Circuit breaker

It is the main object of this invention to provide a circuit breaker in which the movable contacts have sufficient opening force and opening distance, and in which the breaking time is short. In order to attain the object, the circuit breaker of this invention is provided with a dead center link mechanism comprising a movable contact link which is rotatably supported by the arms of a cross bar which is operated so as to be capable of opening-closing the circuit and tripping by means of a trip-free mechanism, and a toggle spring which is bridged between the cross bar and the movable contact link. When a large current such as a short-circuit current flows, a movable steel plate attracted by an electromagnet for instantaneously tripping hits a rod for supporting the movable contacts which is connected to the movable contact link, whereby the dead center link mechanism makes the movable contacts open at high speed from the fixed contacts. As a result of such an operation, the current-limiting effect in this circuit breaker is excellent and breaking performance is improved.

BACKGROUND OF THE INVENTION 
The present invention relates to circuit breakers, more particularly to 
circuit breakers for promoting current-limiting effect and improved 
breaking performance. 
Generally, in some conventional circuit breakers, electromagnetic means are 
adapted to operate a trip-free mechanism by the working of the 
electromagnet for momentary tripping at first, and then to forcibly open a 
contact arm with the remaining stroke of the electromagnet. However, in 
such circuit breakers, the movable contact is normally biased by a 
pressure means such as a compression spring in which the rate of increase 
of force is normally remarkably high, when the contact arm is forced to 
open. Therefore the opening force is not enough and it is impossible to 
get sufficient distance by the action of the electromagnet alone. 
Accordingly, after some time delay by an operation of switching mechanism, 
the circuit breaker effectively begin to open the contact with the 
co-operatio of the trip-free mechanism and the forcibly operating 
electromagnet means. 
But a problem exist in that the breaking time tend to become longer. 
SUMMARY OF THE INVENTION 
Accordingly, in order to avoid the above-noted problem in the prior art, it 
is the general purpose of the present invention to provide a circuit 
breaker in which the action of an electromagnet acts on a movable contact 
device having a dead center link type spring mechanism to get a sufficient 
opening distance overcoming the return force of the contacting spring. 
This invention is the first that can overcome the difficulties inherent in 
applying the conventional arrangement in a breaking mechanism having two 
breaking points in series. 
Other objects of the present invention are: 
1. The provision of a circuit breaker for stongly exhibiting a 
current-limiting effect when breaking a short-circuit current, whereby 
thermal or mechanical damage to its system can be reduced and breaking 
performance can be satisfied with a relatively small arc extinguish 
chamber. 
2. The provision of a circuit breaker in which the force of an 
electromagnet means for actuating the movable contact in addition to the 
electromagnetic force between conductors is produced by a short-circuit 
current in the earliest period of a short-circuit to directly open a set 
of movable contacts, whereby the initial current-limiting effect is 
increased. 
3. The provision of a circuit breaker for obtaining a stable initial 
opening action and a stable opening state. 
4. The provision of a circuit breaker which is of simple structure and, at 
the same time, can easily increase the initial arc voltage. 
5. The provision of a circuit breaker which can obtain a large opening 
distance with the co-operation of the electromagnetic repulsive force 
between the fixed and movable contacts and the electromagnetic means. 
6. The provision of a circuit breaker which can positively operate even 
with small short-circuit currents which contact sufficiently open the 
movable contact in the prior art. 
7. The provision of a circuit breaker which automatically returns a movable 
contact link and a toggle spring from a short-circuit breaking state to 
the normal OFF-state when the short-circuit is cleared, whereby the 
following closing action may be carried out without any hindrance. 
8. The provision of a circuit breaker in which not only the ratio of to 
instantaneous trip setting to each of various rated current is constant, 
but also the movable contacts can be opened by a lower current for a 
smaller rated current. Thus a greater current limiting effect is attained 
with this action and by the inner resistance of a winding coil which is 
larger for a smaller rated current. 
