FORM 2

THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
[See section 10, Rule 13]
A SWITCHING DEVICE;
LARSEN & TOUBRO LIMITED, A COMPANY INCORPORATED UNDER THE COMPANIES ACT, 1956, WHOSE ADDRESS IS L&T HOUSE, BALLARD ESTATE, MUMBAI - 400 001, MAHARASHTRA, INDIA

30 JAN 2006

THE FOLLOWING SPECIFICATION
PARTICULARLY DESCRIBES THE
INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

TECHNICAL FIELD
The present invention relates to a switching device, more particularly to a switching device with enhanced short circuit performance.
BACKGROUND OF INVENTION
A switching device is normally designed to withstand currents having values in the order of 12 times of the rated current of the switch. However, if the closed contact of the switching device is subjected to a high current impulse, an electrodynamic force may open the contact and the occurring arc may damage the device as deformation, welding and other phenomenon due to repulsion or heat.
Therefore, the electric switch may have to be designed to enable to withstand short time high current especially when passing through high current.
US Patent US 4513270 describes a compensation in contactor by means of a piece made from soft magnetic material rigidly secured to contact holder permanently interlocked with the movable part of the contactor electromagnet and so arranged that, as soon as the overload current reaches certain value, it exerts an attraction force on a portion of said movable contact carrier and a second piece made from a soft magnetic material rigidly coupled with a said movable contact carrier or bearing directly on said movable contact carrier, said force of attraction being sufficient to counter balance the effect of repulsion forces.
However, the US patent utilizes two pieces of magnetic material, one of which essentially be U-shaped. The contacts are required to have a 'U' shape so that during passage of overload current, a repulsion forces
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between two limbs of it shaped moving contacts result in increasing the contact pressure between fixed and moving contacts. This puts restriction that the moving contact should be 'U shaped and should bridge a gap between set of fixed contacts.
Therefore, it may not be suitable to install in the existing devices as the configuration of the switching device has to be changed.
Another US Patent US4849590 describes a switch having contacts constructed so that an electro-dynamic force is created by the current flowing through the switch, which tends to maintain the switch in a closed state. This force counteracts other electro-dynamic forces, which tend to open the switch.
However, in the contemporary designs utilizing electro -dynamically compensated contact system is that the contacts are required to have a specific type of construction to achieve the electro-dynamic compensation.
A US Patent US4427957 describes magnetic members are provided operatively in association with the contact members to define a closed magnetic loop with a current path extending through the loop, when the contact members are turned to the make-position. The magnetic members attract each other by the magnetic flux through the closed loop to maintain the contacts in the make-position when the current flowing through the current path is not zero.
However, the prior art requires the current path to be extended through a magnetic loop. Also, the above patent uses more than one magnetic member to form magnetic loop.
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US5815058 describes a contact enforcement apparatus for an electric switch includes two electro-magnetic bars. The first electro-magnetic bar is disposed between a crossbar of the switch and a moveable contact arm. The crossbar carries the moveable contact arm and the first electromagnetic bar such that the electrical contacts of a stationary contact arm and the moveable contact arm are opened and closed by the crossbar. The second electro-magnetic bar is in U-shape and has a front wall and a back wall, between which the moveable contact arm is at least partially inserted when the electrical contacts are closed together. When a "fault" current passes by the electrical contacts, an attractive force between the electro-magnetic bars is largely increased. This attractive force overcomes a repulsive force caused by the "fault" current which tends to open up the electrical contacts, so that the contact between the moveable and stationary contacts remain closed.
However, the contacts in the movable contact arm are partially inserted between the electro-magnetic bars when the switch is closed. Further, there is a force of attraction between electro-magnetic bars.
Although the above prior arts suggest a solution for compensation, the construction of the device is bulky and may not be used in the existing switching devices. Further, more than one magnetic component is used of a particular shape to achieve such compensation.
SUMMARY OF INVENTION
It is an object of the invention to provide means for compensating the effect of electro-dynamic repulsion forces which arise between fixed and
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moving contacts during passage of short circuit current, so as to prevent the contacts from malapropos separating.
Another object of the invention is to achieve the said compensation with minimum restrictions on shapes and placements of the contacts and make it the most economic arrangement of achieving electrodynamic compensation.
According to an embodiment of the present invention provides a switching device comprising a fixed contact; a resiliency held contact member; a fixed terminal; a movable contact bridge having moving contact secured at one end adapted to move translationally between the fixed contact and resiliency held contact member; and a soft magnetic strip fixed on said movable contact bridge facing the resiliency held contact member. The current flows from the fixed terminal to the moving contact to the resiliency held contact member. The soft magnetic material secured to moving contact bridge exerts an attraction force on the resiliency held contact member in the same direction as that of the spring thereby counter balancing the effect of repulsion force exerted between the fixed and movable contact.
The present invention utilizes a single strip of soft magnetic material secured on movable contact bridge. During short-circuit currents, the force of attraction becomes substantial between the said strip of magnetic material and the resiliency held contact member. The increased force helps in compensating for the repulsion forces between fixed and moving contacts.
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The size of switching device is unrestrictive and number of components are lesser as compared with the cited prior art.
