US2283736A - Circuit breaker - Google Patents

Description

May 1 1942 J. G. JACKSON CIRCUIT BREAKER Filed Nov. 3,- 1938 3 Sheets-Sheet 1 Wm @M @J W: M

May 19, 1942. v l JACKSON I 2,283,736

CIRCUIT BREAKER Filed NOV. 3, 1938 3 Sheets-Sheet 2 I 6% i5 z; .7 w =5 1; M 5/ r I A i 45 Z 1" F 44 I i I z flflfl y E 1 1 1 INVENTOR W JM A TTORNEY MaylQ, 1942. J. G. JACKSON CIRCUIT BREAKER Filed Nov. 5, 1938 3 Sheets-Sheet 3 INVENTOR I A TTORNEY Patented May 19, 1942 2,283,736 CIRCUIT BREAKER John G. Jackson, Detroit, Mich assignor to Square D Company, Detroit, Miclnfacorporation of Michigan Application November 3, 1938, Serial No. 238,604

17 Claims.

This invention relates to electric circuit controlling instrumentalities and more particularly to automatic electric circuit breakers adapted for the protection of motors from overloads.

One object of the invention is to provide a device for changing the trip time characteristic of an automatic circuit breaker.

Another object of the invention is to provide a device for tripping an automatic circuit breaker at a predetermined motor overload.

Another object of the invention is to provide an automatic electric circuit breaker having a cover sealably secured thereto and having interchangeable means at the exterior of its sealed enclosure for changing the current rating at which the breaker will automatically trip.

Another object of the invention is to provide an automatic electric circuit breaker bearing a certain current and/or time rating characteristie with means interchangeably mounted at an exterior terminal of the circuit breaker to decrease the tripping current rating thereof.

Another object of the invention is to provide readily interchangeable means for efi'ecting tripping of .an automatic electric circuit breaker at predetermined motor overloads, said means being adapted to engage a terminal means at the exterior'of the circuit breaker.

Another object of the invention is to provide a unitary element including a series resistor for adjusting the trip rating characteristic of an automatic electric circuit breaker and adapted to be secured to an exterior terminal thereof.

Another object of the invention is to provide a series resistance element for varying the current and/or time rating characteristic of an automatic electric circuit breaker and adapted to be supported by an exterior terminal thereof and having a means thereon for the attachment of circuit wiring.

Further objects and features of the invention will be readily apparent to those skilled in the art from the following specifications and the drawings illustrating certain embodiments of the invention in which:

Figure 4 is a view similar to Figure 2 showing the parts in automatic trip position.

Figure 5 is a sectional view taken along the line V--V of Figure 2.

Figure 6 is a sectional view taken along'the line VI-VI of Figure 5.

Figure '7 is a vertical sectional view of a modified form of the invention.

Figure 8 is a schematic diagram of the connection of the device according to the present invention to directly control the circuit of the mo;-

tor.

Figure 9 is a schematic diagram of an addi tional feature of applicants invention wherein the circuit breaker motor overload protection device is combined with a remotely controlled magnetic switch to control the motor circuit.

The circuit breaker comprises aninsulating base I of Bakelite or other material having side walls 2 and 3 and end flanges 4 serving as barriers between the separate terminals and extending to the full height of the circuit breaker. Within the breakers are portions 5 in line with the barriers l, servingpas barriers between the poles and for support of the cover portion and p which also extend to the full height of the breaker. In line with thebarriers 5 and integral therewith but extending for. only a portion of the height of the breaker to provide an opening for an operating member to be hereinafter described are further barrier portions 6 having ribs I to increase the leakage paths between the parts of opposite polarity within the breaker.. By the barrier portions 5 and 6 the interior of the breaker is divided into three portions having common openings above the rib I.

The side walls and barriers 6 are provided with pairs of guiding projections 8 and 9 and with re- Figure l is a top plan view of the circuit break- I. disposed elements duced ledge portions l0 adjacent one end. At this end, the exterior of the breaker between the barriers 4 may be provided with ledges for the support of terminal connectors which may comprise interior metallic sleeves l I having their tops spun over depressed portions in connectors l3 which extend within the interior of the breaker and are there supported upon the ledges III. Terminal studs 14 may be threaded into the sleeves II for attachment of-circuit conductors. To the interior -lower surfaces of the connectors l3 are brazed or otherwise attached the stationary contacts l5.

