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:
VARIABLE THERMAL ARRANGEMENT FOR CIRCUIT BREAKER PROTECTION 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 a circuit breaker and
particularly to thermal adjustments in a circuit breaker. The invention more particularly relates to variation of cantilever length of bimetallic element in the circuit breaker for achieving variable thermal adjustment.
B) BACKGROUND OF THE INVENTION
[0002] Circuit switching devices and circuit interrupters such as circuit
breakers, contactors, motor starters, motor controllers and other load controllers are used to protect electrical circuitry and equipment from damage due to abnormal condition, such as an overload condition or a relatively high level short circuit or fault condition.
[0003] The circuit switching devices and circuit interrupters devices typically
have two positions corresponding to the status of the separable contacts. For example, these positions include an ON position, in which the separable contacts are closed, an OFF position in which the contacts are open. In some other switching devices, there is a tripped position in which the contacts are tripped open.
[0004] These 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.
[0005] Circuit breakers such as moulded case circuit breaker (MCCB)
provide over current protection for various types of electrical equipments. For such protection circuit breakers employ thermal and magnetic elements which trip the breaker under various over current conditions. The thermal component typically includes a bimetallic element which responds to a relatively long duration overload conditions to trip the breaker when a specified current level is exceeded for a period

of time. The goal is to increase the temperature of bimetal when there is change in current. Two methods are typically employed for raising the temperature of bimetal. First is the indirect heating type in which heat is transferred to the bimetal from a heating element by means in which there is minimal or no flow of electric current through the bimetal. Second method is the direct heating type in which there is flow of current through the bimetallic element. In this case, at least a portion of the current flowing through the breaker is channelled through the bimetallic element. The ohmic resistance of the bimetallic element causes it to generate heat in proportion to the square of the level of current flowing through it. In either method, as the bimetallic element becomes warmer, due to differential co-efficient of thermal expansion of the metals comprising the bimetal, the bimetallic element starts bending. When the temperature exceeds a predetermined value, the bimetallic metal comes in contact with the trip mechanism of the breaker and releases the latch due to which the contacts separate.
[0006] Conventional methods of adjusting the distance between the
bimetallic element and the trip bar is by giving a sloped projection on the trip bar at the point where bimetallic element touches the trip bar. But these methods are susceptible to various problems such as unintentional release of the mechanism latch when the trip bar is moved axially for thermal adjustment. This also puts constraints on the latching dimensions which have to be controlled very accurately which in turn increases the manufacturing cost.
[0007] Another inherent drawback of the prior methods is the inconsistency
in tripping time across the adjustment. This is expected since at minimum setting, the current is lower than in comparison to the maximum setting, heat generated will be lesser and thus the time taken by the bimetallic element to deflect will be more.
[0008] Circuit breakers normally comprise of three or more poles. But certain
applications require only two or even single pole. For these applications, single or two pole circuit breaker are also provided. Reduction in no of poles results in the

reduction in width and thus puts constraints on the axial movement of the trip bar. Thus, the range of the thermal adjustment reduces in such case.
[0009] Hence there exists a need to provide a variable thermal arrangement
for a circuit breaker by varying the cantilever length of the bimetallic element.
[0010] The above mentioned problems and disadvantages of the prior
methods indicates a need for an improved method of adjusting the thermal setting which can give benefits over and above by overcoming the problems in prior methods.
[0011] The above mentioned shortcomings, disadvantages and problems are
addressed herein and which will be understood by reading and studying the following specification.
C) OBJECTS OF THE INVENTION
[0012] The primary object of the present invention is to provide variable
thermal arrangement for a circuit breaker by varying the effective cantilever length of the bimetallic element.
[0013] Another object of the present invention is to provide a variable
thermal arrangement to achieve consistency in tripping time across the range of variable thermal adjustment.
[0014] Yet another object of the present invention is to provide a variable
thermal arrangement that provides flexibility to use in compact constructions of the circuit breaker.

