US2491959A

US2491959A - Automatic trip circuit breaker

Info

Publication number: US2491959A
Application number: US642037A
Authority: US
Inventors: Lloyd W Dyer
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1946-01-18
Filing date: 1946-01-18
Publication date: 1949-12-20
Anticipated expiration: 1966-12-20

Description

Dec. 20, 1949 w. DYER AUTOMATIC TRIP CIRCUIT BREAKER 2 Sheets-Sheet 1 Filed Jari. `18, 194e wm R.. ||l Q. @L n w1 v w @n n \Q\ Q nm. In QW mw 2.. frz mh. l MIN.

HHH

jVlTNESSES:

lIMVENTQR L Zag/d Wyer:

ATTORNEY 2 Sheets-Sheet 2 Dec. 20, 1949 w, DYER AUTOMATIC TRIP CIRCUIT BREAKER Filed Jan. 18, 1946 Patented Dec. 20, 1949 AUTOMATIC TRIP CIRCUIT BREAKER Lloyd W. Dyer, Pittsburgh, Pa., assgnor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania 4 Application January 18, 1946, Serial No. 642,037

(Cl. G-88) 11 Claims.

This invention relates to circuit breakers, and more particularly to circuit breakers of the type which are tripped instantaneously on overload currents of predetermined magnitude and after a time delay on lesser overloads.

One object of the invention is to provide a circuit breaker with an improved sealed-in trip device having adjusting means for varying the magnetic tripping point of the breaker and calibrating means for determining the range of adjustment.

Another object of the invention is to provide a circuit breaker with an improved trip device disposed in a sealed casing and having adjusting means accessible externally of said casing for varying the magnetic tripping point of the breaker, and Calibrating means settable before said casing is sealed to determine the range of adjustment .of said adjusting means.

Another object of the invention is to provide a circuit breaker with an improved trip device comprising an electromagnet and an armature operable to trip the breaker, a releasable trip member operable to trip the breaker, a single spring for biasing the armature against operation and for operating said releasable trip member, adjustable means for varying the magnetic tripping point of the breaker, and means for calibrating the range of adjustment, all of said parts being enclosed in a sealed casing and said adjustable means being accessible through said casing.

Another object of the invention is to provide a circuit breaker with an improved trip device according to the preceding paragraphs, wherein the entire trip device is removable and replaceable as a unit without affecting the tripping point of the device.

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 objects and advantages thereof, will be best understood from the following detail description with the embodiment thereof when read in conjunction with the accompanying drawings.

In said drawings:

Fig. 1 is a front elevational view of a circuit breaker embodying the principles of the invention, the cover being partly broken away to more clearly show some of the parts.

Fig. 2 is a front elevational view of a part of the circuit breaker shown in Fig. 1 with a part of the breaker cover and a part of the trip device cover broken away to more clearly show some of the parts of the trip device.

Fig. 3 is a detail view of the trip device adjusting cam; and

Fig. 4 is a vertical sectional view of the circuit breaker taken substantially along line IV-IV of Fig. l, the breaker being shown in closed circuit position.

Referring to Figures 1 and 4 of the drawings, the circuit breaker includes a base I I of moulded insulating material on which the several elements of the breaker mechanism are mounted. A cover I2 also of moulded insulating material is removably secured to the base and cooperates there- With to form an enclosing housing for the breaker mechanism.

The circuit breaker is of the multipole type. each pole comprising a pair of terminals I3 and I5, one disposed at each end of the breaker, a stationary contact I'I, an arc extinguisher I9, and a movable contact 2I. A common operating mechanism indicated generally at 22 is provided for simultaneously actuating the three movable contacts to open or to closed position, and a trip device indicated generally at 23 serves to eect automatic opening of the three sets of contacts in response to predetermined overload conditions in the circuit through any pole of the breaker.

Each of the movable contacts 2| is mounted on a contact arm 25 pivotally supported intermediate its ends on the free end of a pair of switch arms 2l by means of a pivot pin 26. The arms 21 for all of the poles are mounted on a rectangular tie bar 29 of insulating material, and each pair of the arms 2'I is joined by a rectangular sleeve 3I surrounding the tie bar and secured Ilthereto so that said tie bar and all of the switch arms 21 rotate as a unit, the tie bar being pivotally supported in the side walls of the breaker. The tie bar 29 extends transversely across the three poles of the breaker.