Between the guiding projections 8 and 9 are I6 which are U-shaped in cross section and having a flat top surface. The legs of the U are of varying lengths generally where the latching portions as shown and are provided with central slots I1 and 8 aligned with their respective stationary contacts I and have a substantially equal depth so that their ends rest at different levels in view of the variation of the length of the legs. With- 5 contacts I5 and 22 are preferably provided with liners 23 of fibre or other material which may be formed from a sheet notched as at 24 and then bent to the rectangular shape shown fitting within the chambers. The ends of the bars remote from the contact tips 22 are provided with latch- 20 ing surfaces adapted to cooperate with latching portions 26 supported on a, movable structure to be hereinafter described in detail.

Heavy compression springs 21 are guided with.-

in small cup portions 23 integral with the bot- 25 tom of the base I and are received within notches 33 in the bottom edges of the-bars 26 and bias the bars upwardly against the stationary contacts I5 and the latching portions 26.

The side walls2 and 3 at their top are pro- 30 vided with grooves 3| receiving a shaft 32 carrying manual operating members 33 each having an exterior handle 34 projecting through the top of the breaker and each with an interior portion com risin a cam surface 35 disposed above the 35 p place and securely held by the spinning of the inserts 50 within the depressed portions of the top face of the element l6. The cam surface 35 includes a flat portion 36 forming a stable support for the'manual off position of the breaker and a resetting tip 31 to be hereinafter explained. I v

A spring 38 biases the handle for rotation in a clockwise direction as shown and hence maintains cam surface 35 in contact with the top face of the member l6; spring 38 likewise biasing the member l6. firmly the cam Ion! shall position for the operating handle which be different from the overload trip position. as will be hereinafter explained. Above the 50 latching end of the bars 20, the side walls 2 and 3, and the barriers 5, are provided with grooves 33 serving as bearings-for a shaft 40 having integrally depending portions 4| disposed between the side walls 2 and 3 and the barrier portions 5. 55

To the depending portions 4| are rigidly secured generally L-sha'ped elements 42 from the surface of which the latching portions 26 are pressed. The bases of the L-shaped members 42 are provided with upstanding flanges 43 engageable by 0 bimetallic members 44 as they become heated to move the members 42 to an unlatching position 26are disengaged from the latching surfaces 25 on the bars 20. A flexible lead each of the bimetals 44 and its corresponding bar 20 and may be rigidly cdnductingly secured thereto by brazing or in any-other manner.- Leaf springs 46 attached to the members 42, as for instance, by means of a rivet 41 which secures 7 the member 42 to the depending portions 4| on The ends of the 10 bridging blocks yinterconnects the free end of 5 The support and adjusting means for the bimetallic members 44 will now be described. Between the barriers 4 on the exterior of the breaker, at the left hand end as shown, are supported connectors 48 which are rigidly secured in place by having depressed portions into which are spun the tops of metallic inserts 50. Above the connectors 48 are disposed resistor buttons 5| to be hereinafter described in detail. The openings in the bottom of the base I through which the inserts I and 50 are inserted are preferably closed as by insulating wax as at 52. The

' connectors 48 extend into the interior of the breaker and are there bent at right angles to provide vertical portions 53. The connectors 48 are notched as at 54 to clear the projecting portions on the barriers between the breaker poles and the vertical portions 53 are slidably disposed in vertical notches in the barriers and side walls of the breaker. The central portions of the vertical portions 53 of the connector are pressed out as at 56 and to these portions are rigidly welded or brazed the fixed ends of the bimetallic members 44. Between the vertical portions 53 and the pressed out portions 56 aredisposed 51 into which may be threaded adjusting screws 58. The tops of the vertical portions 53 are preferably bent over as at 53 to maintain the blocks in place. These blocks may be loosely disposed between the portions 53 and 56 and are prevented from. vertical removal by the bent over portions 53 and from sidewise removal by engagement with the barrier walls. Due to the notches 54 and the slots 55 each connector and bimetal assembly may be readily slid into connectors.

The ends of the bars 23 adjacent the latching surfacm 25 are provided with inclined surfaces 68 and 6|. The surfaces are provided to cause further opening movement of the latching members 42 upon release of any one of the bars 23.

