FORM 2
THE PATENTS ACT, 1970 (39 of 1970)
& THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
[See section 10, Rule 13]
LATCHING MECHANISM FOR DRAW-OUT CIRCUIT BREAKER; '
LARSEN & TOUBRO LIMITED, A COMPANY INCORPORATED UNDER THE COMPANIES ACT, 1956, WHOSE ADDRESS IS L&T HOUSE, BALLARD ESTATE, MUMBAI - 400 001, MAHARASHTRA, INDIA
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 to circuit breakers, and more particularly, to latching mechanisms for draw-out circuit breakers.
BACKGROUND OF THE INVENTION
A draw-out circuit breaker has a breaker mounted inside a cradle. The cradle which houses the draw-out circuit breaker comprises of a racking cam, a rail pin, a breaker sitting plate and a rail. The breaker can take three positions/operating conditions within the cradle - i) a disconnected position, wherein main contacts as well as control circuit contacts are disengaged from supply; ii) a test position, wherein the main contacts are disengaged and control circuit contacts are engaged; iii) a connected position, wherein the main contacts as well as control circuit contacts are connected to supply. In addition to these positions, there is a maintenance position from which the breaker can be lifted from the cradle for maintenance.
Accordingly, when the breaker travels from disconnected to test to connected position, racking cam 2 rotates in anticlockwise direction, and engages with rail pin 32 as shown through figure 3A to figure 3F, and when the breaker travels from connected to test to disconnect position, racking cam 2 rotates in clockwise direction and rail pin 32 remains engaged with racking cam 2 as shown in figures 4A through 4F.
As per above prior art, during transition from one position to other, especially during test position as shown in figure 3B, there is play between the racking cam 2 and rail pin 32, and hence in test position wherein various activities are carried on the breaker, the breaker tends to move in space available because of the play. The various activities carried on the breaker are: switching ON and OFF the breaker, charging of Main spring{not shown in figures. This results in erroneous testing, and also causes isolating contacts to lose continuity. The loss of continuity at the contacts further results in false indication of breaker position as position indication is given

through isolating contacts. Further, if the circuit breaker is maintained in such a position for a prolonged amount of time, the breaker may snap out of the cradle damaging the cradle and other surrounding equipment.
Also, prior arts require very accurate manufacturing processes to be followed, with no or little scope for variations in the latching mechanism. The prior arts have mechanisms largely dependent on the free rotation of the latching mechanism. This sometimes creates failure of the latching mechanism to operate, in turn, resulting in failure of holding mechanism.
One other prior art depends on the threaded parts on the racking mechanism used to move the breaker in and out of the cradle. The threads can easily get damaged. To prevent this damage, special precaution between the nut and lead screw must be taken and the screw must have a special design to prevent wash out.
[001] Further, one prior art, United States Patent 3783209 by Westinghouse Electric corp. discloses a mechanism for holding and levering draw-out type circuit breakers. The prior art relates to circuit breakers and more particularly to large type draw out circuit breakers and a novel holding and levering mechanism for positioning the breaker in an enclosure. It provides a holding and levering in mechanism which is attached to the front of a draw out circuit breaker. The circuit breaker is held in position by the lead screw and box beam.
[002] Another prior art, United States Patent 2673260 by Eaton Corp. discloses a electrical switching apparatus and securing mechanism. The prior art provides improved carriage racking and interlocking mechanism particularly suitable for enabling truck mounted power circuit breakers to be completely separated from metal enclosing cubicle as well as pushed manually to an intermediate test position in the cubicle having primary circuit terminals mounted at the back and a hinged door for closing the front after the circuit breaker with its main contacts open is racked in out of the connected engagement with the primary circuit terminals.
Another prior art, United States Patent 7486499 by Eaton Corp. discloses a electrical switching apparatus and securing mechanism. The prior art relates generally to electrical

