US3526861A - Circuit breaker with improved trip device - Google Patents

Circuit breaker with improved trip device Download PDF

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US3526861A
US3526861A US3526861DA US3526861A US 3526861 A US3526861 A US 3526861A US 3526861D A US3526861D A US 3526861DA US 3526861 A US3526861 A US 3526861A
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trip
circuit breaker
conductor
trip device
main conductor
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Eugene J Walker
Zelko J Kruzic
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Westinghouse Electric Corp
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Westinghouse Electric Corp
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    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01HELECTRIC SWITCHES; RELAYS; SELECTORS; EMERGENCY PROTECTIVE DEVICES
    • H01H71/00Details of the protective switches or relays covered by groups H01H73/00 - H01H83/00
    • H01H71/10Operating or release mechanisms
    • H01H71/12Automatic release mechanisms with or without manual release
    • H01H71/40Combined electrothermal and electromagnetic mechanisms

Description

Sept. 1, 1970 E. J. WALKER v CIRCUIT BREAKER WITH IMPROVED TRIP DEVICE 3 Sheets-Sheet 1 Filed May 5. 1968 INVENTORS ene J. Walker and BY Zelko J. Kruzic WITNESSES v ATTORNEY E. J. WALKER 3,526,861

CIRCUIT BREAKER WITH IMPROVED TRIP DEVICE 3 Sheets-Sheet 2 Sept. 1, 1970 Filed May 3. 1968 5 J. M B3 F (I u I W 51, ll 7, I'll Q v p Z IL? Ill nu O f A Sept. 1, 1910' 1 E. p. WALKER I 3,526,861

CIRCUIT BREAKER WITH IMPROVED TRIP DEVICE Filed May 5,, 1968 '3 Sheds- Sheet s United States Patent Office 3,526,861 Patented Sept. 1, 1970 3,526,861 CIRCUIT BREAKER WITH IMPROVED TRIP DEVICE Eugene J. Walker, Beaver, and Zelko J. Kruzic, New

Brighton, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed May 3, 1968, Ser. No. 726,474

Int. Cl. H01h 75/12 US. Cl. 33535 6 Claims ABSTRACT OF THE DISCLOSURE A circuit breaker comprises an improved thermal and magnetic trip device. The breaker comprises a pair of contacts for opening and closing a circuit. The trip device comprises a main conductor in electrical series with the contacts and a shunt conductor for shunting current around a portion of the main conductor. A bimetal is connected to the shunt conductor to be heated in response to the current in the shunt conductor to thereby thermally trip the breaker. A magnetic core and armature are supported to be energized by the full current in the circuit to thereby magnetically trip the breaker. The parts are supported so that limited movement of the main conductor does not interfere with calibration or adjustment of the trip device.

BACKGROUND OF THE INVENTION In the design of certain types of circuit breakers it is desirable to provide certain thermal and electromagnetic tripping characteristics in an overall construction that will not lose calibration or adjustment either during assembly of the breaker or thereafter when terminals are tightened and loosened. For example, in some applications the main conductor that carries current through the trip device is provided with terminal portions that are moved whenpressure is applied thereto connecting or disconnecting operations. The parts should be so supported that this limited movement of the main conductor does not interfere with the calibration or adjustment of the thermal and/ or electromagnetic trips. Thus, an object of this invention is to provide a circuit breaker with an improved thermal and magnetic trip device that permits liimted relative movement of the main conductor without disturbing the calibration or adjustment of either the thermal trip or the magnetic trip.

Another object of this invention is to provide a circuit breaker with an improved trip device, comprising a main conductor for energizing the trip device with an electromagnetic trip energized by the full current through the trip device to operate a trip member and with a thermal trip comprising a bimetal connected to a shunt conductor that shunts a portion of the full current past a portion of the main conductor for heating the bimetal to operate the trip member, with the trip device being constructed and arranged such that the main conductor can move under stresses without affecting the calibration or adjustment of either the thermal trip or the electromagnetic trip.

PRIOR ART The patent to Stevenson et al., No. 3,211,860 discloses a circuit breaker comprising a thermal and electromagnetic trip device wherein a bimetal is connected directly to the main conductor to be heated by the fu l current in the main conductor.