Moreover, the present invention is suitable for multiple contact configurations besides possessing the advantage that it can also be used in the present state of the art switches without altering or amending the existing construction of the above said switches.
BRIEF DESCRIPTION OF DRAWINGS
Figure 1 illustrates the front view, depicting the basic components of the switch assembly.
Figure 2 illustrates the side view of the switch assembly showing the compensation piece.
DETAILED DESCRIPTION OF THE INVENTION
According to an embodiment of the present invention, the switching device is provided said switching device a fixed contact; a resiliently held contact member; a fixed terminal; a movable contact bridge having moving contact secured at one end adapted to move translationally between the fixed contact and resiliently held contact member; and a soft magnetic strip fixed on said movable contact bridge facing the resiliently held contact member. The current flows from the fixed terminal to the moving contact to the resiliently held contact member.
As shown in figure 1 the switching device ON condition is illustrated.
According to an embodiment, the switching device of the present invention having a moving contact means (6) and plurality of means for carrying current. The said moving contact means (6) comprises plurality of rollers
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(9) being operatively connected to carriage means (10) for holding the moving contacts (6). The said carriage means (10) for holding the moving contacts (6) has bridge being able to move translationally. The plurality of means for carrying current comprises two linear current carrying members of which one member (4) being elongated and spring loaded adapted to exhibit resilient action and the other member (1) is fixed on housing (12). The moving contacts (6) are pressure engaged or sandwiched between the two linear members (4) and (1) and come out during break operation, undergoing a translational motion.
According to another embodiment of the present invention, a switching having housing assemblies (12) including base (2) and a top (3).The base (2) is economically designed and provided with pre determined grooves and channels, the major purpose being to house the current carrying parts and give required support to them, apart from lending a guided path for the moving entity viz. Moving bridge assembly. The base (2) and top (3) of the housing are preferably rectangular in shape. It has to be understood that the shape of the housing may be altered according to the requirement or need.
The moving assembly (10) and (6) are capable of moving in the guideway having an insulating carriage provided with limbs (not shown in figure) extending outwards in a perpendicular direction to the direction of its travel, which are adapted to carry contact means (6). The contact means (6) in operative connection to the moving assembly/insulating carriage having moving contact means (6) and are mounted on the said limbs with the help of circlips. The carriage or moving bridge (10) sliding in the guideway is adapted to bridge the gaps in the current carrying path
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between the conducting means mounted on the inside surface of the top (3) housing (12) and fixed terminal (1) on the base (2) . The guideway is preferably located at the center of the base (2) of the housing (12) and extends all along the width of the base (2) of the housing (12). The base of the housing comprises pair of current carrying terminals (1a and 1b) rigidly fixed on the base with the help of screws and mounted in front of each other, perpendicular to the guideway in the base housing. The number of the fixed terminals may be decided according to the need or requirement.
The moving contacts (6a and 6b) are mounted on the limbs of the moving bridge (10) with the help of circlips. The rectangular top (3) of the housing is adapted to encompass the whole switch (14) and when placed on the base (2) of the housing (12) with constituent members therein, forms the contact system of the switch (14). The top (3) of the housing has an elongated rectangular conducting element i.e. the shorting link (4) being mounted in the top (3) of the housing (12) with its position just above the fixed terminals (1a) and (1b), and face parallel to them in a manner that it is free to perform lateral movement by virtue of the spring means (5), which are mounted to the top (3) of the housing (12).
The said spring means (5) may have contact springs, one end of which is hooked in the top (3) of the housing, the other resting on the shorting link (4), thus directly exerting force on short link (4) and adapted to keep the moving contacts (6a and 6b) firmly held in between the fixed terminals (1a and 1b) and the shorting link (4), when the switch is in ON condition.
A soft magnetic material (7), rigidly secured to the contact bridge (10) and so arranged that, it exerts an attraction force on the shorting links (4) , said
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force of attraction acting in the same direction as that of the springs (5), thus assisting the springs (5) attached to the shorting links (4). The attraction force counter balances the effect of the repulsion forces which are exerted between the fixed terminal (1) and movable contacts (6), at the time of short circuit; thus avoiding any contact separation. The strip of soft magnetic material (7) is fixed on the surface of movable contact bridge (10), facing shorting link (4). The soft magnetic strip piece (7) is so located as to run parallel along a considerable length of the shorting link (4), spanning almost all the area under it. It occupies a central position between the 2 sets of moving contacts (6a) and 6 (b) and substantially away from the shorting link (4).
The above described is the compensation arrangement and said arrangement maybe replicated at all the poles of the switch (14)
Further, during short-circuit currents, the force of attraction becomes substantial between the said strip of magnetic material and the resiliency held contact member. The increased force helps in compensating for the repulsion forces between fixed and moving contacts.
The size of switching device is unrestrictive and number of components are lesser as compared with the cited prior art.
Moreover, the present invention is suitable for multiple contact configurations besides possessing the advantage that
it can also be used in the present state of the art switches without altering or amending the existing construction of the above said switches.
Figure 1 shows the moving contacts (6) 'sandwiched' between the fixed terminals (1) and shorting link (4), thus bridging the gap between them in
