F0RM2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10; rule 13)
1. Title of the invention. - A MEANS TO DEBILITATE THE EFFECT OF
HOLMS FORCE ON THE CONTACT SYSTEM
2. Applican(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 the field of switchgears and circuit breakers. More particularly the present invention relates to an improved contact arrangement for an air circuit breaker configured to debilitate repulsive forces.
BACKGROUND AND THE PRIOR ART
Circuit breakers are used to safeguard the electrical equipment from over current situations caused or short current situations in equipments. On flow of fault current conditions, the electrical contacts within the circuit breaker will open and it stops the flow of electrical current through the circuit breaker to the equipment.
An air circuit breaker is a high or low voltage circuit breaker in which the tripping and closing mechanisms are operated pneumatically (electro-mechanically). The tripping mechanism is a pneumatic (electro-mechanical), piston-driven (shaft driven), arrangement. In the simplest case, it operates by rotating an enclosed vane which then drives the contacts by means of a rack-and-pinion mechanism.
As the resulting arc stretches between the opening contacts, presence of air across the arc helps to stretch/cool it and, thus, extinguish it. An air circuit breaker is used for transmission, making and interrupting large value of three phase current for safe operation of power systems in case of surges, or short circuit symmetric/asymmetric faults. Moving contact fingers and fixed contact fingers form an essential component of the circuit breaker which helps in maintaining continuous flow of rated current. In the existing condition number of fingers used is more for conduction purpose and involves complex parts to construct the circuit breaker.
US7935902 provides a contact arm assembly including a plurality of substantially parallel plates having a space between each of the plurality of substantially parallel plates and a plurality of finger assemblies, at least one of the plurality of finger assemblies being pivotally attached to the plurality of substantially parallel plates and being located in the space between

each of the plurality of substantially parallel plates, each of the plurality of finger assemblies having a body and an arc runner, the arc runner being locked against the body in at least two
locations.
While the prior art shows a contact arm assembly in a circuit breaking device, this arrangement comprises complex structure and the effectiveness of the circuit breaking device is not achieved and the cost of manufacturing the circuit breaker is high.
There is therefore a need for a contact finger arrangement in a circuit breaker with reduced number of fingers, thus reducing the copper and in turn the cost while retaining the rated current flow ability along with the breaking capacity and withstand and to design a new finger design along with the fixed contact profile and arrangement to debilitate the effect of Holm's force on the contact system. The existing system of contact opening and closing takes place in a plane vertical to the horizontal plane (here horizontal plane would be the base of circuit breaker). In such an arrangement angle between spring force wherein springs are mounted right behind the finger system and Holmes force is nil. In such a scenario the forces acting against the breaker closing (Holmes force) has to be overcome by spring force in which case if number of contact fingers are reduced the spring force has to increase resulting in new spring design providing more spring force. In order to avoid such a scenario and to achieve the desired result of reduction of contact fingers in order to save cost, the closing of the finger system takes place in a certain angle, so as to distribute the force components of Holmes force in a way that forces acting against the breaker closing, reduce, yet net vector sum of Holmes force remains constant.
This way with reduced number of contact arrangements same current carrying capacity can be retained along with the rated breaking capacity and withstand.
OBJECT OF THE INVENTION
A main objective of the invention is to overcome the disadvantages/drawbacks of prior art invention.

Another object of the invention is to reduce the number of fingers and reduce the copper used while retaining the rated current flow ability along with the breaking capacity.
Another object of the invention is to provide a new contact finger arrangement along with fixed contact profile to debilitate the effect of Holm's force on the contact system.
Another object of the invention is to provide a new contact finger arrangement to debilitate the effect of Holm's force on the contact system in an air circuit breaker.
SUMMARY OF THE INVENTION
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the present invention. It is not intended to identify the key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concept of the invention in a simplified form as a prelude to a more detailed description of the invention presented later.
Accordingly in the present aspect of the invention there is provided an improved contact arrangement for an air circuit breaker configured to debilitate repulsive forces.
Other aspect of the present invention provides an improved contact arrangement for an air circuit breaker configured to debilitate repulsive forces, said arrangement comprising a top terminal assembly including a fixed contact; a bottom terminal assembly including a moving contact; a pole cage mounted on a bottom terminal pin means thereby enclosing said bottom terminal assembly; a plurality of contact fingers pivotally mounted on a finger pivot pin means thereby forming a substantially revolute joint with said pin means; a plurality of spring means positioned between said pole cage and said contact finger whereby said spring means connected to said contact finger though a finger spring grip; wherein in the closed position said spring means configured to provide compressive force on said fingers thereby creating moment over said finger pivot pin axis enabling passing of current between said top terminal assembly and said fingers through said bottom terminal assembly.

Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The following drawings are illustrative of particular examples for enabling methods of the present invention, are descriptive of some of the methods, and are not intended to limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description.
Fig. 1 shows the open position.
Fig. 2 shows the closed position.
Fig. 3 shows the geometry of the finger
Fig. 4 shows a general view of the finger with the main contact
Fig. 5 shows the free body diagram of the finger
Fig. 6 shows a general view of the arcing contact
Fig. 7 shows a side-view of the top terminal
Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure.
Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.