The patent to Strobel Pat. No. 3,244,837 discloses a circuit breaker comprising a thermal and electromagnetic trip device with the electromagnetic trip being energized by the current in a looped conductor that provides a shunt path parallel to a portion of the main conductor.

SUMMARY OF THE INVENTION A circuit breaker comprises the pair of contacts for Opening and closing a circuit and a main conductor in electrical series with the contacts. A trip device, comprising a trip member pivotally supported on a fixed pivot and operable to effect automatic opening of the contacts, comprises a shunt conductor connected at opposite ends thereof to the main conductor to provide a parallel shunt path parallel to a portion of the main conductor. A thermal trip comprises a bimetal that is fixedly connected at one end to the shunt conductor with the other end being free to move, when the bimetal flexes, to operate the trip member. An electromagnetic trip comprises a generally U-shaped magnetic core, that is energized by the full current in the circuit, and an armature structure that is pivotally supported on one leg of the core and that is movable to operate the trip member. The supported end of the bimetal and the U-shaped magnetic core are both fixedly supported with relation to the fixed pivot of the trip member so that movement of the trip device or movement of the main conductor will not disturb the calibration or adjustment of either the thermal trip or the electromagnetic trip. The trip device is a removable trip device with the main conductor comprising a terminal at each of the opposite ends thereof so that under certain conditions the main conductor is subjected to stresses that will tend to move the main conductor. The circuit breaker is an insulating-housing or molded-case type three-pole circuit breaker with the removable trip device comprising a separate thermal and electromagnetic trip for each pole unit and a single trip member or trip bar common to all of the pole units.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 of the drawings, a circuit breaker 7 shown therein comprises an insulating housing 9. The Insulating housing 9 comprises a base 11 and a removable -cover 13 suitably secured to the base 11. The circuit breaker is a three-pole circuit breaker with insulating barrier means 15, formed with the base 11 and cover 13, forming three adjacent compartments for housing the three-pole units. A pair of terminals 17 and 19, are provided in cavities at opposite ends of each pole unit for enabling connection of each pole unit in an electric circuit.

The circuit breaker is of the type more specifically described in the patent to Stevenson et a1. Pat. No. 3,260,822. The circuit breaker comprises a stationary contact 21, a movable contact 23 and an arc-extinguisher 25 for each pole unit. A common operating mechanism 27 is provided for simultaneously actuating the three movable contacts to open and closed positions. A trip device indicated generally at 29 serves to effect automatic opening of the breaker contacts in response to overload conditions in a manner to be hereinafter described.

The terminal 17 is disposed at the outer end of a conducting strip 31 which extends into the housing and rigidly supports the stationary contact 21. The movable contact 23 is mounted on a rigid contact arm 33 that is supported on a switch arm 35 that is connected to an insulating tie bar 37. The tie bar 37 extends across all three pole units of the breaker and supports the switch arms for the several pole units for unitary movement. The tie bar 37 is suitably supported in the housing for movement about an axis to open and closed positions. The contact arm 33 is connected, by means of a flexible conductor 39, to a conductor 40 that engages a main conductor 41 that is secured at one end thereof to the base 11 by means of a screw 42. The screw 42 connects the conductor 40 with the main conductor 41. The main conductor 41 extends through the trip unit 29 and is connected at its outer end to the terminal 19. As can be seen in FIG. 5, the main conductor 41 is widened at the opposite ends thereof to provide the terminal portions for receiving the screw 42 and terminal 19 in a well known manner.

The operating mechanism 27 is disposed in the center compartment of the housing and is supported on a pair of spaced frame members 53, (only one being shown), which are secured to the base 11. The operating mechanism comprises an inverted generally U-shaped operating lever 55, a toggle compressing toggle links 57, 59, a pair of overcenter springs 61 and a releasable cradle or trip member 63 that is controlled by the trip device 29. The operating lever 55 is pivotally supported at the inner ends of the legs thereof in U-shapes slots in the spaced supporting plates 53. The cradle or trip member 63 is pivotally supported on a pin 64 that is supported between the spaced plate members 53. An insulating handle 65, that is supported on the outer end of the lever 55, extends through a suitable opening in the front of the cover 9 to permit manual operation of the breaker.