A exible shunt conductor 30 connects the contact arm 25 with an angular terminal 32 secured by a screw 33 to one end of a conductor 35. The screw 33 also serves to secure the conductor 35 to the base II. The other end of the conductor 35 is formed into an energizing winding 36 of a trip magnet (to be described later) and is connected by a flexible conductor 31 to a point near the free end of a thermally responsive bimetal element 39. The other end of the bimetal 39 is rigidly secured to the incr end of a conductor 4I, the terminal I5 being secured to the outer end of said conductor. Each of the stationary contacts I'I is mounted on the inner end of a conductor 43 having its outer end connected to the terminal |3. y

The electrical circuit for each of the poles 1s essentially the same and extends from the terminal |3 through the conductor 43, stationary contact |1, the movable contact 2|, the contact arm 25, the flexible shunt conductor 30, the terminal 32, the conductor 35, winding 36, the iiexible shunt conductor 31, the bimetal clement 39 and the conductor 4| to the terminal |5 Each of the three poles of the breaker is-contained in a separate compartment formed by barriers 42 (Figs. 1, 2, and 4) moulded integral with the base and extending to the trip device. These barriers 42 are in alignment with similar barriers 45, (Fig. 4) moulded integral with the cover |2 so that when the cover is in place the barriers 42 and 45 serve to insulate each compartment from the adjacent compartments and prevent flashovers.

The tie bar 29, which extends across all three poles of the breaker, is provided with annular iianges 48 disposed in recesses in the sides of the barriers 42 to prevent flashover through the openings in the barriers 42 provided to accommodate the tie bar.

The operating mechanism for the breaker 11- lustrated generally at 22 (Fig. 4) is disposed in the compartment housing the center pole and is supported on a pair of frames 44 secured to the center part of the base il. The operating mechanism includes a forked operating lever 41, a toggle comprising links 49 and 5|, a pair of overcenter springs 53, and a releasable cradle 55 controlled by the trip device 23. The cradle 55 is pivotally supported on the frame 44 by a pivot pin 56. The inner ends of the forked operating lever 41 are rounded and are pivotally seated in recesses in the frame 44 as indicated at 51. An insulating shield 59 for covering an opening 6| in the cover I2 is mounted on the outer end of the operating lever 41 and a Inanual operating handle 63 integral with said shield projects through the opening 6| to permit manual operation of the breaker mechanism.

The ltoggle links 49 and 5| are pivotally con- Ynected together by a knee pin 65. The link is pivotally connected by a pivot pin 61 to the pair of switch arms 21 for the center pole, and

the end of the link 49 is bifurcated and pivotally Overcenter springs 11, operatively associated` with the contact arms 25, surround contact arm operating members 19 and are compressed between angular brackets BI secured to the pivoted ends of the arms 21 and shoulders 83 on the members 19. The springs 11 hold the ends of the members 19 in engagement with recesses in the inner ends of the contact arms 25. The members 19 and the contact arms form toggles for operating the contact arms 25.

As previously mentioned, the trip device 23 includes a bimetal element 39 for each of the poles of the breaker, said bimetal elements be ing connected in series in the circuit of their respective poles. The trip device also includes a series overload magnet 86 for each pole, and a'trip bar 81 common to all of the poles. The trip bar 81 is of one piece moulded insulating material and is pivotally supported by means of pins 89 moulded in axial alignment in the ends of the trip bar and which rotatably support said bar in bearings 9| (Fig. 1) in the side walls of the circuit breaker housing.

The trip bar 81 extends across the three poles of the breaker through openings provided therefor in the barriers 42 and 45. The trip bar 81 is provided with a latch member secured to projections 92 moulded integral with the trip bar 81 and normally engages the latch end of the cradle 55, thereby restraining the cradle in operative position. In order to more completely insulate the compartments, the trip bar 81 is provided with a plurality of flanges 88 moulded integral with said trip bar and disposed one on each side of each of the barriers 42 and 45. These flanges are positioned close to the bar.- riers and effectively prevent flashover through the opening provided in the barriers for the trip bar 81.

The trip device 23 is of the renewable type and is enclosed in a sealed housing of moulded insulating material comprising a base and a cover 91 which are suitably secured together and sealed after the trip mechanism is assembled therein and adjusted. The housing 95-91 is divided into three compartments for separately enclosing the individual trip device for each of the three poles of the breaker. The compartments are formed by partition walls 99 moulded integral with the base 95 and matching partition walls ||l| moulded integral with the cover 91.

