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. -

CURRENT-LIMITING CONTACT SYSTEM ARRANGEMENT 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 00 T State of Maharashtra, India

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


TITLE OF THE INVENTION
CURRENT-LIMITING CONTACT SYSTEM ARRANGEMENT FOR
SWITCHING DEVICES
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a current limiting contact system arrangement for switching devices.
BACKGROUND AND THE PRIOR ART
European patent Document no. EP0419324 describes a current-limiting contact arrangement composed of two stationary conductor rails which each have one stationary contact and of a contact bridge having two movable contacts which are brought in and out of the contact with the stationary contacts for closing and opening an electric circuit. The conductor rails each have two current loops extending on both sides along the opening travel of the movable contacts in planes running parallel to the longitudinally running center plane of the contact arrangement. When the contacts are electro dynamically opened in response to a high short-circuit current, the inductive effect of the short-circuit current flowing through the current loops causes the arcs forming between the opening contacts to be deflected in such a way that they are diverted in an accelerated manner towards the splitter plates resulting in arc quenching. This contact system has an inherent problem that even though the contact system provides a component of force which accelerates the arc towards the splitter plates the contribution of this force is not fully utilized and the contribution of the current loops towards the electromagnetic forces which causes the moving contact arm to repel during short circuit conditions is minimal. The fast opening of the contact system and high blow out forces on the arc are requisites for faster arc quenching.
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The document US4491795A1 describes a circuit breaker switch configuration in which a conducting member in the switch for connecting a switch terminal to a switch contact is shaped to form a loop current path, such that when a current flows in the switch, a magnetic field, which is appropriately oriented, serves to confine plasma generated by an arc discharge which occurs between the switch contacts when switch is opened. But the current path in this configuration only partially encircles a region which encompasses the arc extinguishing chamber and the arc zone, which might lead to unbalanced forces acting on the arc and might result in undesirable arc movement within the arc chamber because of unbalanced forces acting on the- arc.
OBJECTS OF THE PRESENT INVENTION
Accordingly one object of the present invention is to overcome the disadvantages / problems of the prior art.
Another object of the present invention is to provide a current limiting contact system arrangement for switching devices.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
The present invention provides a contact arrangement for a current limiting circuit breaker. A current limiting circuit breaker clears the fault before it sees the prospective current. The contact opens up and extinguishes the arc before it reaching the prospective peak.
The contact arrangement consists of fixed contacts which forms a complete current loop which encircles the arcing zone. The fixed contact of this contact system arrangement is designed in such a way that it produces a magnetic field symmetrical about the central plane and this contributes to increase the blow out


force experienced by the arc. This magnetic field also aids in the repulsion of the moving contact by maintaining a relatively constant electromagnetic force on the moving contact as it moves in the space during short circuits.. In the traditional current limiting contact system profile the repulsion force experienced by the moving contact decreases as it starts moving in space during short circuit conditions and this leads to a reduction in the contact opening velocity. This leads to a larger arc quenching time and chances of re-strike due to the contact falling back is also more in this case .The limbs of the fixed contact system are designed in such a way that both the limbs contributes towards the electromagnetic forces which aid in moving the moving contact away from the fixed contact during short circuit conditions and thus contribute to arc quenching. These limbs thus won't allow the contact velocity to decrease during the movement of the fixed contact, thus leading to faster arc quenching and reduces the chances of arc re-strikes.
The current loop in the fixed contact of the present contact system is designed in such a way that there is an element carrying current in anti-parallel direction to the arc. This leads to an increased blow out force on the arc which facilitates better arc movement and thus faster arc quenching. The arc also experiences a continuous blow out force produced by this limb through out the entire course of opening of the moving contact. This leads to lower arc quenching time and let through energy seen by the downstream equipments.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig I illustrates the complete assembly of the contact system in a cassette kind of enclosed construction.
Fig 2 illustrates an exploded view of the whole assembly comprising the contact system and the cassettes.
Fig 3 illustrates an exploded view of the contact system and arching chamber
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Fig 4 illustrates a front view of the contact system and arc chamber
Fig 5 illustrates an isometric view of contact system when the breaker is in ON state.
Fig 6 illustrates the flow of current through the contact system when the breaker is in ON state.
Fig 7 illustrates the flow of current through the contact system under short circuit conditions.
DETAILED DESCRIPTION OF ACCOMPANYING FIGURES
Figl Shows the modular cassette with terminals (3) and (4). There are two cassettes: front (1) and back (2) which are easy to assemble and enclose the contact system and the arc chamber for proper isolation of them from the breaker mechanism.
Fig 2 Shows the exploded view where cassette (1) and (2) separated out and complete contact system and arc chamber assembly is visible.
Fig 3 Shows the exploded view of the assembly showing various components. J-shaped fixed copper contact (5)is brazed with silver contact button (6) which mates with the silver button (7) brazed onto the moving copper contact (8) fixed in rotor(9). Moving contact (8) moves along the rectangular groove in the J-type fixed contact (5). The insulation requirement is met by the plastic component (10) which insulates moving contact (8) from fixed contact (5) as well as enclosing the Arc chute (13) with the help of side walls of cassette (1) and (2) together. Slot