9. The provision of a circuit breaker in which the electromagnetic 
attracting force is assisted by the action for attracting a movable 
contact link, whereby ferromagnetic members are used, produced by the 
short circuit current which is flowing through the fixed contact 
conductor. 
10. The provision of a circuit breaker in which an arc is easily lead into 
an arc extinguish chamber to raise the arc extinguishing effect by the 
action of flux of the electromagnetic means. 
11. The provision of a circuit breaker which can increase the 
electromagnetic repulsive force for directly acting on a movable contact 
arm by the short-circuit current flowing through the electromagnetic coil. 
One of means for attaining the above objects is that the circuit breaker of 
the present invention has two breaking points in series for each phase. 
Other objects and advantages of the present invention will become apparent 
from the following description of embodiments with reference to the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1 to 4 in which a circuit breaker constructed in 
accordance with the present invention is illustrated, the circuit breaker 
has a breaker body 1 which is formed into a box shape from a mold base 2 
made of an insulator, a base cover 2' being a part of the mold base 2, and 
a mold cover 3, and the breaker body 1 covering all of the live parts in 
the circuit breaker except the terminal portions. The mold base 2 and the 
mold cover 3 have comb-like insulating partition walls 201 and 301 as 
shown in FIGS. 2 and 7. In joining the mold base 2 and the mold cover 3, 
each tunnel-like pole chamber 4 in which the main constitutional means of 
each pole and operating mechanism part are accommodated is formed, and in 
which a switch operating mechanism chamber 5 being adjacent to the one 
side of the pole chamber 4 is formed. In the operating mechanism chamber 5 
a switch operating mechanism 5A, a trip mechanism 5B and a rated current 
adjusting mechanism 5C are accommodated. 
For purposes of simplification and illustration, common parts to all poles 
will be described in regard to only one of the poles. 
As shown in FIG. 4, metal terminal pieces 6 and 6' are mounted on terminal 
shelves at both ends of each pole. A fixed contact device includes a first 
fixed conductor 7 and a second fixed conductor 8 being provided opposite 
each other with a space left therebetween, and to the first and second 
conductor 7 and 8 a first fixed contact 9 and a second fixed contact 10 
are respectively attached. One end of the first conductor 7 is led out to 
the terminal shelf to be connected to the metal terminal piece 6, the 
other end is bent into a V-shape, and the first contact 9 is set on the 
bent portion 7' near the point of bending. Also, on the upper portion of 
the first fixed conductor 7, an L-shaped first fixed conductor cover 11 
molded of a formable insulator is fastened by a screw 12. A vertical 
portion 11a of the first conductor cover 11 constitutes one of the end 
walls in each pole chamber 4. A ferromagnetic body 13 is mounted on the 
inside face of the first conductor 7 along the configuration thereof. 
The second fixed conductor 8 is formed to surround three sides of a winding 
15 in an electromagnetic device 14. One end of the second conductor 8 has 
a bent portion 8a which is engaged with a fitting slot 2b formed in a 
partition wall 2a of the mold base 2 for accommodating tripping elements, 
and the other end has a projecting winding connection tab 8b which is bent 
laterally. 
A movable contact device includes a movable contact arm 16. First and 
second movable contacts 17 and 18 are attached to the ends of the contact 
arm 16 so that they may be respectively associated with the first and the 
second fixed contacts 9 and 10 and so that they can bridge between the 
fixed contacts 9 and 10 in closing the circuit and can separate from the 
fixed contacts 9 and 10 in opening the circuit. As shown in FIGS. 4 and 7, 
the movable contact arm 16 is, in the center thereof, inserted into a 
fitting slot 19a formed at a lower enlarged portion of a supporting rod 19 
made of insulator, and the supporting rod 19 and the contact arm 16 are 
connected into one body by means of a pin 20. The upper end of the rod 19 
has forked arms 19b and 19c. 
