FORM 2
The Patents Act 1970
(39 of 1970)
&
The Patent Rules 2003
COMPLETE SPECIFICATION
(See Section 10 and rule 13)
TITLE OF THE INVENTION: CONTACT LOCKING SYSTEM FOR CIRCUIT BREAKERS
APPLICANT:
LARSEN & TOUBRO LIMITED
L&T House, Ballard Estate, P.O. Box No. 278,
Mumbai, 400 001, Maharashtra,
INDIA.
PREAMBLE OF THE DESCRIPTION:
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED

A) TECHNICAL FIELD
[0001] The present invention generally relates to electrical switching apparatus and particularly to circuit breakers. The present invention more particularly relates to locking assemblies for movable contact arms in circuit breakers contact assemblies.
B) BACKGROUND OF THE INVENTION
[0002] Electrical switching apparatuses, such as circuit breakers, are employed in diverse capacities in power distribution systems such as, for example, to provide protection for electrical equipment from electrical fault conditions for instance during current overloads, short circuits, abnormal level voltage conditions or the like.
[0003] Circuit breakers are the mechanical switching devices capable of making, carrying, and breaking currents under normal circuit conditions and also making, carrying for a specified time, and breaking currents under specified abnormal conditions. A circuit breaker basically consists of a pair of separable contacts and an interrupting medium. The function of the contacts is to conduct the electrical current when the breaker is closed and withstand the arcs while interrupting. Generally, the electrical contacts have a stationary part and a moving part. By bringing the moving contact in touch with the stationary contact, an electric current flow through the circuit and the breaker is closed. By driving the moving contact away from the stationary contact, an electric arc is created and by quenching it, the current stops flowing and the breaker is open.
[0004] A circuit breaker typically consists of a current carrying path, a mechanism and an arc quenching chamber. The movable contact arm is spring loaded to exert contact pressure on the fixed contact assembly. The circuit breaker is expected to operate to interrupt the fault current rapidly so as to

minimize damages to the installations resulting from thermal and mechanical stresses.
[0005] A conventionally used molded case circuit breaker operates on the current limiting principle wherein the over current flowing through the circuit breaker contacts results in electro-dynamic repulsive forces between the fixed and moving contacts. The stationary contact being secured to the base of the molded case, repulsive forces exerted cause the moving contact to trace a curvilinear locus as the contacts separate. Consequent to the motion of moving contact, an incremental resistance is introduced between the stationary and moving contacts. The incremental in turn creates an increasing arc voltage that acts opposite to the system voltage and limits the fault current to quench the arc.
[0006J The inherent nature of any spring loaded contact system introduces the probability of "contact bounce-back" even after successful opening. This phenomenon, if unchecked, may eventually result in violent re-strike of the arc between the contacts and consequently, cause serious damage to the circuit breaker and the installation.
[0007] Hence there is a need to provide an improved contact locking arrangement to prevent the contact from bounce back and to limit the fault current in order to achieve a low value of let through energy.
[0008] The above mentioned shortcomings, disadvantages and problems are addressed herein and which will be understood by reading and studying the following specification.
C) OBJECT OF THE INVENTION
[0009] The primary object of the present invention is to develop an improved contact locking arrangement in molded case circuit breaker to prevent contact bounce back.

|0010] Another object of the present invention is to develop an improved contact locking arrangement to limit fault current.
[0011] Yet another object of the present invention is to develop an improved contact locking arrangement to reduce let through energy in case of fault current.
[0012] These and other objects and advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.
D) SUMMARY OF THE INVENTION
[0013] The above mentioned shortcomings, disadvantages and problems are addressed herein and which will be understood by reading and studying the following specification.
[0014] The various embodiments of the present invention provide an improved moving contact locking arrangement for molded case circuit breaker. A moving contact locking mechanism for switching device comprises a driveshaft, a moving contact, contact pin, roller and its pin and two contact springs. A moving contact is assembled with the drive shaft. Moving contact is held in this position by inserting contact pin in drive shaft. Roller and its pin are placed in the channel provided in the driveshaft. Contact springs are hinged on the roller pin and the other end of the contact springs are mounted on the driveshaft.
[0015] Contact springs generate force on the moving contact about the axis of rotation and impart required contact force to the contact. This force assists the moving contact to maintain continuity of the electrical circuit until the threshold current limit is exceeded and restrains the rotary motion of moving contact.