The trip devices for the terminal poles are the same for which reason only the one illustrated, which is for the center pole, will be described. The trip device comprises a trip lever 93 pivotally supported on a pin |03 mounted in the partition walls 99 of the base 91 of the trip device housing. 'I'he trip lever 93 is provided with a latch end |05 which is biased by means of a pair of springs |01 (Fig. 2) into latching engagement with a latch member |09 mounted on the free end of the bmetal element 39. The springs |01 are disposed one on each side of the trip lever 93 and are compressed between projections on the trip member 93 and a bracket 3 mounted on an armature I5 which forms a part of the eiectromagnet 86.

The armature ||5 is pivoted on a pin H1 supported in spaced projections on the upper end of an L-shaped magnet yoke ||9. The magnet yoke ||9 is secured by means of a screw |2| to the base 95 of the trip device housing and is provided with a core member |23 which extends into the winding 36 of the tripping electromagnet 86.

An adjustable stop screw |25 (Fig. 4) is provided in a formed portion |21 of the armature to limit the clockwise movement of the armature ||5. This stop is adjusted before the assembled trip unit is installed in the breaker. y

Means are provided for varying within a predetermined range the minimum overload current required to trip the breaker without removing either the cover of the breaker or the sealed trip device cover. This means comprises an eccentric cam' member |29 (Figs. 3 and 4) provided with spaced notches 3| in its cam surface. The cam |29 is mounted on a pin |33 and is supported by a shoulder on the pin resting upon a U-shaped bracket |35, the two legs of which are secured by means of screws |31 (only one being shown) to the partition walls 99 of the base 95. The reduced lower end of the pin |33 extends through an opening in the bracket |35 to provide a pivot for the cam |29. One end of an angular adjusting lever |39 pivoted on the pin |03 is provided with a small, conical projection |4| (Fig. 3) which is adapted to engage the notches |3| in the cam |29 to retain the cam in its adjusted position. The other end of the lever |39 bears against a projection |43 on the bracket ||3 which supports one end of the spring |01. A spring |44 coiled about the pin |03 having one end hooked over the lever |39 and the other end hooked over the trip lever 93 biases the adjusting lever |39 into engagement with the adjusting cam |29.

The spring |01 is, therefore, common to bot-h the trip lever 93 and the armature ||5 serving to bias the trip lever 93 against the latch |09, and, at the same time, biases the armature |5 against the adjusting lever |39. It will be obvious that rotating the cam |29 will operate the lever |39 and vary the air gap l 5|23 thereby varying the minimum current value required to trip the breaker.

The cam |29 is rotated by means of an adjusting screw |45 which has an opening therein for engaging a serrate hub |41 integral with the cam |29. The adjusting screw |45 extends through an opening in the cover 91 of the trip device and partially through a. corresponding opening in the cover |2 of the circuit breaker, and is accessible for adjusting the tripping point of the trip magnet 86 without removal of either the breaker cover or the trip device cover. An annular shoulder |49 on the screw |45 prevents removal of the screw and changing of the relative position of the screw |45 and cam |29 without removal of both of the covers.

Before the cover 91 of the trip device is assembled and sealed in place` the adjusting screw |45 is removed and the cam |29 is rotated to adjust the armature for the desired tripping current values, that is, the minimum current value at which it is desired that the tripping magnet will function to instantaneously trip the breaker. The adjusting screw |45 is then replaced on the hub |41 of the cam |29 in the proper location corresponding to the low calibration mark on the cover 91. The cover 91 is then assembled and sealed on the base 95 after which the main break er cover I2 is installed.

It will be noted that neither the armature ||5 nor the trip member 93 have any mechanical connection with the trip bar 81. Thus the trip unit may be removed and another unit installed in the breaker without disturbing the position of the trip bar and without affecting the tripping point of the breaker as would be the case if there was a critical latch point between the trip device and r the releasable operating mechanism.