motors (11) and (12) are placed in the grove of the insulating component (10) on both the sides between the cassette and the insulating component (10).
Fig 4 Shows the front view of the arrangement shown in fig3
Fig 5 Shows the isometric view of the contacts when the breaker is in ON state. Silver contact button (7) of the Moving contact (8) mates with the silver button (6) of the fixed contact (5).The rectangular cut along which the moving contact (8) moves when the breaker changes state from ON to OFF is also seen.
Fig 6 Illustrates the flow of current which enters from terminal (3) into the breaker and flows through the fixed contact (5) to the moving contact (8) through the contact buttons (6) and (7). Current entering the moving contact leaves from the terminal (2) through fixed contact at the other end as shown in the figure.
Fig 7 Illustrates the flow of current when the moving contact of the current limiting breaker repels open under short circuit conditions. Under short circuit conditions, magnitude of the current passing through the contacts increases at a rapid rate generating high repulsion forces in the current limiting breaker. When the magnitude of repulsion forces is above the threshold value, the moving contact (8) Rotates about the fulcrum which is the centre of the rotor (9) causing arc to strike between the moving contact (8) and fixed contact (5) as shown in the figure. The arc is a current carrying medium and thus serves as current path till it extinguishes with the help of Arc-chute (13).
GENERAL WORKING
The present invention relates to a contact arrangement for a current limiting circuit breaker. The current limiting circuit breaker clears the fault before it sees the prospective current thus minimizing the let through energy passing to the connected down stream devices.
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Present invention refers to a double break contact system for switching between closed and open state .The moving contact (8) rotates about the fulcrum which is at the centre along the length of the moving contact (8). Such a system is symmetric about the fulcrum. The contact system is modular with an enclosure. This insulating plastic enclosure is in the form of a cassette with two separating components (1) and (2) to achieve ease of assembly. This invention can also be extended to single break contact system.
Present invention consists of fixed contact (5) which resembles forms a current loop as represented in Fig 5 and Fig 6. The Fixed contact (5) has been designed to bend at different points'to form four different limbs labeled as a. b, c and d as represented in the Fig 5. These limbs are bent at particular angles to produce maximum repulsion forces on the arc. A rectangular slot is provided in the fixed contact (5) which is placed on limbs b and c with the purpose of giving room for the movement of the arm of moving contact (8) while the moving contact (8) rotates and switches state from closed to open or vise versa. The arm of the moving contact (8) moves along this slot.
Moving contact (8) is held mating with the fixed contact (5) at the silver contact button(6) of the fixed contact(5) and contact button (7) of the moving contact(8) ( seen in Fig 3) by application of contact force generated using set of springs not seen in the figures.
The arcing chamber which splits and quenches the arc which appears in case of a short circuit , consists of metallic plates stacked together to form the arc chute (13) as seen in Fig 3. It is fixed with the help of slots in the cassette (1) and (2).
The moving contact (8) and the fixed contact (5) are close to each other near the rectangular slot of the fixed contact (5). Thus there is a need for additional insulation which is provided by an additional insulating component (10) as seen in
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Fig 3. This is a plastic insulating component which provides the additional insulation and also encloses the arcing chamber with the cassette (1) and (2) providing with the side walls (Fig 2), to generate high pressure inside the arching chamber at the time of a short circuit.
Slot motor consists of two components (11) and (12). These slot motors are placed on both the sides in the slots of the insulating component (10) as seen in Fig 3. Current carrying conductors exert electromagnetic forces on each other. This force is proportional to the square of magnitude of the current flowing and the nature of the force is given by the direction of the current flow. The force is attractive when the currents are in the same direction and it is repulsive for currents in opposite directions. Current flows through the fixed contacts (5) and moving contact (8) as shown in fig6. Current Flowing in the arm of the moving contact (8) is opposite in direction to the current flow in the limb (a) of the fixed contact (5) as seen in fig6. This gives rise to a force on moving contact (8) which is repulsive in nature. This force tries to rotate the moving contact (8) in counter-clockwise direction in Fig 6. This electromagnetic force decreases as the distance from the limb (a) increases. Current flowing in the same arm of the moving contact(8) is in the same direction as the current flowing in the limb (d) of the fixed contact(5) . This produces a force on the moving contact (8) which is attractive in nature and tries to rotate the moving contact (8) in counter-clockwise direction in Fig 6. These electromagnetic forces decrease as the distance from the limb (d) increases .Both the forces being in the same direction add up and provide an almost uniform force across the full stretch from closed to open state of the moving contact (8) giving a uniform velocity of contact opening . The magnetic field produced is uniformly distributed about the central plane. Magnitude of these forces is directly proportional to the square of the current flowing. Thus under normal current conditions, the forces produced by the current flowing through the contacts are not sufficient enough to counter the spring force which is keeping the contacts matting.


