Claims:WE CLAIM:
1. A single-pole or multi-pole switching device comprising:

for each pole, within a top housing (1) and a bottom housing (2)
at least one first stationary contact (5);
a shaft (9) assembly comprising:
a plurality of mobile contacts (6), arranged in a substantially parallel manner and being pivotally attached to rotate around a fulcrum pin (12), wherein the mobile contact (6) can be coupled or uncoupled to the first stationary contact (5);
compression spring means (11) and extension spring means (10) operably coupled to each mobile contact (6) to provide a contact force required to maintain a pressure between each mobile contact (6) and the first stationary contact (5) for good electrical joint,
a conductor (7) operably coupled to a second stationary contact (8) at one end and the mobile contact (6) at the other end;
a main spring means (20) operably coupled to the shaft assembly and an operating lever (4) to couple or decouple the moving contacts and the first stationary contact to turn the switching device on or off;
wherein the mobile contacts (6) together carry the total current for each pole and
wherein higher contact pressure being applied by the extension springs or the compression springs or a combination of both on extreme contacts or differential contact pressure on multiple contacts.

2. The switching devices as claimed in claim 1, wherein the extension spring means (10) comprises a plurality of extension springs (E1, E2, E3, E4, E5) positioned in between the mobile contacts (6) and a fixed spring plate (13) to apply required contact pressure.

3. The switching device as claimed in claim 1, wherein extension springs of higher spring force are used for the extreme mobile contacts (6).

4. The switching device as claimed in claim 2, wherein the spring plate (13) is further adapted to make higher contact pressure with the extension springs, by provisions (14), in the extreme mobile contacts (6).

5. The switching device as claimed in claim 2, wherein the fixed spring plate (13) having a substantially concave resting surface (17) for extension spring so as to facilitate highest contact pressure in the extreme mobile contacts and gradual reduction in contact pressure towards the innermost mobile contact.

6. The switching devices as claimed in claim 1, wherein the compression spring means (11) comprises a plurality of compression springs (C1, C2, C3, C4, C5) positioned in between a resting surface (16) in the shaft (9) and the mobile contacts (6).

7. The switching devices as claimed in claim 1, wherein compression springs of higher spring force are adapted to be used for the extreme mobile contacts (6).

8. The switching device as claimed in claim 5, wherein the shaft (9) is further adapted to make higher contact pressure with the compression springs, by provisions (15), in the extreme mobile contacts (6).

9. The switching device as claimed in claim 1, wherein extension spring having different spring force is used to apply differential contact pressure on the mobile contacts.

10. The switching device as claimed in claim 1, wherein compression spring having different spring force is adapted to apply differential contact pressure on the mobile contacts.

11. The switching device as claimed in claim 1, wherein the shaft (9) having substantially concave surface (18) ensuring highest contact pressure in the extreme mobile contacts and gradual reduction in contact pressure towards the innermost mobile contact.

12. The switching device as claimed in any one of the preceding claims the mobile contacts (6) of the shaft (9) assembly can be adapted to provide higher contact pressure in the extreme mobile contacts (6) and gradual reduction in contact pressure towards the innermost mobile contact.

Dated this 31st day of March 2018

Abhishek Sen
Of S. MAJUMDAR & CO.
(Applicant’s Agent)

, Description:FIELD OF INVENTION
The present invention relates to a circuit breaker, in general and more particularly, to construction of a shaft to enhance electrodynamic withstand capability of the circuit breaker.

