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: FLIP-FLOP CONTACT LOCKING
MECHANISM

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 a contact locking mechanism for use in switching devices/circuit breakers. More particularly, the invention relates to an improved contact locking mechanism with minimum components having a unique inbuilt feature to flip–flop without having any additional component with cam profile for flip-flop. The mechanism of the present invention enhances the velocity of contact system after toggling during fault clearance by circuit breaker reducing let through energy and at the same time it enhances the life of circuit breaker against short circuit fault clearance.

Background and the prior art

A circuit breaker is a mechanical switching device, capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those of short circuit (Short circuit is an accidental or intentional connection, by a relatively low resistance or impedance, of two or more points in a circuit which are normally at different voltages).

The main function of any electrical switching apparatus is to carry the rated current. Therefore the current carrying capacity of any circuit breaker is so important that if the unit fails to carry the rated current it will cause damage to itself as well as to the electrical equipment to which it is protecting.

Circuit breakers have three positions corresponding to the status of the separable contacts. For example, these positions would include an ON position, in which the separable contacts are closed, an OFF position in which the contacts are open and Trip position in which the contacts are tripped open.

These circuit breakers / switching devices are usually installed in an enclosure so as to have all the control and distribution network laid in form of metallic sheets and or cable wires inside and all operating means from outside so as to prevent access to high voltage and thus a potential hazard.

When a circuit breaker is in ON condition and a short circuit fault condition arises, high amount of fault current flows through the circuit and in response to this the circuit breaker tries to move the separable contacts to open position after sensing the fault condition through short circuit sensing mechanism (such as magnetic arrangement). The time taken from a fault condition initiation and the fault current being interrupted by circuit breaker determines the let through energy / arc energy passed by the circuit breaker to the downstream equipments. A circuit breaker is said to be more efficient when the let through energy is minimal.

Let through energy of circuit breaker depends on many parameters, one among them being the time for opening of the contacts from closed condition. The short circuit sensing mechanism of circuit breaker involves mechanical elements in between and thus takes a definite time before giving a trip open signal to the contacts thus increasing the let through energy.

Short circuit conditions involves high amount of fault current levels which induces high repulsive forces between suitably arranged current carrying contacts. Thus if the contacts can be locked in the repelled open position, the let through energy can be reduced substantially. This improves life of the equipments and systems connected downstream and greatly reduce the hazardous conditions which can damage the downstream equipments.

Thus many of the circuit breakers use such locking systems for contact locking under short circuit conditions. Further, efficiency of a contact locking system is measured by the velocity it can induce to the contacts once they have repelled open. It is also important to have the desired contact force in the ON condition, i.e. when the separable contacts are in closed condition.

In traditional / present circuit breakers additional moving components, cam profile is used to achieve contact locking. This reduces the efficiency of the mechanism as there is some amount of energy lost in overcoming friction. Friction also gets added to the amount of heat generated at the surfaces in contact. It also increases wear and tear of the parts, thereby reducing the overall life of the system. Also, there will be variation in the contact force over a period of time due to such wear and tear.

It is also desirable to make the contacts move faster once it crosses the toggling position resulting in faster cut-off of fault current increasing the current limiting capability of the circuit breaker. It is also very important to achieve optimum contact force in the ON condition to keep the potential drop at the contact buttons to the minimum.

US 4539538 discloses a molded case circuit breaker including a movable upper electrical contact having a base portion with a slot formed therein for releasably receiving a portion of an elongated spring biased locking pin disposed in a rotatable cross bar of an operating mechanism of the circuit breaker. The locking pin is biased in the slot by a pair of tension springs secured to the cross bar, enabling the upper electrical contact to move in unison with the cross bar. Upon the occurrence of a high level short circuit or fault current of sufficient magnitude, the pin is displaced from the slot enabling the independent rotational movement of the upper contact arm. During such movement, an elongated leaf spring is downwardly deflected and then released by the base portion. Subsequently, the upper electrical contact is prevented from contacting the lower electrical contact by the engagement of the leaf spring with the base portion. A subsequent trip operation of the operating mechanism removes the latching feature of the leaf spring.