The toggle links 57 and 59 are pivotally connected together by means of a knee pivot 67. The front end of the link 57 is pivotally connected to the trip member 63 by means of a pin 69. The back end of the toggle link 59 is pivotally connected to the switch arm 35, for the center pole unit, by means of a pin 71. The overcenter springs 61 are connected under tension between the outer end of the operating lever 55 and the knee pivot 67 of the toggle 57, 59 in a well known manner.

The circuit breaker is manually operated to the open position seen in FIG. 1 by counterclockwise movement of the handle 65 to the off position, which movement moves the overcenter springs 61 to cause collapse of the toggle 57, 59 and opening movement of the switch arms 35 for all of the pole units in a well known manner. The breaker is manually closed by reverse or clockwise movement of the handle 65 to the on position which movement moves the overcenter springs 61 to erect a toggle 57, 59 to thereby move the switch arms 35 for all of the pole units to the closed position in a manner well known in the art.

Referring to FIGS. 2-5, the trip device 29 comprises an insulating base or support member 75 that is fixedly secured to a rigid metallic supporting bracket 77 by means of a pair of rivets 79. An insulating cover 81 is removably connected to the base 75 by a pair of screws (not shown) to provide an insulating housing 75, 81 for housing parts of the removable trip device. The housing 75, 81 is separated into three compartments for housing the three thermal and electromagnetic trips of the three pole units. Only the thermal and electromagnetic trip of the centerpole unit will be specifically described, it being understood that the description applies to each of the pole units. As is best seen in FIGS. 4 and 5, a looped conductor 85 is connected at the opposite ends thereof to the main conductor 41 to provide a parallel current path for shunting a portion of current around an intermediate portion 87 of the main conductor 41. A rigid metallic generally U-shaped bracket 89, having a pair of flanges 91 (only one of which is seen in FIG. at the opposite sides thereof, is fixedly secured to the base 75 by means of the two rivets 79 which extend through openings 93 (FIG. 5) in the flanges 91. A bimetal 95 is fixedly secured at the lower end thereof to one leg of the looped conductor and to the bight portion of the bracket 89 by means of four rivets 97. As can be seen in FIG. 5, an insulating wrapper 99 is provided around portions of the conductor 41 and loop conductor 85 to insulate these conductors from adjacent parts. As can be seen in FIG. 4, the free end of the bimetal is disposed opposite a calibrating or adjusting screw 101 that is supported on an extension 103 that is attached to an insulating trip bar 107. A screw 111 is supported on the trip bar to limit counterclockwise (FIG. 4) movement of the trip bar 107. The trip bar 107 is pivotally supported in the housing 75, 81 by means of a pair of supporting arms 113 that are supported on the bracket 77 by means of screws 115. As can be seen in FIG. 3, the trip bar 107 is formed with a pair of insulating pin portions 117 that extend through openings in the arms 113 to support the trip bar 107 for pivotal movement about the pins 117. A spring 119 (FIG. 4) biases the trip bar 107 in a counterclockwise direction. In the normal position of the trip bar 107, a latch member 121, that is rigidly imbedded in the trip bar 107, engages a latch portion 117 of a latching mechanism that is indicated generally at 119 and that restrains the cradle or trip member 63 (FIG. 1) in the latched position shown in FIG. 1.

As can be understood with reference to FIGS. 3-5, the rivets 79 also extend through openings in a laminated U-shaped magnetic core 125. Thus, the rivets 79 of the center-pole unit, fixedly mount the U-shaped magnetic core 125, the rigid bracket 89 and the insulating base 75 to the bracket 77. There is a bracket 77 in only the centerpole unit. It can be understood that in each of the two outer pole units the pair of rivets 79 fixedly mount the associated U-shaped magnetic core 25 and the associated bracket 89 to the base 75. Continuing on with a description of the trip means of the center-pole unit, a laminated magnetic armature 127 is provided with a knife-edge type pivot 129 at one end thereof that is suitably supported in a V-shaped slot in one leg of the associated U-shaped core 125. A pair of springs 131 bias the armature 127 into the open or unattracted position seen in FIG. 3. An elongated arm 135 is pivotally connected to the armature 127 at the lower end thereof, and is provided with a calibrating or adjusting nut 137 at the upper end thereof. The rod or arm 135 extends through an opening in a plate member 141 that is fixed to the insulating trip bar 107. An insulating adjusting knob 143 protrudes through an opening in the front of the trip-device housing 75, 81, and is supported for rotation on a supporting bracket 145 that is fixedly secured to the base 75. The adjusting knob 143 is provided with a cam surface 147 at the lower end thereof that engages the front end of a rod 149. The lower end of the rod 149 extends through an opening in thebracket 133 and engages the front surface of the armature 127. The air gap of the electromagnet can be adjusted by rotating the knob 143 to move the rod 149 longitudinally to thereby pivot the armature 127 to adjust the air gap between the armature 127 and the one leg of the associated magnetic core 125. The armature 125, rod 135 and nut 137 form an armature structure indicated generally at 151 which armature structure is actuated to operate the trip bar in a manner to be hereinafter more specifically described.

