# Patent

## Title
Trip mechanism for circuit breaker

## Abstract

Disclosed is a trip mechanism for a circuit breaker, capable of maintaining a constant characteristic of a normal trip operation in spite of changes in installation angles of the circuit breaker including a horizontal installation, a perpendicular installation and an inclination installation, the trip mechanism including, a coil configured to generate a magnetic absorption force due to a fault current on a circuit, a stationary core fixed to a lower portion of the coil within the coil, a movable core movable down to the stationary core within the coil by the magnetic absorption force generated from the coil, and a trip lever rotatably to connected to the movable core so as to be pulled by the movable core when the movable core is downwardly moved and configured to trigger the switching mechanism such that the movable contactor is moved to an open circuit position.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
Pursuant to 35 U.S.C. §119(a), this application claims the benefit of earlier filing date and right of priority to Korean Application No. 10-2008-0138667, filed on Dec. 31, 2008, the contents of which is incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit breaker, and particularly, to a trip mechanism for a molded case circuit breaker.
2. Background of the Invention
In general, a molded case circuit breaker is an electrical device for protecting an electric load or an electric circuit by breaking an electric circuit upon occurrence of fault current such as an over current, an instantaneous current and a short-circuit current between an electric power source and the electric load.
A molded case circuit breaker includes a terminal part by which a power source and an electric load are connected, a stationary (fixed) contactor electrically connected to the terminal part and disposed within the molded case circuit breaker, a movable contactor having a position contacted with the stationary contactor and a position separated from the stationary contactor, a switching mechanism for operating the movable contactor to contact the stationary contactor or be separated from the stationary contactor, and a trip mechanism for detecting occurrence of an overcurrent or an abnormal large current such as a short circuit current needed to be instantaneously broken, so as to trigger a trip operation of the switching mechanism for circuit breaking.
A representative example of the trip mechanism may be a full electromagnetic type trip mechanism using a thermal expansibility of oil within a cylinder wound by coil.
The construction and operation of the full electromagnetic trip mechanism according to the related art will now be described with reference to FIGS. 1 to 3.
FIG. 1 is a perspective view showing an inner construction of a molded case circuit breaker according to the related art, FIG. 2 is a perspective view independently showing a trip mechanism of FIG. 1, and FIG. 3 is a longitudinal sectional view of the trip mechanism of FIG. 2.
As shown in FIG. 1, a molded case circuit breaker 100 according to the related art includes an outer case 101 in which components are disposed, a switching mechanism 120 for rendering a movable contactor rotated to a position contactable with a corresponding stationary contactor (not shown) or a position separated from the stationary contactor so as to close (or turn on) a circuit or open (or turn off) the circuit, a trip mechanism 110 for detecting an occurrence of a fault current such as short circuit current on an electrical circuit so as to trigger a trip operation of the switching mechanism 120, an arc extinguishing mechanism 130 for extinguishing arc generated between the movable contactor 125 and the stationary contactor (not shown) during the trip operation, and terminals 140 by which an electrical power source or an electrical load of the circuit is connected.
The switching mechanism 120 includes a handle 123 for allowing a user to manually close or open a circuit of the molded case circuit breaker, a stationary contactor (not shown), a movable contactor 125 rotatable in response to the manipulation of the handle 123 and having a position contactable with the stationary contactor or a position separated from the stationary contactor, a crossbar (not shown) rotated by the trip mechanism 110, a latch holder (not shown) locked or released by virtue of the crossbar, and released upon the rotation of the crossbar so as to be rotated by a bias spring (not shown), a latch (not shown) having a position latched by the latch holder or a position released in response to the rotation of the latch holder, a trip spring (not shown) for providing the movable contactor with a driving force to rotate to the separated position upon release of the latch, a link (now shown) for transferring the driving force of the trip spring, and a shaft 124 connected to the link and for supporting the movable contactor 125 and rotating together with the movable contactor 125 so as to drive the movable contactor 125 to an open circuit position or a closed circuit position.
Hereinafter, detailed construction and operation of the trip mechanism 110 according to the related art will be described with reference to FIGS. 2 and 3.
As shown in FIGS. 2 and 3, the trip mechanism 110 includes a coil 111 connected to a circuit for generating an electromagnetic force upon occurrence of a fault current, such as a short circuit current on the circuit, a cylinder 113 inserted into the coil 111, a contactor 117 located at an upper end of the cylinder 113, an armature 115 abutted onto the contactor 117 by the electromagnetic force generated by the coil 111 so as to operate a trip lever 115a, and a frame 119 coupled with the cylinder 113 and the armature 115 and connecting a terminal part 140 to the coil 111.
