FORM2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10; rule 13)
1.Title of the invention: SCISSOR BASED LOCKING
ARRANGEMENT FOR CONTACT SYSTEM
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:

TITLE OF THE INVENTION
Scissor based locking arrangement for contact system
FIELD OF THE INVENTION
The present invention relates locking mechanism in contact systems. More particularly, the invention relates to an improved cam based locking mechanism for contact system adapted to withstand high electrodynamic forces without repulsion which requires a significant amount of compensation to offset ensuing mechanical stresses.
BACKGROUND AND THE PRIOR ART
Circuit breaker is a device employed to protect the equipments in an installation in case of over currents arising out of fault conditions. An electric circuit breaker contact locking mechanism is provided including at least one stationary contact member and at least one movable contact member and biasing means between the movable contact member and a normally stationary support member to provide contact closing pressure during normal condition. It is also required to interrupt the fault current rapidly in one or more circuit's paths upon the occurrence of an over current in any one circuit path so as to minimize damage to the installation resulting from thermal and mechanical stresses.
To provide comprehensive coordination with downstream protection devices, the device in consideration is required to withstand the maximum through fault current. Whilst it should respond instantaneously if the fault current is higher than the pre-set current value designated for withstand. Withstanding such high electrodynamics forces without repulsion requires a significant amount of compensation to offset ensuing mechanical stresses.

Compensation of these forces can be done either through special current path configurations or from the combination of both such configurations and mechanical locking arrangements. These types of compensation require higher space to accommodate the same and require an external actuation or impetus for operation/de-latching.
In the above mentioned scenario it is expected that downstream breaker shall open and clear the fault whereas upstream breaker do not open & remain in closed condition. This ensures that the other downstream feeder (breakers) remains undisturbed and achieves discrimination.
US 6,084,489 discloses the circuit breaker is characterized by separable contacts operable between a closed and an open position a contact arm having a latching surface and a latch arranged to engage the latching surface when the contacts are blown-open under short-circuit conditions. The arrangement of the latch allows for a positive lock under high magnitude short circuit levels while minimizing the force required by a mechanism to unlock the arm.
US 6015959 discloses cam mechanism which is used for switching in Normal conditions where as present invention cam arrangement is used to operate only in abnormal (short circuit) condition.
US 6,448.522 disclose a motor operator mechanism is disclosed for moving a breaker handle of a circuit breaker between off and on positions. The motor operator mechanism comprises of a first pin biased to engage the breaker handle in a direction to close the circuit breaker, a pin latch configured to releasable engage the first pin when the breaker handle is in a position intermediate to the off and on positions, wherein releasing the pin latch allows the first pin to move the breaker handle to the on position.

US 6,590,482 discloses a circuit breaker operating mechanism for separating a pair of electrical contacts within an electrical circuit breaker includes a lower link operatively connected to one of the electrical contacts. An upper link includes first and second legs extending from a central portion. The first leg is pivotally secured to the lower link, and the second leg includes a cam surface formed thereon. A roller is in intimate contact with the cam surface, and the cam surface is configured such that movement of the upper link relative to the roller causes the upper link to pivot about the central portion. Pivoting of the upper link about the central portion moves the lower link causing the second contact to move away from the first contact. An operating spring is configured to provide a force for separating the electrical contacts when the operating mechanism is tripped. An operating handle includes a void disposed therein, and an end of the spring is secured to the operating handle within the void.
US 6,018,284 discloses a circuit breaker pole or poles comprises of movable contact means with a support carrier movable with respect to the frame between an open position and a closed position support carrier and one or more contact fingers movable with respect to the support carrier between a contact position and a retracted position. Electromagnetic compensation means are designed to apply electromagnetic forces on the contact fingers to keep the contact finger in contact with the fixed stationary contact.
The drawbacks of the above mentioned prior art are that in the prior power system the circuit breakers are connected in series. Whenever fault occurs in the circuit the fault current passes through both the downstream and the upstream breaker which sees the fault current. Even though in most case the downstream breaker clears the fault, the upstream breaker opens to some extent and then closes due to the high electrodynamics forces due to the current it carries for a short duration of time. Even-