As can be seen in FIG. 1, the screw 42 fixedly mounts the bracket 77 to the base 11 in the center-pole unit of the circuit breaker to thereby fixedly mount the trip device 29 in position.

Referring to FIG. 1, it will be understood that when the circuit breaker is closed, the circuit through each pole unit extends from the terminal 19 through the main conductor 41, with the loop conductor 85 (FIGS. 4 and 5) shunting a portion of the current around the intermediate portion 87 of the main conductor 41, through the con ductor 40, the flexible conductor 39, the contact arm 33, the movable contact 23, the stationary contact 21, the conducting strip 31 to the other terminal 17.

When the circuit breaker is closed, and a persistent overload, such for example as from 125% of normal current to times normal current, occurs in any of the pole units, the heat generated by the current in the looped shunt conductor 85 is conducted to the bimetal 95 and the bimetal 95 flexes such that the upper end thereof moves to the right (FIG. 4) to engage the screw 101 to rotate the trip bar 107 clockwise (FIG. 4) whereupon the latch 121 releases the latch portion 117 of the latch structure 119 to free the cradle or trip member 63 (FIG. 1). When the circuit breaker is closed, the toggle 57, 59 is erected, and upon release of the cradle 63 the springs 61 operate to rotate the cradle 63 clockwise (FIG. 1) about the pivot 64 to cause collapse of the toggle 57, 59 to open the circuit breaker in a manner well known in the art. During this operation, the handle 65 is moved to an intermediate position intermediate the on and off positions to provide a visual indication that the circuit breaker has been tripped. The breaker cannot be closed following a tripping operation until the mechanism is reset and relatched. The mechanism is reset and relatched by moving the handle 65 to a position slightly past the full off position during which movement an extension 155 on the operating lever 55 engages a shoulder 157 on the cradle 63 pivoting the cradle counterclockwise (FIG. 1). Near the end of this movement, the free end of the cradle 63 Wipes past the latch structure 119, and upon release of the handle 65, the latch structure 119, which is again latched by the latch 121 of the trip bar 107, latches the free end of the cradle 63 in a manner well known in the art. After the cradle 63 has been relatched, the operating handle 65 can be moved to the on position to close the contacts in the same manner as was hereinbefore described.

During the above-described thermal tripping operation the bimetal 95 flexes with a time delay so that if the overload is a momentary overload the circuit breaker will not trip out. Upon the occurrence of a severe overload of 10 times or more of the normal current through any of the pole units, the electromagnet 125, 127, which is energized by the full current in the conductors 41,

85, is operated to effect instantaneous attraction of the armature 127 toward the core 125 and the armature 127 pivots on the one leg of the core 125 moving toward the other leg of the core 125. During this movement, the armature structure 151 is moved downward and the nut 137 engages the associated projection or plate 141 to pivot the trip bar 107 clockwise (FIG. 4) whereupon the cradle 63 is released to effect a tripping operation in the same manner as was hereinbefore described. The circuit breaker is reset and relatched following an electromagnet tripping operation in the same manner as was hereinbefore described.

During assembly of the circuit breaker, the thermal trip is calibrated by adjusting the position of the screw 101 (FIG. 4) on the extension 103 of the trip bar 107 to thereby adjuts the initial position of the air gap between the bimetal 95 and the adjusting screw 101. The electromagnetic trip is also adjusted during the assembly of the circuit breaker. Referring to FIG. 3, the elongated rod 149 comprises a top portion 161 that is threaded into a bottom portion 163. Rotation of either of these portions 161 or 163 with relation to the other portion will thereby vary the length of the rod 149 to thereby adjust the amount of air gap when the adjusting knob 143 is at a particular position. The electromagnetic trip is also adjusted by rotating the nut 137 to provide that the nut 137 will engage the associated plate 141 near the end of the armature tripping movement and to provide that the nut 137 will in fact operate to rotate the trip bar 107 to a tripping position near the end of the armature movement.

