FORM 2
The Patents Act 1970
(39 of 1970)
&
The Patent Rules 2003
COMPLETE SPECIFICATION
(See Section 10 and rule 13)
TITLE OF THE INVENTION:
CURRENT LIMITING TYPE CIRCUIT BREAKER WITH MAGNETIC REPULSION FORCE CONTROL SYSTEM
APPLICANT:
LARSEN & TOUBRO LIMITED
L&T House, Ballard Estate, P.O. Box No. 278,
Mumbai, 400 001, Maharashtra . INDIA.
PREAMBLE OF THE DESCRIPTION:
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED

A) TECHNICAL FIELD
[0001] The present invention generally relates to molded case circuit breaker. Particularly the invention relates to current limiting type circuit breaker. More particularly the invention relates to the current limiting type circuit breaker with controlled electromagnetic repulsion forces at contacts, a lower rise of temperature at contacts and, current discrimination with downstream electrical switching devices.
B) BACKGROUND OF THE INVENTION
[0002] Moulded case circuit breakers are used for protecting power distribution equipment against damage from over current conditions, such as an overload or a short circuit or both. With higher available fault currents, in the moulded case circuit breakers it is desirable to provide the circuit breaker with an increasing current interrupting rating. The current interrupting rating is accomplished using heavier components and by making the case of the circuit breaker larger and stronger so as to withstand forces caused by high current operation. However, space available for installation of the circuit breakers is often limited, and it is desirable to increase the current interrupting rating by regulating flow of current through the circuit breaker. Molded case current limiting circuit breakers limit the current flowing in faulted circuit to a substantially less magnitude.
[0003] Generally, short circuit protection in the circuit breakers is provided by an electromagnet assembly or by magnetic repulsion forces. Electromagnet assemblies