Due'to the inclined. portion 60, when one bar is within the notches 2| in the 45 bars 20. The purpose of maintaining surface against the member 6 is to provide an released the action of this surface upon the corresponding latching portion 26 will cause a further movement of all the latching members 42 and insure that all the bars will be released upon the release of any one of them and will, therefore, insure that all the poles of the breaker will be opened if there is an overload upon any one of them. The inclined surfaces 6| provide ease in resetting of the bars as they will easily slide over the latching portions 26. In enclosing the breaker parts within the interior, the ends are closed by means of two fibre sheets 62 and 63 which are bent at right angles and have one leg disposed beneath the metallic cover 64 and the other leg disposed vertically at the ends of the breaker. Between the barriers 4 and 5 are provided notches 65 into which the vertical legs of the sheet 63 extend and which serve to securely hold them in vertical position. These sheets may in turn be provided with slots receiving the barriers 4 below the notches 65 therein. The sheets are maintained in place by the cover 64 which is in turn sealably secured to the base by means of rivets 66 extending through openlugs 61 in the insulating barriers 5. The sheets '62 and 63 are thick enough to be rigid and when in place serve to seal the mechanism of the breaker from access from the exterior.

The resistor button 5| disposed at the left f hand end of the breaker between the barriers 4 and above the connectors 5|| is formed of'a g I 9,283,738 supporting element 68 having one extremity 8 9 threaded and having the other extremity provided with an opening III. A collar II is disposed generally midway upon the element 68 and is notched as at 12. A mica tubing 13 is disposed about the upper extremity of element 68 and a coil 14 is wound thereupon. Another mica tub-.

ing 15 encircles the lower extremity of the coil H and extends to the notch 12; Enclosing the whole of the upper extremity of element 58 is a cap 16 having a threaded opening in the top.

thereof to receive the stud l1 and provided with a smaller opening within. which the upper extremity of the coil 14 is secured. The cap l6 extends to the notch 12' of the collar H and both the cap and the mica tubing 15 are crimped at their lower extremitiesto provide a sealed rigid attachment to the element 58. Amica cover '58 is disposed above the resistor wire I4 and beneath the under surface of the top of the cap it, said mica cover having openings therein to permit the passage therethrough of the wire I4 and the stud Til, the stud I1 forming a terminal for the reception of circuit wiring. For the better retention of heat by the resistor button heat insulating material is disposed in the space between the coil it and the inner surface of the cap it, The resistor button is rigidly secured to the breaker by its threaded extremity and the metallic insert 5t.

The operation of the circuit breaker will now be described. In the position shown in Figure 2, the parts are in their normal or circuit closed position. through each pole of the breaker is from the connector 33 through the portion 53 thereof tothe bimeta-l i i, irom the free end of the bimetal through the flexible lead %5 to the contact bar and. thence through the contacts iii and 22 to the opposite connector it. If it is desired to manually open the contacts to open the cir cult through the breaker, the operating handle St is rotated. clockwise, as shown, into the position shown in Figure 3, with the fiat part'% of the cam surface to in engagement with the top that surface of the member it. As the center oi rotation of the operating member is now within the engagement of the flat portion 3&3 with the member iii, this is a stable position which will he maintained by the parts.

members have been moved toward the bottom of the circuit breaker compressing the spring 211 and moving the contact tip away the stationary contact it into the position tated about their engagement with the latching portions 265 on the member 32. To place the ts again closed circuit position, the handle rotated counter-clockwise from its position. in are 3 to are moved into engagement by the force rted by the springs ill.

ii, while in the closed circuit position of Figure 2, an overload should. occur through any pole of the breaker, the corresponding bimetal will become heated and will warp in a clockwise direction as shown in Figure 4, whereupon the free end thereof by its engagement with its co operating portion 33 will rotate the members 52 clockwise, thus moving the latching portions 2% away from the latching surfaces ti: on the When a latching portion 26 reaches the edge of a latching surface it on any or the bars that bar will be snapped to the position shown in Figure t and by the engagement of its In this position,

