US2445426A

US2445426A - Circuit breaker

Info

Publication number: US2445426A
Application number: US495416A
Authority: US
Inventors: Gayne D Gamel; Walter S Hawkins
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1943-07-20
Filing date: 1943-07-20
Publication date: 1948-07-20
Anticipated expiration: 1965-07-20

Description

July 20, 1948.`

Filed July 20, 1943 G. D. GAMEI. Erm. l

' CIRCUI 'Ii BREAKER .2 Sheets-Sheet 2 INVENTORS 60 Ynel? ame/ and GWK/.061

Patented July 20, 1948 Ntra sra

araNr carica CIRCUIT BREAKER Application iniy 2t), 19%, Serial No. Litfili n crains. toi. atomes) The invention relates to circuit breakers and more particularly to air circuit breakers of low voltage and high current rating, for example, hav= ing voltage ratings up to 600 volts alternating cur= rent, and current ratings from several hundred amperes upwards to several thousand amperes.

. Circuit breakers of the class described frequently .require overload tripping means which will give a time delay prior t initiating tripping 'of the Vbreaker on overloads up to a predetermined magnitude. A th'ermostatic element of the bi metallic type is particularly suited for the time delay feature as it provides a time delay which varies inversely as the magnitude of the current. However, due to the large currents involved, it is a difficult problem to properly heat the bimetal element in response to the current. The usual methods of heating the bimetal trip elements have not been found satisfactory. Heating the bimetal element by shunting a small portion of the currentA therethrough is unreliable because it is difcult to obtain the proper ratio of current flow throughthe bimetal elements. Heating the bimetal element from a heating portion of a series conductor requires a high temperature bimetal element and results inan undesirable concentration of heat or hot spot in the circuit breaker.

An object of `the invention is the provision oi a circuit breaker of high current rating with an improved overload trip device of simple construction which is accurate and reliable in operation and inexpensive to manufacture.

Another object of the invention is the provision oi a circuit breaker of high current rating with an improved overload trip device comprising a series magnetic tripping means and a bimetal trip element heatedindirectly inA response to the current flow for controlling operation of the magnetic tripping means at least for overloads up to a predetermined magnitude.

Another object of the invention is the provision of a high current circuit breaker with an improved overload trip device comprising a. series magnetic tripping means, the operation of which is controlled, at least for overloads up to a predetermined magnitude, by a low temperature bimetal element heated in response to the current flow by a partial turn conductor linking the magnetic circuit of the magnetic tripping means.

Another object of the invention is to provision oi' a high current circuit breaker having a series magnetic tripping means controlled by a. low temperature vbimetal element heated by a loop conductor which links the magnetic` circuit and h'as a gap therein across which one end of the bimetal element is secured.

Another object of the invention is the provision of a high current circuit breaker with' an improved overload trip device comprising a series magnetic tripping means having a normally latched armature adapted to be released in response to overload currents by an ambient tenteI perature compensated low temperature bimetal trip element heated inductively from the current oi the magnetic tripping means.

Another object of the invention is the provision of a circuit breaker with an improved overload trip device employing a bimetal element heated in response to the current of the circuit for initiating tripping of breaker, and a very simple calibrating and adjusting means for adjusting the minimum overload tripping point of the circuit breaker.

The novel features that are considered characteristic of the invention are set forth in particular in the appended claims. The structure and mode of operation of the invention, together with additional objects and advantages thereof, will be best understood from the following detailed description of one embodiment thereof when read in conjunction with the accompanying drawings in which:

Figure l is a vertical sectional view of a circuit breaker embodying the invention, the view being taken on an irregularsection line along the center pole of the breaker with parts broken away and with' the trip device shown partly in section to illustratethe construction thereof.

Fig. 2 is an enlarged fragmentary sectional viewshowing the trip device. y

Fig. 3 is a bottom view of the lower part of the trip device on the same scale as Fig. 2.

