FORM 2
THE PATENTS ACT, 1970 (39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
[See section 10, Rule 13]
CURRENT LIMITING CIRCUIT BREAKER
WITH CONTACT LOCKING ARRANGEMENT
FOR HIGH ELECTRO DYNAMIC
WITHSTAND;
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 invention relates to circuit breakers and more particularly to a contact locking arrangement for circuit breakers. BACKGROUND OF THE INVENTION
Switching device typically carries, make & break the current under normal and abnormal conditions involving very high over-currents. The Switching device should interrupt the over current arising due to abnormal conditions in the network, as rapidly as possible to minimize damage caused by thermal and mechanical stresses to the equipment installed downstream.
Switching devices may also be designed to withstand high short circuit current, for achieving selectivity or coordination between the switching devices so that the closest switching device upstream of the fault location will isolate the faulted section from the healthy section of the power network. This results in increased availability of the system. One of the main requirements of such a switching device is its ability to withstand high thermal and mechanical stresses developed by the short circuit currents throughout the duration of flow of fault current.
Switching devices like, molded case circuit breaker operates on the current limiting principle typically have a pair of solid stationary Electrical contacts joined by a solid moving Electrical contact, which provides a path to carry the electrical current in the network.
The moving electrical contact should have a locking arrangement to generate enough contact pressure at the joints so that under normal conditions i.e. at rated current, the temperature rise of the current carrying components of the breaker should not exceed the allowable limit set by the relevant product standards. In the over current conditions, high electro dynamic repulsive forces are generated between the stationary and moving contacts. The

stationary contact being secured to the base of the molded case, repulsive forces repel open the moving contact. The above said locking arrangement should also offer very high opposition force to the electro dynamic repulsion forces making the system feasible for selectivity or coordination whilst simultaneously satisfying the requirements necessitated by the current limiting feature of the circuit breaker. As the forces developed in the system by the over current exceeds the high threshold opposition force offered by the locking arrangement contacts repel open in order to clear the fault.
One prior art, United States Patent 7005594 by LS Industrial Systems discloses a movable contactor assembly of circuit breaker. The movable contactor assembly for circuit breaker, has a rotation shaft with holding groove surfaces receiving slide pins on outer surface of shaft, such that motion of movable contactor is delayed to return to fixed contactors.
Another prior art, United States Patent 7394032 by Moeller GmbH discloses a electrodynamically tilting contact system for power circuit breakers. The electrodynamically tilting contact system for power circuit breakers has a tilting snap-action mechanism with two rockers on rotary contact bridge parallel to rotational axis of symmetry.
However, locking arrangement for the moving contact discussed above, 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, serious damage to the circuit breaker and the installation. To prevent the contact from bounce back and to limit the fault current in order to achieve a low value of let through energy, a contact locking arrangement is essential.
The present invention describes an arrangement to provide contact pressure under normal current carrying conditions, lock the moving contact arm for higher threshold values of fault current and to prevent the contact from bounce back.

SUMMARY OF THE INVENTION
Accordingly, the present invention in one aspect provides a contact locking arrangement for a circuit breaker, the contact locking arrangement comprising a drive shaft; at-least one moving contact pivoted at its centre on the drive shaft, the moving contact being capable of rotating and engaging with a fixed contact based upon the operating condition of the circuit breaker; a locking assembly coupled to the moving contact and the drive shaft, the locking assembly comprises: a first upper link extending from a proximal end to a distal end, the first upper link pivotally mounted at its proximal end on the drive shaft, and coupled to the moving contact; a first lower link pivotally mounted on the first upper link at one end and the moving contact at the other end; a second upper link mounted at a distance from the first upper link, the second upper link extends from a proximal end to a distal end, the second upper link pivotally mounted at its distal end on the drive shaft, and coupled to the moving contact; a second lower link pivotally mounted on the second upper link at one end and the moving contact at the other end; and plurality of springs connecting the first upper link to the second upper link wherein, in ON condition locking assembly produces counter-clockwise torque on the moving contact to restrain the movement of the moving contact. 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 1, shows an exploded view of a contact locking arrangement in accordance with
an embodiment of the invention.
Figure 2 shows the contact locking arrangement in ON position in accordance with an
embodiment of the invention.

Figure 3 shows the contact locking arrangement in neutral plane position in accordance with an embodiment of the invention.
Figure 4 shows the contact locking arrangement in OFF position in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the present invention provide a contact locking arrangement for a circuit breaker, the contact locking arrangement comprising a drive shaft; at-least one moving contact pivoted at its centre on the drive shaft, the moving contact being capable of rotating and engaging with a fixed contact based upon the operating condition of the circuit breaker; a locking assembly coupled to the moving contact and the drive shaft, the locking assembly comprises: a first upper link extending from a proximal end to a distal end, the first upper link pivotally mounted at its proximal end on the drive shaft, and coupled to the moving contact; a first lower link pivotally mounted on the first upper link at one end and the moving contact at the other end; a second upper link mounted parallel to the first upper link, the second upper link extends from a proximal end to a distal end, the second upper link pivotally mounted at its distal end on the drive shaft, and coupled to the moving contact; a second lower link pivotally mounted on the second upper link at one end and the moving contact at the other end; and plurality of springs connecting the first upper link to the second upper link wherein, in ON condition locking assembly produces counter-clockwise torque on the moving contact to restrain the movement of the moving contact
Figure 1 shows an exploded view of the contact locking arrangement 100 for a circuit breaker in accordance with an embodiment of the invention. The contact locking arrangement 100 includes a drive shaft 102, a moving contact 104 and a locking assembly 110.
In an embodiment of the invention, moving contact 104 is pivoted on drive shaft 102. According to the present invention, moving contact 104 is pivoted at its centre and is capable of rotating and engaging with a fixed contact (not shown) based upon the operation condition of the circuit breaker.
As shown in figure 1, the locking assembly 110 is coupled to moving contact 104 and drive shaft 102. In an embodiment of the invention locking assembly 110 is coupled to moving