Contact cross bar 21 of molded insulator extends through each of the poles 
from the switch operating mechanism chamber 5, as shown in FIGS. 2 and 4, 
has arms 21a which are integrally molded with the bar 21 and diverge into 
two plates at each pole. A projection 22a which projects from two wing 
portions at one end of a movable contact link 22 is inserted into and 
rotatably supported at the end portions of these arms 21a. The movable 
contact link 22 are made of a ferromagnetic substance and are bent into a 
U-shape. Two wing portion legs 22b formed at the other end of the contact 
link 22 are inserted between the above-mentioned forked arms 19b and 19c 
and are integrally connected to them by a pin 23. A toggle spring 26 is 
held between pins 24 and 25 which are respectively set between the arms 
21a and between the legs 22b. Thus, a dead center link mechanism is formed 
which applies pressure to the movable contacts 17 and 18 in the closing 
state in order that they may be brought into contact with the fixed 
contacts 9 and 10. 
As shown in FIGS. 4, 5 and 8, the electromagnetic device has yokes 28 made 
by laminating a plurality of magnetic steel plates on both ends of a 
cylindrical fixed iron core 27 on to which the winding 15 is wound. Each 
of these yokes 28, have a projection portion 28a formed at a position 
opposite a movable steel plate 29. The movable steel plate 29 is rotatably 
supported by a bent lug portions 29a by a shaft 30. One end portion 29b of 
the steel plate 29 is opposed to the upper end of the supporting rod 19, 
and the other end portion 29c is opposed to a trip bar 46 which is 
mentioned hereinafter. Also, and attraction portion 29d made from 
laminated magnetic steel plates is mounted on a portion of the steel plate 
29 which is opposite the projection portions 28a of the yokes 28. Such a 
movable steel plate 29 is continually biassed clockwise by means of a 
return spring 31. Further, insulating plates 32 and 33 are mounted along 
the insides of both yokes 28 and the second fixed conductor 8 as shown in 
FIG. 8. The winding 15 is wound so that its ampere turn may be constant in 
each different rated current, and thereby the ratio of the instantaneously 
tripping point to each rated current is continually constant. When the 
movable steel plate 29 is attracted, a first portion 29c of the steel 
plates 29 acts on the trip bar 46 in the tripping mechanism 5B, and a 
second portion 29b pushes the supporting rod 19. 
An arc extinguish chamber 34 is provided with arc extinguish devices 36 and 
36' on both sides of the movable contacts 17 and 18. The arc extinguish 
devices 36 and 36' are comprised of a plurality of arc extinguish grids 
35 respectively having notches 35a on the side of the contact 17 or 18, 
the grids 35 being arranged in parallel and leaving a insulating space 
therebetween in the open direction. On the side opposite to the notch 35a, 
the arc extinguish grids 35 which are at the lowest portion of the arc 
extinguish devices 36 and 36' have bent connecting portions 35b. And the 
arc extinguish device 36 and 36' are connected to each other by an arc 
discharging plate 37 which bridges between the associated bent connecting 
portions 35b. 
One end of the winding 15 is connected to the connecting lug 8b of the 
second fixed conductor 8, and the other end is connected to one end of a 
winding leadout terminal plate 38 which is arranged to be astride of the 
upper edge of the partition wall 2a. A base portion of a bimetal 39 which 
is bent into a J-shape as an overcurrent response element is fastened to a 
base wall 2c of a cavity for accommodating the tripping elements by means 
of a screw 40 and a nut 41. One end of the bimetal 39 is connected to the 
other end of the terminal plate 38, and to the upper portion of the other 
end of the bimetal 39 a bimetal adjusting screw 42 is set and one end of a 
flexible lead wire 43 is connected. The other end of the flexible lead 
wire 43 is connected to the terminal metal plate 6' through a conductive 
terminal 44. One end of the mold base 2 is provided with an end plate 45 
of the pole chamber 4 which is made of an insulator. The head end of the 
bimetal adjusting screw 42 is opposite a tongue 46a of a trip bar 46 which 
is molded of insulator. An upper projection lug 46b of the trip bar 46 is 
arranged to associate with a tail portion 29c of the movable steel plate 
29. 