Also, the cylinder 113 may be formed of a non-magnetic material, and provided therein with a plunger 114 movable by the magnetic force generated by the coil 111, and a plunger spring 114s for elastically supporting the plunger 114 such that an elastic restoring force can be applied in an opposite direction to a movement direction of the plunger 114. The cylinder 113 is sealed with containing oil 114a therein. Here, silicon oil is typically used as the oil 114a. 
Further, the armature 115 is elastically supported by an armature spring 115s so as to be restored to its original position after completion of a trip operation.
Hereinafter, the operation of the trip mechanism 110 according to the related art will be described with reference to FIGS. 2 and 3.
First, if an overcurrent larger than a rated current by about 120% is detected, a time-delay trip operation is normally performed. Under this condition, the plunger 114 exceeds the force of the plunger spring 114s by the magnetic force generated from the coil 111 due to the overcurrent, to thereby move toward the contactor 117, thereby attracting the armature 115. The movement of the attracted armature 115 then triggers the trip operation of the switching mechanism 120 shown in FIG. 1.
In the meantime, when a current several or several tens times larger than the rated current flows instantaneously, an instantaneous operation is performed. Under this condition, the magnetic force generated by the coil becomes drastically stronger than that in the time-delay operation. Accordingly, the armature 115 is attracted independently even if the plunger 114 moves or not, thereby triggering the trip operation of the switching mechanism 120 shown in FIG. 1.
Now, the triggered trip operation of the switching mechanism 120 will be described with reference to FIG. 1.
When the trip mechanism 110 detects a fault current such as a short circuit current on the circuit, the armature 115 provided at the trip mechanism 110 is rotated in a clockwise direction in FIG. 1 so as to push the cross bar. Accordingly, the crossbar is rotated and accordingly pushes the latch holder directly or via other components. The rotation of the latch holder releases the latch, and accordingly the trip spring discharges an accumulated elastic energy. The discharged elastic energy is transferred to the shaft 124 via the link (not shown). That the shaft 124 is then rotated in a clockwise direction in FIG. 1 and accordingly the movable contactor 125 supported by the shaft 124 is also rotated in the clockwise direction. As the movable contactor 125 rotated in the clockwise direction is separated from to the corresponding stationary contactor, the trip operation for breaking a circuit is completed.
However, the related art molded case circuit breaker 100 has several problems as follows.
First, because of the movement of the plunger 114 within the cylinder 113, the trip mechanism should be responsive to currents ranging from an overcurrent as a relatively low current compared to a short circuit current to an abnormal large current needed to be instantaneously broken, resulting in increasing a range of a magnetic force required for trip operation upon occurrence of the overcurrent and occurrence of the abnormal large current, such as a short circuit current needed to be instantaneously broken. To implement such wide range, the moving distance of the plunger 114 should be increased. Consequently, the cylinder 113 and the plunger 114 occupy a larger space and simultaneously increase the number of winding the coil 111.
Furthermore, viscosity of the oil 114a contained in the cylinder 113 changes depending on installation angles of the molded case circuit breaker 100, and the moving distance of the plunger 114 depends on the viscosity of the oil 114a. Consequently, upon occurrence of an overcurrent or an abnormal large current such as a short circuit current needed to be instantaneously broken, a characteristic of a trip operation problematically changes.
SUMMARY OF THE INVENTION
Therefore, to obviate the drawbacks of the related art, a first object of the present invention is to provide a compact trip mechanism for a circuit breaker.
A second object of the present invention is to provide a trip mechanism for a circuit breaker, the circuit breaker independently having a trip mechanism for overcurrent on a circuit and another trip mechanism for an abnormal large current needed to be instantaneously broken.
A third object of the present invention is to provide a trip mechanism for a circuit breaker capable of constantly maintaining a trip operation characteristic independent of changes in installation angles.
To achieve the first and third objects in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a trip mechanism for breaking a large current such as a short circuit current needed to be instantaneously broken in a circuit breaker, provided with a stationary contactor, a movable contactor movable to an open circuit position contacted with the stationary contactor and a closed circuit position separated from the stationary contactor, and a switching mechanism configured to drive the movable contactor to the open circuit position or the closed circuit position, the trip mechanism including, a coil configured to generate a magnetic absorption force due to a fault current on an electrical circuit, a stationary core fixed to a lower portion of the coil within the coil, a movable core movable down to the stationary core within the coil by the magnetic absorption force generated from the coil, and a trip lever rotatably connected to the movable core so as to be pulled by the movable core when the movable core is downwardly moved, and configured to trigger the switching mechanism such that the movable contactor is moved to the open circuit position.