if the contacts of the upstream breaker opens by a small amount an arc is struck between the fixed and the moving contact due to which there occurs a disturbance in the system and also the life of the breaker decreases.
Further in the prior art the latch engages when contact is in open position only and not in both closed and open (after reset) position. The arm is unlocked by operating mechanism and cam mechanism used is manually operated which is not as reliable as the self triggering mechanism. In the prior art there in no scheme involved where the mechanical arrangement is used for locking moving contact till particular threshold value and self triggered unlocking beyond threshold value.
Thus there is a need to provide a system in which the upstream breaker will wait and not open till the downstream breaker clears the fault and if the downstream breaker fails to clear the fault then the upstream breaker will open. Here this is achieved with mechanical compensation for the electrodynamics forces on the moving contact.
Further there is a need to provide a locking within the limited space of the rotor assembly by providing cam surfaces on the moving contact and the side plate. Further there is a need to provide a spring to enhance the locking force.
OBJECTS OF THE INVENTION
A basic object of the present invention is to overcome the drawbacks/disadvantages of the prior art.
One of the main object of the invention is to improve the life of the contact system and to provide enhanced discrimination among the circuit breakers.

Another object of the present invention is to provide an improved cam based contact mechanism where the threshold value can be can be tuned to address various threshold requirements to provide flexibility to the function by varying various constructional parameters with very less change in restraining force of energy device.
Yet another object of the present invention is to provide an improved mechanism to compensate the electrodynamic force by auto de-latching of the contact defined by a biased cam arrangement.
Yet another object of the present invention is to provide an improved circuit breaker which is easily adapted to any type of contact system such as single break, double break, and multiple finger system.
Yet another object of the present invention is to provide an intelligent auto de-latching mechanical contact locking arrangement that will ensure contact stability to a pre-defined threshold force calibrated to a pre defined threshold current.
Yet another object of the present invention is to provide an improved mechanism to cause rapid rotation of the contacts, which will ensure fast arc quenching and subsequent clearance of fault.
Yet another object of the present invention is to make the system self sustaining in terms of locking of the contact system to provide discrimination.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided a scissor based locking arrangement for contact system, said mechanism comprising (i) a rotor means;

(ii) plurality of fixed contact;
(iii) plurality of moving contact operatively connected to said fixed contacts in ON operation, said moving contacts comprising plurality of cam profiles, wherein said plurality of cam profile of said moving contact comprising :
(a) plurality of contact pressure cam profile substantially located on the moving contact adapted to provide contact pressure on moving contact and
(b) plurality of locking cam profile substantially located on the moving contact adapted to provide locking pressure on moving contact;
(iv) plate means operatively connected to said moving contact being riveted or screwed or connected together with said rotor means, said plate means having plurality of cam profile, wherein said plurality of cam profile of said plate means comprising :
(a) plurality of contact pressure cam profile substantially located on the plate means adapted to exert contact pressure on plate means and
(b) plurality of locking cam profile located substantially on the moving contact adapted to exert locking force on plate means;
(v) plurality of contact pressure assembly located on the said plate means comprising:
(a) plurality of contact pressure pin substantially connected between said contact pressure cam on side plate and contact pressure cam on moving contact;

(b) plurality of fixed pin means substantially placed on the diametrically opposite side of said contact pressure pin and fixed on the side plate in close proximity with said locking cam profile of said plate means and
(c) plurality of contact pressure element having hook like profile substantially placed in-between said contact pressure pin and said fixed pin adapted to provide contact pressure on moving contact and
(vi) a locking assembly located on the plate means comprising
(a) plurality of locking pin means substantially connected between said locking cam on side plate and locking cam on moving contact adapted to restrict rotary motion of said moving contact up to a predetermined threshold force value and
(b) a locking element having hook like profile substantially placed in-between said locking pin adapted to provide locking pressure on moving contact.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Fig. 1 illustrates Front view of mechanical locking arrangement at ON i.e. Locked Position of contact system.
Fig. 2 illustrates Isometric view (without fixed contact assembly and springs) of mechanical locking arrangement for contact system.
Fig. 3 illustrates Front view (without fixed contact assembly) of mechanical locking arrangement at ON i.e. Locked Position of contact system.