switching apparatus and more particularly to securing mechanisms for electrical switching apparatus such as draw out circuit breakers. The securing mechanism is self adjusting upon insertion of the circuit breaker into a cell of switchgear enclosure in order to secure the circuit breaker within the cell.
Further, though some prior arts provide a mechanism for holding the breaker in test position in a cradle, the same is achieved using large number of components. Thus these arrangements are complex and not very economical.
In view of the above, there is a need to provide a mechanism to hold a draw-out type circuit breaker with minimal play and which overcomes the drawbacks stated above.
SUMMARY OF THE INVENTION
Accordingly, the present invention In one aspect provides a latching mechanism for a draw-out type circuit breaker, the latching mechanism comprising a racking cam pivoted on a cradle of the circuit breaker, the racking cam being capable of rotating and engaging with a rail pin on the cradle based upon the operating condition of the circuit breaker; and a latch pivoted on the racking cam, the latch being capable of rotating and engaging with the rail pin on the cradle based upon the operating condition of the circuit breaker, wherein the latch is pivoted on the racking cam off-centre-of-gravity of the latch, such pivoting restrains rotation the rotation of the latch and enables engagement of the latch with the rail pin.
In another aspect, the invention provides a circuit breaker using such latching mechanism.
BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to embodiments of the invention, examples of which may be illustrated in the accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in the context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.
Figure 1A shows a draw-out circuit breaker in accordance with an embodiment of the invention. Figure 1B shows a latching mechanism adapted to inner side of a cradle of the circuit breaker in accordance with an embodiment of the invention.
Figure 2 shows a latch of the latching mechanism in greater detail in accordance with an embodiment of the invention.
Figures 3 and 4 show positions of a latching mechanism during racking operation as per prior-art.
Figures 5A-E show position of the latching mechanism during racking operation in accordance with an embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the present invention provide a latching mechanism for a draw-out type circuit breaker comprising a racking cam pivoted on a cradle of the circuit breaker, the racking cam being capable of rotating and engaging with a rail pin on the cradle based upon the operating condition of the circuit breaker; and a latch pivoted on the racking cam, the latch being capable of rotating and engaging with the rail pin on the cradle based upon the operating condition of the circuit breaker, wherein the latch is pivoted on the racking cam off-centre-of-gravity of the latch, such pivoting restrains rotation the rotation of the latch and enables engagement of the latch with the rail pin, wherein during transition between various operating conditions of the circuit breaker, the racking cam engages with the rail pin through the latch on the raking cam, the latch thereby reducing the undesired play during the said transitions.

Accordingly, the latching mechanism restricts unwanted movement of breaker in test, connected and disconnected positions, and also during each and every point of racking in and racking out of circuit breaker. The unwanted movement may arise due to switching ON and OFF the breaker, charging of main spring (not shown in figure).
Figure 1A, shows a draw-out circuit breaker 100 in accordance with an embodiment of the invention. Circuit breaker 100 comprises a breaker 102 mounted inside a cradle 104, isolating contact 106 and disconnecting contacts 108. Further, as shown in figure 1B the inner side of cradle 104 comprises of a rail 110, a breaker sitting plate 112, and a latching mechanism 114- Breaker 102, when mounted inside cradle 104, engages with rail 110 and breaker sitting plate 112, and is driven by a racking mechanism to travel in different operating conditions -connected, test, disconnected and maintenance, and is driven by a racking mechanism to travel in different operating positions - connected, test, disconnected and maintenance.
In an embodiment of the invention, latching mechanism 114 is adapted on the inner side of cradle 104 as shown in figure 1B. Latching mechanism 114 includes a racking cam 116, a rail pin 118, and a latch 120.
In an embodiment of the invention racking cam 116 is pivoted on cradle 104 of the circuit breaker. Racking cam 116 is capable of rotating and engaging with rail pin 118 on cradle 104 depending on operating condition of the circuit breaker.
In an embodiment of the invention, latch 120 is adapted to racking cam 116. Latch 120 is pivoted on racking cam 116. Latch 120 is capable of rotating and engaging with rail pin 118 on cradle 104 based upon operating condition of the circuit breaker. In an embodiment of the invention, latch 120 is pivots on racking cam 116 off-centre-of-gravity of latch 120, such pivoting restrains rotation of latch 120 and enables engagement of latch 120 with rail pin 118. In this regard, latch 120 has a pre-determined centre of gravity such that weight of latch 120 restrains rotation and engagement of latch 120 with rail pin 118.

