MOLDED CASE CIRCUIT BREAKER
Field of the invention
[0001] The present invention relates to molded case circuit breakers (MCCB) and more particularly to the contact system used in such MCCBs.
Prior Art [0002] Conventional molded case circuit breakers include at least one pair of separable main contacts which may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. In the automatic mode of operation, the separable main contacts may be opened by either an operating mechanism or by magnetic repulsion forces generated by a reverse electrical current loop formed between upper and lower contact arms.
[0003] It is observed that not all the points on the surface of the upper and lower contact buttons get into contact since even with highly polished contact surfaces; presence of crater like portions on the surface of the contact buttons cannot be eliminated completely. These portions through which current flows are called 'a spots'. In the upper contact, the current in the surrounding region converge to the nearest 'a spot'. Similarly current flows through the 'a spots' in the lower contact and then disperse outwards to flow through a larger area. According to Biot-Savart's law, a force is developed between two current carrying conductors and the nature of force depends on the current direction. If the current flows in the same direction then a force of attraction is developed between the conductors and if the current direction is opposite to each other, a force of repulsion is developed between them. A force of repulsion formed between the contacts of circuit breaker due to the unavoidable irregular contact surfaces is called constriction force.
[0004] When high currents of the order of 50 kilo amperes flow in the circuit breaker, which might happen during a short circuit, huge constriction force is developed which repels open the contacts. This condition may favor the category A circuit breakers as they require quick opening of contacts. On the other hand,

in case of utilization category B MCCBs, the circuit breaker needs to remain in contact for a predetermined time, which may be in the order of one second. For example, according to lEC (International Electro technical Commission) 60947 standard for utilization category B of molded case circuit breakers, the contact button should remain in contact for at least 50 milliseconds even during short circuit conditions. Therefore, there is a need to reduce the huge constriction force generated between the contacts in case of molded case circuit breakers. [0005] Further, when electrical contacts open to interrupt a large current, there is a tendency for an arc to form between the opened contacts. The arc allows the flow of current to continue. Circuit breakers are designed to incorporate various features to divide and extinguish the arc. For example, in air-insulated and miniature breakers an arc chute structure consisting of metal plates or ceramic ridges cools the arc, and magnetic blowout coils deflect the arc into the arc chute. The magnetic field produced by the blowout coils force the electric arc to lengthen and move away from the contact buttons towards the arc chute. This blow out force is proportional to the magnitude of current. In order to reduce the arc, a higher blow out force is desirable when the arc is produced while contacts are being separated.
[0006] In the light of foregoing discussion, there is a need for a molded case circuit breaker, which has reduced constriction force and produces higher blow out force when an arc is produced while contacts are being separated. The molded case circuit breaker should provide greater opening between the upper and lower contacts and should be able to control short circuit withstanding time or current.
SUMMARY OF THE INVENTION
[0007] An objective of the invention is to provide a molded case circuit breaker, which has reduced constriction force.
[0008] Another objective of the invention is to provide a molded case circuit breaker, which produces higher blow out force when an arc is produced while contacts are being separated.

[0009] Yet another objective of the invention is to provide a molded case circuit breaker, which enables greater opening between the upper and lower contacts. [0010] Still yet another objective of the invention is to increase the time for withstanding a short circuit.
[0011] Still yet another objective of the invention is to increase the value of short circuit current for a predetermined value of short circuit withstanding time. [0012] Accordingly, the present invention provides a molded case circuit breaker, comprising a contact system having a stationary contact, a movable contact, and a spring-biased mechanism for enabling the coupling of the stationary and movable contacts, wherein the direction of force due to the spring-biased mechanism is at an acute angle with the surfaces of the contact buttons on the stationary and movable contacts.
[0013] The present invention, therefore, provides a molded case circuit breaker which reduces the constriction force between the contacts and produces higher blow out force when an arc is produced while contacts are being separated. The molded case circuit breaker of the present invention further enables greater opening between the upper and lower contacts and is able to control short circuit withstanding time or current.
BRIEF DESCRIPTION OF THE DRAWINGS [0014] So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to various embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. [0015] FIG. 1A, FIG. IB and FIG. 1C show a contact system of a molded case circuit breaker, wherein:
[0016] FIG. 1A represents the contact system of the molded case circuit breaker in an open position;

