FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10; rule 13)
1. TITLE OF THE INVENTION - IMPROVED LATCH MECHANISM FOR USE IN CIRCUIT BREAKERS
2. APPLICANT(S)
(a) NAME : LARSEN & TOUBRO LIMITED
(b) NATIONALITY: An Indian Company.
(c) ADDRESS: L & T House, Ballard Estate, Mumbai 400 001,
State of Maharashtra, India
3. PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed:

FIELD OF THE INVENTION
The present invention relates generally to circuit breakers and more particularly to an improved latch mechanism utilizing minimal energy thereby achieving maximum efficiency and life span of a circuit breaker.
BACKGROUND OF THE INVENTION
A circuit breaker is a mechanical switching device, 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 circuit conditions such as those of short circuit (Short circuit is an accidental or intentional connection, by a relatively low resistance or impedance, of two or more points in a circuit which are normally at different voltages)
The main function of any electrical switching apparatus is to carry the rated current. Therefore the current carrying capacity of any circuit breaker is so important that if the unit fails to carry the rated current it will cause damage to itself as well as to the electrical equipment to which it is protecting.
Circuit breakers have three positions corresponding to the status of the separable contacts. For example, these positions would include an ON position, in which the separable contacts are closed, an OFF position in which the contacts are open and Trip position in which the contacts are tripped open with mechanism in de-energized condition.
These circuit breakers / switching devices are usually installed in an enclosure so as to have all the control and distribution network laid in form of metallic sheets and or cable wires inside and all operating means from outside so as to prevent access to high voltage and thus a potential hazard.
When a circuit breaker is in ON condition and a short circuit fault condition arises, high amount of fault current flows through the circuit and in response to this the circuit breaker tries to move the separable contacts to open position after

sensing the fault condition through short circuit sensing mechanism (such as magnetic arrangement). The time taken from a fault condition initiation and the fault current being interrupted by circuit breaker determines the let through energy / arc energy passed by the circuit breaker to the downstream equipments. A circuit breaker is said to be more efficient when the let through energy is minimal.
Let through energy of circuit breaker depends on many parameters, one among them being the time for opening of the contacts from closed condition. The short circuit sensing mechanism of circuit breaker involves mechanical elements in between and thus takes a definite time for sensing and giving a trip open signal to the mechanism which in turn separates contacts. The time taken by the mechanism to separate the contact decides the let through energy. More the time more will be the let through energy.
If mechanism components are arranged in such a way to separate the contacts in least possible time then let through energy can be reduced substantially. This improves life of the equipments and systems connected downstream and greatly reduce the hazardous conditions which can damage the downstream equipments.
Further, efficiency of a contact separation by mechanism is measured by the velocity it can induce to the contacts once it starts reacting to the signal. It is important to have the desired contact force in the ON condition, i.e. when the separable contacts are in closed condition. It is also important to have optimum trip force at trip plate where the tripping signals are received to avoid any kind of delay between the generation of signals and response of mechanism.
US 4251702 discloses a spring powered charging mechanism is operatively coupled with a spring powered, circuit breaker contact operating mechanism such that charging of the operating mechanism springs is powered exclusively by the charging mechanism springs as they discharge. The charging mechanism includes a prop controlled by the condition of the operating mechanism to releaseably sustain the charging springs in a fully charged condition, while the operating mechanism includes a hook to releaseably sustain the breaker contacts open against the urgency of fully charged operating mechanism springs. Consequently,

