and load. The rapid separation of contacts introduces a current limiting arc. The arc should preferably be introduced during the first rise of the extreme fault current, so as to prevent its reaching the very high magnitude that would be obtained in the absenceA of the introduction of such current limiting arc.

In aforementioned Edmunds U.S. Pat. No. 3,012,118 and Bodenschatz U.S. Pat. No. 3,164,700, the rapid separation of the contacts upon the occurrence of the severe short circuit condition is obtained by an auxiliary operating mechanism in conjunction with the movable contact. This auxiliary operating mechanism, which may utilize the blowoif forces. generated at the contacts, serves to provide the rapid initial separation .of the contacts with the follow-up operation of the conventional trip serving to release the latch for the main circuit breaker operating mechanism. Bodenschatz U.S. Pat. No. 3,127,488 shows an alternative arrangement, which utilizes a semi-stationary contact in conjunction with the movable contact. Under conditions of moderate overload or short circuit conditions in which the operation of the conventional type thermal or magnetically actuated trip serves to operate the circuit breaker, the movable contact will translate While the semi-stationary contact remains stationary. However, in the event of a severe short circuit condition, the semi-stationary contact will rapidly move away from the stationary contact, typically under the-inliuence of the blowoif forces. After the semi-stationary contact moves away from the stationary contact, follow-up operation of the conventional trip is required to release the main latch of the circuit breaker mechanism and move the movable contact. Reiss et al. U.S. Pat. No. 3,005,073 shows another arrangement, whereby after a blowoff force induced high speed separation of the contacts, it is necessary for the conventional electromagnetic or thermal trip to function in order to actuate the circuit breaker operating mechanism.

In all the aforementioned current limiting circuit breakers, the initial high-speed separation of the contacts must be followed by the operation of the conventional circuit breaker mechanism and the release of its latch, with such latch release serving to permit follow-up movement of the circuit breaker operating mechanism to the automatic trip position.

It should be recognized that inasmuch as current limiting circuit interruption techniques are most apt to be used in conjunction with multi-phase circuit breaker devices, the separation of the contacts in the non-faulted phase will be delayed until the main circuit breaker mechanism has operated.

In accordance with my invention, the rapid separation of the contacts upon the occurrence of a severe shortcircuit condition operates upon the main latch means. Thus, I provide rapid release of the main latch and operation of the circuit breaker operating mechanism, without requiring the attendant follow-up operation of the conventional circuit breaker thermal or mangetically actuated trip.

In the various illustrative embodiments of my invention, the latch means of the circuit breaker operating mechanism includes a spring biased collapsible latch member. Upon the attainment of a predetermined severe short circuit, the blowoif induced forces at the contacts are suicient to oppose the contact pressure forces and the biasing force of the collapsible latch. The latch is thereby released causing movement of the circuit breaker operating mechanism to the automatic trip position.

The collapsible latch (which is referred to as an auxiliary latch release) is provided by a member of the main operating mechanism latch train, comprising a pair of` relatively movable portions. These portions are normally maintained in a certain fixed relationship by an auxiliary biasing means, such that they function as a single integral member. However, in the event of a predetermined severe fault current condition, the extremely high blowoif forces which rapidly separate the contacts and provide the current limiting action, also provide a movement of these relatively movable portions, with the follow-up disengagement of the main operating mechanism latch.

In accordance with one form of my invention, the relatively movable portions of the latch are provided by a two-part cradle mechanism, which includes a biasing spring in association therewith. The force of the biasing spring is operatively related to the contact pressure forces and the blowoff generated forces, such that upon the occurrence of a severe fault current condition of a predetermined nature, the blowoif forces will be sufficient to overcome the contact pressure forces and auxiliary latch spring force to collapse the two-part cradle.

lIn accordance with other alternative forms of my invention, the collapsible multi-portion latch member is provided in conjunction with the latch members intermediate the cradle and the trip unit latch. lIn selecting a particular structure, I have, however, found it desirable to ma-ke the member of the latch train collapsible, which is nearest to the contacts on which blowotf occurs, since the speed of opening after blowoi depends on the restrictive forces along the latch train. Auxiliary latches are usually spring loaded in order to keep the primary latch load to a minimum and in order to return the latches to the basic starting position after the movement of the circuit breaker operating mechanism to the automatic trip position. The latch return spring forces may be `quite small, but due to their position in the overall lever system they may also balance the tripping forces which are available from the operating springs after release of the primary latch. Therefore, the latch return springs, as well as the mass of the latches in the lever system may prevent the optimum rapid operation of the opening cycle. Hence, the latch member closest to the contacts is most suited to be responsive to blowoff forces. Accordingly, for most rapid operation of a circuit breaker constructed in accordance with my invention, the collapsible latch should preferably be provided in conjunction with the cradle.