Fig. 4 illustrates Front view of mechanical locking arrangement at TRIP i.e. Unlocked Position of contact system.
Fig. 5 illustrates Front view of side plate rnentioned in mechanical locking arrangement for contact system
Fig. 6 illustrates Isometric view of moving contact mentioned in mechanical locking arrangement for contact system
Fig. 7 illustrates Front view of mechanical locking arrangement at OFF Position of comact system
DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWING Elements and Construction:
Fixed contact (1): It is the fixed current carrying element of the circuit.
Moving contact (2): It is moving current carrying element of the circuit. The moving contact starts to separate beyond the threshold value set with the help of cam and the springs.
Rotor Coupler (3): It connects operating mechanism of circuit breaker to moving contact through rotor. It acts as coupling element between different poles. Also it acts as a coupling element for the side plate.
Side Plate(5): It is connected to the rotor coupler' through joining arrangement such as riveting, screwing, profile to make it integral part of moving contact. It has cam profiles which are engaged with CP cam pin to exert contact pressure and locking pin to exert locking force. This cam profile can also be on moving contact directly.

Contact Pressure Spring Pin (15): It is connected between the contact pressure cam on side plate and the contact pressure cam on the moving contact to provide contact pressure with the help of contact pressure springs.
Contact Pressure(CP) Spring (6): It is connected between CP pin and the Fixed Pin. It can be any energy storing element such as extension, compression, torsion spring etc. In stated invention, extension spring is used for effective utilization of volume
Locking Pin (7): It restricts rotary motion of moving contact assembly till predefined threshold force value and allows rotary motion of contact assembly beyond the same threshold value. In this case, symmetrical locking arrangement is used to achieve the function but same function can be achieved by nonsymmetrical arrangement as well.
Locking Spring(8): Locking Springs are the energy storing elements which are used to apply anticlockwise(direction opposite to torque applied due to contact arm forces) torque on locking cam on the moving contact at locked position and to restore locking pin at unlocked position. In this case locking spring used is extension spring. It can be conventional extension spring with any type of hook arrangement. Also by appropriate modifications, Compression and torsion spring can also be used for this application. In addition, Operating threshold of the invention mentioned can be adjusted by varying restrain force values associated with springs.
Fixed Pin (4): It works as the fixed end for the CP spring. Also it acts as the spacers between the side plate
Moving Contact Button (9): It is the contact button connected to the moving contact.
Fixed Contact Button (10): It is the contact button connected to the fixed contact.
Arc Runner (11): It is connected to the fixed contact for efficient movement of the electric arc formed during fault condition in the circuit breaker.

Shield (12): It magnetically shields the lower part of the U bend in the fixed contact from the upper part.
Contact Pressure Cam on Side plate (13): It is the cam for the contact pressure on the side plate. The CP pin moves in this cam.
Contact Pressure Cam on Moving Contact (16): It is the cam for the contact pressure on the Moving contact. The CP pin moves in this cam.
Locking Cam on Side plate (14): It is the cam for the locking on the side piate. The Locking pin moves in this cam.
Locking Cam on Moving Contact (17): It is the cam for the locking on the Moving contact. The Locking pin moves in this cam.
Referring to Figure 1 - 7 discloses
ON-OFF Operation:
At ON Position, moving contact arm 2 is at first stable position with locking cam profile 14 on side plate 5 engaged with respective locking cam profile on moving contact 17.
Referring to Fig.l the said contact pressure spring 6 held extended between the fixed pin 4 mounted on rotor coupler 3 and the CP spring pin 15, generates force on each side of contact arm 2. This force generates a torque on the said moving contact 2 about its axis of the rotation and also imparts the required contact force to the contact. The CP cam profiles on moving contact and side plate 16 and 13 resp are designed in such a way that on operating condition, CP spring pin 15 rests on CP cam profile of moving arm 16 and should be isolated from CP cam profile on side plate 13. The generated force through contact pressure spring 6 assists the said contact arm 2 to maintain continuity of the electrical circuit under normal operating conditions.