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ON condition. The contact springs (5) are fully compressed and exert contact pressure on the shorting link (4). The top housing (3) is adapted to encompass the whole switch when placed on the base housing (2).
In figure 2 the side view of the compensated switching device in ON condition of the switch is illustrated. The compensation piece (7), along with the force which it exerts on the shorting link (4) is also shown.
The continuity is established in the electric circuit and the current flows from first fixed terminal (1a) to first set of moving contacts (6a) to shorting link (4) to second set of moving contacts (6b) and finally to second fixed terminal (1b).
When current flows in the circuit, the magnetic field surrounding the shorting link (4) becomes distorted in the vicinity of the soft magnetic piece (7), and the flux lines tend to pass through it. This effect gives rise to compensation force F that draws the shorting link (4) towards the ferromagnetic compensation piece (7), as shown in Figure 2. This force assists the compression springs (5) attached to the shorting link, thus firmly holding the contacts, avoiding any repulsion between them. The nature of this force is dynamic, since it varies as the square of the current value.
At any given point of time the total contact pressure is the addition of this self adjusting, dynamically varying compensating force F and the fixed force provided by the compression springs (5) attached to the shorting links(4). During the operation of the switch at rated or overload currents, the above mentioned generated compensation force is not of significant value and the fixed force of the compression springs takes the upper
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hand, providing the requisite contact pressure. Whereas at high current levels of the short circuit magnitude, the compensation force, being proportional to the square of the current, shoots up exponentially and becomes the dominant one, avoiding the contact repulsion, which is prevalent at short circuit currents.
The advantage of the present invention is that the dimensions of the switch assembly thus remain unaltered even after its integration with the switch. Since the compensation arrangement has a single piece of ferromagnetic material, this becomes by far, the most cost effective compensation solution.
Further, the contact shapes shown in the different figures are not limiting.
The invention has been shown and described in a preferred form only, and by way of example, and many variations may be made in the invention, which will still be comprised within its spirit. It is understood, therefore, that the invention is not limited to the details cited above. There may be a set of fuses, or any other overload safety devices integrated in the said design, which will then act as a switch-disconnector-fuse. Though four-pole design is mentioned, two or three pole design, if required, may easily be designed on the similar lines. While the present embodiment of the invention refers to a quad break switch, it may conveniently be retracted or extended to any number of breaks. Means may also be provided for adjusting the contact pressure. The shape, arrangement and material of the compensation piece or pieces could be varied as well as those of the contact assembly itself.
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WE CLAIM
1. A switching device comprising
(a) a fixed contact;
(b) a resiliency held contact member;
(c) a fixed terminal;
(d) a movable contact bridge having moving contact secured at one end adapted to move translationally between the fixed contact and resiliency held contact member; and
(e) a soft magnetic strip fixed on said movable contact bridge facing the resiliency held contact member;
wherein the current flows from the fixed terminal to the moving contact to
the resiliency held contact member.
2. A switching device as claimed in claim 1 wherein the moving contact bridge is pressure engaged between the resiliency held contact member and the fixed terminal.
3. A switching device as claimed in claim 1 wherein the moving contact comprises plurality of rollers being operatively connected to the moving contact bridge enabling the moving contact bridge to move translationally during break operation.
4. A switching device as claimed in claim 1 wherein the contact member is resiliency held by a compression spring.
5. A switching device comprising:
(a) housing;
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(b) plurality of fixed contacts;
(c) plurality of resiliency held contact members;
(d) plurality of fixed terminals;
(e) a movable contact bridge having moving contact each secured at one end adapted to move translationally between said fixed contacts and resiliency held contact members; and
(f) a soft magnetic strip fixed on each said movable contact bridge facing the resiliency held contact member;
wherein the current flows from the first fixed terminal to the moving contact to the resiliency held contact member to second set of moving contacts and to second fixed terminal.
6. The switching device as claimed in claim 5 wherein the housing comprises base and a top wherein the top housing is adapted to encompass the whole switching device.
7. The switching device as claimed in claim 5 or 6 wherein the resiliency held contact member is mounted on the top housing.
8. The switching device as claimed in claim 7 wherein the contact member is mounted by spring.
9. The switching device as claimed in any one of the preceding claims wherein the soft magnetic material secured to moving contact bridge exerts an attraction force on the resiliency held contact member in the same direction as that of the spring thereby counter balancing the effect of repulsion force exerted between the fixed and movable contact.
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10. A switching device as herein in described and illustrated in the accompanying figures.

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ABSTRACT
The present invention provides a switching device comprising a fixed contact; a resiliently held contact member; a fixed terminal; a movable contact bridge having moving contact secured at one end adapted to move translationally between the fixed contact and resiliently held contact member; and a soft magnetic strip fixed on said movable contact bridge facing the resiliently held contact member. The current flows from the fixed terminal to the moving contact to the resiliently held contact member. The soft magnetic material secured to moving contact bridge exerts an attraction force on the resiliently held contact member in the same direction as that of the spring thereby counter balancing the effect of repulsion force exerted between the fixed and movable contact.
30 JAN 2006
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