The fixed means, such as said first fixed conductor 7, the second fixed 
conductor 8, the electromagnetic device 14 and the arc extinguish devices 
36 and 36', are integrally mounted between a pair of insulating plates 47 
and 47' in each pole chamber 4. As shown in FIGS. 7 and 8, the insulating 
plates 47 and 47' are spaced from each other so that their outer faces can 
be closely inserted between sidewalls of a tunnel-like accommodating slot 
in each pole chamber 4, the groove being formed by the mold base 2 and the 
mold cover 3. These insulating plates 47 and 47' have a plurality of 
perforactions. Projections 7c, 8c and 35c etc. which are integrally formed 
at both side edges of the first and the second fixed conductors 7 and 8 
and the arc extinguish grids 35 are respectively inserted into and caulked 
to their associated perforations 48, 49, 50 etc. The fixed iron core 27 in 
the electromagnetic device 14 extends through the winding 15, the 
insulating plates 32 and the yokes 28, and projecting cylinders 27a at its 
ends are inserted into and caulked to bores 51 in the insulating plates 47 
and 47'. The movable steel plate 29 is supported by shaft 30 which is 
inserted into the lugs 29a and bores 52 in the insulating plates 47 and 
47'. 
The supporting rod 19 having the movable contact arm 16 which is fitted 
into the fitting slot 19a at the lower portion of the rod is inserted from 
a space between the insulating plates 47 and 47', and the pin 20 extends 
between guide slots 53 in the plates 47 and 47' through the movable 
contact arm 16 and the supporting rod 19, and thereby the arm 16 and the 
rod 19 are supported so as to be movable in the opening and closing 
direction. 
Upon assembly, it is desirable as mentioned above to set the first and the 
second fixed conductors 7 and 8, the electromagnetic device 14 and the arc 
extinguish devices 36 and 36' between a pair of the insulating plates 47 
and 47', and to hold the arc discharging plate 37 between the lower edges 
of the insulating plates 47 and 47' and between the bent connecting 
portions 35b of the lowest arc extinguish grids 35, and to insert a pole 
chamber unit, to which the winding leadout terminal plate 38, the bimetal 
39, the flexible lead wire 43 and the conductive terminal 44 are 
integrally connected, into the accommodating slot in the pole chamber 4, 
and then to set the first fixed conductor cover 11 on the first fixed 
conductor 7, to next insert the one end of the movable contact link 22 
between the forked arms 19b and 19c of the supporting rod 19 to join them 
with the pin 23, and to set the contact cross bar 21 connected to the 
movable contact link 22 in the predetermined position, and to screw and 
fix a screw 54 through the bent connecting portion 35b to the tapped bore 
in the arc discharging plate 37, and further to set the leading plate 38 
on the upper edge of the partition wall 2a, to attach the bimetal 39 to 
the base wall 2c by means of the screws 40 and 41 and lastly, to fit the 
end 44a of the conductive terminal 44 to a slot in the mold base 2. 
The trip bar 46 extends from the operating mechanism chamber 5 over each of 
the poles through the vicinity of the joint portion of the mold base 2 and 
the mold cover 3. The tongue 46a which is formed in the trip bar 46 in 
each pole has an inclined face 55 formed on one side thereof, and the 
inclined face 55 can be disposed in a suitable position by the operation 
of adjusting the gap between a bimetal adjusting screw 42 and the tongue 
46a. That is, because the trip bar 46 is supported so as to be movable in 
a direction intersecting the poles, the position of the inclined face 55 
can be varied by the rated current adjusting mechanism 5C in the operating 
mechanism 5, and therefore by varying the gap between the inclined faces 
55 and their associated bimetal adjusting screws 42 the rated current can 
be adjusted. 