Locking spring 8 connected between symmetric locking pins 7 exerts torque on moving contact arm 2 through locking cam profiles on moving contact 17.This will decide threshold force value of stated arrangement.
During ON-OFF operation, input to the rotor coupler 3 is given through the operating mechanism connected to rotor coupler 3. When extension springs of the operating mechanism get extended up to dead centre, potential energy is stored in the springs. This Stored potential energy gets converted into kinetic energy after dead centre of the operating mechanism and rotor coupler 3 is rotated along with moving contact arm 2 from ON to OFF position.
OFF-ON Operation:
At OFF Position, moving contact arm 2 is at second stable position with locking cam profile on side plate 14 engaged with respective locking cam profile on moving contact 17.During OFF-ON operation also, Input to the rotor coupler 3 is given through the operating mechanism connected to rotor coupler 3. Extension springs of the operating mechanism get extended up to dead centre to store potential energy in the springs. This Stored potential energy gets converted into kinetic energy after dead centre of the operating mechanism and rotor 4 is rotated along with Contact arm 2 from OFF to ON position.
ON-TRIP Position:
Referring to Figure 4 and 5 discloses ON-TRIP Operation: During ON-TRIP Operation, contact arm 2 is not displaced till the predefined threshold force value of stated arrangement is reached. Threshold value is the mechanical compensation force essential to withstand the electro-dynamic forces exerted on the moving contact arm 2

because of high flow of current through contact elements. When Electro-dynamic forces go beyond the pre-defined threshold force, locking pin 7 will translate along the locking cam profile on side plate 5 so as to allow rotary motion of moving contact 2 in opposite direction of locking spring torque. Hence contact arm 2 will move to the OFF Position.
Resetting Operation:
When Operating mechanism drives the rotor coupler 3, locking pin 7 and contact arm 2 are resettled to locking position. Now, breaker is ready for next OFF-ON operation.
Threshold point of contact arm displacement and Slope of contact arm displacement can be changed by varying various constructional parameters such as locking pin diameter, perpendicular distances of cam force from contact arm pivot point and locking cam profile angle.
Definitions of the technical terms
Centre Pin: This element connects moving contact to the side plate to allow rotary motion of contact about Z axis. Also position of centre pin determine dead centre position of contact system
Operating mechanism: It is the circuit breaker operating mechanism which provides energy to rotor to achieve rotary motion of contact arms.
Threshold value: It's the mechanical compensation force essential to withstand the electro-dynamic forces exerted on the moving contact because of the high fault current. Electro-dynamic forces generated beyond the pre-set value will enable moving contact to rise over the threshold value thereby operating the same.

Compensation mechanism can be tuned to achieve desired threshold value by changing the constructional parameters.
Dead centre- Dead centre is an equilibrium condition of actuating mechanism. In this condition the mechanism spring is fully charged and beyond this point mechanism flips & operates independently due to energy discharged by mechanism spring.
Advantages:
♦ It is self sufficient for locking the contacts for rotation, the external mechanism is only used only for giving position to the rotor system.
♦ Easily adapted to any type of contact system such as single break, double break, and multiple finger system.
♦ Threshold value can be can be tuned to address various threshold requirements to provide flexibility to the function by varying various constructional parameters with very less change in restraining force of energy device.
♦ The locking force on the moving contact can be changed by changing the cam on
the moving contact as well as the cam on the side plate, so for same value of the
energy storing device higher locking force can be obtained.
♦I Negligible resistance during opening of contact arms and assisting torque on moving contact after dead centre rotation of contact.
♦ Life of the breaker increases. Because during fault condition, upstream breaker
only opens if downstream breaker fails to clear the fault. This significantly
reduces occurrence of arcing at button which in turn reduces contact button
erosion.

The invention has been described in a preferred form only and many variations may be made in the invention which will still be comprised within its spirit. The invention is not limited to the details cited above. 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.

WE CLAIM
1. A scissor based locking mechanism for contact system, said mechanism comprising
(i) a rotor means;
(ii) plurality of fixed contact;
(iii) plurality of moving contact operatively connected to said fixed contacts in ON operation, said moving contacts comprising plurality of cam profiles, wherein said plurality of cam profile of said moving contact comprising :
(a) plurality of contact pressure cam profile substantially located on the moving contact adapted to provide contact pressure on moving contact and
(b) plurality of locking cam profile substantially located on the moving contact adapted to provide locking pressure on moving contact;
(iv) plate means operatively connected to said moving contact being riveted or screwed or connected together with said rotor means, said plate means having plurality of cam profile, wherein said plurality of cam profile of said plate means comprising :
(a) plurality of contact pressure cam profile substantially located on the plate means adapted to exert contact pressure on plate means and