[0017] FIG. IB represents the contact system of the molded case circuit breaker
in a just closed position;
[0018] FIG. 1C represents the contact system of the molded case circuit breaker
in a closed position in accordance with an embodiment of the invention.
[0019] FIG. 2 depicts the resolution of forces in the contact system of the molded
case circuit breaker, in accordance with an embodiment of the invention.
[0020] FIG. 3 is an isometric view of the molded case circuit breaker contact
assembly, in accordance with an embodiment of the present invention.
DETAILED DESCRIPTION OF INVENTION [0021] Various embodiments of the present invention provide a molded case circuit breaker which reduces the constriction force between the contacts and produces higher blow out force when an arc is produced while contacts are being separated. The molded case circuit breaker further enables greater opening between the upper and lower contacts and is able to control short circuit withstanding time or current.
[0022] FIG. 1A shows a contact system 100 of a Molded Case Circuit Breaker (MCCB) in an open position, in accordance with an embodiment of the invention. Contact system 100 includes a stationary contact 102, a movable contact 104, and a spring-biased mechanism 106. Each of stationary contact 102 and movable contact 104 has a plurality of contact buttons. For example, stationary contact 102 has contact buttons 108a and 108b, and movable contact 104 has contact buttons 110a and 110b.
[0023] In accordance with an embodiment of the invention, spring-biased mechanism 106 includes a cup 112, a shaft 114, a plurality of springs 116, and a pin 118. Spring-biased mechanism 106 enables the coupling of stationary contact 102 and movable contact 104 through contact buttons 108a, 108b, 110a and 110b. Contact button 108a meets contact button 110a and contact button 108b meets contact button 110b. The force between contact button 108a and 110a, and 108b and 110b is determined by the properties of springs 116 - spring constant, spring length, and the like. Cup 112 holds movable contact 104 through pin 118 and shaft 114 such that horizontal movement of movable contact 104

with' respect to cup 112 is restricted. Springs 116 connect cup 112 and movable contact 104 through a plurality of pin rods fixed in the grooves at the top of movable contact 104 and bottom of cup 112.
[0024] In accordance with an embodiment of the invention, contact system 100 is designed to produce an over ride of 5 mm (millimeters), i.e., the gap between movable contact 104 and stationary contact 102 in open position (FIG.1A) is 30 mm but cup 112 has a total movement of 35 mm. FIG. IB represents contact system 100 in a just closed position, where cup 112 along with movable contact 104 has moved 30 mm. As shown in FIG. IB, contact button 108a is just in contact with contact button 110a and contact button 108b is just in contact with contact button 110b and springs 116 are not stretched. Cup 112 has a 5 mm over ride movement relative to movable contact 104. The 5 mm movement stretches springs 116 to produce a contact pressure of 24 kgf at each pole as shown in FIG. 1C. It may be apparent to a person skilled in the art that springs 116 may be designed to produce a pressure of 24 kgf for the five mm movement. The contact pressure of 24kgf reduces the resistance and increases the area of contact at each pole. It may be apparent to a person skilled in the art that the over ride value may vary and depends upon the design constraints. [0025] With reference to FIG. 1A, a line 120 lies in the plane of the surface of contact button 108a. Also, line 122 is parallel to the direction of force due to springs 116. In accordance with an embodiment of the invention, line 120 is at an acute angle with line 122. Therefore, the surface of contact button 108a is at an acute angle with respect to the direction of force due to springs 116. Hence, the meeting surfaces of contact buttons 108a, 108b, 110a, and 110b are at an acute angle with the line of direction of force due to springs 116. In an embodiment, the acute angle is 60 degrees. In various embodiments, the acute angle may vary between zero and 90 degrees. A more detailed description on the acute angle and the effect of change in the same on the contact pressure force between the poles is provided with reference to FIG. 2.
[0026] In accordance with an embodiment of the invention, lower faces 124 of contact buttons 110 are at a predetermined distance from lower face 126 of