[0016] High electrodynamics forces exerted on the moving contact tends to cause the moving contact to rotate against the contact force. When the electrodynamics forces acting on the moving contact exceed the threshold value (the opposing force exerted by contact spring), the contact starts rotating about its axis of rotation. The increasing force gradient between the force exerted on the moving contact due to a current flowing through the contact (electrodynamics forces) and contact spring force acting on the moving contact leads to achievement of a dead center. The point at which the roller surface aligns with tip of the moving contact surface is the dead center or toggling position of the moving contact. The moment the contact assembly achieves its dead center (about 10° rotation), there will be a rapid transition of the moving contact from the toggle position to a locked position. The moving contact stops on the stopper surface of the driveshaft. The contact locking avoids any form of a contact bouncing phenomenon during an arcing time in an arc-chamber assembly and this leads to a drastic reduction in let-through energy.
[0017] During short circuit, the contact locking phenomenon is immediately followed by the driveshaft movement assisted by the circuit breaker mechanism. The movement of the driveshaft facilitates the moving contact arm to reset so that it is ready for next ON operation,
E) BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The other objects, features and advantages will occur to those skilled in the art from the following description of the preferred embodiment and the accompanying drawings in which:
[0019] FIG.l illustrates the assembly of a drive-shaft provided with a moving contact locking arrangement according to one embodiment of the present invention.

[0020] FIG.2 illustrates the exploded view of a drive-shaft provided with a moving contact locking arrangement according to one embodiment of the present invention.
[0021] FIG.3 illustrates the ON position of the driveshaft assembly according to one embodiment of the present invention.
[0022] FIG.4 illustrates the TOGGLE and LOCKED positions of driveshaft according to one embodiment of the present invention.
[0023] FIG.5 illustrates the LOCKED position of a moving contact according to one embodiment of the present invention.
[0024] FIG.6 illustrates the three pole driveshaft assembly according to one embodiment of the present invention.
[0025] Although specific features of the present invention are shown in some drawings and not in others. This is done for convenience only as each feature may be combined with any or all of the other features in accordance with the present invention.
F) DETAILED DESCRIPTION OF THE INVENTION
[0026] In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which the specific embodiments that may be practiced is shown by way of illustration. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments and it is to be understood that the logical, mechanical and other changes may be made without departing from the scope of the

embodiments. The following detailed description is therefore not to be taken in a limiting sense.
[0027] The various embodiments of the present invention provide an improved moving contact locking arrangement for molded case circuit breakers. The locking arrangement according to the present invention is adapted to provide a contact pressure under normal current carrying conditions and lock the moving contact arm at a predetermined angular position subsequent to its repulsion upon exceeding a threshold value of the current.
[0028] FIG.l illustrates a contact locking assembly according to one embodiment of the present invention. The contact locking assembly includes a drive shaft 2, a moving contact 1, contact pin 6, roller 3, roller pin 4, and two contact springs 5. The moving contact 1 is assembled with the drive shaft 2 by aligning the moving contact hole with the corresponding drive shaft hole 8. The moving contact 1 is held in the locking position by inserting the contact pin 6 in the drive shaft hole 8. The roller 3 and the roller pin 4 are placed in the channel provided in the driveshaft 2 below the moving contact sliding surface. One end of each of the contact spring 5 is hinged on the roller pin 4 and the other end of the contact spring 5 is mounted on the driveshaft 2.
[0029] FIG.2 illustrates an exploded view of the drive-shaft having a moving contact locking arrangement according to one embodiment of the present invention. The moving contact 1 is assembled with the drive shaft 2 by inserting the contact pin 6 in the drive shaft hole 8. The roller pin 4 is connected to the roller 3. The contact springs 5 are hinged on the roller pin 4 and the other end of the contact springs 5 are mounted on the driveshaft 2 as shown in FIG. 2.
[0030] FIG. 3 illustrates the ON position of a driveshaft assembly according to one embodiment of the present invention. The moving contact 1 is in contact with the fixed contact 7 and the electrical circuit is in ON condition. The contact