It is, therefore, a principal object of my invention to provide an improved circuit breaker of the current limiting variety.

A further object of my invention is to provide a current limiting circuit breaker, wherein the rapid disengagement of the contacts, in response to a severe fault current condition, serves to release the main circuit breaker latch without requiring follow-up operation of the conventional circuit breaker trip unit.

Another object of my invention is to provide a current limiting circuit breaker which includes a rst fault responsive means to release the main operating mechanism latch upon the occurrence of relatively moderate overload currents of a first predetermined nature, and a second fault responsive operating means for rapidly separating the contacts and releasing the main operating mechanism latch upon the occurrence of severe fault currents of a second predetermined nature.

Ari additional object of my invention is to provide a current limiting circuit breaker, whereupon the occurrence of a severe fault current condition is accompanied by blowotf forces which rapidly move the contacts apart, and transmit the blowoif forces to a release means for the circuit breaker main operating mechanism latch.

Still a further object of my invention is to provide such a current limiting circuit breaker in accordance with the previous objects, wherein the latch means of the circuit breaker operating mechanism includes an auxiliary spring loaded collapsible latch member in which the blowoff generated forces in the event of a severe short circuit condition serve to collapse the auxiliary latch means, and thereby effect disengaement of the main circuit breaker operating mechanism.

These, as well as other objects of my invention, will become apparent upon a consideration of the following descriptions and drawings in which:

FIG. 1 shows a iirst embodiment of a circuit breaker, constructed in accordance with my invention, in the contact engaged condition.

FIGS. 2 and 3 sequentially show the operation of the circuit breaker of FIG. 1, upon the occurrence of a moderate overload or fault condition, causing operation of the conventional type trip unit mechanism.

|FIG. 4 shows initial movement of the circuit breaker mechanism upon the occurrence of a severe short circuit condition, with the contacts being blown apart to obtain current limiting action, and the auxiliary latch release commencing its operation.

FIGS. 5 and 6 sequentially follow FIG. 4 and show the release of the circuit breaker operating mechanism by the auxiliary latch.

FIG. 7 shows another embodiment, generally similar to lFIG. 1, but with a modified biasing arrangement for the collapsible latch.

FIGS. 8 and 9 show other modifications of my invention, wherein the collapsible portion of the latch is provided in conjunction with the intermediate auxiliary latch. In FIG. 8, a compression spring is shown for biasing the relatively displaceable collapsible portions and in FIG. 9 a tension spring is shown.

FIGS. 10 and 11 show still other embodiments of my invention, wherein the collapsible member of the latch is provided in conjunction with the latch linkage intermediate the tripper bar and auxiliary latch. In FIG. .10 a compression spring is shown for biasing the operative portions of the collapsible linkage member, and in FIG. 1l a modification is shown utilizing a tension spring.