As shown in FIG. 3, the switch operating mechanism chamber 5 is provided 
with the switch operating mechanism 5A and the trip mechanism 5B in 
relation to the contact cross bar 21 and the trip bar 46. The operating 
mechanism 5A is adapted to be free to trip by the working of the trip 
mechanism 5B regardless of its position in the process of the closing 
operation. 
The operating mechanism 5A has a handle 58 molded of an insulator which is 
supported by a pair of fixed frames 56 fixed to the mold base 2 with a 
handle shaft 57, and an operating lever 58a of this handle 58 projects 
from an operating portion 3b in the mold cover 3 so that it can rotate the 
handle 58. When this operating lever 58a of the handle 58 is inclined to 
the left, the circuit breaker represents the ON-state, and a handle spring 
59, which is bridged between the fixed frame 56 and the handle 58, makes 
the handle 58 usually deflected in the OFF-direction, that is, clockwise. 
To the handle 58 one end of an operating link 61 is connected by a pin 60, 
and the other end is slidably connected to a slotted aperture 63a in a 
trip lever 63 by a pin 62. Also, the trip lever 63 is rotatably supported 
by means of a trip lever shaft 64 fixed on the fixed frame 56. One end of 
a contact link 65 is connected to the operating link 61 by the pin 62, and 
the other end is connected to by a pin 66 a cross bar operating arm 21b 
which is integrally molded with the contact cross bar 21. One end of a 
breaking spring 67 is held on the pin 6, and the other end is held on a 
spring supporting lug 68 attached to the mold base 2, and therefore this 
spring 67 biases the cross bar 32 clockwise. 
When the trip mechanism 5B is in its normally closed circuit state, a claw 
63b at one end of the trip lever 63 is engaged with a semicircular notch 
portion 69a of a trip pin 69 which is supported on the fixed frame 56. A 
trip pin arm 70 has a portion curved along the outer face of the trip pin 
69, and is fastened to the trip pin 69 by a screw 71 so that its angle of 
rotation with the trip pin 69 can be adjusted. 
One end of a bimetal 72 for ambient temperature correction is connected to 
a side portion 70a of the trip pin arm 70 and the other end is mounted at 
a position opposed to a projection 46c of the trip bar 46 in the operating 
mechanism chamber 5. A trip pin return spring 73 is disposed to 
continually urge the trip pin arm 70 clockwise. The clockwise motion of 
the arm 70 is limited by the engagement with a projection 56a formed in 
the fixed frame 56, and as a result of this limitation, a stationary 
position of the arm 70 in a steady state can be determined. 
The rated current adjusting mechanism 5C has a rated current adjusting knob 
75 of an insulator which is rotatably supported between fitting means 74 
and 74' fixed on the fixed frame 56. This knob 75 is adapted so that its 
head portion is exposed from an aperture in the mold cover 3, and so that 
its lower end can rotate a working lever 76 with the aid of an eccentric 
cam 75a. A working pin 76a which is set in one end of the working lever 76 
is engaged with a slotted bore 46d in the trip bar 46. When rotating the 
knob 75 from the outside, the trip bar 46 is axially moved, the position 
of the inclined face 55 on the tongue 46a in each pole chamber 4 is 
shifted and the gap between the inclined face 55 and the screw 42 is 
varied, thus enabling the rated current to be adjusted. 
As with the rated current adjusting knob 75, a trip button 77 is arranged 
so that its head portion is exposed from an aperture 3d in the mold cover 
3, and so that its lower end pushes a projection 46e of the trip bar 46 
through a U-shaped button return spring 78 which is mounted on the lower 
side of the fitting means 74'. At the head portion of the contact cross 
bar 21 at the end opposite to the cross bar operating plate 21b, a switch 
indication plate 21c is integrally formed, to indicate the ON-OFF state 
through an indication window 3e in the mold cover 3. 