US 5633483 discloses a circuit breaker having a stationary contact, a movable contact and a blade which has a first end and a second end. The movable contact is secured to the first end of the blade. The blade is rotatable about a pin between a BLOWN OPEN position and a CLOSED position, wherein the movable contact is separated from the stationary contact when the blade is in the BLOWN OPEN position and the movable contact engages the stationary contact when the blade is in the CLOSED position. The blade further has a protrusion portion extending from its second end. The circuit breaker further includes a catcher rotatably coupled to a pivot pin and having a nose portion for engagement with the blade protrusion portion. A spring is mounted about the pivot pin and engages the catcher for biasing the catcher towards the blade whereby the nose portion rests against the protrusion portion of the blade when the blade is in the CLOSED position and the nose portion overlaps the protrusion portion when the blade is in the BLOWN OPEN position thereby holding the blade in the BLOWN OPEN position.

US 5874699 discloses a molded case circuit breaker having a moving conductor assembly which includes a modular carrier assembly incorporating a plurality of spring biased contact cam members each of which engages contact pressure lobes on a pair of contact arm laminations to provide contact pressure. In response to the magnetic repulsion forces generated by a short circuit, the springs compress allowing the cam members to pivot and side step the contact pressure lobes so that the contact arm blows-open with little resistance. The cam members and springs are preassembled in a transverse channel in the carrier body and laterally retained in place by side plates for easier pivotal attachment of the contact arm laminations to the lobes on the side plates. The side plate lobes raise the contact arm relative to the mass of the carrier to reduce eddy current heating and provide a positive off indication for welded contacts by restricting movement of lower toggle links of the operating mechanism which are coplanar with the side plates. A modular crossbar includes molded bearings engaged by or made integral with hexagonal metal shaft sections which extend into hex openings in the carrier bodies of adjacent poles.

US 5844455 discloses a current-limiting apparatus for a circuit breaker which makes it possible to effectively protect an electric load element by quickly blocking an electric flow path when an excessive current flows due to a ground, shortage, etc. at an electric flow path and preventing a re-formation of an electric flow path after the electric flow path is blocked. The apparatus includes a guide pin passing through the second pin hole and being extended from both side surfaces of the movable contact member, a holder, a portion of which support both ends of the movable contact member rotation center pin, for rotatably supporting the movable contact member, a current-limiting latch having a curved guide surface formed on one outer circumferential surface of the same on which the guide pin slidably moves, a support groove formed on another outer circumferential surface of the same, and a pin hole for supporting the holder, a rotation center pin inserted into the pin hole of the current-limiting latch, both ends of which rotation center pin are inserted into portions of the holder, and a contact spring, both ends of which are supported by the support groove of the current-limiting latch and the support surface of the holder, for generating a variable force which prevents the movable contact member from being rotated.

US 7217895 discloses a contact assembly for a circuit breaker includes a fixed contact, a movable contact, and a movable contact arm. The movable contact arm includes a first end carrying the movable contact, a second end, and a pivot portion proximate the second end. A moving arm portion extends from the first end toward the pivot portion. The moving arm portion has a width, an upper edge, a lower edge, and a height defined by the distance between the upper edge and the lower edge. In response to a trip condition, the movable contact separates from the fixed contact and the movable contact arm pivots open at an angular opening velocity. The height of the moving arm portion of the movable contact arm is at least four times the width of the moving arm portion, thus minimizing the moment-of-inertia of the movable contact arm, and increasing the angular opening velocity.

These and other limitations and needs of the conventional mechanisms necessitate an improved contact locking mechanism. The present inventors have found such a locking mechanism which could overcome the limitations over and above the improvements and flexibility provided additionally. It has been found by the inventors that a unique design and arrangement of the moving contact means and driveshaft means provides enhancement of the velocity of contact system after toggling during fault clearance by circuit breaker reducing let through energy and at the same time it enhances the life of circuit breaker against short circuit fault clearance.

Objects of the invention

One object of the present invention is to overcome the drawbacks/disadvantages of the prior art.

A basic object of the present invention is to provide a contact locking mechanism with minimum components having a unique inbuilt feature to flip–flop without having any additional component with cam profile for flip-flop.

Another object of the present invention is to provide a contact locking mechanism having higher velocity of moving contact after toggling resulting in faster cut-off of fault current increasing the current limiting capability of the circuit breaker.