Referring initially to the embodiments of FIGS.-16, circuit breaker 10, which is shown in generally functional form, may basically be of the same type shown in my United States Patent Application Ser. No. 690,878, tiled Dec. 15, 1967 and entitled Trip Unit Latch Positioning Means for Constant Latch Bite. Circuit breaker 10` `includes a molded housing, including a main body portion 12 and cover 14 secured thereto by suitable fastening means (not shown). The circuit breaker may typically 1nclude a separate and replaceable trip unit assembly 20, which may be of the same general type shown in U. S. Pat. No. 3,319,195 issued May 9, 1967 and entitled Circuit Breaker Trip Unit Assembly. The circuit breaker includes a stationary contact 22 connected to line terminal 24. The cooperating movable contact 26 is mounted on a contact arm 28, which in turn is pivotally mounted at 30 to contact carrier 32. Contact pressure forces, urging the cooperating contact pair 22, 26 in intimate electrical engagement are typically obtained by the spring force of the contact operating mechanism 50 and the contact pressure means 34, which may be of the type shown in U.S. Pat. No. 3,268,702 entitled Loosely Mounted Independent Coupling Means for Adjusting Contact Pressure on Contact Arm Position. The contact carrier 32 is pivotally mounted to an operating mechanism frame (deleted for the purposes of simplicity) at pivot 36. The circuit breaker 10 may be a multi-phase device, with the various phases interconnected by tie bar 38, with reference being made to the above discussed patents and applicants for a further description of the coordinated multi-phase operation.

The movable contact arm 28 is electrically connected to the trip unit assembly 20 by means of a conductor `40. In actual practice, separate contact carrier straps are provided in conjunction with each phase `of the main circuit breaker mechanism and the replaceable trip unit assembly, with ilexible braid-type conductor 40 merely being shown for simplification. The trip unit assembly is shown including a magnetically actuated overload responsive means including a stationary magnet 42 and armature 44, with braid 40 serving as the energization for the magnet assembly. The opposite end of the braid 40 is connected at 42 to the load terminal 45. The trip unit assembly is appropriately calibrated, such that upon the occurrence of a predetermined overload condition, the armature 44 will be drawn down to the stationary magnet 42 (as shown in FIG. 2), moving down armature stem 46 and rotating the tripper bar 48 counterclockwise so as to defeat the engagement between the tripper bar and the latch member 49.

The coventional tripping mechanism 20 may also include the well-known type of thermally responsive bimetal element (not shown), which similarly serves to rotate the tripper bar 48 counterclockwise under conditions of a moderate sustained overload condition.

The circuit breaker operating mechanism 50 includes an overcenter toggle means having lower toggle link 52, upper toggle link 54 and spring biasing means 56. Spring biasing means 56 is connected at one end to the toggle knee 58 and at its Opposite end to the manual operating handle 1S, in the generally well known manner. A latchable cradle is provided, which in accordance with the present embodiment comprises two portions, 58 and 60` Relatively movable portions 58 and 60 are pivoted about the upper toggle bearing 62, and normally maintained in the position of FIG. l by compression spring 64. That is, compression spring 64 serves t-o move portions 58 and 60- apart, as limited by stop 59, so that during the normal operation of the circuit breaker (that operation other than the occurrence of a severe fault condition causing blowoff force induced separation of the contacts and release of the main cradle latch), cradle portions 58 and 60 operate as a single integral member. Cradle portion 58 includes a latch tip 65, which is in latching engagement at 63 with auxiliary latch 66, which in turn is in engagement at 67, with latch tip 68 of the trip unit latching link 49.

In accordance with my invention, the force of cornpression spring 64 is operatively related to the contact blow-off forces accompanying a severe fault condition of a predetermined nature, such that the blowoif forces are sufficient to overcome the contact pressure forces and collapse cradle portions 58, 60 against the restraining force of spring 64. The minimum force requirement of spring 64 is given by the relationship F106 b COS a-l-Fzc cos B dtan 'y F64 (min.)

where the angles and distances are indicated in FIG. l, and

For practical applications, the load of spring 64 will be considerably higher than the minimum value given by (l) above, in order that the circuit breaker will be nonsensitive to normal shock and vibration forces, and the current limiting feature of the breaker will respond only at and above a predetermined crossover value in the overcurrent tripping characteristic.