springs 5 generate a force F on the dead end of the moving contact 1 which generates a torque on the moving contact 1 about the axis of rotation and imparts required contact force to the contact. This force assists the moving contact 1 to maintain a continuity of the electrical circuit until the threshold current limit is exceeded.
[0031] FIG.4 illustrates the TOGGLE and LOCKED positions of the driveshaft according to one embodiment of the present invention. High electrodynamics forces exerted on the moving contact 1 because of the flow of high fault currents through the unique current limiting contact profile tends to cause the moving contact 1 to rotate against the contact force. When the electrodynamics forces acting on the moving contact exceeds the threshold value the contact 1 starts rotaling about its axis of rotation.
[0032] The increasing force gradient between the force exerted on moving contact 1 due to a current flowing through the contact (electrodynamics forces) and the contact spring forces acting on the moving contact 1 leads to achievement of dead center 9. The point at which the roller surface aligns with the tip of the moving contact surface is the dead center 9 or toggling position of the moving contact. The moment the contact assembly achieves its dead center 9 (about 10° rotation), there will be a rapid transition of a moving contact from the dead center 9 or toggle position to a locked position.
]0033] FIG. 5 illustrates the LOCKED position of a moving contact according to one embodiment of the present invention. The moving contact 1 stops on the stopper surface of the driveshaft 2. The arrangement causes the moving contact 1 to remain in the locked open position until the circuit breaker is tripped. The contact locking avoids any form of contact bouncing phenomenon during the arcing time in an arc-chamber assembly, which in turn leads to a drastic reduction in let-through energy.

[0934} F1G.6 illustrates the three pole driveshaft assembly according to one embodiment the present invention. The three drive shaft assemblies are connected and placed parallel to each other. A projection 10 is provided on both the sides of the each of the driveshaft assembly to connect adjacent driveshaft assemblies. A trip condition is detected in at least one of the driveshaft assembly and the compression spring 5 is energized under the trip condition to displace the moving contacts 1 to simultaneously trip all the driveshaft assemblies.
G) ADVANTAGES OF THE INVENTION
[0035] A moving contact locking arrangement of the present invention provides high reliability and ease of assembly because of less number of components.
[0036] A moving contact locking mechanism of the present invention offers a fast acting "Dead center" crossing type of locking arrangement. The fast acting arrangement reduces the let through energy in case of fault currents
[0037] The present invention provides high dielectric strength across contact gap and facilitates rapid arc voltage development which limits the peak current resulting in low let-through energy.
[0038] The present invention offers robust design to achieve high mechanical stability and consistency in contact system.
[0039] A design of the moving contact locking arrangement assembly of the present invention is compact and economical.
[0040] Although the invention is described with various specific embodiments, it will be obvious for a person skilled in the art to practice the invention with modifications. However, all such modifications are deemed to be within the scope of the claims.

[0041] It is also to be understood that the following claims are intended to cover all of the generic and specific features of the present invention described herein and all the statements of the scope of the invention which as a matter of language might be said to fall there between.

CLAIMS
What is claimed is:
1. A contact locking system for Circuit breakers comprising:
a driveshaft;
a moving contact attached to the driveshaft;
a contact pin fastened to the moving contact;
a roller assembled to the drive shaft;
a roller pin mounted to the roller; and
at least two contact springs hinged between the roller pin and the drive shaft;
wherein the contact springs exert a force at a dead center of the moving contact to
generates a torque on the moving contact about an axis of rotation to provide a
contact force to restrain a rotary motion of the moving contact and to prevent
contact bouncing phenomenon.
2. The locking system arrangement according to claim 1, wherein the contact springs generates a force at the dead center of the moving contact to generate a torque on the moving contact about the axis of rotation to impart a required contact force to the moving contact to maintain continuity of the electrical circuit until the threshold current limit is exceeded and restrains the rotary motion of moving contact.
3. The system according to claim 1, wherein the dead center of the moving contact is a point at which the roller surface aligns with tip of the moving contact surface.
4. The system according to claim 1, wherein the dead center is achieved, when the force gradient between the electro dynamic force exerted on the moving contact due to a current flowing through the moving contact and the contact spring force

acting on the moving contact increases and provides for a toggling of the moving contact.
5. The system according to claim 1, wherein the moving contact stops on the
stopper surface of the driveshaft in a locked position.
6. The system according to claim 1, wherein the moving contact is moved from the dead center position to a locked position based on the principle of toggling by the surface sliding thereby avoiding a contact bouncing phenomenon during arcing time in an arc-chamber assembly in the circuit breaker.
7. The system according to claim 1, wherein the contact springs provide a contact pressure under normal current carrying conditions and lock the moving contact at a predetermined angular position subsequent when the current passing through the moving contact exceeds a threshold value.