(b) plurality of locking cam profile located substantially on the moving contact adapted to exert locking force on plate means;
(v) plurality of contact pressure assembly located on the said plate means comprising:
(a) plurality of contact pressure pin substantially connected between said contact pressure cam on side plate and contact pressure cam on moving contact;
(b) plurality of fixed pin means substantially placed on the diametrically opposite side of said contact pressure pin and fixed on the side plate in close proximity with said locking cam profile of said plate means and
(c) plurality of contact pressure element having hook like profile substantially placed in-between said contact pressure pin and said fixed pin adapted to provide contact pressure on moving contact and
(vi) a locking assembly located on the plate means comprising
(a) plurality of locking pin means substantially connected between said locking cam on side plate and locking cam on moving contact adapted to restrict rotary motion of said moving contact up to a predetermined threshold force value and
(b) a locking element having hook like profile substantially placed in-between said locking pin adapted to provide locking pressure on moving contact.

2. Mechanism as claimed in claim 1 wherein said rotor means adapted to act as a coupling element for the plates.
3. Mechanism as claimed in claim 1 wherein said plurality of fixed contact means having a substantially U-shaped profile having upper and lower arms, said fixed contact comprising :

(a) a fixed means operatively connected with the upper arm of said fixed contact;
(b) a moving means operatively connected in-between said moving contact and said fixed means adapted to associate said fixed contact with moving contact;
(c) an arc means extending from said fixed means adapted for efficient movement of the electric arc formed during fault condition in the circuit breaker and
(d) a shield means substantially placed in-between the said upper and lower arms of the fixed contact means adapted to magnetically shields the lower arm of the fixed contact means from its upper arm.

4. Mechanism as claimed in claim 1 wherein said contact pressure pin operatively moves on said contact pressure cam of moving contact.
5. Mechanism as claimed in claim 1 wherein said contact pressure pin operatively moves on said locking cam of moving contact.

6. Mechanism as claimed in claim 1 wherein said locking pin operatively moves on said locking cam of moving contact.
7. Mechanism as claimed in claim 1 wherein said locking pin operatively moves on said locking cam of plate means.
8. Mechanism as claimed in claims 1 wherein said plurality of locking cam profile of plate means is engaged with locking cam profile of moving contact on ON operation .
9. Mechanism as claimed in claims 1 wherein said plurality of locking cam profile of plate means is engaged with locking cam profile of moving contact on OFF operation .
10. Mechanism as claimed in claims 1 wherein said contact pressure pin being adapted to rest on contact pressure cam profile of moving contact and remain isolated from contact pressure cam profile on side plate during ON operation.
11. Mechanism as claimed in claim 1 wherein said fixed pin works as the fixed end for the contact pressure element.
12. Mechanism as claimed in claim 1 wherein said fixed pin is adapted to act as a spacer between plate means.

13. Mechanism as claimed in claim 1 wherein said moving contact means optionally comprises a locking cam and contact pressure cam on side plate cam profile.
14. Mechanism as claimed in claim 1 wherein contact pressure elements comprises contact pressure spring means.
15. Mechanism as claimed in claim 14 wherein said spring means is an extension spring or a compression spring or torsion spring or any other energy storing device.
16. Mechanism as claimed in claim 14 wherein said contact pressure spring means is a extension spring adapted for effective utilization of volume,
17. Mechanism as claimed in claim 1 wherein said locking element optionally comprises symmetrical locking arrangement.
18. Mechanism as claimed in claim 1 wherein said locking element comprises energy storing elements.
19. Mechanism as claimed in claim 18 wherein said energy storing elements comprise locking springs.

20. Mechanism as claimed in claim 19 wherein said locking spring is an extension spring.
21. Mechanism as claimed in claims 19 and 20 wherein said locking spring is optionally a compression spring or a torsion spring.
22. Mechanism as claimed in claims 19-21 wherein said locking spring is substantially placed in-between said contact pressure cam profile.
23. Mechanism as claimed in any of the preceding claims further comprising a centre pin adapted to operatively connect said moving contact to said rotor so as to allow rotation of said moving contacts.
24. An improved cam based locking mechanism for contact system as herein substantially described and illustrated with reference to the accompanying drawings.