the circuit breaker is capable of executing multiple contacts opening and closing operations without recharging.
US 4,129,762 disclose the mechanism enables normal separation of the contacts when the handle is moved, even if the contacts are lightly welded together. This prevents the handle from moving if they are solidly welded together. A toggle joint is included between the drive member for the moving contacts and the automatic trip mechanism. The shaft of the toggle is engaged in an opening in an articulated arm.
US 6,259,340 discloses an exemplary embodiment of the invention, a dual test mechanism is presented for use in a circuit breaker. More specifically, the dual test mechanism includes a dual test button which comprises a single switch for testing both the AFCI and GFCI circuits of the breaker. The test mechanism includes a circuit board, which forms a part of the circuit breaker, and a test button assembly which includes a test button and signaling components which are electrically connected to the circuit board.
US3621189 discloses a pivotally mounted contact arm and releasable latch lever, retained by a current-responsive latch system, are interconnected by a toggle linkage, the knee of which is connected by tension springs to a manually movable handle operable to collapse and straighten the toggle to effect opening and closing movement, respectively, of the contact arm. A cam follower is pivotally suspended from the connection joining the toggle linkage and latch lever and has a free end interposed between a fixed cam and one link of the toggle, the follower being drawn along the cam by movement of the latch lever when released to break the knee over center of the linkage, accelerate the collapse of the toggle, and restrain the upper toggle link to prevent contact-arm rebound.
In some prior art circuit breakers typically include an electrical contact on a movable arm which rotates away from a stationary contact in order to interrupt the current path. In response to an over current condition, circuit breakers generally move the arm to break the current path by tripping a spring-biased latch

mechanism. The latch mechanism includes a bearing surface for supporting a cradle which, in turn, is coupled to the movable arm.
Tripping the latch mechanism causes the bearing surfaces to release the cradle, thereby forcing the arm and its contact away from the fixed contact.
A drawback of some existing latch mechanisms is that the bearing surface of the latch mechanism may fail to release the cradle in response to the latch mechanism being tripped, thereby preventing interruption of the current path during an over current condition. Also, the bearing surface of the latch mechanism may improperly release the cradle without being tripped so as to interrupt the current path during normal operating conditions. In an effort overcome this drawback and achieve proper operation of the latch mechanism, the bearing surface may be lubricated or buffed. This solution, however, is unsatisfactory because it results in relatively large manufacturing tolerances and because it increases the cost of production due to the addition of a separate process step for lubrication or buffing.
In prior art circuit breakers, mechanism components are arranged to achieve the intended function. Contacts separating velocity was observed to be less than that of optimum speed. The resetting forces were also found to be higher than the optimum force which can be operated by average personnel.
There is therefore a need to provide an improved latch mechanism for use in a circuit breaker to maintain the latch bracket in stable ON or OFF condition that is minimum so as to reduce the trip force required to rotate the trip plate and cause it to de-latch the mechanism with minimal energy and achieving maximum efficiency.

OBJECTS OF THE INVENTION
One object of the present invention is to overcome the disadvantages/drawbacks of the prior art.
A primary object of the present invention is to provide an improved latch mechanism for a circuit breaker.
Another object of the present invention is to provide a resetting force required with the profile of main latch is least.
Another object of the present invention is to provide the higher velocity of moving contact after separation resulting in faster cut-off of fault current increasing the current limiting capability of the circuit breaker.
Yet another benefit is to provide additional benefit is the least trip force required to de-latch the mechanism.
These advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.
SUMMARY OF THE INVENTION
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the present invention. It is not intended to identify the key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concept of the invention in a simplified form as a prelude to a more detailed description of the invention presented later.
In accordance with the present embodiment of the invention, there is provided an improved latch mechanism for an electrical circuit breaker.
In accordance with another embodiment, the invention provides a minimal resetting force for the main latch to operate.

In accordance with a further embodiment of the invention, there is provided a main latch arrangement, to separate the contacts at higher velocity for quickly cutting off the fault current and preventing the circuit from damages due to flow of fault current.
In accordance with a further embodiment of invention, there is provided an improved trip mechanism, with least trip force to de-latch the mechanism.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Other features as well as the advantages of the invention will be clear from the following description.
In the appended drawings:
Figure 1 (a) illustrates the front view of the main latch of the mechanism indicating resetting surface, latching surface and stopper surface with marking a, b & c respectively.
Figure 1 (b) illustrates the front view of the mechanism in wireframe
Figure 2 illustrates the front view of the start of the resetting process with hidden lines
Figure 3 illustrates the front view of the end of the resetting process with hidden lines
Figure 4 illustrates the front view of the start & end of the tripping process in wireframe
Figure 5 illustrates the front view of mechanism in ON, OFF & Trip condition
Figure 6 illustrates the exploded / dismantled front view of the mechanism in wireframe.