When the bimetal element 39 for any one of the poles is heated a predetermined amount in response to an overload current, it flexes in a direction to move the free end thereof toward the left (Fig. 4) and after a time delay unlatches the trip lever 93, whereupon the spring |01 rocks the trip lever sharply clockwise. During this movement, the end of the lever 93 which is disposed above a projection |5| moulded integral with the trip bar, strikes said projection and rocks the trip bar 81 counterclockwise causing the latch 90 to release the cradle 55. This permits the overcenter springs 53 to rotate the cradle clockwise about its pivot 55 to effect opening of the contacts. At the start of the operation of the cradle 55 a projection 68 thereon engages a stud 66 on of the toggle link 49 to move to the right of the line of action of the overcenter springs 53 which Athen act to complete the collapse of the toggle. This rocks the arms 21 counterclockwise carrying the contact arms 25 counterclockwise therewith to open the circuit of the breaker. This operation of the mechanism also causes the handle 63 to be moved to an indicating position intermediate the open and closed positions.

In the closed circuit position (Fig. 4) the line of action of the overcenter spring 11 is above the pivot 26 of the contact arm 25 and biases the contact arm in a clockwise direction about its pivot providing pressure for the contacts. `When the arms 21 are rotated counterclockwise, as above described, a vtail |53 on the toggle member 19 strikes a projection |55 on the base causing the toggle 19-25 to move overcenter. This rocks the contact arm 25 counterclockwise about its pivot 26l which opens the contacts a greater distance than would be possible if the movable contacts were mounted directly on the arms 21.

The trip device is also operated electromagnetically by means of the previously described electromagnet 86 (Fig. 4) to instantaneously trip the breaker upon the occurrence of a heavy overload, for instance, 1000% or more of rated current, or on short circuits in the circuit of any pole of the breaker.

Upon the occurrence of such an overload current, or a short circuit, the armature ||5 is attracted with sufficient force to engage an arm |51 thereof with a projection |59 on the trip bar 81 and operate the trip bar to release the cradle 55 and eiect opening of the contacts in the man ner previously described.

Following each automatic operation of the breaker due to operation of the trip device 23 in response to an overload current, it is necessary to reset the mechanism to operative condition before the contacts can be reclosed. This is effected by rotating the manual lever 41 from its tripped position to its extreme counterclockwise or open position. This movement of the lever 41 is transmitted to the cradle 55 by a hooked projection |63 engaging a portion of the cradle. As the cradle 55 is rotated in a counterclockwise direction, the latch end thereof passes the latch member 90 and engages a projection |6| molded integral with the trip bar 81, rocking the trip bar clockwise slightly past its normal position to relatch the cradle 55. During the clockwise resetting movement of the trip bar 81, the projections |5| thereon engage the ends of the trip levers 93 and rock said levers counterclockwise to relatch them with the latches |09 on the bimetal elements 39.

. After the cradle 55 has been relatched, the contacts can be closed manually by moving the lever 41 clockwise to its closed circuit position. During this movement, the line of action of the overcenter springs 53 crosses to the right of the pivot 69 of the toggle link 49 causing the toggle 49-5I to move overcenter and rock the arms 21 quickly clockwise to close the contacts. As the switch arms 21 approach the closed position, the arm 25 strikes a projection |65 on the base at a point to the left of the pivot 26 (Fig. 4) causing clock-wise rotation of the contact arm 25 about its pivot 26 moving the spring 11 overcenter above the pivot 26. This causes the contacts to close with a snap action and applies' pressure to the closed contacts after they are closed.

The contacts are opened manually by moving the lever 41 in a counterclockwise direction about its pivot to the open position. This movement carries the line of action of the springs 53 to the left of the pivot 69 of the toggle link 49 and the force exerted by said springs now biases the toggle 49-5! away from its overcenter position. Before the lever 41 reaches its extreme counterclockwise or open position, this force becomes sufficient to cause the toggle to quickly collapse, and since the toggle link I is pivotally connected to the switch arms 21 for the central pole, these arms are rotated counterclockwise causing opening of the contacts for all of the poles. The contacts are closed manually in the manner previously described.

The circuit breaker is trip free, i. e., operative automatically in response to predetermined abnormal circuit conditions in any one of the poles thereof irrespective of the position of the operating lever 41.

The arc extinguishers I9, there being one provided for each pole of the breaker, are of the spaced plate type and comprise, generally, a stack of slotted plates of magnetic material positioned adjacent the paths of movement of the movable contacts 2|. The presence of these plates causes the arc resulting from the rupture of the circuit to be drawn in the direction toward the ends of the slots where the arc is broken up into a plurality of short arcs, and these short arcs are quickly cooled and extinguished.

