F O R M 2

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

1. Title of the invention: AN ELECTRICAL CONTACT SYSTEM HAVING SHORT CIRCUIT CURRENT WITHSTANDING CAPABILITY

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:


FIELD OF THE INVENTION

The present invention relates to electrical switching devices. More particularly, the invention is concerned about an electrical contact system which would be able to withstand the abnormal condition of short circuit in an electrical circuit without imparting abnormality to the switching device, mainly contact welding.

BACKGROUND OF THE INVENTION

US 4513270 discloses a contactor capable of having a very high breaking power comprises a movable contact carrier bridge (3) having a central part in the form of a blade (3c) and conducting J-shaped pieces (2a-2b) supporting fixed contacts. A non-magnetic blade bent in the form of a U is mounted slidable inside a hollow movable insulating contact holder (1) and guides and holds at its upper end a magnetic plate (8) cooperating with said blade (3c) and forming an air-gap with another U-shaped magnetic piece (6), disposed between the legs of the bent blade (5) and integral (at 7) with the contact holder (1). A contact pressure spring (4) bears on the basis of the U-shaped magnetic piece (6) and on the bottom of the bent blade (5).

US 3887888 discloses an electric switch of the sort capable of carrying high amperage currents, especially under short circuit conditions is provided with a ferromagnetic U-shaped yoke piece around the contacts and an armature mounted on the movable contact carrier in position to be attracted by the magnetic force created in the yoke when current flows through the contacts. The fixed and movable contact members act like a one-turn electromagnetic coil especially under severe short circuit conditions to hold the contacts together against electromagnetic forces which otherwise would cause contact separation under such conditions. Rocking of the movable contact on the fixed contact is prevented by a leaf spring which acts in closed circuit position of the contacts and in cooperation with the holding means on short circuit conditions,

US 6150909 discloses an electromagnetic switchgear unit has an electromagnetic drive appliance which acts as an auxiliary magnet system and by means of which the contact force is first increased on the occurrence of a short-circuit current, before the contacts are abruptly torn apart. For this purpose, the drive appliance actuates a lever rotatably supported on the contact bridge carrier, which lever displaces the contact bridge carrier and, by this means, compresses a contact compression spring and a further compression spring.,

EP 0974997A2 discloses a conductor housing (16) has a pair of stationary contacts (18). Movable contact carrier (12) has movable contacts (20). Magnetic components (26,30) are movably located near contact (20) and rigidly mounted on the carrier, respectively. Magnetic force caused by fault current drives the magnetic components to attract mutually by which movable and stationary contacts are separated. An Independent claim is also included for the contact weld preventing method.

US 4162467 discloses a circuit interrupter with improved current withstand capability characterized by stationary and movable contact means, the stationary contact means comprising first and second conductors being substantially parallel and having an air gap therebetween, the first conductor being pivotally mounted on the second conductor and being in the zone of pivotal movement of the first conductor so that when a fault current of predetermined conditions occurs a magnetic flux between the first and second conductors urges the first conductor toward the movable contact means.

US6377143 discloses a system and method for preventing contact weld under various fault current conditions is disclosed. The system includes a contactor having stationary and movable contacts biased towards each other and switchable between an open and closed position. Energization of an electromagnetic coil engages the contacts creating an electric path for current flow through the contactor. Pulse width modulation is used to lower the power to the coil and maintain the contacts in the closed position. The contactor is equipped with safeguards to prevent contact welding. Under low fault currents, welding is prevented by contact material composition. Under intermediate fault currents, the contacts are blown open and remain open using magnetic components until the arc dissipates and the contacts have cooled sufficiently. Under high fault currents, the arrangement causes the contacts to blow open and separate the armature from the coil preventing re-engagement of the contacts until the coil is energized again.

The conventional methods provide capability to the switching device to withstand the abnormal condition of short circuit in an electrical circuit under such conditions include:
1. Use of weld resistant material
2. Use of current limiting modules to lower Let thru energy during short circuit
3. Use of magnetic pushers / plungers to intentionally open or prevent opening of moving contact in case of flow of short circuit current.
4. Use of magnetic opening device which acts at higher currents of short circuit to open the contact and holds the contact in open condition so as to delay re-closure and hence allow cooling of molten metal and hence welding of contacts
5. Use of mechanical latching means to keep the contact in open condition after the contact repels open under electro-dynamic / blow open force.