These and advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.

Summary of the invention

According to one aspect of the present invention there is provided an improved contact locking mechanism comprising :
a rotating member;
moving contact means pivotally engaged to said rotating member such that rotation of said rotating member allows movement of said moving contact means so as to come in contact with other intended/fixed contact means,
wherein said moving contact means comprises a slot means;
pin means slidably accommodated in said slot means projecting outwardly on both sides of said slot means; and
spring means operatively biased with said pin means on one end and said rotating member on the other end.

Brief description of the accompanying drawings

Other features as well as the advantages of the invention will be clear from the following description.
In the appended drawings:

Figures 1 to 6 illustrates the mechanism with two spring means whereas Figures 7 to 12 with single spring means.

Figure 1(a) illustrates a top orthographic view of the mechanism of the present invention.

Figure 1(b) illustrates a front orthographic view of the mechanism of the present invention.

Figure 2 illustrates a front view of the mechanism with hidden lines.

Figure 3 illustrates a front view in wireframe.

Figure 4 illustrates a front view of the mechanism explaining the position of upper contacts during ON & OFF condition

Figure 5 illustrates an exploded isometric view of the mechanism of the present invention.

Figure 6 illustrates an isometric view of the mechanism.

Figure 7 (a) illustrates an orthographic top view of the mechanism

Figure 7 (b) illustrates an orthographic front view of the mechanism

Figure 8 illustrates the front view with hidden lines.

Figure 9 illustrates the front view in wireframe.

Figure 10 illustrates the front view explaining the position of upper contacts during ON & OFF condition.

Figure 11 illustrates an exploded isometric view.

Figure 12 illustrates an isometric view.

Detailed description of the accompanying drawings

In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and illustrate the best mode presently contemplated for carrying out the invention. Further functioning of the mechanism has been discussed below to describe the way the mechanism operates. However, such description should not be considered as any limitation of scope of the present unit. 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.

This invention talks about an improved contact locking mechanism with minimum components with an inbuilt feature to flip–flop without having any additional component with cam profile for flip-flop. This feature enhances the velocity of contact system after toggling during fault clearance by circuit breaker as the friction required for additional component is reduced

Locking system disclosed herein is shown in Figure 1 and Figure 7. Referring to Figure 5 and Figure 11, the mechanism comprises a rotating member of the circuit breaker mechanism, driveshaft 2 to which the upper/moving contact 1 is pivoted at pin 4. Rotation of this driveshaft will cause upper moving contacts to come in contact with the lower contacts and thereby complete the electrical circuit. Spring 5 is connected at one end to a slot provided in driveshaft 2 and the other end to pin 3 which slides in the slot 6 provided in the Upper moving contact 1. The slot 6 is substantially L shaped and pin 3 accommodated in this slot moves corresponding to the movement of the upper moving contact means 1. The slot 6 comprises a bent 7 which defines point of toggling of the upper moving contact.

Figure 6 and Figure 12 shows the 3D view of the arrangement from the front end of the driveshaft for a single pole of the circuit breaker. For multiple poles, multiple such assemblies will be used side by side using a common connecting driveshaft across all the poles. It is to be understood that one can use any number of mechanisms for any number of poles without departing from the true spirit of the present invention. Figure 2 is the front view of the arrangement with hidden lines and Figure 5 &11 shows an exploded view of the internal arrangement with springs.

The working of the invention is shown in Fig 4 & 10 with different positions. This is the position in which the moving contact cannot go further down in ON condition due to mechanism linkages and it cannot go further up in tripped condition due to stopper provided on mechanism and cover.

When the contact locking mechanism is assembled in the circuit breaker along with the mechanism and operated, in ON condition, the moving contact will be slightly upward when compared to the normal position. This is due to the intended over travel provided in the circuit breaker mechanism to achieve a certain minimum contact force. The position of moving contact is shown in Figure 4 & 10 in green color.

When the fault occurs, there is an upward force on the moving contact 1 thereby it will rotate about the pivot 4. This movement of the contact will cause the sliding movement of slider pin in upward direction while extending the tension spring. As soon as the toggling point is crossed, the extended spring will collapse, thus the spring which is stretched to its maximum deflection reaches an equilibrium state by reducing its length and releasing energy in the process to move the moving contact and it will rotate the moving contact in upward direction till it stops over mechanism. If the repulsion force on the upper contact is less than that offered by tension spring, then the contact will not get locked and the moving contact will again come to it ON position.