The switching operation of the circuit breaker of the above-mentioned 
embodiment in accordance with the present invention, is as follows: 
The states shown in FIGS. 3 and 4 are an ON-state where the operating lever 
58a of the handle 58 is inclined to the left. If the operating lever 58a 
is manually moved to the right in order to change from this ON-state to 
the normal OFF-state, the handle 58 moves the pin 62 to the left along the 
slotted aperture 63a in the trip lever 63 by means of the operation link 
while keeping the claw 63b and the notch portion 69a engaged, while at the 
same time both the contact cross bar 21 is rotated clockwise and the 
breaking spring 67 is compressed by means of the contact link 65, and a 
line linking the handle shaft 57 and the pins 60 and 62 shift to an 
L-shaped over center link state, whereby the handle 58 and the operating 
link 61 are kept statical. Accordingly, the arm 21a of the contact cross 
bar 21 and the movable contact link 22 are integrally rotated clockwise, 
and then, the supporting rod 19 and the movable contact arm 16 connected 
to the rod 19 are shifted. Thus, the pin 20 is slid down along the guide 
slots 53, and then the first and second movable contacts 17 and 18 are 
separated from the first and second fixed contacts 9 and 10, whereby the 
movable contact arm 16 becomes an OFF-state as shown in FIG. 5. 
When manually moving the operating lever 58a to the left from the 
OFF-state, the pin 62 is moved to the right along the slotted aperture 63a 
by means of the operating link 61, and the contact cross bar 21 is rotated 
counterclockwise by the contact link 65, and then the arms 21a and the 
movable contact link 22 are integrally rotated counterclockwise, therefore 
pulling the movable contact arm 16 up by means of the supporting rod 19 to 
become an ON-state in that the first and second movable contacts 17 and 18 
are pressed upon the first and second fixed contacts 9 and 10. 
The following detailed description deals with the case of automatic 
breaking by the operation of the tripping device when normal overcurrent 
flows in the ON-state. The bending of the bimetal 39 causes the adjusting 
screw 42 to press the inclined face 55 of the tongue 46a, so that the trip 
bar 46 is rotated clockwise. At this moment, the bimetal 72, the trip pin 
arm 70 and the trip pin 69 are integrally rotated counterclockwise by 
means of the projection 46c of the trip bar 46 in the operating mechanism 
chamber 5, causing the engagment of the notch portion 69a and the claw 63b 
to be released, whereby the trip lever 63 is rotated counterclockwise. 
Further, the force of the breaking spring 67 makes the pin 63 move to the 
left along the slotted aperture 63a with the aid of the contact link 65, 
and this results in the spring 67 being compressed and at the same time 
the contact cross bar 21 being rotated clockwise, and finally the first 
and the second movable contacts 17 and 18 are separated from the fixed 
contacts 9 and 10 to cut off the current. 
At this time, as the handle 58 is simultaneously rotated clockwise by means 
of the handle spring 59, the operating link 61, the trip lever 63 and the 
contact link 65 are pulled up. Therefore, passing through the automatical 
breaking state, it becomes an OFF-state so that the claw 63b is engaged 
with the notch portion 69a. 
With reference to cases where the overcurrent is above 8 to 13 times as 
great as the rated current, that is, above the determined current for the 
instantaneously trip point, first the electromagnetic device 14 is excited 
by that overcurrent, whereby the initial attracting action of the 
attraction portion 29d to the projection portions 28a of the yokes 28 
makes one end 29b of the movable steel plate 29 hit the upper end of the 
supporting rod 19. And so, keeping the contact cross bar 21 in the closed 
circuit state, the movable contact link 22 is rotated clockwise on the 
shaft 22a against the toggle spring 26, and then the movable contacts 17 
and 18 begin to separate at high speed from the fixed contacts 9 and 10, 
and the arc voltage consequently increases creating a sufficient initial 
current-limiting effect. While the movable steel plate 29 is completely 
attracted to the yokes 28, the movable contact link 22 is rotated until 
the working line of the toggle spring 26 approaches the shaft 22a, causing 
the working force of the toggle spring 26 to the movable contact link 22 
to decrease greatly. When a large current such as a short-circuit current 
flows in such a state, the electromagnetic repulsive force which arises 
between the movable contacts 17, 18 and the fixed contacts 9, 10, or the 
attracting action of the movable contact link 22 by the first fixed 
conductor 7 is added, whereby the movable contact link 22 is rotated in 
the direction where the first and the second movable contacts 17 and 18 
are opened, and then the working line of the toggle spring 26 goes beyond 
the shaft 22a to the opposite side. Accordingly, as its working force 
reverses, the movable contact link 22 is rapidly rotated clockwise, and 
the break open circuit state shown in FIG. 6 is reached in which he 
opening distance is larger than the normal opening distance shown in FIG. 