[0015] Yet another object of the present invention is to provide a variable
thermal arrangement to increase the flexibility of the achievable range of thermal adjustment.
[0016] Yet another object of the present invention is to provide a variable
thermal arrangement which eliminates the movement of trip bar of the circuit breaker and thus avoid its associated problems such as unintentional release of mechanism latch member.
[0017] 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.
D) SUMMARY OF THE INVENTION
[0018] The various embodiments of the present invention provide a variable
thermal arrangement for circuit breakers. The arrangement comprising of a heating element, a bimetallic element connected to the heating element, a slider adapted to move in slots provided in the heating element and the bimetallic element, an adjustment knob connected to the slider, and a calibration screw provided in the bimetallic element. The movement of the adjustment knob varies the cantilever length of the bimetallic element thereby adjusting the deflection of the bimetallic element required for tripping the circuit breaker.
[0019] According to an embodiment of the present invention, the
arrangement further comprises a connecting means to connect the bimetallic element to the heating element. The connecting means is a rivet. The sliding movement of the slider is controlled by a screw member, which is adapted to rotate about its axis.
[0020] According to an embodiment of the present invention, the screw
member includes external threads and the slider includes internal threads in a hole

provided in the slider. The dimensions of the slider are selected such that the slider forms a close fit when assembled with the bimetallic and heating element. The slider is moved to the extreme of the slot towards a fixed end of the bimetallic element such that the length of the cantilever is maximum. The slider is moved to the extreme of the slot away from the fixed end of the bimetallic element such that the length of the cantilever is minimum.
[0021 ] According to an embodiment of the present invention, the deflection
of the bimetallic element is varied to provide for a predetermined thermal setting. Here, the adjustment of the slider is vertical along the bimetallic element.
E) BRIEF DESCRIPTION OF THE DRAWINGS:
[0022] 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:
[0023] FIG. 1 illustrates an isometric front view of the variable thermal
arrangement in a circuit breaker protection system according to one embodiment of the present invention.
[0024] FIG. 2 illustrates the isometric rear view of the variable thermal
arrangement in a circuit breaker protection system according to one embodiment of the present invention.
[0025] FIG. 3 illustrates a schematic front view of the variable thermal
arrangement in a circuit breaker protection system according to one embodiment of the present invention.
[0026] FIG. 4 illustrates a perspective side view of the variable thermal
arrangement in a circuit breaker protection system according to one embodiment of

the present invention.
[0027] FIG. 5 illustrates the sectional side view of the variable thermal
arrangement in a circuit breaker protection system according to one embodiment of the present invention.
[0028] FIG. 6 illustrates an exploded view of the variable thermal
arrangement in a circuit breaker protection system according to one embodiment of the present invention.
[0029] FIG. 7a illustrates the position of the slider at maximum setting of the
variable thermal arrangement in a circuit breaker protection system according to one embodiment of the present invention.
[0030] FIG. 7b illustrates the position of the slider at minimum setting of the
variable thermal arrangement in a circuit breaker protection system according to one embodiment of the present invention.
[0031] FIG. 8 illustrates the first step in the assembly of the slider into the
slot of a heating and bimetallic element according to one embodiment of the present invention.
[0032] FIG. 9 illustrates the second step in the assembly of the slider into the
slot of a heating and bimetallic element according to one embodiment of the present invention.
[0033] FIG. 10 illustrates the third step in the assembly of the slider into the
slot of a heating and bimetallic element according to one embodiment of the present invention.

