FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES, 2003 COMPLETE SPECIFICATION [See section 10, Rule 13] CIRCUIT BREAKER MITSUBISHI ELECTRIC CORPORATION, A CORPORATION ORGANISED AND EXISTING UNDER THE LAWS OF JAPAN, WHOSE ADDRESS IS 7-3, MARUNOUCHI 2-CHOME, CHIYODA-KU, TOKYO 100-8310, JAPAN THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED. DESCRIPTION Field [0001] The present invention relates to a circuit breaker including a fixed contact, a movable contact, and an electromagnetic operation mechanism for bringing the movable contact into contact with the fixed contact. Background [0002] As described in Patent Literature 1, a conventional circuit breaker typically includes a toggle mechanism and a closing spring, and uses the energy discharged when the closing spring with stored energy is released to close the movable contact beyond the dead point of the toggle mechanism. This type of circuit breaker is able to increase the speed of closing the movable contact by closing the movable contact beyond the dead point of the toggle mechanism. However, this type of circuit breaker also needs to open the movable contact beyond the dead point for interruption, and thus requires a large number of components and a complicated circuit breaker structure so as to ensure the opening speed. [0003] As a circuit breaker that does not use the closing spring described above, Patent Literature 2 discloses an electromagnetically operated circuit breaker in which the movable contact is closed by an electromagnetic operation mechanism and the movable contact is opened by an opening spring. Citation List Patent Literature [0004] Patent Literature 1: Japanese Patent Application Laid-open No. 2000-209719 Patent Literature 2: Japanese Patent Application Laid-open No. S49-113167 Summary Technical Problem [0005] The conventional electromagnetically operated circuit breaker can have a simplified circuit breaker structure because the movable contact is closed by the electromagnetic operation mechanism. However, the conventional electromagnetically operated circuit breaker needs to close the movable contact against the opening spring, and thus requires a large number of components so as to ensure the speed of closing the movable contact, which is a problem. [0006] An object of the present invention is to obtain a circuit breaker that closes the movable contact using an electromagnetic operation mechanism and opens the movable contact using an opening spring, and more specifically to a circuit breaker which can ensure the speed of closing the movable contact with a small number of components. Solution to Problem [0007] In order to solve the above-described problem and achieve the object, a circuit breaker according to an aspect of the present invention includes a switching contact to open and close an electric circuit; an opening spring; a toggle mechanism; and an electromagnetic operation mechanism. The opening spring urges the switching contact in a separating direction that separates the switching contact and increases an urging force that is a force to urge the switching contact in the separating direction when the switching contact moves from an open state to a close state. The toggle mechanism puts the switching contact from the open state into the close state. The electromagnetic operation mechanism includes a movable iron core and a fixed iron core. The movable iron core is configured to move against the urging force to cause the toggle mechanism to put the switching contact from the open state to the close state. The fixed iron core includes a first attracting surface to attract the movable iron core, and an inter-surface distance between the first attracting surface and the movable iron core is shorter than a distance that the movable iron core moves. Advantageous Effects of Invention [0008] The circuit breaker according to the present invention can achieve the effect of ensuring the speed of closing the movable contact with a small number of circuit breaker components. Brief Description of Drawings [0009] FIG. 1 is a view illustrating an exemplary configuration of a circuit breaker according to a first embodiment. FIG. 2 is a view illustrating an exemplary internal configuration of a housing of the circuit breaker according to the first embodiment. FIG. 3 is an enlarged view of an electromagnetic operation mechanism and the portion located above the electromagnetic operation mechanism illustrated in FIG. 2. FIG. 4 is an enlarged view of the electromagnetic operation mechanism illustrated in FIG. 2. FIG. 5 is a view illustrating the circuit breaker in a completely closed state according to the first embodiment. FIG. 6 is a diagram illustrating the relationship between the stroke of a movable iron core of the electromagnetic operation mechanism and the amount of load on a drive shaft