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include an electromagnet disposed in series with a load conductor and include a cooperating armature. During the short circuit condition, the short circuit current passes through the electromagnet which generates attraction forces to attract the armature and unlatch trip bar which, in turn causes the circuit breaker to trip. However, the time constant of the operation of electromagnet assemblies are usually quite high and substantial damage to the circuit loads can occur due to the delay in tripping characteristics. This emphasises the need for tripping mechanism with much faster response times.. In one of the magnetic repulsion force technique, magnetic repulsion members include set of contacts featuring two depending arms. The depending arms are referred to as dual pivot contacts, which separate more quickly than single pivot contacts. The dual pivot contacts are connected in series with the single pivot contacts. Current flows in opposite directions through the depending arms, creating a magnetic repulsion. As current increases, the magnetic repulsion force increases. During the short circuit condition the magnetic repulsion force causes the depending arms to blow open, in turn causing the circuit breaker to trip rapidly.
[0004] The molded case current limiting circuit breakers are available in various frame sizes. The frame size refers to a number of important characteristics of the circuit breaker, examples of the characteristics include, but are not limited to maximum allowable voltage, maximum allowable current, interrupting capacity, and physical dimensions of the molded case current limiting circuit breaker. However, the molded case circuit breaker arid in particular, current limiting molded case circuit breakers are relatively compact, a problem exists in providing current limiting capabilities for the circuit breaker of relatively smaller frame sizes. More specifically,
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the components in a relatively larger frame size current limiting molded case circuit breaker are difficult to further downsize to provide a current limiting circuit breaker in a smaller frame size.
[0005] In light of foregoing discussion, there exists a need for a molded case current limiting circuit breaker with control of magnetic repulsion force between electrical contacts of the circuit breaker. Also there exists a need for higher short circuit withstand capability in the higher rated molded case current limiting circuit breakers and, reducing temperature rise between the electrical contacts of the molded case current limiting circuit breaker. Further there is a need for providing current discrimination between upstream and downstream devices within the circuit breakers of same frame size.
[0006] The above mentioned shortcomings, disadvantages and problems are addressed herein and which will be understood by reading and studying the following specification.
C) OBJECT OF THE INVENTION
[0007] These and other objects and advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.
[0008] The primary object of the invention is to develop a magnetic repulsion force control system for molded case current limiting circuit breaker with controlled
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reduction of magnetic repulsion force between moving and fixed contacts of the molded case current limiting circuit breaker leading to high short circuit withstand capability.
[0009] Yet another object of the invention is to develop a magnetic repulsion force control system for molded case current limiting circuit breaker, with higher contact force between moving and fixed contacts to reduce the amount of temperature rise at the contacts , which is further used to reduce designed physical size of the molded case current limiting circuit breaker.
[0010] Yet another object of the invention is to develop a magnetic repulsion force control system for molded case current limiting circuit breaker, to provide current discrimination between upstream and downstream electrical switching devices placed in electrical distribution line by using two molded case current limiting circuit breakers of same frame size and different current ratings.
[0011] Yet another object of the invention is to develop a magnetic repulsion force control system for molded case current limiting circuit breaker, to provide difference between trip threshold level of upstream and downstream electrical switching devices placed in electrical distribution line such that only faulty part of electrical network gets disconnected.
[0012] Yet another objective of the invention is to develop a magnetic repulsion force control system for molded case current limiting circuit breaker, with a ferromagnetic material between the moving and the fixed contacts of molded case
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current limiting circuit breaker leading to intermittent reduction of magnetic linkage between the contacts leading to reduction of magnetic repulsion force, and delaying of circuit tripping.
[0013] Yet another object of the invention is to develop a magnetic repulsion force control system for molded case current limiting circuit breaker, to reduce magnetic repulsion force between moving and fixed contacts of the molded case current limiting circuit breaker by varying amount of insertion of any ferromagnetic material of suitable shape and size, between the moving and fixed contacts.
[0014] Yet another object of the invention is to develop a magnetic repulsion force control system for molded case current limiting circuit breaker, to appropriately match with load and selectivity requirements of electrical distribution system.
D) SUMMARY OF THE INVENTION
[0015] The various embodiments of the present invention provide a magnetic repulsion force control system for current limiting circuit breaker.
[0016] According to one embodiment of the present invention, a current limiting circuit breaker device with magnetic repulsion force control system is provided with an upper contact arm and a lower contact arm.
A laminated sheet of ferromagnetic material is inserted between the upper contact arm and the lower contact arm at any location along the contact arm. Two insulating plates are provided on the upper surface and on the lower surface of the laminated sheet of
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ferromagnetic material. The laminated sheet of ferromagnetic material reduces the magnetic repulsion force appearing between the upper contact arm and the lower contact arm due to the flow of currents in the upper contact arm and the lower contact arm in opposite directions.
[0017] The laminated sheet of ferromagnetic material is attached to the lower contact arm. The upper contact arm is movable. The lower contact arm is fixed. The laminated sheet of ferromagnetic material regulates the repulsion force to change the current withstanding capability based on the thickness of the laminated sheet inserted between the upper contact arm and the lower contact arm. The upper contact arm is a moving contact. The lower contact arm is a fixed contact.
E) BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The other objects, features and advantages will occur to those skilled in the art from the following description of the preferred embodiment and the accompanying drawings in which:
[0019] FIG. 1 is a side sectional view of a current limiting circuit breaker contact structure according to one embodiment of the present invention.
[0020] FIG. 2 illustrates a two dimensional graph showing magnetic repulsion
force and current characteristics in a current limiting circuit breaker according to one
embodiment of the present invention.
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[0021] Although specific features of the present invention are shown in some drawings and not in others. This is done for convenience only as each feature may be combined with any or all of the other features in accordance with the present invention.
F) DETAILED DESCRIPTION OF THE INVENTION
[0022] In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which the specific embodiments that may be practiced is shown by way of illustration. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments and it is to be understood that the logical, mechanical and other changes may be made without departing from the scope of the embodiments. The following detailed description is therefore not to be taken in a limiting sense.
[0023] The various embodiments of the present invention provide a magnetic repulsion force control system for current limiting circuit breaker.
[0024] According to one embodiment of the present invention, a current limiting circuit breaker device with magnetic repulsion force control system is provided with an upper contact arm and a lower contact arm.
A laminated sheet of ferromagnetic material is inserted between the upper contact arm and the lower contact arm. Two insulating plates are provided on the upper surface and on the lower surface of the laminated sheet of ferromagnetic material. The
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laminated sheet of ferromagnetic material reduces the magnetic repulsion force appearing between the upper contact arm and the lower contact arm due to the flow of currents in the upper contact arm and the lower contact arm in opposite directions.
[0025] The laminated sheet of ferromagnetic material is attached to the lower contact arm. The upper contact arm is movable. The lower contact arm is fixed. The laminated sheet of ferromagnetic material regulates the repulsion force to change the current withstanding capability based on the thickness of the laminated sheet inserted between the upper contact arm and the lower contact arm. The upper contact arm is a moving contact. The lower contact arm is a fixed contact.
[0026] FIG. 1 is a side sectional view of a current limiting circuit breaker according to one embodiment of the present invention. With respect to FIG. 1, a current limiting circuit breaker is provided with an upper contact arm 101 which is movable and a lower contact arm 102 which is fixed. The circuit breaker has a laminated piece of ferromagnetic material 103 inserted between the fixed and movable contacts 101,102. The ferromagnetic material 103 is attached to the lower contact arm 102 which is fixed. Eddy current losses resulting to heating in the ferromagnetic material 103 are reduced by providing laminations in the ferromagnetic material 103.Two insulating plates namely an upper insulating plate 104 and a lower insulating plate 105 are provided on upper and lower surface of the ferromagnetic material 103 respectively to provide electrical insulation from moving and fixed contacts 101,102 respectively. The fixed and movable contacts form an electrical connection at the contact button 106.