To achieve the second object in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a trip mechanism for a circuit breaker further including a bimetal assembly serving as a trip mechanism for an overcurrent breaking, the bimetal assembly configured to break an electrical circuit upon occurrence of the overcurrent on the circuit, in addition to the trip mechanism for breaking the large current needed to be instantaneously broken.
The foregoing and other objects, features, aspects and advantages of the is present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.
In the drawings:
FIG. 1 is a perspective view showing an inner construction of a molded case circuit breaker according to the related art;
FIG. 2 is a perspective view independently showing a trip mechanism of FIG. 1;
FIG. 3 is a longitudinal sectional view of the trip mechanism of FIG. 2;
FIG. 4 is a perspective view showing an inner construction of a molded case circuit breaker having a trip mechanism in accordance with one embodiment of the present invention;
FIG. 5 is a perspective view independently showing an assembled state of to the trip mechanism in accordance with the one embodiment of the present invention;
FIG. 6 is an exploded view of the trip mechanism in accordance with the one embodiment of the present invention; and
FIG. 7 is exploded view of a coil assembly of the trip mechanism in accordance with the one embodiment of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a trip mechanism for a circuit breaker in accordance with one embodiment of the present invention will be described with reference to FIGS. 4 to 7.
First, description will be given with reference to FIG. 4 which is a perspective view showing an inner construction of a molded case circuit breaker having a trip mechanism in accordance with one embodiment of the present invention.
As shown in FIG. 4, a circuit breaker 200 may include terminal parts 240, an arc extinguishing mechanism 230, a switching mechanism 220, and a trip mechanism 210.
Here, the terminal parts 240, the arc extinguishing mechanism 230, the switching mechanism 220 and the trip mechanism 210 may be provided within a single case 201, which is molded by use of synthetic resin with an electrically insulating property.
The terminal parts 240 may be made of an electrical conductor piece such as copper, and provided at both ends in a lengthwise direction of the case 201. An electrical power source side and an electrical load side may be electrically connected to both the terminal parts 240, respectively. FIG. 4 exemplarily shows a three-phase alternating current (AC) circuit breaker, in which the power source side terminal part and the load side terminal part each has three terminals in correspondence with the three-phase AC.
The arc extinguishing mechanism 230, as well known, is to extinguish arc, which is generated between contacts upon a trip operation, namely, circuit breaking, and provided for each of three phases. The arc extinguishing mechanism 230 may be implemented as a plurality of U-shaped arc shielding plates, which are perpendicularly stacked by each other with a predetermined gap.
The switching mechanism 220 is to connect or break the power source side (i.e., the power source side terminal part 240) to or from the load side (i.e., the load side terminal part 240). The switching mechanism 220 may include, for example, a handle 223, a nail 215, a crossbar 213, a latch (not shown), a latch holder (not shown), a trip spring (not shown) and the like.
The handle 223 may have an externally exposed upper portion such that a user can manually manipulate it to an opened or closed position (i.e., OFF or On position) of a circuit between the power source side and the load side, and also have a lower portion rotatably supported within the case 201.
The nail 215 may have one side coupled by a hinge so as to be rotatable, and lock the latch holder to maintain a state of the ON position when the handle 223 is present at the ON position.
The crossbar 213 may be rotated by an external force due to the trip mechanism 210 and also configured to rotate the nail 215 so as to release the latch holder.
Hereinafter, a trip operation due to triggering of the trip mechanism 210 will be briefly described.
Upon occurrence of a fault current on a circuit, such as short circuit current or overcurrent, the trip mechanism 210 detects such fault current and accordingly pushes the crossbar 213 to be rotated. Consequently, the nail 215 is rotated in interlocking with the rotation of the crossbar 213, thereby releasing the latch holder.
As the latch holder is released, the latch is movable freely. Accordingly, the trip spring discharges an charged elastic energy toward the shaft via a link member (not shown). The shaft is thereby rotated so that the movable contactor supported by the shaft is separated from the corresponding stationary contactor, thereby completing an automatic circuit breaking, namely, the trip operation.
Here, the crossbar 213 may be rotated by being pressed by virtue of a triggering protrusion 216a or a protruding member 218a included in the trip mechanism 210.
Hereinafter, description will be given of a detailed construction and operation of the trip mechanism for the circuit breaker in accordance with the preferred embodiment of the present invention, with reference to FIGS. 4 to 7.
The trip mechanism 210 for the circuit breaker according to the preferred embodiment of the present invention may include a coil 211, a stationary core 211c, a movable core 214 and a trip lever 216.