Figure 5 shows position of latching mechanism when breaker travels from one operating condition to other. When breaker 102 travels from one operating condition to other along cradle 104, racking cam 116 rotates corresponding to movement of breaker 102 and accordingly engages with rail pin 118.
Accordingly, when breaker 102 travels from disconnected to test to connected position, racking cam 116 rotates in anticlockwise direction, and engages with rail pin 118 as shown in figures 5A through 5D, and when breaker 102 travels from connected to test to disconnect position, racking cam 116 rotates in clockwise direction and rail pin 118 remains engaged with racking cam 116 as shown in figures 5E through 5H.
During racking operation - transition from one operating condition to other, especially during connected and test position latching mechanism 114 engages and blocks rail pin 118 and breaker 102. Latch 120 is pivoted on racking cam 116 off-centre-of-gravity enabling latch 120 to engage with rail pin 118 such that breaker 102 can be locked in desired position, and if breaker 102 tries to move out of position, latch 120 will further block latch 120 on rail pin 118 and offers a direct normal force on breaker 102.
Latching mechanism 114 thus ensures that breaker 102 is held in position and no play exists in test position as illustrated in figure 5G. Also, during racking operation, when breaker 102 is held in one of the positions, the circuit breaker may be turned ON or OFF which produces vibrations. These vibrations pass through axis of rotation of latch 120. Latch 120 is mounted off-centre-of-gravity such that weight of latch 120 holds the latching mechanism in place during these vibrations. Latching mechanism 114 thus during this operation holds breaker 102 in position and restricts any unwanted movement which may be caused due to vibrations generated while turning the circuit breaker ON or OFF.
The present invention in another aspect provides accurate indication of breaker position. In this regard, the latching mechanism of the present invention ensures that there is no loss of continuity at the isolating contacts of the breaker when the breaker is held in test position. No

loss of continuity at the isolating contacts during test position facilitates accurate position indication of the breaker as there is no loss of continuity at the contact.
Advantageously the latching mechanism of the present invention provides:
• No play when breaker is held in testing position
• Accurate position indication
• No play, and hence breaker can withstand the vibrations generated in the switchgear assembly e.g. due to turning a switching apparatus ON or OFF
• Cost effective mechanism as minimum number of components are required
• East adaptability to existing circuit breakers
While the present invention has been described with respect to certain embodiments, it
will be apparent to those skilled in the art that various changes and modification may be made
■ without departing from the scope of the invention as defined in the following claims.

WE CLAIM:
1. A latching mechanism for a draw-out type circuit breaker, the latching mechanism
comprising:
a racking cam pivoted on a cradle of the circuit breaker, the racking cam being capable of rotating and engaging with a rail pin on the cradle based upon the operating condition of the circuit breaker; and
a latch pivoted on the racking cam, the latch being capable of rotating and engaging with the rail pin on the cradle based upon the operating condition of the circuit breaker, wherein the latch is pivoted on the racking cam off-centre-of-gravity of the latch.
2. The latching mechanism as claimed in claim 1, wherein such pivoting restrains the rotation of the latch and enables engagement of the latch with the rail pin.
3. The latching mechanism as claimed in claim 1, wherein during transition between various operating conditions of the circuit breaker, the racking cam engages with the rail pin through the latch on the raking cam, the latch thereby reducing the undesired play during the said transitions.
4. The latching mechanism as claimed in claim 1, wherein the operating conditions of the circuit breaker are - switching ON and OFF the breaker, charging of main spring when the breaker is in connected, test or disconnected position in cradle.