this movement the bars fill have roto the position in Figure 2 and the con- 1 In this position the current path ill that vthe other bars will also bereleased and moved to the same position. In moving to this position, the bars, under the bias of the compression springs 21, rotate about the engagement or the notches 2| with the top of the slots l8 and their latching ends move upwardly until the topedges of the bars engage the tops oi! the slots I! in the left hand legs of the member I as shown in Figure 4. The parts are so related that in the closed circuit position of Figure 2 the handle 34 will be in the position sbowndue to the bias of the spring 38 pressing against members It. As the springs 21 are very much stronger than'the spring 38, when the bars are released'on overload, the members l6 will move upwardly under their bias until the interior portion of the manual operator engages the underside of the cover as shown in Figure 4, thus rotating the handle in a counter-clockwise direction and indicating the tripped condition or the breaker. In the automatic tripping, it is noted that the members 42 are rigidly connected to the depending portions 4| on the shaft and move together as a unit. When only a single one of the bimetals 44 is carrying an overload, it will move as shown in Figure 4. It the other bimetals are not warped the members 42 corresponding thereto will move toward these bimetals compressing their leafv springs ii.

To reset the breaker, after an automatic opening, the handle is rotated clockwise from the position shown in Figure 4. llhe blades at then move in straight lines toward thebase until the back edges adjacent to the contact tips 22 engage the bottoms oi the arc chambers whereupon further movement of the handle causes the blades 26 to rotate about the pivot formed by the engagement of the notches ill with the ends of slots it. As the movement of the handde continues, the inclined surfaces ti will engage the tops of the latching portions 2% and will move the latches clockwise to permit the latching ends of the hars to pass beyond the latching surfaces. if the hirnetal has cooled snficiently to permit the members 32 to resume the r normal v position, release of the handle will their cause the latching tips 25 to be engaged with the latch ing portions 226 to latch the ends or the in position.

the movement to resetting position. is necessary to move the handle beyond posh tion shown in Figure 3 so. that the tips 53? of the handle is moved'counter-clockwise as from manual-oil position and the parts are moved into engagement by the bias of springs 23.

in the calibrating of the poles of the breaker,

adjustment is secured through the adjusting screws 58. By moving these screws inwardly, they press against'the portions 56' serving to bend there and change the positions of the free ends of the bimetals thus regulating the amount of movement that is-necessary for each bim'etal torelease the latching portions as from the latching surfaces 25 on the bars 2 3. When the resistor within the button I the bimetal operating at breaker poles have been properly calibrated, the

screws 58 may be locked by means of a drop of solder or other sealing material and with the fibre sheet 62 in place and'the cover 84 'sealably attached to the base, access to the interior of the breaker will be prevented so that calibration can- .not be tampered with.

The resistor button 5| provides a means for readily lowering the rating of the breaker, and the buttons 5i may be very readily interchanged with others having difierent values of resistance to secure, if desired, lower rating of the tripping point for the breaker, while it is impossible to raise the original calibration of the breaker by any such means external to it.

In the normal connection of the circuit breaker, as heretofore known, in the art, there is a good conducting path from the bimetal 44 through the connector 48 to the exterior circuit 1 wiring. As the bimetal 44 will, when at or near the length of the bimetal between its attached point and its latch releasing portion. Whether the resistor within. button Ii will be operating at a higher or lower temperature than the bimetal 44 will depend upon the change in current rating desired in'the circuit breaker, and from this upon the value of the resistance within the button andthe current flowing therethrough. It can readily be seen that the results obtainable by the present invention in lowering the rating of the bimetal will be attained even though the resistor at the exterior circuit breaker terminal should not be at a higher temperature than the bimetal ll, since any rise in temperature at the terminal will act to inhibit the fiow of heat from the bimetal to the circuit wiring by creating a hot spot at the terminal-which will lessen the fiow of heat from the bimetal. This is obviously true, since the fiow of heat from the bimetal to the terminal will be a direct function of the difference of temperature between them. For certain rating change values, it may be that the will be operating at a greater temperature than the bimetal 44, so that there will be an actual flow of heat from the resistor to the bimetal; so that the temperature of the bimetal will not only be increased by the stoppage of ilow of heat therefrom, but that there will be an actual input .Irom the bimetal to the resistor. Either of these-cases results in a higher temperature than previously for a given flow of current and, hence, means that there will be a greater unlatching movement of the bimetal at a given current than heretofore, so that the current at which the bimetal will move sufiiciently to unlatch the breaker will be greatly reduced.