As was previously described, the trip device 29 is a removable trip device mounted in the circuit breaker by means of the screw 42 (FIG. 1), and the main conductors 41 are subjected to stresses that can move the main conductors 41 when the screws 42 are tightened and loosened and when the terminals 19 (FIG. 1) are tightened or loosened. With the construction of the trip device as disclosed, the movement of the main conductors 41 under stresses will not move the magnetic cores 125, the armature structures 151, the elongated rods 149, the bimetals 95, or the trip bar 107. As can be seen in FIG. 4, the rivets 79 fixedly secure the magnetic coil and bracket -89 to the base 75 which is fixedly secured to the supporting bracket 77 so that these parts are all fixed with relation to each other. The trip bar 107 is mounted on the supporting arms 113 which are in turn mounted on the bracket 77 so that the pivots 117 of the trip bar are fixed with relation to the supporting bracket 77. The rod 149 engages the armature 127 and the cam surface of the adjusting knob 143 that is mounted on the bracket 145 that is fixed to the housing base 75 so that the adjusted position of the adjusting rod 149 is fixed with relation to the supported bracket 77. Thus, although there can be movement of the rod 149 during adjustment, and although there is movement of the armature 127 during a tripping operation, and although there is movement of the bimetal 95 during a thermal tripping operation, these movements are not movements that will adversely affect the calibration or adjustment of the trip device, and movement of the trip device as a unit or movement of the main conductors 41 under the above-mentioned stresses will not adversely affect the calibration or adjustment of either the thermal trip or the electromagnetic trip. With the bimetal 95 mounted on a portion of the looped conductor 85, and with the looped conductor 85 shunting current around a portion of the main conductor 41, the desirable thermal tripping characteristics are achieved. With the full pole-unit current energizing the electromagnet 125, 127, the desirable electromagnet tripping characteristics are achieved.

We claim:

1. A circuit breaker comprising a pair of contacts for opening and closing a circuit, a trip device comprising a main conductor in electrical series with said contacts, said trip device comprising a trip member operatively movable to effect automatic openings of said contacts, said trip device comprising a shunt conductor connected to provide a parallel current path parallel to a portion of said main conductor for shunting current around said portion of said main conductor, said trip device comprising a thermal trip and an electromagnetic trip, said thermal trip comprising a bimetal connected to said shunt conductor in a heat-conducting relationship with said shunt conductor and being in an unactuated position, said thermal trip being calibrated such that upon the occurrence of an overload current above a first predetermined value in said circuit said bimetal is heated by the heat generated in said shunt conductor and flexes to operatively move said trip member, said electromagnetic trip comprising a magnetic core and. a magnetic armature structure supported to be energized by the full current in said circuit, said core and armature structure being in an unactuated position, said electromagnetic trip being calibrated such that upon the occurrence of an overload current in said circuit above a second predetermined value said armature structure moves toward said core to operatively move said trip member, said trip device comprising support means, said support means supporting said trip member and said bimetal such that said trip member and said bimetal cannot move relative to each other in such a Way as to destroy the calibration of said thermal trip, said support means supporting said trip member and said core and armature structure such that said trip member and said core and armature structure cannot move relative 7 to each other in such a way as to destroy the calibration of said electromagnetic trip.

2'. A circuit breaker according to claim 1, said circuit breaker comprising an insulating housing with said contacts being supported within said housing, said trip device comprising a removable trip device removably supported within said housing, and said main conductor comprising terminal means at opposite ends thereof to permit connection and disconnection of said trip device in electrical series with said contacts in said circuit to be controlled by said contacts.

3. A circuit breaker according to claim 2; said circuit breaker comprising a multi-pole circuit breaker with a pair of said contacts for each pole; said trip device comprising a multi-pole tripdevice with a separate main conductor, shunt conductor, bimetal, magnetic core and magnetic armature structure for each pole; and said trip member comprising a common trip mem'ber common to all of said poles and being supported for pivotal movement about a fixed pivot on said support means.