of the electromagnetic operation mechanism from the open state to the closed state according to the first embodiment. FIG. 7 is a view illustrating an exemplary configuration of a circuit breaker according to a second embodiment. FIG. 8 is an exploded perspective view of an electromagnetic operation mechanism according to the second embodiment. FIG. 9 is an external perspective view illustrating the electromagnetic operation machanism according to the second embodiment in an assembled state. FIG. 10 is a plan view of the electromagnetic operation mechanism according to the second embodiment. FIG. 11 is a side view of the electromagnetic operation mechanism according to the second embodiment. FIG. 12 is a view illustrating an exemplary configuration of a magnetic plate according to the second embodiment. FIG. 13 is a view for explaining a method of coupling a first divided iron core and a second divided iron core using a first coupling member, a second coupling member, a third coupling member, and a fourth coupling member according to the second embodiment. FIG. 14 is a view illustrating how the electromagnetic operation mechanism is fixed to supports protruding from a partition wall of a housing according to the second embodiment. FIG. 15 is a view illustrating an exemplary configuration of a magnetic plate constituting a fixed iron core of an electromagnetic operation mechanism according to a third embodiment. FIG. 16 is a plan view of the electromagnetic operation mechanism according to the third embodiment. FIG. 17 is a view illustrating how the electromagnetic operation mechanism is fixed to supports protruding from a partition wall of a housing according to the third embodiment. FIG. 18 is a plan view illustrating another configuration of the electromagnetic operation mechanism according to the third embodiment. Description of Embodiments [0010] Hereinafter, circuit breakers according to embodiments of the present invention will be described in detail based on the drawings. The present invention is not limited to the embodiments. [0011] First Embodiment. A circuit breaker according to a first embodiment is a circuit breaker that opens and closes an electric circuit such as a low-voltage distribution line, and interrupts the electric circuit in response to detecting at least one of an overcurrent and a leakage current. In the following description, for convenience of explanation, the positive Z-axis direction is regarded as upward, the negative Z-axis direction is regarded as downward, the positive X-axis direction is regarded as right, the negative X-axis direction is regarded as left, the positive Y-axis direction is regarded as front, and the negative Y-axis direction is regarded as rear. In the following description, clockwise and counterclockwise directions mean clockwise and counterclockwise directions in the drawings described later. [0012] FIG. 1 is a view illustrating an exemplary configuration of the circuit breaker according to the first embodiment. FIG. 2 is a view illustrating an exemplary internal configuration of a housing of the circuit breaker according to the first embodiment. FIG. 3 is an enlarged view of an electromagnetic operation mechanism and the portion located above the electromagnetic operation mechanism illustrated in FIG. 2. FIG. 4 is an enlarged view of the electromagnetic operation mechanism illustrated in FIG. 2. FIG. 5 is a view illustrating the circuit breaker in a completely closed state according to the first embodiment. [0013] As illustrated in FIG. 1, a circuit breaker 1 according to the first embodiment includes a housing 2, a first fixed conductor 10, a second fixed conductor 11, a mover 20, and a flexible conductor 30. The housing 2 is formed of an insulating member. The first fixed conductor 10 is connected to a power supply side conductor (not illustrated). The second fixed conductor 11 is connected to a load side conductor (not illustrated). The mover 20 includes a movable contact 21. The flexible conductor 30 electrically connects the second fixed conductor 11 and the mover 20 and has flexibility. The movable contact 21 is an example of a switching contact. [0014] A first space 7 and a second space 8 separated by an insulating wall 4 are formed inside the housing 2. The first fixed conductor 10 penetrates a wall 3 of the housing 2 from the outside of the housing 2 to the first space 7. One end 10a of the first fixed conductor 10 protrudes outward and is connected to the power supply side conductor (not illustrated). The other end 10b of the first fixed conductor 10 is placed in the first space 7 and includes a fixed contact 13. [0015] Similarly to the first fixed conductor 10, the second fixed conductor 11 penetrates the wall 3 of the housing 2 from the outside of the housing 2 to the first space 7. One end 11a of the second fixed