The coil 211 may be magnetized by a fault current on a circuit so as to generate a magnetic absorption force. The coil 211, as shown in FIG. 7, is wound on an outer circumferential surface of a bobbin 211d having a hollow central portion and also supported by the bobbin 211d. One end portion of the coil 211 to may be connected to a yoke 211b, which is formed of an electrically conductive material for supporting the bobbin 211d at a lower side of the bobbin 211d. The yoke 211b may be electrically connected to the power source or the load of the circuit, as shown in FIG. 6, via a lower supporting plate 212b connected to support a lower portion of the yoke 211b, a lower supporting portion (see FIG. 6) of a bimetal 218 of a bimetal assembly and a terminal 240. Therefore, the coil 211 may be electrically connected to the power source side or the load side of the circuit via the yoke 211b, the lower supporting plate 212b, the lower supporting portion of the bimetal 218 and the terminal 240. As shown in FIG. 7, an assembly of the coil 211 and the bobbin 211d may be disposed between an upper cover 211a having a through hole 211a1 through which the movable core 214 is inserted and the yoke 211b, which is coupled to the upper cover 211a and supports the bobbin 211d at the lower side of the bobbin 211d. 
Still referring to FIG. 7, the stationary core 211c may be fixed in a lower portion of the coil 211 and extend upwardly within the coil 211. Alternatively, the stationary core 211c may be, for example, welded onto a bottom surface of the yoke 211b, or integrally molded with the yoke 211b. According to the preferred embodiment, the stationary core 211c may be formed of a magnetic material or steel so as to serve to increase a magnetic absorption force of the coil 211 attracting the movable core 214.
As shown in FIG. 6, the movable core 214 may be movable down to the stationary core 211c (see FIG. 7) within the coil 211 by the magnetic absorption force from the coil 211. The movable core 214 may be provided with two circular plate portions 214h outwardly protruding from an outer surface of its upper end in a radial direction and a neck portion 214n having a diameter shorter than that of the circular plate portions 214h. The movable core 214 may be schematically configured in a shape of a long cylindrical steel bar.
The trip mechanism for the circuit breaker according to the embodiment of the present invention may further include a bias spring 214s installed in the hollow central portion of the bobbin 211d for applying an elastic force to the movable core 214 to be pushed away from the stationary core 211c. 
As shown in FIGS. 5 and 6, the trip lever 216 may be rotatably connected to the movable core 214 so as to be pulled by the movable core 214 when the movable core 214 is downwardly moved. Also, the trip lever 216 may trigger the switching mechanism 220 (see FIG. 4) such that the movable contactor (not shown) is moved to an open circuit position. Referring to FIG. 6, the trip lever 216 may include hinge portions 216b at both ends thereof, and a channel portion 216c in which the neck portion 214h 214n of the movable core 214 is inserted (engaged). The trip lever 216 may further include a triggering protrusion 216a upwardly extending from the channel portion 216c for pushing the switching mechanism 220 (see FIG. 4), especially, the crossbar 213 of the switching mechanism 220 so as to trigger the switching mechanism 220. The hinge portions 216b of the trip lever 216 may be rotatably supported by a pair of hinge supporting opening portions 212a disposed at upper ends of a pair of supporting plates 212 to be explained later.
A width (interval) of the channel portion 216c of the trip lever 216 is greater than a diameter of the neck portion 214n of the movable core 214 but smaller than a diameter of the circular plate portion 214h of the movable core 214. Consequently, the movable core 214 and the trip lever 216 may interlock each other in a state where the neck portion 214h 214n of the movable core 214 is engaged with the channel portion 216c of the trip lever 216. Therefore, an up-and-down movement distance of the movable core 214 can be limited by the trip lever 216. Also, in this structure, when the movable core 214 is moved upwardly due to the bias spring 214s, the separation of the movable core 214 from the coil 211 can be prevented.
The trip mechanism for the circuit breaker, as also shown in FIG. 6, may is further include a lower supporting plate 212b configured to support the yoke 211b at the lower side thereof and electrically connected to a circuit between the power source and the load. The lower supporting plate 212b may preferably be formed of a conductive material.
The trip mechanism, as shown in FIGS. 5 and 6, may further include a bimetal assembly (217, 218, 218a) electrically connected to a circuit between the power source and the load, and thermally displaced upon occurrence of an overcurrent on a circuit so as to push the switching mechanism 220, thereby triggering a trip operation of the switching mechanism 220.
The bimetal assembly, as shown in FIG. 6, may include a heater 217, a bimetal 218 and a protruding member 218a. 
Referring to FIGS. 5 and 6, the heater 217 may be electrically connected to the circuit between the power source and the load so as to be heated up by the overcurrent on the circuit.
Also, the bimetal 218 may have an upper end portion as a free end portion and a lower end portion fixed to a lower supporting plate (reference numeral not given). Therefore, in view of the bimetal 218 wrapped by the heater 217, the lower end portion thereof is fixed and only the upper end portion is bent by the heating of the heater 217. Here, the terminal 240 may be electrically connected to the circuit between the power source and the load, and the heater 217 may be electrically to connected to the terminal 240.