Fig. 4 is an enlarged front elevational view of the bimetal trip element and heater.

Fig. 5 is a. plan view of the bimetal trip element and heater.

Figs. 6 and '7 are enlarged views of the scale plate of th'e trip adjusting means.

Fig. 8 is an enlarged view of the notched washer 'with which the scale plate cooperates, and

Fig. 9 is a fragmentary front elevational view showing the adjusting knob and scale plate.

The invention is illustrated as applied to a. circuit breaker of the type disclosed in Patent No. 2,414,526, issued Jan. 21, 1947 to Merl E. Horn et al. and assigned to the assignee of th'e present invention. Some of the features of the trip device disclosed in this application are disclosed and claimed in the above mentioned copending application.

Referring to Fig. `1 ofthe drawings, the high current circuit breaker illustrated comprises a base I I of insulating material on which is mounted three identical pole units, only th'e center pole unit being shown. The contact means of all of the pole units are adapted to be simultaneously operated by a single operating mechanism indicated generally at I3 located at the center pole.

Each pole of the circuit breaker has terminal members I5 and l1, and high current capacity contact means comprising stationary main contacts i3 and 2| mounted respectively on terminal 5 and on a block 23 otconducting material, a tiltably mounted spring biased auxiliary contact member 25, a bridging contact 21 for bridging main contacts I9 and 2|, and an auxiliary contact member 23 i'or engaging auxiliary contact member 25. The latter contact members have intermediate contacts 3|33 and arcing contacts 35--31 thereon which open in sequence shortly after parting of the main contacts upon opening of the circuit breaker. The bridging contact 21 and the auxiliary contact member 29 are carried by a yoke shaped switch member 39 which is pivoted at its lower end on a main frame 4| between the sides thereof by a pivot pin 43. The frame 4|, the terminals |5 and |1. and the conducting block 23 are secured to the base by screws. 'I'he bridging contact 21 is mounted for limited movement on the switch member 39 and is backed by' a spring to provide contact pressure in the closed position oi' the breaker, A ilexible shunt conductor 45 connects the auxiliary contact member 25 to the upper terminal I5, and another flexible shunt conductor 41 connects the movable auxiliary contact member 29 and the bridging contact 21 to the conducting block 23. This block 23 is in turn electrically connected through the energizing winding of the trip device of the corresponding pole to the lower terminal i1.

An arc extinguisher 5| preferably of the spaced plate de-ionizing type is associated with the auxiliary contact means of each pole of the circuit breaker for quickly extinguishing arcs drawn by the arcing contacts upon opening of the circuit breaker.

The common operating mechanism I3 for simultaneously operating the contact means of all of the poles is mounted on the frame 4| of the l center pole unit. This mechanism includes a. lever 53 pivoted by a pin 55 on the upper end of the frame and connected by a link 51 to the center pole switch member 39. The lever 53 is releas`ably connected by a series oi' latches 59 and 6| to a closing lever 63 which is also pivoted on the frame by the pivot pin 55.- The latches 59 and 6| are pivotally supported on the lever 53 between the sides thereof, and the latch 6| is normally held by lthe trigger latch 59 in latching engagement with a latch roller 55 on the closing lever. A latch 51 pivoted on a shaft 99 supported on the frame 4| engages the latch roller 65 to hold the closing lever latched in closed position. The latch 61 is biased to latching position by a spring 1 I.

'I'he two outer pole units (not shown) each have a lever similar to the lever 53 of the center pole except not provided with any latching means. These levers of the two outer poles are pivoted on the individual frames of the two outer pole units similar to the frame 4| and are connected to the outer pole switch members by links similar to the link 51. The levers are all mechanically connected for simultaneous movement together by means of a tie bar 13 of insulating material fastened to the three levers. It will thus be seen that the movable contact means of all three poles are adapted to be simultaneously operated together by the single operating mechanism.