5. The larger the current such as a short-circuit current is, the more the 
electromagnetic repulsive force between the movable and the fixed 
contacts, or the attraction force of the magnetic substance 13 to the 
movable contact link 22 increases, whereby the reversal of the working 
force of this toggle spring 26 is accelerated. Also, the attracting action 
of the movable contact link 22 through the first fixed conductor 7 is 
strengthened by means of the magnetic substance 13. Further, the magnetic 
substance 13 reinforces the first fixed conductor 7. 
On the other hand, when one end of the movable steel plate 29 is attracted 
by the yokes 28, the other end 29c hits the projection 46b of the trip bar 
46 to rotate the trip bar 46 clockwise, and the engagement of the trip pin 
69 and the trip lever 63 is released, whereby through the aid of the 
contact link 65 the contact cross bar 21 is rotated clockwise by the 
working force of the breaking spring 67 to move the contact cross bar 21 
to the closed circuit position. Upon this movement of the contact cross 
bar 21, the shaft 22a goes beyond the working line of the toggle spring 26 
to the opposite side, this spring keeping the movable contact link 22 in 
the break open circuit position, and the working force of the spring 
returning to normal resulting in the open circuit state shown in FIG. 5. 
In this preferred embodiment of the present invention, automatic breaking 
can be safely attained when in an overcurrent range which is just a little 
over the rated current for instantaneous tripping where the 
electromagnetic repulsive force between the movable and the fixed 
contacts, or the attraction force of the movable contact link 22 to the 
magnetic substance 13 can not be fully exhibited even if it is impossible 
for one end 29b of the movable steel plate 29 to be attracted by the yokes 
28 to push the supporting rod 19 down in the opening direction, so that 
the position of the movable contact link 22 reverses the working force of 
the toggle spring 26, because the other end 29c of the movable steel plate 
29 turns the trip bar 46 to collapse the switch operating mechanism, 
whereby the contact cross bar 21 is rotated to continuously break the 
circuit. Further, the movable steel plate 29 may have such a stroke that, 
when the movable steel plate 29 is attracted by the yoke 28, the contact 
cross bar 21 is kept in the closed-circuit position and one end 29b of the 
movable steel plate 29 reverses the working force of the toggle spring 26, 
the steel plate 29 pushing the supporting rod 19 down until the movable 
contact link 22 can be rotated by the force within toggle spring 26. 
Referring to FIG. 9, the relation between the working force of each element 
when automatically breaking a relatively large current which is over the 
instantaneous tripping setting current is explained as follows. FIG. 9 is 
a diagram showing the mutual relationship among the attraction force of 
the electromagnetic device 14 which arises out of the current for starting 
the opening of the movable contacts 17 and 18, the electromagnetic 
repulsive force between the fixed contacts 9, 10 and the movable contacts 
17, 18, the attraction force of the movable contact link 22, and the 
return or opening force generated in the movable contact arm 16 from the 
dead center link mechanism. As the short-circuit current varies with the 
passage of time in not only alternating current but also direct current, 
real analysis must contain a factor of time, but for convenience, the 
value of the current is constant. 