The protruding member 218a may be disposed at an upper portion of the bimetal 218 to protrude from the bimetal 218 toward the switching mechanism 220, especially, the crossbar 213 of the switching mechanism 220. As the bimetal 218 is bent, the protruding member 218a may push the switching mechanism 220, especially, the crossbar 213, thereby triggering the trip operation of the switching mechanism 220.
Still referring to FIGS. 5 and 6, the trip mechanism for the circuit breaker according to the embodiment of the present invention may further include a pair of supporting plates 212 for supporting the coil 211, the bimetal assembly (217, 218, 218a) and the trip lever 216 at both sides thereof. The pair of supporting plates 212 may have hinge supporting opening portions 212a for supporting the hinge portions 216b at both ends of the trip lever 216.
Hereinafter, description will be given of the trip operation of the trip mechanism for the circuit breaker according to the preferred embodiment of the present invention, with reference to FIGS. 4 to 7.
First, if an overcurrent higher than a rated current by about 120% is detected, a time-delay trip operation is performed by the trip mechanism for the circuit breaker according to the preferred embodiment of the present invention. Now, the time-delay trip operation will be described.
If an overcurrent higher than a rate current by about 120% flows on the circuit, the heater 217 electrically connected to the circuit via the terminal 240 is heated up due to the overcurrent and accordingly the bimetal 218 is bent. The protruding member 218a located at the upper end portion of the bimetal 218 then pushes the switching mechanism 220, especially, the crossbar 213 (see FIG. 4). Cooperatively, the crossbar 213 is rotated to trip the switching mechanism 220 such that the movable contactor is separated from the stationary contactor, resulting in completion of the trip operation. That is, the crossbar 213 releases a nail (not shown) of the switching mechanism, and the nail is rotated by a bias spring (not shown). Accordingly, the rotating nail pushes up a latch holder (not is shown) to be rotated. Consequently, a latch (not shown) locked by the latch holder is released such that a trip spring (not shown) discharges an charged elastic energy. The elastic energy renders the movable contactor rotated and thereby the movable contactor is separated from the stationary contactor, thereby completing the trip operation.
In the meantime, if a current several times or several tens times higher than a rated current flows on the circuit, an instantaneous trip operation is performed, as follows, by the trip mechanism for the circuit breaker according to the preferred embodiment of the present invention.
If the current several times or several tens times higher than the rated current flows on the circuit, a strong magnetic absorption force is generated by the coil 211 and the stationary core 211c electrically connected to the circuit, and thereby the movable core 214 is attracted (absorbed) toward the stationary core 211c so as to be moved downwardly. In response to the downward movement of the movable core 214, the trip lever 216 is pulled downwardly to be rotated based upon the hinge portions 216b as a rotation shaft. Cooperatively, the triggering protrusion 216a of the trip lever 216 is rotated to press the crossbar 213, which is thereby rotated. As mentioned in the description of the time-delay operation with reference to FIG. 4, the rotation of the crossbar 213 trips the switching mechanism so that the movable contactor is separated from the stationary contactor, resulting in completion of the trip operation.
As described above, the trip mechanism for the circuit breaker according to the present invention with the construction does not need an oil cylinder and a plunger, which occupy large areas, and also independently has a trip mechanism for an overcurrent on a circuit and a trip mechanism for an abnormal large current, such as short-circuit current needed to be instantaneously broken. Hence, in comparison with the related art trip mechanism which causes an increase in the size of the oil cylinder and the number of winding a coil because one oil cylinder and a coil are used to detect both overcurrent and short-circuit current and perform a trip operation, the trip mechanism for the circuit breaker according to the present invention can become compact in size by preventing the increase in the size thereof and the number of unnecessarily winding a coil.
Also, the trip mechanism for the circuit breaker according to the present invention does not use an oil cylinder, which is advantageous for constantly maintaining a trip operation characteristic in spite of changes in installation angles and installation environments of the circuit breaker.
The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present disclosure. The present teachings can be readily applied to other types of apparatuses. This description is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. The features, structures, methods, and other characteristics of the exemplary embodiments described herein may be combined in various ways to obtain additional and/or alternative exemplary embodiments.
As the present features may be embodied in several forms without departing from the characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalents of such metes and bounds are therefore intended to be embraced by the appended claims.