With applicant's invention, a standard automatic electric circuit breaker having a rating sufficient to protect the circuit wiring with which it is connected may be readily modified by a re.- sistor element such as at II to give to the circuit breaker a lower effective rating suitable for the protection of a motor supplied through the circuit breaker and results in the provision of both short circuit and motor protection in a single device. g

It has furthermore been found that with a construction according to applicants invention that the effective rating of the standard circuit breaker may be so modified by means of the external terminal resistors as to produce a trip time characteristic within the normal requirements for motor protection. That is, with the circuit breaker time current rating characteristic being such as to produce tripping within a time element upon a sustained overload for a given size of motor, that the circuit breaker time lag will be suillcient using the button to carry the starting current load oi the motor without tripping out. As an example of this, on a single pole circuit breaker rated at 15 amperes in accordance with the circuit breaker standard of the Underwriters Laboratories, and having a short circuit capacity sufilcient to interrupt the short circuit current of a circuit having 9, capacity of 5,000 amperes was modified by means of a button 5| so that it would trip ultimately at a current of about 6 amperes which would be suitable for the protection of a $4 H. P. motor in accordance with the standards of the National Electrical Code. It was further found that this arrangement corresponded to the requirements for motor overload protection in tripping within the limiting times at both 200% and 600% of normal rating and that on actual test it would hold in in ample length of time to permit the motor to start with its heavy inrush current. By using resistances of different values, at the external terminal, it is possible with the same circuit breaker protection to secure any desired motor overload protection up to the rating of the circuit breaker.

Applicants invention, therefore, provides a very economical means for providing a tamperproof circuit breaker protection with readily interchangeable values of motor overload protection and permits the ready varying of a standard circuit breaker to produce a combination circuitbreaker motor starter having all the necessary characteristics of such a device.

In Figure 7 therehas been illustrated a single pole circuit breaker provided with "the external resistance for providing the desired motor overload protection and while there are detailed changes in the design of the circuit breaker parts having no bearing on the present invention, as, for example, the change in the mounting of the stationary contact connector II, the addition of the fibre arc chamber sheet 19, and the change in the handle spring II, as well as the different manner of connecting the end barrier sheets and the interposition of a further insulating fibre'sheet beneath the metal cover, the main difference lies in the bimetallic latch 02 which is shown as corrugated inaccordance with the invention described in the claims in the patent to Olav Mas.- ing No. 2,122,693, and which directly latches the movable contact bar since, in a single pole, there is no necessity for the use of the latching members 42 in the multipole construction which may be moved in Figure 8 of a circuit according to motor where circuit to start and stop the motor is desired.

Figure .9 is a schematic representation of a motor starter circuit breaker combination for remote control of the motor circuit in which a remote push button is used to control the operating coil of a magnetic switch to effect opening and closing of the motor circuit. The combination of applicant's improved circuit breaker and motor overload protection device with the magnetic switch is a further inventive conception of the concept of this invention in view of the fact that with the device according to the present invention the magnetic switch is not equipped with overload protection, but the overload protection for the motor is embodied in the combination circuit breaker and overload protective device. In previous arrangements, an overload in the motor had been used to interrupt the energizing current or the magnetic switch to efiect opening of the circuit at the contacts of the magnetic switch. In applicauts arrangement, the motor overload current is interrupted by the circuit breaker contacts and the magnetic switch is not equipped with overload means as heretofore provided, so that the magnetic switch is called upon to interrupt only the normal current of the motor while overload currents, as well as short circuit currents, are interrupted by the circuit breaker mechanism.

While the construction of the particular circuit breaker illustrated in the drawings has been specifically described in accordance with the patent statutes, it is understood that applicants invention is equally applicable to any thermal circult breaker wherein it will have the eiTect of modifying the normal current-temperature characteristic oi the thermal current responsive element, While applicants invention is entirely independent of any particular circuit breaker mechanism, it will, however, have its greatest efiect in a thermal breaker where the thermal element is closely associated with the exterior temnnal so that the exterior resistance will have its greatest efiect upon the thermal element within the circuit breaker interior.

