FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
PROVISIONAL SPECIFICATION
(See section 10; rule 13)

1. Title of the invention. -

ASSEMBLY OF MOVING CONTACTS FOR SWITCHING DEVICES

2. Applicant(s)
(a) NAME :
(b) NATIONALITY :
(c) ADDRESS :

LARSEN & TOUBRO LIMITED
An Indian company
L & T House, Ballard Estate, Mumbai 400 001, State of Maharashtra, India

3. PREAMBLE TO THE DESCRIPTION
The following specification describes the invention


TITLE OF THE INVENTION
ASSEMBLY OF MOVING CONTACTS FOR SWICTHING DEVICES TECHNICAL FIELD OF THE INVENTION
The present invention relates to a switching device i.e.
current limiting circuit breaker. More particularly the
invention relates to locking of the contacts used
herewith.
BACKGROUND AND THE PRIOR ART
US 6259048 Bl employs two pairs of off-center extension springs directly engaging the contact housing at once end and rotary contact arm via a linkage arrangement at the opposite end . The arrangement is ofleast one pair of extension springs between the link and the associated contact housing provides the optimum contact force by compensating for component tolerance. and contact erosion and wear while maintaining a reliaible means for locking the contact arm open in the event of an over current condition. It was not known from this prior art that moving contacts can be designed based on dead center crossing such that forces required are less and number of physical components are reduced.


OBJECTS OF THE PRESENT INVENTION
Accordingly. One object of the present invention is to overcome the disadvantages of the prior art.
Another object of the present invention is to provide a switching device comprising improved locking arrangement.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
The present invention relates to switching device i.e. current limiting circuit breaker, and more particularly to the locking of the contacts used herewith.
The circuit breaker comprises of various components and contact system forms one part of the same. The contact system comprises of a moving contact and fixed contacts. Moving contact can rotate about the . fulcrum. The moving contact is held to mate with fixed contacts at both the ends by application of force using a set of springs. The contacts are designed in such a way that current flowing through the contacts exerts electromagnetic forces which has a tendency to rotate open the moving contact. These electromagnetic forces are proportional to the square of the current flowing through the contacts. This repulsion force is counteracted by the force exerted by the springs to make the contacts mate.


Under normal operating conditions the electromagnetic forces are not high enough to overcome the spring force thus contacts remain mating for a smooth flow of current through them. Under fault conditions, current rises to a very high magnitude. When high current flows through the system, the contacts experience a very high repulsion force as the force increases in proportion to the square of the current flow. Magnitude of this repulsion force is high enough to overcome the counteracting force by the springs and the moving contact rotates about the fulcrum. As the moving contact opens, it reduces the current through the contacts and thus reducing the repulsive forces .Since the contact system of a switching device is spring loaded , under such conditions moving' contact has a tendency to bounce back and re-make the circuit which gives rise to stresses in the system and damages the circuit breaker particularly the contacts. The innovative feature of the present invention is the smooth and consistent locking of contact when the contact repels open in case of high current values. The contact gets locked in the open position and prevents it to bounce back and re-make the contact.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 illustrates the complete assembly of the movable contact system in two different views.
Fig la Side view
Fig lb Top view


Fig. 2 illustrates the isometric view of the whole assembly.
Fig. 3 illustrates an exploded view of the contact system, springs, links & connecting pin arrangement.
Fig 4. illustrates the cut section view of the assembly and shows positions of various components when the contact is in closed position.
Fig 5. illustrates the contacts in locked position.
DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.la illustrates the contact assembly in two views, Fig la is the side view which shows the current carrying moving contact (2) assembled inside the insulating contact housing (1) with the help of springs (3) , links (4) and pins (5) (both not seen in figure3) . Centre of the contact housing (1) acts as the fulcrum for the moving contact (2).
Fig. lb is the top view of the same assembly.
Fig. 2 is the isometric view of the.same. In this figure the contact housing body is visible along with the links (4), springs (3) and pins (5) which will be fixed in the contact housing (1) and the movable contact (2) .