The circuit breaker is biased to open position by means of a plurality of accelerating springs 15 connected to the switch member 39 and to a portion oi the frame. The circuit breaker is held latched in the closed circuit position by virtue of the main latch 61 which holds the closing lever 4 63 in position. Since the lever 53 is latched up to the closing lever, and is mechanically connected to the three movable switch members 39. these switch members and the contacts are thus held latched in closed position until the circuit breaker is tripped.

A trip bar-11 of insulating material is mounted on the shaft 69 and extends across the three poles of the circuit breaker. This trip bar has three spaced arms 19, one for each pole of the breaker. Each arm 19 extends to a position directly above the trip plunger of the corresponding overload trip device indicated generally at 8| so that when any one of the three trip devices is operated in response to an overload. the trip bar will be rotated to a tripping position to trip the circuit breaker. A trigger 83 on the center arm 19 of the trip bar is disposed to engage a curved tail piece of the trigger latch 59 and release the latches 59, 3| when the trip bar is actuated to tripping position by any one of the trip devices. Release of the latches 59 and 3| releases the lever 53 from the closing lever 53, and the opening springs 15 thereupon actuate the three movable switch members 33 and movable contacts of the circuit breaker to open position by rotating the lever 53 counterclockwise about pivot 55. This opening movement of the circuit breaker will occur even if the closing lever should be held down in its closed position. During the opening movement of the circuit breaker lever 53, a projection 85 thereon moves the main latch 61 to released position thereby releasing the closing lever. The closing lever 63 is biased upwardly by a spring (not shown) so that upon release of this lever it is moved in a counterclockwise direction about the pivot 55 to open position. During the latter part of the opening movement of the closing lever, the latch roller re-engages the latch 5| and this latch and the trigger latch 59 are reset to latching position thereby re-establishing the connection between the closing lever 63 and the breaker lever 53. The circuit breaker may then be closed manually, or electrically by a solenoid (not shown), by rotating the closing lever 63 in a clockwise direction back to its closed position, whereupon it is again latched by the main latch 51 to hold the breaker in closed position. The trip bar 11 may be provided with a handle socket 81 for receiving a handle to manually trip the circuit breaker.

Referring to Figs. 1 and 2 and 3 each of the trip devices 8|, there being one for each pole of the circuit breaker, comprises an electromagnet consisting of a stationary magnetic core 9|, two movable magnetic cores or

armatures

93 and 95, and an energizing conductor 91 forming one or more turns around the core members. The stationary core 9| has a threaded portion which is screwed into a. threaded opening provided in a portion of the frame 4| to secure the core member to the frame. The larger core member or armature 93 slidably extends through an opening in the lower portion of the frame 4| and through a guide opening in the cross portion of a U-shaped frame 99. The frame 99 is rigidly secured to the bottom of the main frame 4| by a plurality of screws |0|. This large armature 93 is normally held down by a latch |03 which is pivotally mounted upon a pivot pin |05 supported by the sides of the frame 99. The latch |03 engages an annular latch element |01 which is screwed into an openingiin the lower end of the core 93. The latch insert |01 by virtue of its screw-threaded engagement with the core stationary core 9 I.

member 93 provides a means for adjusting the air gap between the movable core 93 and the Bcrewingthe element |01 in or out relative to the core 93 serves to lower or raise the core 93 to change the air gap of the electromagnet. The upper end of the insert ,provides a circular latch shoulder engaged by the latch |03.

The smaller movable core member or armature 95 serves for instantaneous tripping of the circuit breaker upon the occurrence of overioads in excess of a predetermined magnitude, for example, above eight to ten times the normal rated

lmovable core member

93 or 95 is pulled up to attracted position against the stationary core upon actuation of the trip device.