[0027] During nominal conduction of rated current by the current limiting circuit breaker normal operation of the current limiting circuit breaker takes place. During fault conditions in the current limiting circuit breaker, for example when circuit current conditions are above magnetic threshold limit of the current limiting circuit breaker, the moving and fixed contacts 101,102 remain closed due to reduced magnetic repulsion force between the contacts 101.102. As a result, maximum limit of current magnitude which could pass through the contacts 101,102 without causing them to repel increases, thereby higher current withstand capability in the current limiting circuit breaker is achieved.
[0028] The ferromagnetic material 103 intermittently reduces magnetic flux linkage between the movable and fixed contacts causing reduction in magnetic repulsion force appearing between the movable and fixed contacts 101,102. The reduction in the magnetic flux leakage between the contacts 101,102 is due to fact that the ferromagnetic material 103 has much higher magnetic permeability than air. The reduction in the magnetic repulsion force between the movable and fixed contacts 101,102 is due to opposite direction of current flow in the upper contact arm 101 and the lower contact arm 102 respectively. Further, the reduction of the magnetic repulsion force between the movable and fixed contacts 101,102 leads to delaying of circuit tripping in the current limiting circuit breaker. As a result, the contacts do not repel open due to reduction in repulsion forces, though repulsion threshold current has reached in the current limiting circuit breaker hence, higher short circuit withstanding capability is offered to the current limiting circuit breaker.

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[0029] Due to reduction in the magnetic repulsion force higher force is exerted on the movable and fixed contacts 101,102 which lead to lower temperature rise between the contacts due to better conduction of current through surface of the contact button 106. Reduction in the magnetic repulsion force is achieved by adjusting amount of ferromagnetic material 103 inserted between the contacts and accordingly, current withstanding capability of the current limiting circuit breaker is changed. Trip threshold level of the current limiting circuit breaker is increased due to reduction in the magnetic repulsion force between the contacts 101,102.
[0030] Current discrimination is provided between any of upstream and downstream switching devices due to increase in the current withstanding capability of the current limiting circuit breaker. The current discrimination is provided between the upstream and downstream switching devices of same or different frame sizes and current ratings. The current discrimination is ensured if maximum threshold of trip unit for the downstream switching device is less than minimum threshold of that for the upstream switching device.
[0031] Further increase in short circuit current causes the ferromagnetic material 103 to reach magnetic saturation limit. Hence, reduction in the magnetic repulsion force minimizes and the magnetic repulsion force gradually increases up to normal repulsion force levels and the contacts 101.102 repel open causing the current limiting circuit breaker to trip.