Fig 3 shows the exploded view of the assembly showing the various components like
1) the body of the contact housing referred as contact housing
2) the movable contact
3) the springs which are placed between the pins (5) on both the ends.
4) the connecting links riveted to the moving contact(2) on one end and the other end placed along the guided grove on the contact housing(l) with the help of pins(5)
5) The connecting pin which slides along the guided groove on the contact housing with one end of the spring resting on it.
Fig 4 shows the contacts in closed position. The position of the springs (3) and links (4) in the closed state can be seen. Also the position of the contact with respect to the contact housing is visible. Springs (3) exerts a force on the moving contact (2). through the connecting pin (5) and links (4), which rests on the support of the contact housing (1) .
Fig 5 shows the contacts in open position. While moving from closed to open state, connecting pins (5) which are guided along the groove of the contact housing (1) / crosses the dead centre the springs (3) and gets locked in the open state.


GENERAL WORKING
Under Fault condition when the moving contact (2) has been repelled open by the repulsion forces inside the breaker, the arrangement of the present invention holds the contacts in the open state till it has been reset by the mechanism of the breaker preventing the moving contact (2) to bounce back and re-make the circuit and cause the damage to the system
This has been achieved by an arrangement which uses of a pair of extension springs (3} . Both the ends of the extended springs (3) are assembled to rest on the connecting pins (5) as can be seen in Fig 2. The ends of connecting pins (5) rests on the groves on both the sides of the contact housing (1) and thus its movement is guided by the profile of the groove.
Link (4) is a flat component with holes on both the ends. Links (4) connect the moving contact (2) with the spring loaded connecting pins (5) as can be seen from fig 2. Links (4) are assembled on both sides of the moving contact (2) with the help of pins (6). Links (4) are free to rotate about the pins (6) . Connecting Pins (5) pass through the other end of the links (4). The whole arrangement can be clearly seen in Fig 3.


The basic principle of the contact locking is the spring crossing the dead center. This Dead centre crossing is achieved with the help of links (4) and the profile of the groove on both the sides of contact housing (1).
The spring is of normal length at two points, one when the moving contact (2) in closed position and other one when it is in repel open position. When the movable contact (2) is closed the connect pin s (5) to which the springs (3) are connected is in one end_ of the groove as seen in Fig 4. The pair of springs (3) exerts a force on the moving contact (2) through connecting pins (5) and links (4) holding the moving contact in the required position and it opposes any force trying to rotate the moving contact (2) to open position.
Under fault the current flowing through the contacts increase to a high value thus generating a high magnitude of repulsive forces which try to repel open the moving contact (2) to open position as shown in fig ' 5. When repelled, the contacts travel and because the. repulsion force is same on both sides of the contact, the contact rotates about the fulcrum.
As the moving contact (2) moves from the closed state to the open state, the links (4) exerts a force on the connecting pins (5). As the connecting pins (5) have only one direction to move, they slide and are guided by the groove on the contact housing (1) .Springs (5) connected


to these pins (5) thus experiences extension till the connecting pins (5) crosses the threshold of the groove. The groove is designed such that the threshold of the groove coincides with the dead centre of the spring. After the connecting pins (5) cross the threshold and thus the dead centre, extension in the springs (3) starts reducing, causing the spring to pull the connecting pins (5) over to the other side of the threshold and continuously reducing the extension in the springs (3) to the normal length as in reaches the closed state as seen in fig 5. This prevents the springs (3) to force the moving contact (2) to fall back to the closed position.
Moving contact (2) remains in a stable state of open position till it has been reset by tripping mechanism of the circuit breaker.
ADVANTAGES :
1. The efficiency of the arrangement is better so the forces involved are reduced. This makes the contact system compact and simple.
2. The angle at which the contact gets locked can be varied by varying the profile on the contact housing. This gives flexibility to the design in terms of changing requirements.


3. It is possible to achieve contact locking lower angles. This considerably reduces the time required to open and lock the contact.
4. Use of links as a force transferring member from springs to contact increases the component of force utilized for creating contact pressure.
5. Links assembled at an angle make contact locking possible without springs going to fully extended length.
6. The construction makes it possible to fully isolate the springs from the live parts.
7. Use of links gives liberty to design moving contact of required shape.