The disadvantages of the conventional methods are:
1. Use of weld resistant materials are effective for limited range of fault currents
2. Use of current limiting modules not only add to the size of the product, but add to the overall resistance of the switching device if not disconnected during normal duty and hence increase the temperature rise of the device at normal current. This method is generally limited to lower rating devices.
3. Use of magnetic pushers / plungers or magnetic opening device for moving contact generally restrict the arc movement during normal current breaking by attracting the arc towards them and hence affect the electrical life of the device. Erosion of contacts is much higher in such methods during short circuit conditions. This is also true for mechanical latching device and it also needs complex latching mechanism.

Thus, there is a need to provide for an electrical contact system for withstanding the abnormal condition of short circuit in an electrical circuit without imparting abnormality to the switching device, mainly contact welding.

The inventors have found that blow opening of moving contacts can be restricted by way of a unique configuration around the moving contact by means of unique design of the stationary contacts such that desired current directions can be achieved in certain portions of stationary contacts giving rise to compensation forces higher than the repulsive force acting on the moving contacts during short circuit condition.

OBJECTS OF THE INVENTION
An object of the present invention is to overcome the problems/disadvantages of the prior art.

Another object of the present invention is to provide an electrical contact system having short circuit current withstanding capability.

SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided an electrical contact system comprising:
pair stationary contact means; and
moving contact means being proximally located in between said pair stationary contact means;
wherein said stationary contact means being operatively surrounding the moving contacts means wholly or partially;
wherein said stationary contact means comprising
a substantially U shaped modular profile located almost at the middle of said stationary contact means having parallel portions I and parallel portions II such that said parallel portions I being operatively located substantially above said moving contact means and said parallel portions II being operatively located substantially below said moving contact means;
wherein said portion I and portion II having an appropriate longitudinal relationship; and
wherein said parallel portions I and parallel portions II being substantially symmetrically located on both sides of said moving contact means such that current direction in said parallel portions I and parallel portions II being opposite to said moving contact means thereby preventing opening of said moving contact means when high current flows through the system.

DETAILED DESCRIPTION OF THE INVENTION

According to a preferred embodiment of the invention, there is provided an electrical contact system having short circuit current withstanding the abnormal condition of short circuit in an electrical circuit without imparting abnormality to the switching device, mainly contact welding.

The present invention utilizes the method of selectively restricting the blow opening of moving contact by way of unique contact configuration around the moving contact. The invention does not need any magnetic material in the vicinity of moving contact and works on the principle of Lorentz force between parallel conductors. This is achieved by placing two conductors below the moving contact which produce repulsion force (opening) on moving contact and two conductors are placed above moving contact which produces compensation force (closing) on moving contact. This is achieved by current direction in these conductors by way of their orientation. The orientation is designed in such a way that compensating force is always higher than repulsion force. The values of these forces are decided by magnitude of the current carried by the device. The net force on moving contact at normal breaking currents is very insignificant due to differential effect and as the contact opens, the net force reverses the direction after certain travel of moving contact which is referred to as magnetic dead center and hence assists opening of the moving contact after this point. At short circuit current carried by the device, the net differential compensating force on the moving contact increases sharply so as to compensate the opening forces on the moving contact due to high current, thus preventing it from opening and hence welding. This contact configuration also enhances the blow-out force on the arc column during normal breaking operation thus improving the arc movement hence enhancing electrical life of the switching device. The invention does not need any complex mechanism. The invention does not have limitation of fault current as the compensation force increases in proportion to the blow open force.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1 illustrates isometric view of an electrical contact system.
Figure 2 illustrates isometric view of stationary contact and terminal.
Figure 3 illustrates isometric view of moving contact.

DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS

As shown in figures 1, 2 and 3, the present invention shows configuration of current carrying parts of the subject invention and does not show the associated insulations between Line & Load Terminals and associated operating mechanisms.
As shown in figure 1, an electrical contact system comprises pair stationary contact means and
moving contact means (5) is proximally located in between said pair stationary contact means and is operatively surrounding the moving contacts means (5) wholly or partially. The stationary contact means further comprising stationary terminal means (3) which are placed on either sides of moving contact (5). Moving contact (5) can be moved along with contact buttons (6) away from contact buttons (7) of stationary terminal by suitable operating mechanism of the switching device so as to disconnect it from stationary contacts and thus disconnecting stationary terminals from each other. Moving contact (5) is covered by insulation (4) and silver alloy button (6) is fixed on both ends symmetrically. Silver alloy button (7) is fixed on the stationary terminal (3). A support means (8) is adapted to support the stationary contact means and the moving contact means (5) and further adapted to separate the stationary terminal means (3). Thus, it is acted as a support as well as a separator (8) which is arranged between two stationary terminals (3).