DETAILED DESCRIPTION OF THE INVENTION
In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and illustrate the best mode presently contemplated for carrying out the invention. Further functioning of the mechanism has been discussed below to describe the way the mechanism operates. However, such description should not be considered as any limitation of scope of the present unit. The structure thus conceived is susceptible of numerous modifications and variations, all the details may furthermore be replaced with elements having technical equivalence. In practice the materials and dimensions may be any according to the requirements, which will still be comprised within its true spirit.
This invention relates to the profile of main latch, one of the most critical components of mechanism, which governs important features of mechanism such as contact velocity, resetting force and trip force. The profile of main latch enhances the velocity of contact system, reduces the resetting force and trip force too.
Accordingly in the present invention, FIG. 1(a) illustrates the front view of the main latch of the mechanism indicating resetting surface, latching surface and stopper surface with marking a, b & c respectively. FIG. 1 (b) illustrates the front view of the mechanism in wireframe.
FIG. 2 illustrates the start of the resetting process of an electrical circuit breaker with improved latch mechanism in hidden lines
FIG. 3 illustrates the end of resetting process of an electrical circuit breaker with improved latch with hidden lines.
FIG.4 illustrates the start & end of the tripping process of an electrical circuit breaker with improved latch in wireframe.
FIG.5 illustrates the mechanism of ON, OFF & Trip condition in an electrical circuit breaker with improved latch.

FIG.6 illustrates dismantled view of mechanism in a circuit breaker. As illustrated in figure the circuit breaker consists of one end of main latch (1) which is pivoted to mechanism plates (7). One end of upper link (2) is pivoted on the other end of main latch (1). One end of lower link (6) is pivoted on the other end of upper link (2). Drive shaft (12) is placed below mechanism plates (7) and it is hinged such a way that the other end is free to rotate in available space. The other end of lower link (6) is attached to this end of driveshaft (12) which is free to rotate Resetting pin (3) is attached to fork (4) such a way that it is free to rotate while resetting. Fork (4) is pivoted on mechanism plate (7) so that the other end is free to rotate. One end of tension springs (13) are attached to the free end of fork (4) and the other end are attached to a pin (11) having upper link (2) and lower link (6) joined together. This pin is generally called floating pin (11).
OPERATING PROCEDURE:
In tripped condition of circuit-breaker, the main latch (1) is rotated at pivot point using fork(4) till latching surface on main latch goes (1) below latch bracket. Once the latching surface of main latch (1) goes below the latch bracket, it holds the main latch (1) in the rotated position. During this process the upper link (2) is pushed below extending the tension spring (13) with the help of floating pin (11), hence the tension spring (13) attached to fork (4) is extended increasing the spring energy. As soon as the fork axis crosses the spring axis, toggling of mechanism takes place and fork (4) moves to OFF condition keeping tension spring (13) in energized condition. This position of mechanism is called as OFF condition.
From OFF condition, when the fork (4) is rotated toward contacts side, as soon as the fork axis crosses the spring axis the floating pin (11) moves forward. During this the tension spring (13) is extended and then the tension spring energy is released and the lower link (6) pushes the drive shaft (12) having contact system downwards and contacts are closed. Since main latch is held by latch bracket, it does not rotate. Upper link (2) rotates with the help of pivot point on main latch (7) and stops on the stopper surface of main latch (7) with the help of roller. This position of mechanism is called as ON condition.