While certain preferred embodiments oi the invention have been specifically disclosed, it is understood that the invention is not limited thereto as many variations be readily apparout to those skilled in the art and the invention is to be given its broadest possible inte pretation within the terms or the following claims.

is claimed is;

l. in an automatic electric circuit breaker, relatively movable contacts, current responsive means for en'ecting separation of said contacts, when the current exceeds a predetennined value for a predetermined period of time, and means for lowering the value of current at which said current responsive means will operate to effect said separation, including readily interchange able resistance unit connected in series circuit with the current responsive means, said resistance unit being insulated from relative heat transfer with respect to said current responsive means except by conduction through said series connection.

2. In combination, relatively movable contacts, serially connected, current responsive, thermal means for eilfecting separation oi said contacts when the current exceeds a predetermined value for a predetermined period of time, and readily interchangeable auxiliary resistance means con nected in series circuit with said thermal means and adapted to be heated by the current flowing through the contacts and current responsive means, to supply an additional increment of heat to the current responsive means, said resistance means being connected with said thermal means in electrical and heat conducting relation and being so disposed with respect to said thermal means as to transmit heat thereto by conduction alone.

3. In combination, relatively movable contacts, serially connected, current responsive means for effecting separation of said contacts when the current exceeds a predetermined value .for a predetermined period of time, and resistance means interposed between said current responsive means and circuit wiring electrically connected thereto and adapted to be heated by the flow of current through the contacts and current responsive means, said contacts and current responsive means being mounted within a sealable enclosure preventing access thereto and said resistance means being readily interchangeably mounted at the exterior of said enclosure.

4. In an automatic electric circuit breaker, a scalable enclosure having relatively movable contacts and current responsive means for efiecting separation of said contacts sealed within said enclosure from outside access, said means efiecting said separation when the current exceeds a predetermined value for a predetermined period of time, and a resistance element readily interchangeably mounted at the exterior of said enclosure in series circuit with said current responsive means for lowering the value of current at which said current responsive means ofiects said separation.

5. in an automatic electric circuit breaker, a scalable enclosure, stationary and movable contacts and current responsive means for eiiecting separation of said contacts mounted within said enclosure, manually operable means for effecting separation of said contacts and for resetting said circuit breaker mechanism after an automatic contact separation, including an operating handle extending to the exterior of the enclosure, and a resistance unit readily interchangeably mounted at the exterior of the enclosure and connected in series circuit with the current responsive means to lower the value or current at which said current responsive means becomes effective to effect such separation.

6, In an automatic electric circuit breaker, a scalable enclosure, relatively movable contacts and current responsive means for effecting sepa ration of said contacts mounted within said enclosure and sealed from outside access, a terminal for said circuit breaker mounted at the ex terior of said enclosure and connected to said current responsive means in electrical and heat conducting relation therewith, and a readily in terchangeable resistance unit mounted on said terminal at the exterior of the enclosure and heated by the passage of current therethrough to lower the value of current at which said cur rent responsive means efiects such separation.

Z. In an automatic electric circuit breaker, a sealable enclosure, relatively movable contacts and current responsive means for eilecting separation of said contacts mounted within said enclosure and sealed from outside access, a terminal for said circuit breaker mounted at the exterior of said enclosure and connected to said current responsive means in electrical and heat conducting relation therewith, and a readily interchangeable resistance means directly mounted upon said terminal and having a further terminal thereon for the reception of circuit wires to be connected to the circuit breaker, said resistto lower the value of current ance being heated by the passage of current therethrough to lower the value of current at which said current responsive means will effect such separation.

8. In an automatic electric circuit breaker, separable contacts, serially connected, current responsive, thermal means for eifecting separation of said contacts when the current therethrough exceeds a predetermined value for a predetermined period of time, a terminal for said circuit breaker in electrical and heat conducting relation with said current responsive means, and a resistance unit mounted directly on said terminal by a threaded screw connection therebetween, and having disposed on said unit auxiliary terminal means adapted for the connection of exterior circuit wiring.

9. In an automatic electric circuit breaker, separable contacts, serially connected, current responsive, thermal means for effecting separation of said contacts when the current therethrough exceeds a predetermined value for a predetermined period of time, a terminal for said circuit breaker in electrical and heat conducting relation with said current responsive means, an opening through said terminal, a resistance unit mounted on said terminal by an element threaded into said opening, and auxiliary terminal means on said resistance unit adapted for the connection of exterior circuit wiring.