BACKGROUND

A circuit breaker is an electrical protection device which can make, break & carry current in normal condition. When there is a fault in the system due to overload, short circuit etc. circuit breaker can break and clear the fault.
Circuit breaker typically consists of at least one stationary contact and at least one movable contact in each pole. When circuit breaker is switched ON by means of an operating mechanism movable contact mates with the stationary contact. To hold the movable contact at the mating position high contact pressure is applied on the movable contact using springs. Current flowing through the stationary contact and movable contact generates electrodynamic and constriction forces, which are repulsive in nature and acts against the applied contact pressure. The magnitude of the electrodynamic and constriction forces is directly proportional to the square of the current flowing through the contacts. So, when there is a fault in the system electrodynamic and constriction forces increase rapidly. When the combined electrodynamic and constriction forces are higher than the applied contact pressure, the movable contact repels open.
Moulded case circuit breakers are basically classified in to two major categories namely ‘Category A’ & ‘Category B’. Category A breakers are designed to have a high short circuit breaking capacity to clear higher fault levels while the ‘Category B’ breakers are designed to have a capability to withstand fault current allowing the selectivity of the breakers, where breaker immediate upstream to the fault location detects the faulty branch and isolates the faulty branch from the whole system ensuring continuity of supply to other feeders or branches. For category B breaker, having high current withstand capacity, the contact arrangement is designed to compensate or counteract the electrodynamic forces generated during the flow of short circuit current allowing the stationary and movable contacts mating with each other. The contact configuration should be designed efficiently to sustain mechanical and thermal stresses built up during the passage of the fault current, making the system bulkier and heavier.
Category B breakers generally consist of plurality of movable contacts in each pole. The said movable contacts are parallel to each other. Due to dimensional limitations of circuit breaker, the movable contacts are very close to each other. Similarly, each pole of the circuit breaker is also at very close proximity to each other to ensure compact size of circuit breakers. The current flowing in the individual movable contacts are not uniformly distributed in density due to proximity effect. Any one of the outermost movable contact in any of the poles carry higher current. In case of a pole consisting of five movable contacts, generally any one of the outermost movable contacts in a certain pole carries 35% higher current than the normal current in the absence of proximity effect, resulting in higher electrodynamic and constriction forces. As a result, movable contact repels open, thus decreasing the fault current withstand capacity of the breaker. The phenomena of non-uniform distribution of current in the movable contacts should be limited to enhance fault current withstand capacity of the breaker.
Reference has been made to US7989721 B2 disclosing a single-pole or multipole device for low voltage systems. In particular a circuit breaker or a disconnector, which comprises an outer casing containing for each pole at least one fixed contact and at least one mobile contact that can be coupled to/ uncoupled from one another, a rotating element that comprises a shaped body made of insulating material comprising at least one seat for each pole of said switch, said seat being designed to house at least one mobile contact of a corresponding pole, a control mechanism operatively connected to the rotating element for enabling movement thereof and one or more elements made of ferromagnetic material set in a position corresponding to at least one portion of the inner surface of said at least one seat of the mobile contact. The ferromagnetic material limits the phenomena of non-uniform distribution of current between the various contacts of the individual poles to enable the internal contacts to provide a significant contribution to the electrodynamic strength.
Reference has been further made to US7935902 B2 relating to 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. The exemplary embodiments provide a rigid and robust contact movement arrangement which can withstand and account for large electrodynamic repulsion forces created by current flowing in the circuit breaker.
Thus there is a need for a robust, compact, cost effective and easy to assemble shaft assembly to improve the fault current withstand capacity of a circuit breaker, thereby ensuring higher fault current breaking capacity.