`Reference is now made to FIG. 2, which illustrates the operation of the circuit breaker upon the occurrence of a moderate overload or fault current condition above the normal load carrying capacity of the circuit breaker, but within its conventional trip rating. Armature 44 is shown drawn to the stationary magnet 42, thereby moving down the armature stem 4-6, rotating the tripper bar 48, and releasing lthe tip 51 of the latch lever 49. This in turn serves to permit rotation of auxiliary latch 66, releasing latch tip 65 of the cradle portion 58. The operating mechanism 50` will now move the toggle linkage 52, 54 overcenter,v under the force of spring 56, such that the contacts 22, 26 are in the process of opening. The contact operating mechanism sequentially continues to the position shown in FIG. 3, wherein the cradle 58, 60 has completed its movement to the cradle stop 61. It should be noted that the latch train comprising elements '48, 49* and 66 have now returned to their original positions (as shown in FIG. l) and the tripping cycle has been completed. The circuit breaker may now be manually reset in the well known manner. In following the sequence from FIGS. 1`-3, it should be noted that the two-part cradle lSiS-60 has moved as an integral member during this tripping operation of the circuit breaker following the actuation of the trip unit assembly 20. Similarly, the cradle portions 'S8-60 will be maintained as an integral member should the circuit breaker be moved to its manual off position (not shown).

Reference is now made to FIGS. `4-16, lwhich sequentially indicate the operation of the circuit breaker when subjected to a severe fault condition substantially in excess of that discussed in conjunction with the operation of FIGS. 2, and 3. The magnitude of the severe fault condition necessary to initiate the sequence shown in FIGS. 4-6 depends upon the spring force 64 in accordance with Equation 1 above, such that the blowofi forces accompanying such a fault condition will be sufcient to overcome the counteracting forces of the circuit breaker operating mechanism. In FIG. 4 such a severe fault current condition has been initially experienced and the contacts 2.2, 26 begin to part under the blowoff induced forces. This parting of the contacts will be extremely rapid, so as to provide a current limiting arc therebetween, serving to reduce the let-through current flowing through the circuit breaker and the load from the maximum available current that would exist in the system in the absence of such a current limiting arc. The toggle lin=k ages 52, 5'4 have transmitted this sudden increase in force to the cradle 58-60, and in opposition to the compression spring '64, thereby moving cradle portion 58 about pivot 62 in the direction shown by arrow 70 (FIG. 4). This serves to release the latchable cradle tip 65 from the auxiliary latching member 66. It should be noted that the auxiliary latch 66, latch lever 49 and tripper bar 48 vhave remained stationary during this release of the latchable cradle tip 165. Once the cradle tip 65 has been released from latching engagement with 63, the cradle rotates as an integral member clockwise about pivot 69 under the influence of the main operating spring 56, while at the same time the contact arm 2'8 is moving further apart from stationary contact 22, to the condition shown in FIG. 5. In FIG. 6, the cradle has completed its movement to the cradle stop 61, the circuit breaker toggle mechanism has moved overcenter, and the tripping cycle is now completed. It should be recognized at this point that the condition of the circuit breaker of FIG. :6 corresponds to that shown in FIG. 3.

It should, therefore, be appreciated that upon the occurrence of the severe fault condition, the blowoff forces generated at the contacts operate through the circuit breaker operating mechanism 50 to release the latched I engagement of the cradle tip 65, and therefore permit movement of the circuit breaker operating mechanism to its tripped open position, without necessitating the followup actuation of the circuit breaker trip 20, as typically required in the prior art current limiting circuit breakers.

Reference is now made to FIG. 7, which shows an alternative form of my invention, utilizing a tension spring 64' in conjunction with relatively movable cradle portions 58', 60', their limit of relativel motion being dened by stop 59. The operation of this embodiment is generally similar to that shown for the previous embodiment, =with the similar components being indicated by corresponding numerical designations.

Reference is now made to FIG. 8, |which shows another form of my invention in which the collapsible portion of the latch means is provided by forming the auxiliary latch, (previous member 166), of twoy operative portions =8(l, l82 which are normally maintained in the position as shown by a compression spring 84. The cradle member y86 of this embodiment is novi formed of a single element (as compared to portions 58, 60` and '58', '60 of FIGS. l-7), inasmuch as the auxiliary release of the latch is now provided by the collapsible members 82. This embodiment operates in a manner generally conforming to that above discussed. Namely, lwhen the circuit breaker is moved from the contact engaged condition of FIG. 8 to the automatic trip, by operation of the conventional tripping means 20, the elements 80, `82 'will remain as shown, moving as a single integral element to release the latch tip y65 of the cradle. However, in the event of a severe short circuit condition of a predetermined nature, the generated blowotf forces at the contacts 22, 26 are transmitted through the toggle linkage and cradle, against the force of spring 84, to collapse the portion 80, 8-2, allowing the release of latch tip 65 from under the latchable portion 83 o-f the auxiliary latch. This is followed by collapse of the main toggle linkage 5-2, 54 and movement of the cradle 86, under the iniiuence of the main biasing spring 56, to the contact disengaged condition.