In ON condition, when the protection system of circuit-breaker responds to overload or short circuit condition it initiates signal to de-latch the mechanism and separate the contact system. When the signals are received by the mechanism, latch bracket releases the main latch. The main latch (7) then rotates to the other side as the tension spring (13) releases the energy moving the main latch (7) upward. This upward movement of main latch (7) pulls the upper link (2) upward and hence the lower link (6) also moves upwards, while pulling the free to rotate point of drive shaft (2). In this way the contacts get separated.
As explained, in tripped condition of circuit-breaker, when the main latch (7) is rotated at pivot point with fork (4) using resetting pin (3), till latching surface on main latch (7) goes below latch bracket (Please refer Figure 2 and 3). To achieve maximum force utilization in rotating profile, the surface on which the force is applied should be parallel to the line passing through the centre of rotating profile. As explained in Figure 1(a), the resetting surface (refer line 'a') is passing through the centre of main latch (7). In addition to this the resetting pin (3) which exerts force on main latch (7) is free to rotate within the holes of fork (4), hence it experiences least friction while resetting and in turn least resetting force.
With the same principle, the latching surface (refer line 'b') is passing through centre of main latch (7) so that the least force is required to keep the main latch (7) engaged with latch bracket. This necessitates least trip force generation required to trip the mechanism (disengage the main latch with latch bracket)
With the same principle, the roller surface (refer line 'c') is passing through centre of main latch (7) so that the least force is required to move the roller in upward direction so that the floating pin (11), lower link (6) and drive shaft (12) move upward which in turn separates the contact quickly with minimum force and hence it requires minimum time to separate the contact increasing the velocity of contact separation.
The energy required to carry out the three functions described above decides the efficiency of the mechanism.

As we get total applied energy utilized with zero losses to carry out the functions of mechanism, the profile is very useful. With this profile the time required to separate the contacts is least & hence let through energy can be reduced substantially. This improves life of the equipments and systems connected downstream and greatly reduce the hazardous conditions which can damage the downstream equipments.
The profile of main latch also helps to attain optimum trip force at trip plate where the tripping signals are received to avoid any kind of delay between the generation of signals and response of mechanism.
ADVANTAGES
In the present invention, Profile of main latch of mechanism having features which utilizes applied forces with maximum efficiency without loss. Least the energy required to operate the mechanism, maximum is the efficiency of the mechanism and maximum is the life span of the circuit-breaker.
Although the embodiments herein are described with various specific embodiments, it will be obvious for a person skilled in the art to practice the embodiments herein with modifications. However, all such modifications are deemed to be within the scope of the claims.
It is also to be understood that the following claims are intended to cover all of the generic and specific features of the' embodiments described herein and all the statements of the scope of the embodiments which as a matter of language might be said to fall there between.

WE CLAIM:
1. An improved latch mechanism for use in a circuit breaker, wherein said mechanism comprising:
a housing means;
a latch bracket;
a pair of contacts;
a main latch with one end pivoted to a mechanism plate;
an upper link with one end pivoted on other end of said main latch;
a lower link, with one end pivoted to other end of said upper link;
a drive shaft, with one end hinged below the mechanism plate, and other end of said drive shaft attached with other end of said lower link;
a resetting pin attached with a fork, and said fork pivoted on mechanism plate, with a free end to rotate;
a tension spring, with one end attached with free end of said fork, and other end of said tension spring attached with a floating pin; and said floating pin connecting said upper link and said lower link together.
2. An improved latch mechanism as claimed in claim 1, wherein said main latch rotates at its pivot point using said fork beyond said latch bracket thereby locking said main latch and further energizing said tension spring in OFF condition.
3. An improved latch mechanism as claimed in claim 2, wherein said latch bracket releasing said main latch and moving said main latch upward, upon release of energy from spring due to flow of a fault current.

4. An improved latch mechanism as claimed in claim 3, wherein said main latch by its upward movement causing said lower link to pull said drive shaft upward.
5. An improved latch mechanism as claimed in claim 4, wherein said drive shaft by its upward movement separating contacts in said circuit breaker and preventing flow of fault current
6. An improved latch mechanism for use in circuit breakers as herein described and illustrated with reference to accompanying drawings.