The energizing conductor 91 of the electro. magnet is electrically connected at its upper end to a side of the conducting block 23 and at its lower end to the lower terminal member i1. Coil 91 is thus kconnected in series with the contacts of the corresponding pole of the circuit breaker, so 'that the current flow through the breaker energizes the electromagnet. The design of the electromagnet is such that the normal rated current produces sufficient magnetic pull to move the larger movable core member 93 upwardly to attracted position if this core member is released by the latch. The core 93, however, is normally held down by the latch |03, and this latch is adapted to be moved to released position to release the core member after a time delay in response to overload currents up to the aforementioned predetermined magnitude, by means of a current response thermal trip element. Although the small core member 95 is free to move upwardly independently of the larger core member 93. the magnetic pull of the electromagnet is insufiicient to move the smallcore upwardly to attracted position except upon the occurrence of heavy overload currents in excess of the predetermined magnitude at which instantaneous tripping of the circuit breaker is desired.

The circuit breaker and the electromagnet structure as thus far described in detail above is substantially similar to that disclosed in the aforementioned Patent No. 2,414,526.

. In accordance with the present invention, time delay tripping operation of the circuit breaker is. controlled by a bimetal element which is inductively heated from the current iiow through the energizing conductor 91 of the electromagnet through the agency of a loop conductor linking the magnetic circuit of the electromagnet. The loop conductor (Fig. 5) comprises a flat incomplete washer or ring of conductingmaterial having a. gap ||3 between its opposed ends. The conductor encircles kthe stationary core 9| of the electromagnet as shown in Fig. 2 and is clamped or secured to the intermediate cross portion ||5 of the frame 4| between this portion of the fra-me and the energizing conductor or winding 91 of the electromagnet. The conductor is insulated from the frame and from the conductor 91 by a pair of insulating Washers and H9. The bimetal element ||0 comprises a strip of bimetallic Inaterial of the low temperature type composed of two or more strips of metal having dissimilar coeiiicients of expansion, bonded together into one strip which warps or deflects upon being 'heated a predetermined amount. The bimetal element is ofthe so-called low temperature type which will warp in response to a relatively moderate heating thereof as compared to high temperature bimetals which must be heated to a high temperature before deflection will occur. The upper end of the bimetal element I0 is bent angularly and is rigidly secured by a plurality of rivets |2| to the opposed ends of the loop cond uctor l in bridging relation across the gap I I3, thereby completing a path for current flow through the loop conductor and through the portion of the bimetal element which bridges the gap H3. Adjacent its upper end, the bimetal element is held in clamped relation against a portion of the frame by a clamp |23 and a pair of screws |25, thev bimetal being insulated from the clamp and the frame by spacers |21 and |29 of insulating material.

The bimetal element H0 is heated in response to overload current through the circuit breaker by current flow through the loop conductor and a portion of the bimetal element which bridges the gap ||3 therein induced from the magnetic flux of the electromagnet. When heated a predetermined amount in response to overload current, the bimetal trip element ||0 warps or deects inwardly to engage and move an ambient temperature compensating bimetal element |3| fastened to a pivoted U-shaped lever |33 which is connected to the latch |03 by a connecting link |35. The de flecting movement of the bimetal trip element ||0 is thus transmitted to the latch |03 causing the latch to release the movable core 93 whereupon core 93 is moved upwardly to attracted position to trip the circuit breaker. Vllin insulating button |31 secured to the upper end of the ambient temperature compensating bimetal element serves to insulate these elements from one another. The bimetal element I3| serves to compensate the device for changes in ambient temperature so that the overload tripping point of the breaker will not be affected by changes in ambient temperature.