FIG. 9 shows still another modification of my invention, generally similar to that shown in FIG. V8, wherein the collapsible portion of the togg-le linkage is provided by a pair of relatively displaceable portions 90, 92 of the auxiliary latch member. However, in this embodiment, these operati-ve portions are biased by a tension spring 94, in place of the compression spring shown in FIG. 8. Otherwise, the construction and operation of the embodiment shown in FIG. 9 is identical to that above discussed in conjunction rwith lPIG. 8.

FIGS. 10 and 1l show still a further form of a current limiting circuit breaker constructed in accordance with my invention, wherein the collapsible portion of the latch train is provided by a spring biased collapsible auxiliary linkage arrangement in place of the single element 49 shown in the previous embodiments. That is, in both of the embodiments shown in FIGS. l and 11, the cradle is provided by a single integral element 86 and the auxiliary latch is provided by a single integral element 66. In FIG. 10, the collapsible latch linkage is provided by a pair of operative portions 95, 96, normally biased as shown about pivot 97 by a compression spring 98. In the embodiment of FIG. a tension spring 99 is used in conjunction with a similar operative pair of link portions 101, 103, pivotable about pivot 104. As in the previous embodiments, these relatively displaceable link portions operate as integral members under the force of their biasing springs except upon the occurrence of an excessive fault condition of a predetermined nature. In the event of such an occurrence, the blowoff forces generated at the contact faces are transmitted through the operating mechanism, and portions of the latch train to the collapsible assembly, such that the force transmitted thereto is suicient to overcome the retention force of the biasing spring 98 or 99, thereby serving to release auxiliary latch member 66. The release of the auxiliary latch member is followed up by the release of the cradle 86 and the movement of the operating mechanism under the influence of the main biasing spring 56 to the contact disengaged condition.

It is, therefore, seen that the various embodiments of my invention illustrate the concept of utilizing the blowoff forces at the contact pair to release the latch train which maintains the main circuit breaker operating mechanism. Such release of the latch train is accomplished without the need to actuate the relatively slower moving conventional trip unit, and thereby serves to permit rapid movement of the circuit breaker to the Contact disengaged condition. The latch train associated Iwith the circuit breaker operating mechanism includes a unique collapsible portion normally maintained in a predetermined fixed relationship by a biasing means of a predetermined calibrated force. Upon the occurrence of a severe short circuit condition, the blowoff forces generated at the contact pair are transmitted to the collapsible portion of the latch train and the forces are suicient to overcome this latch-biasing force, to thereby collapse the latch, defeating the latched engagement of the circuit breaker cradle, and permitting overcenter movement of the circuit breaker toggle mechamsm.

Although there has been described a preferred embodiment of this novel invention, many variations and modifications will now be apparent to those skilled in the art. Therefore, this invention is to be limited, not by the specific disclosure herein, but only by the appending claims.

The embodiments of the invention in which an exclusive privilege or property is claimed are defined as follows.

1. A circuit breaker comprising at least one pair of cooperating contacts;

an operating mechanism for moving said contacts between an engaged and disengaged condition;

said operating mechanism including contact pressure means for urging the contacts of said contact pair towards each other in the contact engaged condition and providing a predetermined contact pressure force therebetween;

said operating means including a latch means, having a latchable means and a latching means, normally in latched engagement against a biasing force when said contacts are in said engaged condition;

said latchable means normally maintained by said latching means in a rst position;

said latchable means, when unlatched movable towards a second position, under the influence of said biasing force to maintain contact disengagement;

a rst fault responsive operating means responsive to -overload currents of a first predetermined nature for automatically defeating said latched engagement, such that said latchable means moves to said second position and thereby maintains said contact pair in said disengaged condition;

a second fault responsive operating means responsive only to fault currents of a second predetermined nature substantially in excess of said first predetermined nature, for automatically defeating said latched engagement and moving said latchable means to said second position and thereby maintaining said contact pair in said disengaged condition;