movable contact 104, as shown in FIG.1B. For example, lower face 124a of contact button 110b is at a predetermined distance from lower face 126 of movable contact 104. When high current is passed and during the time of arcing, the predetermined distance determines the blow out force, which pushes the arc away from contact buttons 108a, 108b, 110a, and 110b towards an arc chute -the higher the predetermined distance, the higher the blow out force. It may be apparent to a person skilled in the art that the predetermined distance is in the order of the length of contact buttons 110 and is also governed by the design principles. A typical value of the predetermined distance may be 8-12 mm (millimeters).
[0027] FIG. 2 depicts the resolution of forces in contact system 100 of the molded case circuit breaker, in accordance with an embodiment of the present invention. FIG. 2 shows contact buttons 108b and 110b of contact system 100. FIG. 2 also shows *a spots' 202. A curve 204a represents the direction of current in contact button 108b and a curve 206a represents the direction of current in contact button 110b. The force of repulsion due to currents 204a and 206a flowing through contact buttons 108b and 110b is represented by a ray 208. It may be apparent to a person skilled in the art that the magnitude of force 208 is governed by the Biot-Savart's law. Force 208 is a repulsive force, which acts against the force due to springs 116 so as to move contact buttons 108a and 110a away from each other and is proportional to the magnitude of currents 204a and 206a.
[0028] It may be apparent to a person skilled in the art that force 208 may be resolved along the X-Y axis and the components, force 208a along the X-axis and force 208b along the Y-axis, may be obtained. Since the horizontal movement of movable contact 104 is restricted, force 208a will be balanced by pin 118 and shaft 114. Further, force 208b acts in the direction opposite to the direction of force due to springs 116 and acts as a constriction force in case currents 204a and 206a become very high when a short circuit takes place. [0029] It may be observed that the angle between force 208 and X-axis is same as that (represented by alpha (a) in FIG. 1A) between the meeting surfaces of

contact buttons 108a, 108b, 110a, and 110b and the line of direction of force the
to springs 116.
[0030] In accordance with an embodiment of the invention, magnitude of force 208b is determined by the angle a. Force 208b may be written as
F = Fm * cosine a
Where,
F = Force 208b,
F = Force 208,
a = acute angle between Force 208 and X-axis, and Cosine is the standard trigonometric function. [0031] It may be observed that as the value of a varies between zero and ninety degrees, the magnitude of force 208b varies between that of force 208 and zero. In an embodiment, the value of a is sixty degrees and magnitude of force 208b is half that of force 208. Hence, the constriction force is reduced to half. [0032] FIG. 3 is an isometric view of the assembly of a molded case circuit breaker 300 along with contact system 100 in a just closed position, in accordance with an embodiment of the present invention. FIG. 3 also shows an elongated handle 302, which is carried by cup 112. Handle 302 can be used to manually operate molded case circuit breaker 300.
[0033] Thus, the molded case circuit breaker of the present invention has a reduced constriction force and produces a higher blow out force when an arc is produced while contacts are being separated. Further, the circuit breaker of the present invention enables greater opening between the upper and lower contacts. As the repulsion is reduced to half, the time for withstand or the value of short circuit current for a particular withstand time can be increased. [0034] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

We claim:
1. A circuit breaker comprising a contact system, the contact system
comprising a stationary contact, a movable contact, a spring-biased
mechanism, each of the stationary and movable contacts having a
plurality of contact buttons, the spring-biased mechanism enabling the
coupling of the stationary and movable contacts at the contact buttons,
wherein the direction of force due to the spring-biased mechanism is at an
acute angle with surfaces of the contact buttons.
2. The circuit breaker according to claim 1, wherein the spring-biased
mechanism comprises a cup, a shaft, a pin, and a plurality of springs, the
cup holding the movable contact through the shaft and the pin, the pin
restricting the horizontal movement of the movable contact with respect to
the cup.
3. The circuit breaker according to claim 2, wherein the springs produce a
predetermined contact pressure force at the contact buttons.
4. The circuit breaker according to claim 1, further comprising an elongated
handle, the elongated handle being carried by the cup, the handle being
used to manually operate the circuit breaker.
5. The circuit breaker according to claim 1, wherein the acute angle varies
between 45 degrees and 75 degrees.
6. The circuit breaker according to claim 1, wherein the lower face of the
contact buttons on the movable contact are at a predetermined distance
with respect to the lower face of the movable contact.