DETAILED DESCRIPTION OF THE INVENTION
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.
The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
It is to be understood that the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.
By the term "substantially" it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide.
Accordingly the present invention provides for an embodiment that consists of a top terminal assembly 1, and bottom terminal 7. The pole cage 5 is mounted on the bottom terminal pin 71 forming a revolute joint. The finger 2 is mounted on the finger pivot pin 21, such that it forms a revolute joint with the pin. The pole cage 5 is also connected with the pole cage pivot pin 51, forming a revolute joint. The axes of all of these mentioned joints and profiles are preferably parallel to each other. Springs 6 are placed between the pole cage and the finger. The spring is connected to the finger through the finger spring pip. The spring is compressive in nature, and tends to push the finger away from the pole cage.

The current embodiment speaks of two positions of the system viz. the open position and the closed position. FIG.l shows the open position. FIG. 2 shows the closed position.
In the open position the finger does not make physical contact with the top terminal assembly. The finger 2 makes physical contact with the top terminal assembly 1 in the closed position. In this position, the springs 6 are compressed and create a compressive force Fs on the fingers 2. A normal force N is created between the fingers 2 and the top terminal assembly 1. The normal drawn on the contact face of the top terminal assembly 1 makes an angle x with the line joining the centre of the finger pivot pin axis 21. The distance between the point of contact and the finger pivot pin axis is Rl. The moment created by the compressive force Fs over the finger pivot pin axis 21 is Ms. A current I surges through the embodiment passing between the top terminal assembly 1 and the fingers 2, and then through the bottom terminal 7. An electromagnetic field is created in the system. A moment Md is created acting on the ringer pushing it towards the top terminal assembly 1. Due to constriction created at the point of contact, a repulsive force Fr is created on the finger 2. This force creates a moment equivalent to the component of the force acting perpendicular to the perpendicular drawn to the line joining the contact point 33 and the finger pivot pin axis 21. The component of this force is proportional to the sine of the relief angle 'xFIG.5 shows the free body diagram of the finger during the current surge in case of withstand.
For Relief angle =90 degrees, Fr * Rl = Fs * R2 + Md
For relief angle = x < 90 degrees:
Fr * sin (x) * Rl = Fs * R2 + Md
Fr - Holmes force
Rl - distance between point of contact and the finger pivot pin axis
R2 - distance between position of spring system and the finger pivot pin axis
Fr = constant * I2 (Holmes force)

Thus it can be safely concluded from the above equation that, the repulsion force (Holmes force) is reduced by a component of sin (x). Thus by a factor of sin (x), the current can be increased for the same system.
Hence greater current can be transferred through the breaker in closed condition with the same set of springs arrangement.
It is seen that the normal force acting on the top terminal assembly because of the finger is increased by a factor of cosec x.
Thus, lesser amount of additive force needs to be provided to withstand such a surge.
The additive force is directly proportional to the length of the finger introduced in the electromagnetic field created by the bottom terminal. The mass of the body depends on the length. A decrease in the required length thus, can yield in a lower mass of the body. Thus material reduction can be achieved.
During the transition from the open position to the closed position, it is imperative that the moving arcing contacts touch the fixed arcing contact, before the moving main contact touches the fixed main contact. During the transition from the closed position to the open position, the moving arcing contacts touch the fixed arcing contact, before the moving main contact touches the fixed main contact. This process ensures that in case of an arc, it would pass only through the arcing contacts. This process is called commutation. The present embodiment presents a means to achieve commutation. The profile of the finger is created such that it is an arc of a circle, the fixed arcing contact too has a similar profile. These circular profiles ensure that the arcing contacts 'roll' over each other during commutation, this inhibits wearing of the arcing contacts over time.
With the reduced number of fingers, the present arrangement debilitates the repulsive force on contact system.
The Md component is directly proportional to the current flowing through the finger.
The Biot-Savart law of electromagnetism says that
F = BxIxL. In this case the field generated by the bottom terminal B, and the Length of the finger L, both remain constant in either cases. The intended angles between the combinations

for the vector product too remain constant. However, the current flowing through each finger increases in the case of reduction in the number of fingers.
The Holms force generated Fr is proportional to the square of the current flowing through the joint.
Hence, if m number of fingers are used to distribute current, the Holm's force is
Fr = constant x Itotal2/m2
The moment created by the Holms force is Fr*Rl
The Holms force component creating a turning moment about the pivot axis is Fr x sin (x)
The moment created by the spring force remains constant in either cases.
The moment balance equation for the use of n fingers, per finger is:
Fs x R2 = (K x I2/n2 x Rl) - (C x I/n) ....equation 1
The moment balance equation for the use of m fingers and the inclined terminal, per finger is:
Fs x R2 = (K x I2/m2 x Rl x sin (x)) - (C x I/m) ..equation 2
Now, if m < n, a value of x can be chosen such that 0