[0034] 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
[0035] 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.
[0036] The various embodiments of the present invention provide a variable
thermal arrangement for a circuit breaker protection system. According to one embodiment of the present invention the circuit breaker mechanism comprises of a heating element which is part of the conducting path of the circuit breaker, a bimetallic element which is connected to the heating element, a slider is provided which can slide in the slots provided in the heating element and the bimetallic element, sliding movement of the slider is controlled by the screw member which can rotate about its axis, an adjustment knob is provided which is directly exposed to the outside of the circuit breaker, a calibration screw is provided mainly for the purpose of calibration of the thermal protection and rivets connects bimetallic element to the heating element.
[0037] Variable thermal arrangement for circuit breaker consists of a heating
element which is part of the conducting path of the circuit breaker. Thermal and magnetic protection system is typically surrounded around the heater. For magnetic system, the heating element acts as the medium carrying current to induce magnetic

flux. For thermal system, the heating element acts as the heat generating source from which the bimetallic element absorbs heat and there after deflect to trip the circuit breaker. The above arrangement is typical to an indirect type of heating method used in thermal protection system. In case of direct heating type of arrangement, heating element is typically absent since the heat is generated within the bimetallic element due to passage of current through it.
[0038] The thermal protection system in the present invention consists of a
bimetallic element which is connected to the heating element using variety of means including the one shown here using rivets. An adjustment slider is provided which can slide in the slots provided in the heating element and the bimetallic element. The sliding movement of the slider is controlled by the screw member, which rotates about its axis.
[0039] Screw member has threads externally and slider has internal threads in
the hole provided in it. Thus, slider and screw member move relative to each other due to a screw action. The screw is connected to an adjustment knob which is directly exposed to the outside of the circuit breaker. Additionally, the bimetallic element is provided with calibration screw at its tip. This screw is mainly for the purpose of calibration of the thermal protection. The movement of screw adjusts the distance between the bimetallic element and the trip bar of the circuit breaker thereby adjusting the deflection of the bimetallic element required for tripping the circuit breaker.
[0040] The dimensions of the slider are such that it forms a close fit with the
bimetallic element. This avoids the movement of the slider along with the bimetallic element when it deflects. This also helps in creating a close contact area between the heating element and the bimetallic element such that conduction is facilitated.

[0041 ] The principle behind the invention is the variation in cantilever length
to achieve the desired deflection at the tip of the cantilever for the desired load. Maximum deflection of the cantilever i.e. at the tip where the load is applied is given by the equation:

It is understood from the equation that the only variables in the equation for a given material and cross sectional area are load P and length 1. For a constant force, the deflection varies cube times the length of the cantilever. Thus, small variation in length is used to get a given deflection.
[0042] For a given current flowing through the heating element, certain
definite amount of heat is generated. Heat transfer from the heating element to the bimetallic element is both by conduction and convection. Conduction is the region where the bimetallic element is fixed to the heating element. Convection is the region where there is overlap between bimetallic and heating element i.e. at the region just above the fixed end till the heating element bends to the other side.
[0043] The rate of heat generation by the heating element is constant wherein
the heat transferred to the bimetallic element varies across the adjustment range of the thermal setting. In the present invention, the portion below the slider is responsible for the conduction of heat to the bimetallic element due to its close contact with the heating element and that above it for convection. When the thermal

setting is at maximum, the bimetallic element takes certain time. When it is at minimum setting, the conduction area increased in comparison to the maximum setting. Thus, heat transfer rate from the heating element to the bimetallic element is higher than that in case of maximum setting. Thus, the tripping time of the thermal element remains more consistent across the range of thermal adjustment.
[0044] FIG. 1 illustrates an isometric front view of the variable thermal
arrangement in a circuit breaker protection system according to one embodiment of the present invention. The various embodiments of the present invention provide a variable thermal arrangement for a circuit breaker protection system. According to one embodiment of the present invention a variable thermal arrangement for circuit breaker disclosed herein is shown in FIG 1. The variable thermal arrangement comprises of a heating element 101 which is part of the conducting path of the circuit breaker. Thermal and magnetic protection system is typically surrounded around the heater. For magnetic system it acts as the medium carrying current to induce magnetic flux. For thermal system it acts as the heat generating source from which the bimetallic element can absorb heat and there after deflect to trip the circuit breaker. Thermal protection system in the present invention consists of a bimetallic element 102 which is connected to the heating element using variety of means including the one shown here using rivets 107.
[0045] An adjustment slider 103 is provided which can slide in the slots
provided in the heating element 101 and the bimetallic element 102. Sliding movement of the slider 103 is controlled by the screw member 104 which can rotate about its axis. Screw member 104 has threads externally and slider 103 has internal threads in the hole provided in it. Thus, slider 103 and screw member 104 move relative to each other due to screw action. Screw 104 is connected to an adjustment knob 105 which is directly exposed to the outside of the circuit breaker. Additionally, the bimetallic element is provided with a calibration screw 106 at its tip. This screw is mainly for the purpose of calibration of the thermal protection. Movement of screw 106 adjusts the distance between the bimetallic element and the trip bar of the