Once the moving contact gets locked, it is brought back to the normal condition when the circuit breaker protection system subsequently trips the mechanism due to fault. When the signal is given, the driveshaft rotates in the upward direction. The moving contact then comes in contact with the cover / top portion of the circuit breaker and experiences a reaction force in the downward direction in a way as to make the moving contact cross the toggling position. Once the moving contact crosses the toggling position, it goes to the normal position by virtue of the force exerted by the spring. Therefore, process of unlocking of moving contact is just the opposite of the locking process.

In the prior methods there will be an additional component with cam profile, between moving contact and driveshaft which reduces the efficiency of the mechanism as there is some amount of energy lost in overcoming friction. Friction also adds to the amount of heat generated at the surfaces in contact. It also increases wear and tear of the parts, thereby reducing the overall life of the system. In the present invention, the inherent disadvantages are eliminated as the additional component is removed. Avoiding such an additional component there will be consistency in the contact force over a period of time as there is reduction in wear and tear between the additional component and the moving contact which is an important advantage that the prior methods cannot achieve.

Since moving contact moves after toggling without repulsive force, there is faster energy release by the spring after toggling, resulting in higher velocity of moving contact. This results in faster cut-off of fault current increasing the current limiting capability of the circuit breaker.

Toggling position is that instant at which the pins 3, 4 & spring 5 come in the same line. The orientation of the line is fixed on various parameters by design. The effectiveness of the toggling position is decided by the closeness of the toggling position to the ON position. Since, in such case the amount of rotation required by the moving contact to reach the toggling position and get locked will be lesser. This will reduce the let through energy of the mechanism to the downstream equipments.

However it will reduce the contact force between the moving and fixed contacts in the breaker due to lesser angle between spring and moving contact. In order to achieve the same contact force with toggling position closer to the ON position, higher load spring can be used. This however will increase the stress levels in the spring and thus reduce the overall factor of safety in the spring and thus the whole mechanism. Also, since the higher load value spring is greater in size, the cost will be higher and the size of the locking mechanism will increase which is undesirable. In present invention, the toggling point can be controlled by changing the angle of the “L” shaped slot profile of contact with respect to spring force thereby striking the balance between toggling point and spring volume.

Advantages:

1. Removal of additional component with cam profile / reduced friction method for flip-flop
2. Another benefit is the higher velocity of moving contact after toggling resulting in faster cut-off of fault current increasing the current limiting capability of the circuit breaker.
3. Improved efficiency of the system due to reduction in friction in the system.
4. Increased life due to absence of wear and tear between the components.
5. Consistency in the contact force throughout the life of the system.
6. Reduce the consumption of raw material
7. Reduce the energy / power lost in the device

WE CLAIM:

1. An improved contact locking mechanism comprising:
a rotating member;
moving contact means pivotally engaged to said rotating member such that rotation of said rotating member allows movement of said moving contact means so as to come in contact with other intended / fixed contact means,
wherein said moving contact means comprises a slot means;
pin means slidably accommodated in said slot means projecting outwardly on both sides of said slot means; and
spring means operatively biased with said pin means on one end and said rotating member on the other end.

2. Mechanism as claimed in claim 1 wherein said rotating member comprising a drive shaft.

3. Mechanism as claimed in claim 1 wherein said slot means is substantially L shaped.

4. Mechanism as claimed in claim 1 wherein said slot means comprises a bent portion adapted for toggling said moving contact means.

5. Mechanism as claimed in claim 1 wherein said spring means are tension springs.

6. Mechanism as claimed in claim 1 wherein said rotating member further comprising a slot means.

7. Mechanism as claimed in any preceding claims wherein force of said tension spring is less in magnitude than force of repulsion of predetermined / accepted magnitude of current on said moving contact.

8. Mechanism as claimed in any of the preceding claims adapted to be used in a single pole or multi-poles of circuit breakers.

9. An improved contact locking mechanism as herein substantially described and illustrated with the accompanying drawings.