10. In a multipole electric circuit breaker, a plurality of sets of relatively movable contacts, a plurality of current responsive thermal means connected in series with said sets of contacts, means cooperating with said current responsive thermal means for effecting separation of all of said sets of contacts when the current through any one or more of said current responsive means exceeds apredetermined value for a predetermined period of time, and a plurality of readily interchangeable resistance means mounted in series circuit with said current responsive means at which said separation will be eflected.

ii. In a multipole electric circuit breaker, the plurality of setsof relatively movable contacts, a plurality of current responsive thermal means connected in series with said sets of contacts, means cooperating with said current responsive thermal means for effecting separation of all of said sets of contacts when the current through any one or more of said current responsive means exceeds a predetermined value for a predetermined period of time, and readily interchangeable resistance means connected in series circuit with each of said current responsive means and adapted to modify the current-temperature characteristics of the current responsive means to lower the value oicurrent at which said separa-' tion will be eflected.

12. In a multipole electric circuit breaker, the plurality'ot sets of relatively movable contacts, a plurality of current responsive thermal means connected in series with said sets of contacts, means cooperating with said current responsive means for eflecting separation oi all 01' said sets of contacts when the current through any one or more of said current responsive means ex-' ceeds a predetermined value for a predetermined period 01 time, connectors from said current responsive means leading to the exterior of the circuit breaker, and readily interchangeable resistance units mounted upon said connectors at theexterior oi the circuit breaker'to modify the adapted for both circuit and motor overload provalue of current at which said separation will be effected.

13. In an automatic electric circuit breaker, relatively movable contacts serially connected, current responsive thermal means for effecting separation of said contacts when the current therethrough exceeds a predetermined value for a predetermined period of time, and resistance means serially connected with said thermal'means between the thermal means and the external circuit wiring elect ically connected thereto, said resistance means being' heated by the current flow through said contacts and thermal means, and establishing a point of higher temperature than that of the exterior circuit wiring to inhibit the flow of heat from said thermal means normally dissipated to the exterior circuit wiring, said resistance means being insulated from radiation of heat to said thermal means.

14. In a multipole automatic electric circuit breaker, a plurality of sets of relatively movable contacts, a plurality of current responsive thermal means connected in series with said sets of contacts so as to be traversed by the current flowing therethrough, means cooperating with said current responsive thermal means for effecting separation ofall of said sets of contacts when the current through anyone or more of said current responsive means exceeds a predetermined value for a predetermined period of time, a scalable enclosure for said circuit breaker preventing access to said thermal means, and a plurality of readily interchangeable resistor means mounted at the exterior of said enclosure in series circuit with said thermal means so as to. be traversed by the current flowing therethrough and through the contacts, and operating to lower the value of current at which said thermal means function to eii'ect separation of contacts.

15. In a multipole electric circuit breaker, a plurality of sets of relatively movable contacts, a plurality of current responsive thermal means connected in series with said sets of contacts, means cooperating with said current responsive thermal means for effecting separation or all 01' the sets of said contactswhen the current exceeds a predetermined value for a predetermined period of time, a sealed enclosure for said circuit breaker preventing access to said thermal means, connectors from said current responsive means leading to the exterior of the circuit breaker to form terminal means for the breaker, and readily interchangeable resistance units mounted upon said terminals at the exterior of the circuit breaker and functioning to modify the value of current at which the contact separation will be effected, said resistance units carrying additional terminal means forthe connection of circuit wiring. v

16. In an automatic electric circuit breaker 'tection, a scalable enclosure, separable contacts and current responsive thermal means for effecting separation of said contacts sealed within said said thermal enclosure from exterior access, means operating to effect separation 01' the contacts when the current therethrough exceeds a predetermined value for, a predetermined period or time, these predetermined values being fixed against variation and corresponding to the desired protection for the circuit wiring, and readily interchangeable resistance units mounted in series relation with said thermal means at the exterior of the sealed enclosure and being interchangeable to provide for the desired motor overa predetermined period of time, said predetermined values being relatively flxed by the inaccessibilityof the thermal means within the sealed enclosure, and means for lowering the currenttimerating for the tripping of the circuit breaker comprising readily interchangeable resistance means mountedat the exterior of the sealed enclosure in series circuit relation with the thermal means, whereby a circuit breaker having a fixed maximum current-time rating may have its rating readily lowered to desired values without permitting the exceeding of the predetermined maximum setting. 7 a

JOHN G. JACKSON.

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