The structural details of the circuit breaker operating mechanism and the operation thereof are fully described in Patent No. 2,318,298, issued May 4, 1943, to H. D. Dorfman and Gerald J.

greese and assigned to the assignee of this inven- Having described the preferred embodiment of the invention in accordance with the patent statutes, it is to be understood that various changes and modifications may be made in the structural details disclosed without departing from some of the essential features of this invention. f

I claim as my invention:

1. In a circuit breaker comprising relatively movable contacts and actuating means for opening said contacts, a trip bar releasably restraining said actuating mechanism, electromagnetic means responsive to overload currents above a predetermined value, an armature operable by said electromagnetic means to engage and operate said trip bar to cause release of said actuating means, a releasable trip member operable to engage and operate said trip bar independently of said armature to cause release of said actuating means, spring means compressed between said releasable trip member and said armature for actuating said trip member upon release thereof and for biasing said armature to unattracted position, and a bimetal element operable when heated a predetermined amount to release said trip member.

2. In a circuitV breaker comprising relatively movable contacts and actuating means therefor, a trip bar operable to cause automatic operation of said actuating means, a trip device responsive to overload currents for operating said trip bar, said trip device including a sealed casing, an electromagnet including an armature provided with a portion extending outside of said casing and operable to engage and operate said trip bar, a releasable trip member extending outside of said casing, spring means common to said arma- 8 ture and said trip member for biasing said armature to unattracted position and for actuating said trip member independently of said armature to engage and operate said trip bar, means for calibrating said trip device before said casing is sealed, and an adjusting means operable with-A,

out removing said casing to adjust the tripping point of said breaker within a predetermined range.

3. In a circuit breaker comprising relatively movable contacts and operating means therefor, a trip bar releasably restraining said operating means and operable to cause automatic opening operation of said operating means, trip means operatively associated with said trip bar but having no mechanical connection therewith for engaging and moving said trip bar to effect release of said operating means upon the occurrence of a predetermined overload current, means for calibrating said trip means, a sealed cover enclosing said trip means and said Calibrating means, means accessible without removing said sealed cover for adjusting the tripping point of said breaker within a predetermined range, and said trip means and said Calibrating means being removable and replaceable as a unit without affecting the tripping point of the breaker and without disengaging any connection with said trip bar.

4. In a circuit breaker comprising relatively movable contacts and operating means therefor, a trip member releasably restraining said operating means and operable to release said operating means to cause automatic operation of said operating means, a removable trip unit comprising a sealed casing, trip means mounted in said casing and having a portion extending outside of said casing separably engageable with said trip member but having no connection thereto, calibrating means disposed in said sealed casing and adjustable before said casing is sealed to calibrate said trip means, and adjusting means accessible through said casing for varying the tripping point of said breaker, said trip unit being removable and replaceable as a unit without disengaging any connection with said trip member and Without disturbing the tripping point of said breaker.

5. In a circuit breaker comprising relatively movable contacts and operating means therefor, a removable trip unit comprising a sealed casing, a trip member disposed outside of said casing operable to cause automatic opening movement of said operating means, trip means disposed in said casing and having a part extending externally of said casing for engaging and operating said trip member, said part having no mechanical connection with said trip member to permit removal of said trip unit without disengaging any connection with said trip means, adjusting means accessible externally of said sealed casing for varying the tripping point of said breaker within a predetermined range, Calibrating means settable said trip bar, a releasable trip member extending; externally of said casing for engaging and operating said trip bar, spring means common to said armature and said releasable trip member biasing said armature to unattracted position and for operating said releasable trip member, means accessible externally of said casing for adjusting said armature to vary the tripping point of the breaker within a predetermined range, ca1ibrat ing means settablebefore said casing is sealed to determine the range of adjustment of said adjusting means and said armature and said releasable trip member having no connection to said trip bar to permit removal of said trip unit without disengaging any connection with said trip bar and without disturbing the adjustment and calibration of said trip means.

7. In a multipole circuit breaker comprising a plurality of sets of relatively movable contacts and operating means therefor, a trip unit comprising a sealed casing having partition walls therein forming separate compartments, a trip bar disposed outside of said casing and common to all of the poles of said breaker for releasably restraining said operating means, trip means disposed in each of compartments associated with said trip bar outside of said casing for engaging and operating said trip bar upon the occurrence of an overload current in the associated pole, said trip means being separable from said trip bar without disengaging any connection with said trip bar to facilitate removal and replacement of said trip unit, separate adjusting means for each of said trip means accessible through said casing for varying the tripping point of the associated trip means within a predetermined range, and Calibrating means associated with each adjusting means, each of said Calibrating means being settable before said casing is sealed to determine the range of adjustment of the associated adjusting means.