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.
An object of the present invention is to provide a switching device with higher electrodynamic withstand capacity.
An object of the present invention is to provide a robust shaft assembly to improve the fault current withstand capacity of a switching device.
In accordance with an aspect of the present disclosure, is to provide a single-pole or multi-pole switching device comprising for each pole, within a top housing and a bottom housing, at least one first stationary contact; a shaft assembly comprising, a plurality of mobile contacts, arranged in a substantially parallel manner and being pivotally attached to rotate around a fulcrum pin, wherein the mobile contact can be coupled or uncoupled to the first stationary contact; compression spring means and extension spring means operably coupled to each mobile contact to provide a contact force required to maintain a pressure between each mobile contact and the first stationary contact for good electrical joint, a conductor operably coupled to a second stationary contact at one end and the mobile contact at the other end; a main spring means operably coupled to the shaft assembly and an operating lever to couple or decouple the moving contacts and the first stationary contact to turn the switching device on or off; wherein the mobile contacts together carry the total current for each pole and wherein higher contact pressure being applied by the extension springs or the compression springs or a combination of both on extreme contacts or differential contact pressure on multiple contacts.
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 ACCOMPANYING DRAWINGS
The above and other aspects, features and advantages of the embodiments of the present disclosure will be more apparent in the following description taken in conjunction with the accompanying drawings, in which:
Figure 1 illustrates an isometric view of the 4 pole circuit breaker in accordance to the invention.
Figure 2 illustrates a cross-section view of the current carrying path of the circuit breaker in ON condition in accordance to the invention.
Figure 3 illustrates an isometric view of the shaft assembly consisting of plurality of mobile contacts in each pole in accordance to the invention.
Figure 4 illustrates 1-Phase current distribution per mobile contact due to proximity effect in accordance to the invention.
Figure 5 illustrates current distribution per mobile contact due to proximity effect in accordance to the invention.
Figure 6 illustrates an isometric view of the pole assembly in accordance to the invention.
Figures 7 (a)-(b) illustrates an isometric view of modified spring plate in accordance to the invention.
Figure 8 illustrates a cut view of the shaft assembly in accordance to the invention.
Figure 9 illustrates a cut view of the shaft assembly with provisions to improve electrodynamic withstand in accordance to the invention.
Figure 10 illustrates an isometric view of the shaft with provision to improve electrodynamic withstand in accordance to the invention.
Figure 11 illustrates a side view of a mobile contact in accordance to the invention.
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