circuit breaker thereby adjusting the deflection of the bimetallic element required for tripping the circuit breaker.
[0046] FIG. 2 illustrates the isometric rear view of the variable thermal
arrangement in a circuit breaker protection system according to one embodiment of the present invention. The FIG.2 shows the heating element 101, the bimetallic element 102, the slider 103, and the screw member 104. The arrangement also includes the adjustment knob 105, the calibration screw 106 and rivets 107.
[0047] FIG. 3 illustrates the front view of the variable thermal arrangement in
a circuit breaker protection system according to one embodiment of the present invention.
[0048] FIG. 4 illustrates the side view of the variable thermal arrangement in
a circuit breaker protection system according to one embodiment of the present invention.
[0049] FIG. 5 illustrates the sectional side view of the variable thermal
arrangement in a circuit breaker protection system according to one embodiment of the present invention.
[0050] FIG. 6 illustrates the exploded isometric view of the variable thermal
arrangement in a circuit breaker protection system according to one embodiment of the present invention.
[0051] FIG. 7a illustrates the position of the slider at maximum setting of the
variable thermal arrangement in a circuit breaker protection system according to one embodiment of the present invention. In fig 7a the slider 103 is to the extreme of the slot towards the fixed end of the bimetallic element 102. In this condition, the length of the cantilever is maximum and thus the deflection will be maximum. This state

can be used to achieve the maximum thermal setting for the circuit breaker.
[0052] FIG. 7b illustrates the position of the slider at minimum setting of the
variable thermal arrangement in a circuit breaker protection system according to one
embodiment of the present invention. In fig 7b the sliderl03 is to the extreme
of the slot away from the fixed end of the bimetallic element 102. In this condition the length of the bimetallic element 102 is minimum and thus the deflection produced will be lesser in comparison to the other extreme condition. This state can be used to achieve the minimum thermal setting for the circuit breaker. If the slider 103 is in any other intermediate position, the thermal setting of the circuit breaker will be accordingly in between the extreme settings.
[0053] FIG. 8 illustrates the first step in the assembly of the slider 103 into
the slot of a heating 101 and bimetallic element 102 according to one embodiment of the present invention. The dimensions of slider 103 are such that it forms a close fit with the bimetallic element 102. This avoids the movement of the slider 103 along with the bimetallic element 102 when it deflects. This also helps in creating a close contact area between the heating element 101 and the bimetallic element 102 such that conduction is facilitated. The heating element 101 and bimetallic strip 102 are primarily linked together by rivets 107. The area where the heating element 101 and bimetallic strip 102 are primarily linked together by rivets 107 forms the conduction area. The slider 103 is inserted into the common cut through slot of heating element 101 and bimetallic strip 102 vertically.
[0054] FIG. 9 illustrates the second step in the assembly of the slider 103 into
the slot of a heating element 101 and bimetallic element 102 according to one embodiment of the present invention. The slider 103 is inserted into the common cut through slot vertically bringing in contact together the heating element 101 and bimetallic element 102. The area above the conduction area i.e. the overlapping area between heating element 101 and bimetallic element 102 is known as convection