Under short circuit condition, current flowing through the contacts increases at a very rapid rate thus increasing the magnitude of the forces generated. When the forces generated is more than the opposing contact force, the moving contact (8) repels open causing an arc to strike between the contact buttons(6) and (7) This arc acts as the current bridge between the Buttons (6) and (7) till it is extinguished by the Arc chute (13).
During the initial opening of the contacts, direction of the current through arc is opposite to the current flowing through the limb (b) of the fixed contact (5). This generates a high repulsive force known as the blow out force on the arc. This pushes the arc towards the arc chute (13) As the contact further opens, the current in the arc becomes anti-parallel to current in the limb (c) of the fixed contact (5), generating blow out forces. Thus the combination of limb (b) and (c) of the fixed contact (5) ensures a high blow out force sustained throughout the opening of the moving contact (8) from closed to open state.
Advantages:
1. Compared to the traditional contact systems this contact system assists in faster arc quenching, as it leads to faster opening of contacts and exerts a much larger blow out force on the arc.
2. In traditional contact system there is a component of magnetic field which would oppose the arc motion and the motion of the moving contact. Hence the net resultant force acting on the moving contact is Lesser leading to slower opening of the contacts and the blow out force experienced by the arc also is reduced. This leads to larger arc quenching time.
All the components of the magnetic field produced by the proposed contact system are aligned in such a way that it either contributes to increase the blow out forces experienced by the arc or it aids in the faster repulsion of the moving contact by serving to increase the electromagnetic forces experienced by the moving contact during short circuit condition.
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3. The magnetic field produced by the proposed contact system is uniformly distributed about the central plane. So the only unbalanced force the arc would experience is in the outward direction and not in the lateral directions. Hence there would be no unwanted arc motion in the arc chamber. This would lead to faster arc quenching.
4. Usually all the current limiting circuit breakers come with a contact locking arrangement, so that the moving contact which gets thrown up due to the electromagnetic forces generated during short circuit. These contact system always exerts a force in a direction to oppose the motion of the moving contact. This leads to reduction in the opening velocity and leads to larger arc quenching time.
But in the proposed contact system the current loop structure exerts a force in such a way that it compensates for the opposing force exerted by the contact locking system. This leads to a faster contact opening and faster arc quenching
5. The proximity of a current carrying element, which carries a current in an
opposite direction of the arc, to the arc column will exert an
electromagnetic force on the arc column during initial duration of contact
opening and thus significantly reduce the time for which the arc will
remain immobile on the contact buttons. This leads to a much lower
erosion of the contact buttons and better performance under short circuit
conditions;
Dated this 29lh day of August 2008