The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the present disclosure 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 various embodiments described herein can be made without departing from the scope and spirit of the present disclosure. In addition, descriptions of well-known functions and constructions may be 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 present disclosure. Accordingly, it should be apparent to those skilled in the art that the following description of various embodiments of the present disclosure is provided for illustration purpose only and not for the purpose of limiting the present disclosure 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. Thus, for example, reference to "a component surface" includes reference to one or more of such surfaces.
All terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which various embodiments belong. Further, the meaning of terms or words used in the specification and the claims should not be limited to the literal or commonly employed sense, but should be construed in accordance with the spirit of the disclosure to most properly describe the present disclosure.
The terminology used herein is for the purpose of describing particular various embodiments only and is not intended to be limiting of various embodiments. As used herein, the singular forms "a," "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising" used herein specify the presence of stated features, integers, steps, operations, members, components, and/or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, members, components, and/or groups thereof. Also, Expressions such as "at least one of," when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list.
The present disclosure will now be described more fully with reference to the accompanying drawings, in which various embodiments of the present disclosure are shown. The present disclosure may, however, be embodied in many different forms and should not be construed as being limited to the various embodiments set forth herein, rather, these various embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the present disclosure. Furthermore, a detailed description of other parts will not be provided not to make the present disclosure unclear. Like reference numerals in the drawings refer to like elements throughout.
The subject invention lies in providing a shaft assembly to enhance electrodynamic withstand capacity of a moulded case circuit breaker.
An embodiment of the invention describes single-pole or multi-pole switching device comprising, for each pole, within a top housing (1) and a bottom housing (2), at least one first stationary contact (5); a shaft (9) assembly comprising, a plurality of mobile contacts/fingers (6) (the words “mobile contact” and “mobile finger” have been used in the present document interchangeably), arranged in a substantially parallel manner and being pivotally attached to rotate around a fulcrum pin (12), wherein the mobile contact (6) can be coupled or uncoupled to the first stationary contact (5), compression spring means (11) and extension spring means (10) operably coupled to each mobile contact (6) to provide a contact force required to maintain a pressure between each mobile contact (6) and the first stationary contact (5) for good electrical joint, a conductor (7) operably coupled to a second stationary contact (8) at one end and the mobile contact (6) at the other end; a main spring means (20) operably coupled to the shaft assembly and an operating lever (4) to couple or decouple the moving contacts and the first stationary contact to turn the switching device on or off; wherein the mobile contacts (6) together carry the total current for each pole
The switching device of the present invention herein afterwards refer to a circuit breaker, in particular to a moulded case circuit breaker (MCCB).
Referring to figure 1, illustration has been made to show a four-pole configuration of moulded case circuit breaker (MCCB). The MCCB has a top cover (1) and a bottom cover (2) housing the current carrying conductors operated by a mechanism (3) through an operating lever (4).
As shown in figure 2, referring to cross section view of circuit breaker in ON condition, another embodiment of the invention describes the current conducts through the first stationary contact (5) and the mobile contacts (6), a second stationary conductor (8), a flexible copper conductor (7), connecting the mobile contact (6) to a second stationary conductor (8) and vice versa. The mobile contacts (6) rotate around the fulcrum pin (12) located in the shaft (9). The compression springs (11) and/or extension springs (10) are attached to each moving contact inside the shaft (9) to provide the contact force required to maintain the pressure between moving contact (6) and the first stationary contact (5) for good electrical joint. The torque due to the contact springs act against the electrodynamic & constriction forces during overload & short circuit conditions. In ON condition main spring (20) acts against the contact springs.
Another embodiment of the invention describes the shaft (9) housing plurality of mobile contacts (6) as shown in figure 3. Five mobile contacts are housed in each pole. All the five parallel mobile contacts together carry the total current in each pole. The mobile contacts in each pole are designated as M1, M2, M3, M4 and M5. The present invention has been described with five mobile fingers, however, it is to be noted that the invention can be performed preferably with more than three mobile fingers.
Referring to figure 4 showing 1-phase current distribution per mobile contact. When current is flowing through only one phase the alternating magnetic field in the adjacent mobile contacts alters the current density of the mobile contacts depending on their position due to proximity effect. The graph shows the current distribution of respective mobile contact as percentage of the normal current in the absence of proximity effect. As depicted in the graph, the extreme mobile contacts (M1) and (M5) carries 5% higher current than the normal current.
Referring to figure 5 showing the current distribution per mobile contact due to proximity effect. When current is flowing through all the three phases, due to close proximity of the poles, the alternating magnetic field alters the current density of the mobile contacts depending on their position. The graph shows the current distribution of respective mobile contact as percentage of the normal current in the absence of proximity effect. As depicted in the graph, mobile contact (Y1) and (B1) carries 35% & 21% higher current respectively than the normal current. The graph also depicts that only extreme mobile contacts are carrying the higher current. Depending on switching phase angle on the voltage waveform, the mobile contacts carrying highest current is shifted between R, Y & B poles. Higher current passing through any of the extreme mobile contacts results in higher repulsive torque due to combined electrodynamic and constriction force, thus affecting the fault current withstand capacity of the breaker which decreases by 25% (from 25 kA rms to 18.5 kA rms).