The purpose of the present invention is to prevent opening of moving contact (5) under effect of Lorentz and Holms force when abnormally high current flows through the contact system in closed condition thus avoiding damage and welding of contact buttons (6), (7). This is achieved by shape of stationary terminal (3) and its placement around moving contact (5) such that the stationary contact means comprises a substantially U shaped modular profile located almost at the middle of the stationary contact means having parallel portions I and parallel portions II which are having an appropriate longitudinal relationship. The parallel portions I comprises two parallel portions (2) which are operatively located substantially above moving contact means (5). The parallel portions II comprises four parallel portions (1) which are operatively located substantially below the moving contact means (5)..By such configuration, current direction in portions (1) and (2) is always opposite to that of moving contact (5), thereby producing Lorentz force of repulsion on Moving contact (5). Since portions (1) and (2) are symmetrically placed on sides of moving contact (5), they produce resultant force on moving contact (5) which is aligned to the motion of moving contact. Since portions (2) are above moving contact (5), they tend to push moving contact (5) in downwards direction keeping it in closed condition. Since portions (1) are below moving contact (5), they tend to open the moving contact. However, distance between moving contact (5) and portions (1) & (2) as well as lengths of (1) & (2) are kept in such a way that force of closing due to (2) is higher than force of opening due to (1).

As Lorentz force between parallel conductors is proportional to product of currents through them and hence square of current in this case, the difference between closing force due to (2) and opening force due to (1) increases sharply as current increases and overcomes the opening forces on moving contact (5), thus preventing it to open. This avoids creation of arc between contact buttons (6) & (7) thereby avoiding melting and subsequent welding.

The contact configuration is designed in such a way that at normal interrupting currents of the switching device, the said difference of forces is negligible and can be reversed due to square relation of the force to current, thus it does not affect the opening mechanism and thus does not affect the opening speed. Additionally, as the moving contact (5) starts moving away from stationary contacts, at certain distance, with the current continued through arc, both forces become equal, beyond which, the net force purely becomes favorable to opening of moving contact thus assisting opening of moving contact.

Additionally, the stationary contact means comprising pair of vertical limb portions (9) linking portion I and portion II . When the moving contact (5) is moving in such condition and there is arc between contact buttons (6) & (7), the direction of arc current is always opposite to current in portions (9) of stationary contact thereby enhancing the blowout force on the arc column and hence improving arc extinction capability of the switching device.

ADVANTAGES OF THE INVENTION

1. Differential electromagnetic force compensation on moving contact.
2. Insignificant effect on force as well as opening speed of moving contact at normal operating currents as well as breaking currents.
3. Magnetic force dead center concept to reverse the force direction after certain opening of moving contact.
4. Increased blow-out force on arc at breaking conditions in spite of compensating contact system due to unique looping of conductors

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. An electrical contact system comprising:
pair stationary contact means; and
moving contact means being proximally located in between said pair stationary contact means;
wherein said stationary contact means being operatively surrounding the moving contacts means wholly or partially;
wherein said stationary contact means comprising
a substantially U shaped modular profile located almost at the middle of said stationary contact means having parallel portions I and parallel portions II such that said parallel portions I being operatively located substantially above said moving contact means and said parallel portions II being operatively located substantially below said moving contact means;
wherein said portion I and portion II having an appropriate longitudinal relationship; and
wherein said parallel portions I and parallel portions II being substantially symmetrically located on both sides of said moving contact means such that current direction in said parallel portions I and parallel portions II being opposite to said moving contact means thereby preventing opening of said moving contact means when high current flows through the system.

2. System as claimed in claim 1 wherein said stationary contact means comprising pair of vertical limb portions linking portion I and portion II.

3. System as claimed in claim 1, wherein said plurality of stationary contact means further comprising stationary terminal means.

4. An electrical contact system with short circuit current withstand capability as herein substantially described and illustrated with the accompanying drawings.