## Claims

1. A trip mechanism for a circuit breaker, provided with a stationary contactor, a movable contactor movable to an open circuit position separated from the stationary contactor and a closed circuit position contacted with the stationary contactor, and a switching mechanism configured to drive the movable contactor to the open circuit position or the closed circuit position, the trip mechanism comprising:
a coil configured to generate a magnetic absorption force due to a fault current on a circuit;
a stationary core fixed to a lower portion of the coil within the coil;
a movable core movable down to the stationary core within the coil by the magnetic absorption force generated from the coil; and
a trip lever rotatably connected to the movable core so as to be pulled by the movable core when the movable core is downwardly moved, and configured to trigger the switching mechanism such that the movable contactor is moved to the open circuit position,
wherein the movable core comprises a pair of circular plate portions outwardly protruding from an upper end thereof in a radial direction and a neck portion formed between the pair of circular plate portions and
said trip lever comprises hinge portions at both ends thereof and a channel portion engaged with the neck portion of the movable core, and
wherein the trip lever further comprises a triggering protrusion extending from the channel portion and configured to push the switching mechanism for triggering the switching mechanism to trip.

2. The mechanism of claim 1, further comprising:
a bobbin having a hollow central portion on which the coil is supported; and
a yoke configured to support the bobbin, the stationary core being installed on a bottom surface of the yoke to upwardly extend within the coil.

3. The mechanism of claim 2, further comprising a lower supporting plate electrically connected to a circuit between a power source and an electric load and configured to support the yoke at a lower side of the yoke.

4. The mechanism of claim 2, further comprising a bias spring located in the hollow central portion of the bobbin and configured to apply an elastic force to the movable core to be pushed away from the stationary core.

5. The mechanism of claim 1, further comprising a bimetal assembly electrically connected to a circuit between a power source and an electrical load, and thermally displaced upon occurrence of an overcurrent on the circuit so as to push the switching mechanism, thereby triggering a trip operation of the switching mechanism.

6. The mechanism of claim 5, wherein the bimetal assembly comprises:
a heater electrically connected to the circuit between the power source and the load and configured to be heated up by the overcurrent on the circuit;
a bimetal having the heater wrapped thereon and bent by the heating of the heater; and
a protruding member disposed at an upper portion of the bimetal to protrude from the bimetal toward the switching mechanism, wherein the protruding member pushes the switching mechanism to trigger the trip operation of the switching mechanism as the bimetal is bent.

7. The mechanism of claim 6, wherein the bimetal is electrically connected to a terminal electrically connected to the circuit.

8. The mechanism of claim 7, wherein the supporting plates comprise hinge supporting opening portions, respectively, configured to support the hinge portions located at both sides of the trip lever.

9. The mechanism of claim 5, further comprising a pair of supporting plates configured to support the coil, the bimetal assembly and the trip lever at both sides thereof.