escasas helical compression spring |55 disposed between the nut |53 and the front portion of the frame 99 serves to bias the connected knob and nut |53 inwardly and to hold these elements in the position shown in Fig. 2. thereby holding the pivot shaft |39 in the position to which it has been adjusted. The shaft |'5| of the nut is slidable and rotatable in the opening of they frame 99 through which it extends, and the knob is rotatable through substantially a single revolution to adjust the minimum overload tripping point of the trip device. Rotation of the knob |49 in a counterclockwise direction moves the nut |53 and the pivot shaft |39 outwardly away from the base that is. toward the right as viewed in Fig. 2, due to the left-hand thread on the knob shaft. This outward movement of the nut |53 and pivot shaft |39 causes the lever |33 to pivot in a counter-clockwise direction about the pivot |43 as viewed in Fig. 2, thereby moving the upper end of the ambient temperature compensating bimetal element |3| farther away from the free end of the bimetal trip element lili. This increases the overload setting of the trip device due to the fact that the bimetal element ||5 will have to be heated a greater amount and deflect a greater distance before it will release the latch |93 and thereby initiate tripping of the circuit breaker. Conversely, rotation of *he manual adjusting knob |49 in a clockwise direction will effect movement of the nut |53 and pivot shaft |39 inwardly toward the base of the breaker thereby decreasing the distance between the free ends of the bimetal elements and |3i. This will decrease the minimum overload current .at which the bimetal trip element ||6 will effect release of the latch |93 to initiate tripping of the breaker. During adjustment, the pivot |49 remains fixed.

An emular scale plate |51 is disposed on the shaft |5| of the knob between the knob |49 and the front of the frame 99. This scale plate is normally locked to the knob as will be described later but is capable of being rotated relative to the knob to permit calibration of the breaker. The scale plate |51 is provided with a scale marked in percentages of overload current and has a projection |59 (Figs. 6 and 7) secured thereto which is adapted to engage a stop screw |6| mounted in the front portion of the frame 99. Since the scale plate is normally locked for rotation with the knob, the screw |6| and projection |59 serve to limit rotation of the knob to substantially one revolution. A washer |63 is disposed on the shaft |5| between the scale plate |51 and the front portion of the frame 99. This washer as shown in Fig. 8 has a hexagonally shaped opening which slidably engages the hexagonally shaped portion of the shaft |5| so that the washer is slidable relative to the shaft but is rotatable therewith. The periphery of the washer yis provided with a series of notches |55 which extend completely around the circumference of the washer. These notches cooperate with a locking element or projection |61 secured to the inner side of the scale plate |51 to lock the scale plate to the washer for rotation with the knob |49. The projection |61 engages in one of the notches, thereby locking the washer and scale plate for rotation together. The notches |65 also cooperate with a small rounded projection |69 (Fig. 2) on the front side of the frame 99 to retain the adjusting means comprising the knob |49 in adjusted position. The spring |55 presses the knob |49 firmly against the scale plate i5? and washer |63 and presses these elements against the front portion of the frame 99. The spring |55 and notched washer |63 thus serve as a resilient notching device to retain the knob in adjusted position.

To calibrate the circuit breaker a given value of overload current is passed through the breaker and the knob |49 is rotated to a point where the bimetal trip element l0 releases the latch |93 in a given period of time. The knob |49 is then pulled outwardly against the tension of the spring |55 and the scale plate moved outwardly to disengage it from the locking washer |63. The scale plate is then rotated relative to the knob until the given overload current marking on the scale plate is in line with the index mark comprising the stop screw |S| on the frame 99. The knob is then released and the spring returns the parts to the position shown in Fig. 2. The projection |61 then engages one of the notches in the washer |63 and is thereby locked to the knob |49. As previously explained, adjustment of the minimum overload current at which the trip device will trip the breaker is accomplished by rotating the knob |49 in one direction or the other until the proper overload value on the scale lines up with the index |6i. The pitch of the screw thread on the knob shaft is such as to provide a wide range of minimum overload trip settings.

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

We ciaim as our invention:

i. In an electrical circuit breaker, an overload trip device comprising an electromagnet energiaed by the current through the circuit breaker having a centrally disposed fixed core member and an axially movable core member operable when moved to attracted position to trip the circuit breaker, a latch normally restraining said movable core member, a thermally responsive eiement fixedly mounted at one end and operable when heated a predetermined amount in response to overload current to cause said latch to release said movable core member, means for heating said element comprising a ilxedly mounted loop conductor surrounding said core members of said electromagnet and having a gap therein bridged by the flxedly mounted end of said thermally responsive element whereby said element is heated by current induced in said loop flowing through the bridging portion of said thermally responsive element.