area.
[0055] FIG. 10 illustrates the third step in the assembly of the slider 103 into
the cut through slot of a heating element 101 and bimetallic element 102 according to one embodiment of the present invention. Once the slider 103 is inserted into the common cut through slot vertically bringing in contact together the heating element 101 and bimetallic element 102 the slider is rotated 90 degree in clock wise direction. Thus the opening between the bimetallic element 102 and heating element lOlis adjusted with the help of slider by sliding up and down in the cut through slots provided in the heating elementlOl and bimetallic element 102. Further the sliding movement of the slider 103 is controlled by the screw member 104 which can rotate about its axis.
G) ADVANTAGES OF THE INVENTION
[0056] The various embodiments of the present invention provide a variable
thermal arrangement for a circuit breaker protection system. Conventionally the thermal adjustment is made by varying the distance between trip bar and bimetallic element. The present invention provides variable thermal arrangement mechanism where the deflection of the bimetallic element is varied in cantilever length of the bimetallic element to achieve the desired thermal setting. This increase the consistency in tripping time achieved across the variable thermal range. Also, since the method of thermal adjustment is vertical along the bimetallic element rather than in the direction of width of the circuit breaker, the amount of space required for such arrangement of thermal variable setting will be minimum in comparison to the prior methods.
[0057] 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.

[0058] 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.

CLAIMS
What is claimed is:
1. A variable thermal arrangement for circuit breakers, the arrangement
comprising:
a bimetallic element;
a slider adapted to move over the bimetallic element; an adjustment knob connected to the slider; and a calibration screw provided in the bimetallic element;
Wherein the adjustment knob is moved to vary the cantilever length of the bimetallic element thereby adjusting the deflection of the bimetallic element required for tripping the circuit breaker.
2. The arrangement according to claim 1, further comprises a heating element connected to the bimetallic element such that heat is transferred from heating element to bimetallic element.
3. The arrangement according to claim 1, further comprises a supporting element connected to the bimetallic element so that a heat is generated within the bimetal.
4. The arrangement according to claim 1 further comprises a connecting means to connect the bimetallic element the heating element.
5. The arrangement according to claim 1 further comprises a connecting means to connect the bimetallic element to the supporting element.
6. The arrangement according to claim 1, wherein the connecting means is a rivet.
7. The arrangement according to claim 1, wherein the heating element is provided with plurality of slots.

8. The arrangement according to claim 1, wherein the slider is adapted to move in the plurality of the slots provided in the heating element to vary the effective length of the bimetallic element
9. The arrangement according to claim 1, wherein the adjustment knob is moved to control the sliding movement of the slider.
10. The arrangement according to claim 1, wherein the movement of the adjustment knob is translational.
11. The arrangement according to claim 1, wherein the movement of the adjustment knob is rotational.

12. The arrangement according to claim 1, wherein the movement of the slider is translational.
13. The arrangement according to claim 1, wherein the movement of the slider is
rotational.
14. The arrangement according to claim 1, wherein the adjustment knob comprises a screw member.
15. The arrangement according to claim 1, wherein the screw member is adapted to rotate about an axis to control the sliding movement of the slider.
16. The arrangement according to claim 1, wherein the screw member includes external threads.
17. The arrangement according to claim 1, wherein the slider includes internal
threads in a hole provided in the slider.

18. The arrangement according to claim 1, wherein dimensions of the slider are selected such that the slider forms a close fit with the bimetallic element.
19. The arrangement according to claim 1, wherein the slider is moved to the extreme of the slots towards a fixed end of the bimetallic element such that the length of the cantilever is maximum.
20. The arrangement according to claim 1, wherein the slider is moved to the extreme of the slot away from the fixed end of the bimetallic element such that the length of the cantilever is minimum.
21. The arrangement according to claim 1, wherein a deflection of the bimetallic element is varied to provide for a predetermined thermal setting.
22. The arrangement according to claim 1, wherein the method of adjustment of the slider is vertical along the bimetallic element.