contact 104 and drive shaft 102 via a pin 124 and hole 126 arrangement, wherein each pin is an anchor pin.
Locking assembly 110 includes a first upper link 112, a second upper link 114, a first lower link 116, a second lower link 118 and a first spring 120(not shown in figure 1) and a second spring 122 (not shown in figure 1).
In an embodiment of the invention, first upper link 112 extends from a proximal end 112a to a distal end 112b. At proximal end 112a, first upper link 112 is pivotally mounted on drive shaft 102, and coupled to moving contact 104. At distal end 112b, first upper link 112 is coupled to second upper link 114 via second spring 122. In an embodiment of the invention, second spring 122 couples second upper link 114 off its centre to first upper link 112 near distal end 112b.
In an embodiment of the invention, second upper link 114 is mounted at a distance from first upper link 112. Second upper link 114 extends from a proximal end 114a to a distal end 114b. At distal end 114b, second upper link 114 is pivotally mounted on drive shaft 102, and coupled to moving contact 104. At proximal end 114a, second upper link 114 is coupled to the first upper link 112 via first spring 120. In an embodiment of the invention, first spring 120 couples the first upper link 112 off its centre to the second upper link 114 near proximal end 114a.
In an embodiment of the invention, first lower link 116 is pivotally mounted on first upper link 112 at one end, and moving contact 104 at the other end
In an embodiment of the invention, second lower link 118 is pivotally mounted on second upper link 114 at one end, and moving contact 104 at the other end.
Figure 2 shows the contact locking arrangement 100 in ON position. In ON position, locking assembly 110 coupled to moving contact 104 produces anti-clockwise torque pushing moving contact 104 against fixed contact (not shown) maintaining contact pressure to maintain continuity in circuit breaker. In this regard, when the circuit breaker is in ON condition, first

spring 120 and second spring 122 coupling the upper links produce a clockwise torque on the upper links. The clockwise torque on the upper links produce a force on the lower links generating a counter clock-wise torque on moving contact 104. This counter clock-wise torque is responsible for providing necessary contact pressure to maintain continuity in the circuit breaker and thereby electrical circuit. In an embodiment of the invention in ON condition, torque experienced by moving contact is proportional to torque on upper link.
Figure 3 shows the contact locking arrangement 100 in neutral plane position in accordance with an embodiment of the invention. The neutral plane position is a transitory position, wherein moving contact 104 moves from ON position to OFF position. During this transition, direction of the torque on moving contact 104 due to the locking arrangement changes after crossing the neutral plane, i.e torque on moving contact due to locking assembly which opposes the electrodynamic torque on moving contact earlier, starts aiding after moving contact crosses the neutral plane. In this regard, when moving contact is in neutral plane as shown in figure 3, torque on moving contact is zero, and this is referred as dead centre position.
Figure 4 shows the contact locking arrangement 100 in an OFF position. The contact locking arrangement 100 moves from the ON position to the OFF position during a fault condition, wherein moving contact 104 moves from position shown in figure 2 to position shown in figure 3. During the fauit condition, an electro-dynamic force is exerted on moving contact 104. The electro-dynamic force generates a clock-wise torque on moving contact 104. The clock-wise torque on moving contact 104 is countered by a counter clockwise torque of moving contact 104. The counter clockwise torque is due to springs 120, 122 and frictional torque at pins 124 coupling the links. As the clockwise torque produced on moving contact 104 by the electro-dynamic force exceeds the counter clockwise torque of moving contact 104, the moving contact 104 repels and breaks continuity in the circuit breaker and thereby electrical circuit.

Advantageously the present invention provides a contact locking arrangement with high opening velocity, high mechanical stability, consistency and optimum utilization of contact spring force, and dead center crossing type of locking arrangement.
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 contact locking arrangement for a circuit breaker, the contact locking arrangement comprising: a drive shaft;
at-least one moving contact pivoted at its centre on the drive shaft, the moving contact being capable of rotating and engaging with a fixed contact based upon the operating condition of the circuit breaker;
a locking assembly coupled to the moving contact and the drive shaft, the locking assembly comprises:
a first upper link having a proximal end and a distal end, the first upper link pivotally mounted at its proximal end on the drive shaft, and coupled to the moving contact;
a first lower link pivotally mounted on the first upper link at one end and the moving contact at the other end;
a second upper link mounted at a distance from the first upper link, the second upper link extends from a proximal end to a distal end, the second upper link pivotally mounted at its distal end on the drive shaft, and coupled to the moving contact;
a second lower link pivotally mounted on the second upper link at one end and the moving contact at the other end ;and
plurality of springs connecting the first upper llink to the second upper link wherein, in ON condition locking assembly Produces counter-clockwise torque on the moving contact to restrain the movement of the contact.

2. The contact locking arrangement as claimed in claim 1, wherein in fault condition when the electrodynamic repulsive torque on the moving contact exceeds the locking torque produced by the locking assembly contact repel opens and is locked in OFF condition.