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[0032] FIG. 2 illustrates a two dimensional graph showing magnetic repulsion force and current characteristics in a current limiting circuit breaker according to one embodiment of the present invention. With respect to Fig. 2, substantial measurements of "CURRENT" in Amperes and "MAGNETIC REPULSION FORCE" in Newtons are taken along abscissa and ordinate of the two dimensional graph respectively. Curve 201 represents characteristics of current in amperes versus magnetic repulsion force in newtons for a normal breaker in prior art. Curve 202 represents characteristics of current in amperes versus magnetic repulsion force in newtons for the current limiting circuit breaker of the present invention. It is identical from the graph that the curve 202 between points A and B, magnetic repulsion force between contacts of the current limiting circuit breaker is negative.
[0033] For certain current range between the points A and B, the contacts are attracted each other indicating reduction in the magnetic repulsion force between the contacts. Hence, a greater amount of contact force is generated between the contacts of the current limiting circuit breaker. The greater amount of force between the contacts of the current limiting circuit breaker results in lower amount of temperature rise between the contacts and, better conduction of current through the surface of contact button. As current increases between the contacts, ferromagnetic material saturates. Further, due to saturation of the ferromagnetic material the magnetic repulsion force between the contacts increases rapidly and the curve 202 reaches point C. When the curve 202 approaches point C, tow curves 201,202 tend to merge indicating normal characteristic operation of the normal breaker in the prior art.
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G) ADVANTAGES OF THE INVENTION

[0034] Thus the various embodiments of the present invention provides a magnetic repulsion force control system for current limiting circuit breaker, to control reduction of magnetic repulsion force between moving and fixed contacts of the current limiting circuit breaker. The current limiting circuit breaker provides high short circuit withstand capability in current limiting devices. The current limiting circuit breaker provides higher contact force between the contacts to reduce temperature rise between the moving and fixed contacts which is further used to reduce designed physical size of the circuit breaker. The current circuit breaker provides current discrimination between upstream and downstream switching devices of same frame sizes and different current ratings in a distribution line.
[0035] Although the invention is described with various specific embodiments, it will be obvious for a person skilled in the art to practice the invention with modifications. However, all such modifications are deemed to be within the scope of the claims.

[0036] It is also to be understood that the following claims are intended to cover all of the generic and specific features of the present invention described herein and all the statements of the scope of the invention which as a matter of language might be said to fall there between.
Place: Bangalore RAKESH PRABHU
Date: 31 July 2009 Patent Agent

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CLAIMS
What is claimed is:
1. A current limiting circuit breaker device with magnetic repulsion force control
system comprising:
An upper contact arm;
A lower contact arm;
A laminated sheet of ferromagnetic material placed between the upper contact
arm and the lower contact arm; and
Two insulating plates provided on the upper surface and on the lower surface
of the laminated sheet of ferromagnetic material;
Wherein the laminated sheet of ferromagnetic material reduces the magnetic
repulsion force appearing between the upper contact arm and the lower
contact arm due to the flow of currents in the upper contact arm and the lower
contact arm in opposite directions.
2. The device according to claim 1, wherein the laminated sheet of ferromagnetic material is attached to the lower contact arm.
3. The device according to claim 1, wherein the upper contact arm is movable.
4. The device according to claim 1, wherein the lower contact arm is fixed.
5. The device according to claim 1, wherein the laminated sheet of ferromagnetic material regulates the repulsion force to change the current withstanding
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capability and continuous current carrying capacity based on the thickness and location of the laminated sheet inserted between the upper contact arm and the lower contact arm.
6. The device according to claim 1 wherein the upper contact arm and the lower contact arm are interchangeable as fixed or moving contact arm.
7. The device according to claim 1, wherein the laminated sheet of ferromagnetic material regulates the repulsion force to change the threshold tripping current based on the thickness and location of the laminated sheet inserted between the upper contact arm and the lower contact arm.
8. The device according to claims 1, wherein the laminated sheet of ferromagnetic material discriminates the flow of currents between any two or more circuit breakers provided in a distribution line.

RAKESH PRABHU Patent Agent,
ALMT Legal,
#2, Lavelle Road, Bangalore-560 001
To,
The Controller of Patents, The Patent Office, Mumbai

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