2, In an electrical circuit breaker, an overload trip device comprising an electromagnet energized by the current through the circuit breaker having an armature operable when moved to attracted position to trip the circuit breaker, a latch normally engaging and restraining said armature, a thermally responsive element of bimetallic material having one end xedly mounted and the other end deflectable when heated a predetermined amount in response to overload current to cause said latch to release said armature, means for heating said element comprising a loop conductor linking the magnetic circuit of said electromagnet and having a gap therein bridged by the xedly mounted end of said bimetal element, and a second bimetal element operatively relating said thermally responsive element with said latch for compensating said trip device for variations in ambient temperature.

3. In an electrical circuit breaker, an overload trip device therefor comprising an electromagnet energized by the current through the circuit breaker havinga magnetizable core structure and at least one armature operable when moved to attracted position to trip the circuit breaker, a latch for positively restraining said armature, a thermally responsive bimetal trip element operable when heated a predetermined amount in response to overload current to cause said latch to release said armature, means for heating said bimetal element comprising a loop conductor surrounding a portion of said core structure, said loop conductor being xedlymounted relative to said core structure and having a gap therein, one end of said bimetal element being rigidly secured to the ends of said loop conductor across said gap, and means independent of said electromagnet for adjusting the minimum overload current at which said bimetal trip element will cause said latch to release said armature with-out aiiecting said bimetal trip element.

4. In an electrical circuit breaker, an overload energized by the ported and the other end deilectable when heated a predetermined amount in response to overload current -to cause said latch to release said movable core, means for heating said bimetal trip element comprising a iixedly mounted ring o! conducting materialy surrounding one of said cores and having element being rigidly secured the ends of said ring across the gap.

7. In an electrical circuit breaker, an overload trip device therefor comprising an electromagnet current through the circuit breaker and having a magnetizable core structure and at least one armature c:operable when moved to attracted position to trip the circuit breaker. a latch normally restraining said armature, a thermally responsive bimetal trip element operable when heated a predetermined amount in response to overload current to cause said latch to release `said armature, means for heating said l bimetal element comprising a loop conductor having a gap therein, one

trip device therefor comprising an electromagnet energized by the current through the circuit breaker having a magnetizable core structure and at' least one armature operable when moved to attracted position to trip the circuit breaker. a latch normallyirestraining said armature, a thermally responsive bimetal trip element operable when heated a predetermined amount in response to overload current .to cause said latch to release said armature, means for heating said bimetal element comprising a loop conductor surrounding a portion of said core structure, said loop conductor being iixedly mounted on said core structure and having a gap therein, one end of said bimetal element being rigidly secured to the ends of said loop conductor across said gaps and a second bimetalielement operatively relating the latch to said bimetal trip element for compensating said bimetal trip element for variations in ambient temperature.

5. In an electrical circuit breaker, an overload trip device comprising an electromagnet energized by the current through the circuit breaker and having a magnetizable core structure and,

operable when moved to attracted position to trip said circuit breaker, a-

at least one armature latch normally restraining saidarmature against tripping movement, metal trip element deflectable when heated a predetermined amount in response to overload ourrent to cause said latch to release said armature, means for heating said bimetal trip element comprising a loop conductor surrounding a portion of said core structure, said loop conductor being xedly mounted relative to said core structure and having a gap therein, one end of said bimetal trip element being rigidly secured to the ends of said loop conductor across said gap, and a second bimetal element operatively relating said latch with the free end of said bimetal trip element for compensating the device for variations in ambient temperature.