In FIG. 9, the abscissa shows the position of the supporting rod 19, the 
movable contact 17 and 18, and the movable contact arm 16, and the 
ordinate shows the force which each of the elements generate, and the 
return or opening force of the movable contacts 17 and 18. The point of 
origin O is dead center of the dead center link mechanism, and the 
position G.sub.1 is the starting position of the movable steel plate 29, 
and the positions G.sub.2 and G.sub.3 are the starting position and the 
maximum opened position respectively. The curved line A shows the 
attraction characteristic of the electromagnetic device 14, which shows 
that the attraction force increases with the movement of the movable steel 
plate 29 from the position G.sub.1 to the position O. The curved line B 
shows the repulsive force of the return spring 31, and the difference 
between the line A and the line B indicates the working force of the 
electromagnetic device 14. The curved line C shows the electromagnetic 
repulsive force generated between the fixed and the movable contacts, 
which is maxmal at the position G.sub.2 and decreases with the separation. 
The curved line D shows that the force which attracts the movable contact 
link 22 by the current passing through the fixed contacts 9 and 10 
increases with approaching the position G.sub.3. The curved line E is the 
force generated by the dead center link mechanism, this line showing that 
the force is a positive return force at the right of the position O and 
that the force is a negative opening force at the left. FIG. 9 shows the 
example where the position O which is the dead center matches the last 
attraction position of the movable steel plate 29, but it is a matter of 
course that there may be a little difference between the positions. 
In such a relation as in FIG. 9, the working force F acting on the movable 
contact arm 16 and the supporting rod 19 is represented by the following 
equation; 
EQU F.sub.1 =A-B+C+D 
and, the difference between the action force F.sub.1 and the curved line E 
is the dissociating force F.sub.2 of the movable contacts 17 and 18. 
Accordingly, it can be understood from FIG. 9 that the attracting force 
(A-B) of the movable steel plate 29 works only within the range of E&gt;0, 
that is, between the position O and the position G.sub.2, and generated a 
strong opening force in cooperation with the electromagnetic repulsive 
force C. Between the position O and the position G.sub.3 the movable 
contacts 17 and 18 receive the force of E&lt;0, whereby they can 
automatically open themselves, and additionally increasing the attracting 
force D of the movable contact link 22 can generate sufficient opening 
force even if the electromagnetic repulsive force C attenuates. Thus, in 
accordance with the present invention, it is possible that the forces 
generated in each portion can cooperate to develop the preferred opening 
action. 
In the above-mentioned embodiment, although various rated current are 
applied to the circuit breaker, the circuit breaker can be adapted to 
equalize the ratio of the instantaneous tripping point to each rated 
current, because the winding 15 is wound so that its ampere turns may be 
constant for each rated current. Accordingly, the movable contacts 17 and 
18 can be opened by a current lower than the smaller rated current, and, 
since the inner resistance becomes larger, a greater current-limiting 
effect can be attained the smaller the rated current is. Further, it is 
possible that the magnetic flux leakage from the electromagnetic device 14 
acts on the arc which arises between the second fixed contact 10 and the 
second movable contact 18 in breaking the circuit, causing the arc to be 
led into the arc extinguish chamber 34 in accordance with Fleming's 
left-hand rule, and that the magnetic flux acts on the movable contact arm 
16 to strengthen the electromagnetic repulsive force. 
The movable portion which is opposite to a set of fixed core means 15, 27 
and 28 may be separated as shown in FIGS. 10 and 11 into a first movable 
steel plate 29' which has a first portion for acting on the trip bar 46 
when being attracted, and a second movable steel plate 29" which has a 
second portion for hitting the supporting rod 19. These first and second 
movable steel plates 29' and 29" are supported by a common shaft 30' so 
that the second steel plate 29" can be attracted to the fixed core means 
15, 27 and 28 so as to overlap the first steel plate 29' from above. 
Another modification is to completely divide the first steel plate 29' and 
the second steel plate 29" and to support each of them on separate shafts. 
Obviously many modifications and variations of the present invention are 
possible in light of the above teachings. It is therefore to be understood 
that within the scope of the appended claims the invention may be 
practiced in ways other than as specifically described.