8. In a multipole circuit breaker comprising a plurality of sets of relatively movable contacts and operating means therefor, a trip unit comprising a sealed casing having partition walls therein forming separate compartments, a trip bar disposed externally of said casing and common to all of the poles of said breaker for releasably restraining said operating means, trip means vdisposed in each of compartments and having portions extending outside of said casing associated with said trip bar for engaging and operating said trip bar upon the occurrence of an overload current in the associated pole, said portion of said trip means having no mechanical connection with said trip bar, separate adjusting means for each of said trip means accessible through said casing for varying the tripping point of the associated trip means within a predetermined range, and Calibrating means associated with each adjusting means, each of said calibrating means being settable before said casing is sealed to determine the range of adjustment of the associated adjusting means, and said trip unit being removable and replaceable without disengaging any connection with said trip bar and without aiTecting the tripping point of any of said trip means.

9. In a multiple circuit breaker comprising a plurality of sets of relatively movable contacts and operating means therefor, a trip unit comprising a sealed casing having partition walls therein forming separate compartments, a trip bar common to all of the poles of said breaker disposed externally of said sealed casing, trip `overload current in the associated pole, said trip means having no mechanical connection with said trip bar, separate adjusting means 'for each of said trip means accessible through said casing for varying the tripping point of the associated trip means within a predetermined range, callbrating means associated with each adjusting means, each of said Calibrating means being settable before said casing is sealed to determine the range of adjustment of the associated adjustingT means, the range of adjustment of each of said adiusting means being xed when said casing is sealed, and said sealed casing together with said trip means and said adjusting means being removable as a unit without disengaging any connection with said trip bar and without disturbing the adjustment and calibration of said trip means.

10. In a multipole circuit breaker comprising a plurality of sets of separable contacts and operating means therefor, a trip unit comprising a Sealed casing having partition walls therein forming separate compartments, a trip bar disposed outside of said sealed casing and common to at least two poles of the breaker and operable to cause automatic opening operation of said operating means, an electromagnetic trip device including an armature disposed in each compart-' ment and having a portion extending outside said casing for engaging and operating said trip bar, a releasable trip member disposed in each compartment and having a portion extending outside said casing for engaging and operating said .trip bar independently of said armature, spring means in each compartment compressed between the associated armature and said trip member for biasing the associated armature against attraction and for operating the associated releasable trip member, means disposed in each compartment and accessible externally of said casing for individually adjusting said armatures to vary the tripping point of the associated trip device within a predetermined range, and separate calibrating means for each trip device settable before the casing is sealed to separately determine the range of adjustment for each of said adjusting means;

11. In a multipole circuit breaker comprising a plurality of sets of separable contactsiand operating means therefor, a trip unit comprising a sealed casing having partition walls therein forming separate compartments, a trip bar disposed outside of said sealed casing and common to at least two poles of the breaker and operable to cause automatic opening operation of said operating means, an electromagnetic trip device including an armature disposed in each compartment and having a portion extending outside of said casing for engaging and operating said trip bar, a releasable trip member disposed in Ieach compartment and having a portion extending outside of said casing for engaging and operating said trip bar independently of said armature, spring means in each compartment compressed between the associated armature and trip member for biasing the associated armature against attraction and for operating the associated releasable trip member, means disposed in each compartment and accessible externally of Said casing for individually adjusting said armatures to vary the tripping point of the associated trip device within a predetermined range, and separate calibrating means for each trip device settable before the casing is sealed to separately determine the range of adjustment .for each of said adjusting means, said trip unit being removabieand interchangeable without affecting the tripping point of any of said trip devices.

' v LLQYD w. DYER.

REFERENCES CITED l The following references are of record in the me of this patent:

Number 12 i UNITED STATES PATENTS Name Date Wheeler Dec. 24, 1901 Stolz June 16, 1936 Graves, Jr. Mar. 23, 1937 Jennings Dec. 13, 1938 Tusing Oct. 10, 1939 Sachs Dec. 5, 1939 Dorfman Dec. 2, 1941 .Baxter et a1. July 6, 1943 FOREIGN PATENTS Country Date Number Germany Nov. 21. 1939