said second fault responsive operating means conlstructed to respond to its operative fault current condition and defeat'said latched engagement in a substantially more rapid manner than required for the operation of said first fault responsive means;

the rapid operation of said second fault responsive operating means and movement of the contacts to their disengaged condition imparting current limiting characteristics to said circuit breaker;

said latch means including a retention means of a predetermined force for maintaining a certain fixed relationship between the operative latch members;

said -rst fault responsive operating means moving said latching means to defeat said latched engagement without altering said certain fixed relationship;

said second fault responsive operating means providing a release force in opposition to said retention means, said release force, when it reaches a predetermined magnitude, overcoming the force of said retention means to alter said certain fixed relationship in a manner causing defeat of said latched engagement.

2. In a circuit breaker as set forth in claim 1:

said latch means including a member having at least two relatively movable operative portions;

said retention means urging said operative portions to a first position, whereby said member operatively functions as a single integral member, under conditions of contact engagement and the movement of said contacts other than by said second fault responsive operating means;

said second fault responsive operating means generating a latch release force in opposition to said retention force for causing one of said operative portions to move relative to the other operative portion in a manner serving to defeat said,latched engagement.

3. In a circuit breaker as set forth in claim 1:

said second fault responsive operating means utilizing the blowofr forces at the contact pair to provide said latch release force;

said predetermined contact pressure force operatively related to a specific magnitude of the blowoif forces accompanying fault current magnitudes of said second predetermined nature, such that the blowoff forces will be insuicient to overcome said contact pressure force and latch retention means until the fault current magnitude reaches said second predetermined nature.

4. In a circuit breaker as set forth in claim 2:

said second fault responsive operating means utilizing the blowoff forces at the contact pair to provide said latch release force;

said predetermined contact pressure force operatively related to a specific magnitude of the blowoff forces accompanying fault current magnitudes of said sec- 9 Vond predetermined' nature, such that the blowoff forces will be insufiicient to overcome said contact pressure force 'and latch retention means until the fault current magnitude reaches said second predetermined nature.

l ,l 5. In a circuit breaker as set forth in claim 1:

vsaid second fault responsive operating means causing one of said operative portions to move relative to the other operative portion in a manner serving to defeat said latched engagement;

said second fault responsive operating means utilizing the blowoff forcesvat the contact pair to provide said latch release force;

u said predetermined contact pressure force operatively related to a specific magnitude ofthe blowoff forces accompanying fault current magnitudes of said second predetermined nature, suchv that the blowoff forces will be insufiicientl to overcome `said contact pressure force and latch retention means until the fault current magnitude reaches said second predetermined nature.

6. In a circuit breaker as set forth in claim 1:

said operating mechanism including a cradle means having first and second operative portions;

said first operative portion including a latchable cradle tip, and said latching means including a latching member in engagement with said latchable tip;

said first fault responsive means initiating movement of said latching member to defeat the latched engagement with said cradle tip;

said second fault responsive means initiating relative movement of said first cradle operative portion with respect to said second cradle operative portion;

said relative movement causing movement of said cradle tip out of latching engagement with said latching member to defeat said latched engagement.

7. In a circuit breaker as set forth in claim 6:

said second fault responsive operating means utilizing the blowoff forces at the contact pair to provide said latch release force;

said predetermined contact pressure force operatively related to a specific magnitude of the blowoff forces accompanying fault current magnitudes of said second predetermined nature, such that the blowoff forces will be insufficient to overcome said contact pressure force and latch retention means until the fault current magnitude reaches said second predetermined nature.