4. A circuit breaker comprising a pair of contacts for opening and closing a circuit, a trip device comprising a main conductor in electrical series with said contacts, said trip device comprising a trip member pivotally supported on a fixed pivot and operable'to effect automatic opening of said contacts, said trip device comprising a shunt conductor connected at opposite ends thereof to said main conductor and comprising an intermediate portion spaced from said main conductor to provide a'parallel shunt path parallel to a portion of said main conductor for shunting current around said portion of said main conductor, said trip device comprising a bimetal fixedly connected at one end thereof .to said intermediate portion of said shunt conductor and having the other end free to move when the bimetal flexes, upon the occurrence of an overload current above a first predetermined value said bimetal being heated by the current in said shunt conductor and flexing to operatively move said trip member, said trip device comprising a generally U-shaped magnetic core supported to be energized by the full current in said main conductor and shunt conductor, said trip device comprising an armature structure pivotally supported on one leg of said core and pivotally movable about the pivot thereof toward and away from the other leg of said core, upon the occurrence of an overload above a second predetermined value said armature structure moving toward the other leg of said core to operatively move said trip member, and said trip device comprising support means supporting said flexed pivot of said trip member said one end of said bimetal and said core in positions fixed with relation to each other on said support means.

5. A circuit breaker according to claim 4, said circuit breaker comprising an insulating housing with said contacts being supported within said housing, said trip device being a removable trip device removable as a unit from said housing, said main conductor extending between the legs of said magnetic core and comprising terminal means at the opposite ends thereof to enable connection and disconnection of said contacts and said main conductor in said circuit to be controlled by said contacts.

6. A circuit breaker according to claim 5; said circuit breaker comprising a multi-pole circuit breaker with a pair of said contacts for each pole; said trip device comprising a multi-pole trip device with a separate main conductor, shunt conductor, bimetal, magnetic core and armature structure for each pole; and said trip member comprising a common trip member common to all of said polesand being supported for pivotal movement about a fixed pivot on said support means.

References Cited UNITED STATES PATENTS 3,211,860 10/1965 Stephenson 337-50 3,244,837 4/1966 Strobel 33750 BERNARD A. GILHEANY, Primary Examiner H. BROOME, Assistant Examiner

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3711748A (en) * 1971-05-07 1973-01-16 Square D Co Electric plug-on circuit breaker with means for assuring tripping of the breaker before unplugging
US4181906A (en) * 1977-11-23 1980-01-01 Westinghouse Electric Corp. Circuit interrupter with interlocked removable trip unit
US4603312A (en) * 1985-03-21 1986-07-29 Westinghouse Electric Corp. Circuit breaker with adjustable trip unit
US4698606A (en) * 1986-06-20 1987-10-06 Westinghouse Electric Corp. Circuit breaker with adjustable thermal trip unit
US5828276A (en) * 1996-01-22 1998-10-27 Nitto Electric Works, Ltd. Molded-case multicircuit breakers

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3211860A (en) * 1960-03-02 1965-10-12 Westinghouse Electric Corp Circuit breaker with improved trip-device enclosure
US3244837A (en) * 1962-12-17 1966-04-05 Ite Circuit Breaker Ltd Heating means for bimetal trip element

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3211860A (en) * 1960-03-02 1965-10-12 Westinghouse Electric Corp Circuit breaker with improved trip-device enclosure
US3244837A (en) * 1962-12-17 1966-04-05 Ite Circuit Breaker Ltd Heating means for bimetal trip element

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3711748A (en) * 1971-05-07 1973-01-16 Square D Co Electric plug-on circuit breaker with means for assuring tripping of the breaker before unplugging
US4181906A (en) * 1977-11-23 1980-01-01 Westinghouse Electric Corp. Circuit interrupter with interlocked removable trip unit
US4603312A (en) * 1985-03-21 1986-07-29 Westinghouse Electric Corp. Circuit breaker with adjustable trip unit
US4698606A (en) * 1986-06-20 1987-10-06 Westinghouse Electric Corp. Circuit breaker with adjustable thermal trip unit
US5828276A (en) * 1996-01-22 1998-10-27 Nitto Electric Works, Ltd. Molded-case multicircuit breakers

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