Referring to figure 6 showing arrangement of extension springs in the shaft, in accordance to another embodiment of the invention, the extension springs (E1, E2, E3, E4, E5) are assembled between mobile contacts (M1, M2, M3, M4, M5) respectively and a fixed spring plate (13) to apply required contact pressure and act against the repulsive electrodynamic and constriction forces. To compensate the 35% increase in current density in the extreme mobile contacts, in the invention contact spring force of E1 and E5 are increased by 35% to improve the withstand capability from 18.5 kA rms to 21.5 kA rms. The overall reduction in withstand capacity is improved from 25 % to 15% (25 kA rms to 21.5 kA rms).
Withstand capacity can be further increased if extension springs applying highest contact pressure is used in extreme contacts and gradual reduction in innermost extension spring force is achieved. In case of five mobile contacts system contact spring force of E1 and E5 are increased by 35%, followed by E2 and E4 which are increased by 13% and contact spring force of E3 is decreased by 3%. To apply different contact pressure in different mobile contacts totally different extension springs can be used. Else different hook lengths (with all other spring design parameters remaining same) can be used to apply different contact pressure in different mobile contacts. Also initial tension can be varied in the different extension springs to ensure differential contact pressure.
Referring to figure 7 (a) showing a modified spring plate according to another embodiment of the invention. Another way of achieving higher contact pressure in extreme mobile contacts is shown. Instead of using different extension springs in extreme mobile contacts, provisions (14) can be made in the fixed spring plate (13) such that extension springs attached to the extreme mobile contacts are extended more.
In another embodiment of the invention, these provisions are not only limited to the extension spring plate (13). Similar provisions shall be made in extreme mobile contacts to obtain higher contact pressure.
Referring to figure 7 (b) another embodiment illustrates in the fixed spring plate (13), extension spring resting surface (17) is substantially concave. This concave shape facilitates highest contact pressure in the extreme mobile contacts and gradual reduction in contact pressure towards the innermost mobile contact.
Another embodiment of the invention describes a switching device where instead of having separate elements as shown in figures 7 (a)- (b), similar feature can be inbuilt in shaft (9).
Referring to figure 8 showing an arrangement of compression springs in the shaft in another embodiment of the invention, the compression springs (C1, C2, C3, C4, C5) are assembled between mobile contacts (M1, M2, M3, M4, M5) respectively and a surface (16) in the shaft (9) to apply required contact pressure and act against the repulsive electrodynamic and constriction forces. Similar to E1 and E5 as shown in figure 6, force applied by compression springs C1 and C5 are increased to improve the withstand capability from 18.5 kA rms to 21.5 kA rms. The overall reduction in withstand capacity is improved from 25 % to 15% (25kA rms to 21.5 kA rms). To further increase the electrodynamic withstand capacity different compression springs C1 and C5 (applying 35% higher contact pressure), C2 and C4 (applying 13% higher contact pressure) and C3 (applying 3% lesser contact pressure) can be used.
In another embodiment of the invention another way of achieving higher contact pressure in extreme mobile contacts is shown in figure 9. Instead of using different compression springs (C1 and C5) in extreme mobile contacts, provisions (15) can be made in the shaft (9) such that compression springs attached to the extreme mobile contacts are compressed more. These provisions are not only limited to the shaft (9). Similar provisions shall be made in extreme mobile contacts to obtain higher contact pressure.
In another embodiment, referring to figure 10 the shaft has provision to improve electrodynamic withstand. The surface (18) in shaft (9) is also substantially concave. This concave shape also ensures highest contact pressure in the extreme mobile contacts and gradual reduction in contact pressure towards the innermost mobile contact. To obtain required surfaces depicted in figure 9 and figure 10, a separate supported element can also be used in the shaft assembly.
In another embodiment of the invention, referring to figure 11 provision (19) in mobile contact (6) facilitates assembly of extension springs such that highest contact pressure in the extreme mobile contact and gradual reduction in contact pressure towards the innermost mobile contact is achieved to ensure enhanced electrodynamic withstand capability. The position, angle and shape of provision (19) can be designed as per performance requirement. Similar provision can be made for compression springs also.
The switching device of the invention also ensures lesser load on operating mechanism (3) and lesser impact on contact mating surfaces during switching operations. As differential contact pressure is applied on the mobile contacts, total contact pressure acting against mechanism main spring (20) is less. Thus mechanism main spring (20) of lesser force is used in the present invention. In case of a five fingered contact system force of mechanism main spring (20) is reduced by 12%. This invention ensures lesser load and lesser stress on operating mechanism (3).

Some of the non-limiting advantages of the present invention are:
a) Higher electrodynamic withstand capacity

b) Fast fault interruption.

c) Less cost of manufacturing.

d) Performance enhancement.

Although a switching device for higher electrodynamic withstand capacity has been described in language specific to structural features, it is to be understood that the embodiments disclosed in the above section are not necessarily limited to the specific methods or devices described herein. Rather, the specific features are disclosed as examples of implementations of a switching device for higher electrodynamic withstand capacity.