conductor 11 protrudes outward and is connected to the load side conductor (not illustrated). The other end lib of the second fixed conductor 11 is placed in the first space 7. [0016] The movable contact 21 is provided at one end 20a of the mover 20. The other end 20b of the mover 20 is connected to one end 30a of the flexible conductor 30. The other end 30b of the flexible conductor 30 is connected to the other end 11b of the second fixed conductor 11. [0017] The circuit breaker 1 also includes a holder 40, a contact pressure spring 41, and a mover pin 42 . The holder 4 0 is rotatably attached to the other end lib of the second fixed conductor 11. The contact pressure spring 41 is held by the holder 40. The mover pin 42 is rotatably held by the holder 40. The contact pressure spring 41 urges and rotates the mover 20 clockwise about the mover pin 42, and applies a contact pressure between the fixed contact 13 and the movable contact 21 when the movable contact 21 provided on the mover 20 is connected to the fixed contact 13. [0018] The circuit breaker 1 includes a toggle mechanism 50, an electromagnetic operation mechanism 60, a transmission mechanism 70, and a tripping mechanism 80. The toggle mechanism 50 is coupled to the mover 20. The electromagnetic operation mechanism 60 moves the mover 20 via the toggle mechanism 50. The transmission mechanism 70 couples the toggle mechanism 50 and the electromagnetic operation mechanism 60. The tripping mechanism 80 maintains a completely closed state and releases a completely closed state in the circuit breaker 1. Note that the toggle mechanism 50 is placed across the first space 7 and the second space 8, and the electromagnetic operation mechanism 60, the transmission mechanism 70, and the tripping mechanism 80 are placed in the second space 8. A configuration including the toggle mechanism 50 and the tripping mechanism 80 is also referred to as a switching device. [0019] Here, a closed state means that the fixed contact 13 and the movable contact 21 are in contact with each other, and a completely closed state means that the movable contact 21 is completely closed and the contact between the fixed contact 13 and the movable contact 21 is maintained. Closing action or closing operation indicates the action or operation of moving the movable contact 21 into contact with the fixed contact 13. Tripping action or tripping operation indicates the action or operation of moving the movable contact 21 away from the fixed contact 13. [0020] As illustrated in FIG. 2, the toggle mechanism 50 includes an operation arm 51, a coupling plate 53, a shaft 54, and a lever 55. The operation arm 51 is rotatably coupled at one end 51a to the mover 20 by the mover pin 42. The coupling plate 53 is rotatably coupled at one end 53a to the other end 51b of the operation arm 51 by a link pin 52. The shaft 54 is fixed to the other end 53b of the coupling plate 53 and rotates about a shaft center 56. The lever 55 is firmly fixed to the shaft 54 and rotates about the shaft center 56 together with the shaft 54. The operation arm 51 is an example of a first link, and the coupling plate 53 is an example of a second link. [0021] As illustrated in FIG. 1, the electromagnetic operation mechanism 60 is placed below the lever 55, and fixed to supports 5 and 6 protruding from the insulating wall 4 of the housing 2 toward the second space 8. [0022] As illustrated in FIG. 2, the electromagnetic operation mechanism 60 includes a fixed iron core 61, an electromagnetic coil 62, a movable iron core 63, and a drive shaft 64. The fixed iron core 61 is formed of a magnetic body. The electromagnetic coil 62 is fixed inside the fixed iron core 61. The movable iron core 63 can reciprocate linearly in the vertical direction. The drive shaft 64 is fixed to the movable iron core 63. The drive shaft 64 reciprocates in the vertical direction at a position spaced from the shaft center 5 6 in the left direction. Note that any method of fixing the movable iron core 63 and the drive shaft 64 may be used as long as the movable iron core 63 and the drive shaft 64 can be fixed. [0023] The drive shaft 64 is placed in the internal space of the fixed iron core 61 through a gap (not illustrated). When the electromagnetic coil 62 is energized, the drive shaft 64 moves in the vertical direction in the internal space of the fixed iron core 61. [0024] As illustrated in FIG. 3, the transmission mechanism 7 0 that couples the toggle mechanism 50 and the electromagnetic operation mechanism 60 includes coupling pins 71 and 72 and a coupling link 73. The coupling pin 71 passes through one coupling hole 7 4 of the coupling link 7 3 and a coupling hole 65 formed at the distal end of the drive shaft 64. The coupling pin 72 passes through the other coupling hole 75 of the coupling link 73 and a coupling hole (not illustrated) formed in the middle of the lever 55. [0025] The tripping mechanism 80 includes a frame 81 and an opening spring 82. The frame 81 is fixed to the housing 2. The opening spring 82 is stretched between the frame 81 and one end 55a of the lever 55. The opening spring 82 urges the mover 20 and the movable contact 21 in a separating direction, and increases a force to urge the movable contact 21 when the movable contact 21 moves from the open state to the close state, i.e. the contact state. Note that the force by which the opening spring 82 urges the movable contact 21 is referred to as an urging force. The tripping mechanism 80 also includes a trip bar 83, a trip lever 84, and a reset spring 85. The trip bar 83 is rotatably supported by the frame 81. The trip lever 84 is rotatably supported by the frame 81 through a shaft 84c provided at one end 84a. The reset spring 85 is stretched between the frame 81 and the trip lever 84. [0026] The frame 81 is coupled to the insulating wall 4 by a fixing member (not illustrated). The fixing member that fixes the frame 81 to the insulating wall 4 is, for example, a pin. The frame 81 can be coupled to the insulating wall 4 by swaging the pin. When viewed in the direction in which the shaft center 56 extends, which is the axial direction of the coupling plate 53, the frame 81 faces a part of the toggle mechanism 50 and at least a part of the transmission mechanism 70 and covers a part of the toggle mechanism 50 and at least a part of the transmission mechanism 70. [0027] One end 82a of the opening spring 82 is held by a coupling pin 86. The coupling pin 8 6 is inserted into a coupling hole (not illustrated) formed in the one end 55a of the lever 55. The other end 82b of the opening spring 82 is held by a coupling pin 87. The coupling pin 87 is inserted into a coupling hole (not illustrated) formed in the frame 81. Consequently, the opening spring 82 is stretched between the frame 81 and the lever 55. [0028] The opening spring 82 is a tension spring that stores energy when the lever 55 is rotated clockwise about the shaft center 5 6 of the shaft 54 while the opening spring 82 is stretched between the frame 81 and the lever 55. The opening spring 82 applies, to the lever 55, a force in the counterclockwise direction about the shaft center 56. [0029] A shaft 83c extending in the front-rear direction is fixed to the trip bar 83. The shaft 83c is rotatably inserted into a rotation hole (not illustrated) provided in the frame 81. The trip bar 83 includes a semicircle 83a formed in a semicircular shape. As will be described later, when the circuit breaker 1 is in the completely closed state, the trip lever 84 is locked by an arc 83b of the semicircle 83a of the trip bar 83. [0030] The trip lever 84 is engaged with an engagement pin 55c provided at the other end 55b of the lever 55 on an engagement surface 84d provided at the other end 84b. One end 85a of the reset spring 85 is locked in a hole 84e provided between the shaft 84c of the trip lever 84 and the engagement surface 84d. Note that the other end 85b of the reset spring 85 is held on the frame 81 by an engagement pin (not illustrated). Consequently, the reset spring 85 is stretched between the frame 81 and the trip lever 84, and the trip lever 84 is urged by the reset spring 85 in the clockwise direction about the shaft 84c. However, in the state illustrated in FIG. 3, the engagement pin 55c of the lever 55 touches the engagement surface 84d, whereby the trip lever 84 is prevented from rotating clockwise about the shaft 84c. Between the shaft 84c and the engagement surface 84d, there is provided an engagement recess 84f with which the engagement pin 55c is engaged in the completely closed state. [0031] Next, the electromagnetic operation mechanism 60 will be described in detail. As illustrated in FIG. 4, the fixed iron core 61 includes a first attracting surface 61a and a second attracting surface 61b. The first attracting surface 61a attracts the movable iron core 63. The inter-surface distance between the second attracting surface 61b and the movable iron core 63 is longer than the inter-surface distance between the first attracting surface 61a and the movable iron core 63. The movable iron core 63 includes a first attracted surface 63a and a second attracted surface 63b. The first attracted surface 63a faces the first attracting surface 61a. The second attracted surface 63b faces the second attracting surface 61b. [0032] The inter-surface distance A between the first attracting surface 61a that attracts the movable iron core 63 and the first attracted surface 63a of the movable iron core 63 and the distance B, i.e. the stroke that the movable iron core 63 moves so as to close the movable contact 21 against the urging force of the opening spring 82, satisfy a relationship of A