8. A circuit breaker comprising at least one pair of cooperating contacts;

an operating mechanism for moving said contact pair between an engaged and disengaged condition;

said operating mechanism including a latchable cradle interconnected between said contact pair, and biasing means tending to move said contact pair to the disengaged condition;

a latchable means operatively connected to said cradle and normally in latched engagement with a latching member during contact engagement;

said latchable means, when disengaged from said latching member, allowing movement of said cradle under the influence of said biasing means to a contact disengaged condition;

a first fault responsive operating means responsive to overload currents of a first predetermined nature for automatically moving said latching member for defeating said latched engagement and permit movement of the cradle to the contact disengaged condition; s

a second fault responsive operating means responsive only to fault currents of a second predetermined `nature substantially in excess of said first predetermined nature, for automatically moving said latchable means for defeating said latched engagement and permit movement of the cradle to the disengaged condition;

said second fault responsive operating means constructed to respond to its operative fault current condition and defeat said latched engagement in a substantially more rapid manner `than required for the operation of said first fault responsive means;

said latchable means including a member having at least two relatively movable operative portions;

` said biasing means urging said operative portions towards first relative locations, whereby said member normally functions as a single integral member, under conditions of contact engagement and movement of said contacts other than by said second fault responsive operating means;

said second fault responsive operating means causing one of said operative portions to move relative to the other portion into a release position serving to move said latchable means out of latched engagement with said latching member;

said latchable means including a retention means of a predetermined force for maintaining a certain fixed relationship between said two Operative portions;

said first fault responsive operating means moving said latching member to defeat said latched engagement without altering said certain fixed relationship;

said second fault responsive operating means providing a release force in opposition to the force of said retention means, said release force, when it reaches a predetermined magnitude, overcoming said retention means to permit relative movement of said operative portions to said release position;

said second fault responsive operating means utilizing the blowoff forces at the contact pair to provide said latch release force;

said contacts being urged together by a predetermined contact pressure force operatively related to a specific magnitude of blowoff forces at the contact pair to provide said latch release force;

said contacts being urged together by a predetermined contact pressure force operatively related to a specific magnitude of blowoff forces accompanying fault current magnitudes of said second predetermined nature, such that the blowoff forces will be insufficient to overcome said latch retention means until the fault current magnitude reaches said second predetermined nature.

9. A circuit breaker comprising at least one pair of cooperating contacts;

an operating mechanism for moving said contact pair between an engaged and disengaged condition;

means associated with said contact pair for generating a predetermined contact pressure force urging said contacts together in intimate electrical engagement while in the contact engaged condition;

said operating mechanism including a latch means having a latchable member and a latching means for maintaining said latchable member in latched engagement against a first biasing force;

the defeat of said latched engagement moving said contact pair to their disengaged condition under the influence of said first biasing means;

a first fault responsive operating means responsive only to fault conditions of a first predetermined nature for automatically releasing said latched engagement and thereby permitting movement of said latchable member to a contact disenaged condition under the influence of said first biasing means;

said latch means including an auxiliary release means;

said auxiliary release means including a second biasing means for maintaining a certain xed relationship of the individual elements forming said auxiliary latch means;

a second fault responsive means responsive only to fault conditions of a second predetermined nature substantially in excess of said rst predetermined nature, for modifying the relationship of said auxiliary latch means from said certain fixed relationship, said modied relationship serving to release said latched en gagement and permit movement of said latchable member to its Contact disengaged condition under the influence of said biasing means;

said second fault responsive means utilizing the blowoli forces at the contact pair;

said predetermined contact pressure force and second bias means operatively related to a specific magnitude of the blowoi forces accompanying fault conditions .of said second predetermined nature, such that the blowof forces will be insuicient to overcome the contact pressure force and second bias force until the fault magnitude reaches said second predetermined nature;

said second fault responsive operating means constructed to respond to its operative fault current condition and defeat said latched engagement in a substantially more rapid manner than required for the operation of said rst fault responsive means;

the rapid operation of said second fault responsive operating means and movement of the contacts to their disengaged condition imparting current limiting characteristics to said circuit breaker.

10. In a circuit breaker as set forth in claim 9:

said auxiliary release means including a latch member having at least two relatively movable operative portions;

said second bias means urging said operative portions to a irst position, whereby said member operatively functions as a single integral member, under conditions of contact engagement and the movement of said contacts other than by said second fault responsive operating means;

said second fault responsive operating means generating a latch release force for causing one of said operative portions to move relative to the other operative portion in a manner serving to defeat said latched engagement.

References Cited UNITED STATES PATENTS 25 BERNARD A. GILHEANY, Primary Examiner H. BROOME, Assistant `Examiner

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