6. In an electrical circuit breaker, an overload trip device comprising an electromagnet energized by the current through the circuit breaker and having a magnetizable core structure including a stationary core and an axially movable core `operable when moved to attracted position to trip the circuit breaker, a pivoted latch normally engaging and restraining said movable core, said latch having a xed pivot, a thermally responsive bimetal trip element having one end iixediy supa thermally responsive bi-L surrounding a portion of said core structure and end of said bimetal element being rigidly secured to the ends of said loop conductor across said gap, and adjusting means comprising an adjusting member separate from said bimetal element and insulated therefrom rotatable through substantially a single revolution to adjust the tripping point oi said bimetal element without changing the position of said bimetal element. .v 8. In an electric circuit breaker, an overload trip device comprising an electromagnet energized by the current through the circuit breaker and having a movable armature operable when moved to attracted position to trip the circuit breaker, a pivoted latch normally restraining said armature against tripping movement, a pivoted lever, a connecting link connecting said latch and lever pivotally connected to each, a bimetal element separate from said lever heated in response to the current of the circuit and operable when heated a predetermined amount in response to overloads to engage and move said lever and thereby release said latch, and adjusting means comprising a manually operable adjusting member separate from said bimetal element rotatable through substantially a single revolution for adjusting the minimum overload current at which said bimetal element will release said latch, and a spring lock for retaining said adjusting member in adjusted position. v

9. In an electric circuit breaker, an overload trip devicecomprising an electromagnet energized by the 'current through the circuit breaker and having la movable armature operable when moved to attracted position to trip the circuit breaker, a pivoted latch normally restraining said armature against tripping movement, a pivoted lever, a connecting link connecting said latch and lever pivotally connected to each, a bimetal element heated in response to the current oi the ciring means comprising a manually operable adthrough substantially a single revolution for shifting the position of the pivot of said lever to adjust the minimum overload tripping point of said bimetal element. l0. In an electric circuit breaker, an overload a gap therein, said bimetal trip at said one end to a pivoted latch normally restraining said armature against tripping movement, a pivoted lever, a link connecting said latch to said lever pivotally connected to each, an ambient temperature compensating bimetal element carried by said lever, a bimetal trip element heated in response to the current ofthe circuit and operable when heated a predetermined amount in response to overload current to move said compensating bimetal element and lever to release said latch, and adjusting means for adjusting the position of the compensating bimetal element relative to said bimetal trip element without changing the position of said latch to thereby adjust the overload tripping pointof said circuit breaker.

11. A trip device for a circuit breaker comprising tripping means operable to trip the circuit breaker, a bimetal element heated in response to the current through the breaker and operable when heated a predetermined amount in response to overload current to initiate operation of said tripping means, a support, adjusting means comprising an adjustable element movable to vary the value of overload current at which said bimetal element will initiate operation of said tripping means, a rotatable knob having a shaft extending through an opening in said support and screw portion engaging said adjustable element, said knob'being rotatable through substantially a single revolution to vary the position of said advjustable element, an annular scale plate on said shaft between said knob and support slidable and rotatable relative to said shaft, said scale plate having an overload current scale cooperating with an index on said support, a washer on said shaft between said scale plate and said support siidable relative to but rotatable with said shaft, said washer having a series of openings therethroulh disposed -ccncentrically around said shaft cooperating with a projection on said scale plate to normally lock said scale plate for rotation with said shaft and knob and also cooperating with a rounded projection on said support to retain said shaft and knob in adjusted position, and spring means biasing said shaft and knob inwardly to maintain said Washer, scale plate and knob pressed against said support, said knob and shaft being manually movable outwardly in axial direction against the tension of said spring means to unlock said scale plate and permit rotatable adjustment thereof relative to said shaft for calibrating the device.

GAYNE D. GAMEL. WALTER S. HAWKINB.

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

UNITED STATES PATENTS Number Name Date '168,334 Lloyd et al. Aug. 23, 1904 2,140,362 Kintzing Dec. 13, 1938 2,321,704 Robson June l5, 1943 2,331,740 Smith et al. Oct. 12, 1943