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 heat generation in heating element and subsequently, heat transfer to bimetallic element 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. In a typical thermal trip unit, 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. As the bimetallic element becomes warmer, due to differential coefficient 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.

Also there is a need to provide a variable thermal arrangement which avoids inconsistence in tripping time across the adjustment.
[0010] The above mentioned problems and disadvantages of the conventional
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 conventional
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 amount of heat transferred from heating element to 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
[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, an adjustment knob and a screw member. The screw member is connected on top to the adjustment knob exposed outside of the circuit breaker. The movement of adjustment knob varies the heat generated and thus heat transferred to the bimetallic element resulting in tripping the breaker at different settings.
[0019] According to an embodiment of the present invention, the
arrangement further comprises a calibration screw provided in the bimetallic element. The movement of the screw adjusts the distance between the bimetallic element and the trip bar thereby adjusting the deflection of the bimetallic element required for tripping the circuit breaker. The screw member is connected to the adjustment knob for thermal adjustment. The screw member is threaded on a bottom portion when placed in along its length.
[0020] According to another embodiment of the present invention, the
threaded portion of screw member engages with a threaded hole provided in the heater. The screw member is made of an electric conductor with a preset resistivity.
[0021] According to another embodiment of the present invention, the
comprising a connecting means to connect the bimetallic element to the heating

element. The connecting means being a rivet. When the screw member is inserted completely in the heating element such that sufficient heat is generated to deflect the bimetal element and to trip the circuit breaker. A reduction in the cross sectional area when the screw is not completely inserted in the heating element increases the heat transferred from the heating element to the bimetallic element thereby increasing the deflection of the bimetal.
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 unit according to one embodiment of the present invention.
[0024] FIG. 2 illustrates an isometric rear view of the variable thermal
arrangement in a circuit breaker protection unit according to one embodiment of the present invention.
[0025] FIG. 3 illustrates a front view of the variable thermal arrangement in a
circuit breaker protection unit according to one embodiment of the present invention.
[0026] FIG. 4 illustrates a rear view of the variable thermal arrangement in a
circuit breaker protection unit according to one embodiment of the present invention.
[0027] FIG. 5 illustrates a side view of the variable thermal arrangement in a
circuit breaker protection unit according to one embodiment of the present invention.

[0028] FIG. 6 illustrates a top view of the variable thermal arrangement in a
circuit breaker protection unit according to one embodiment of the present invention.
[0029] FIG. 7 illustrates an exploded view of the variable thermal
arrangement in a circuit breaker protection unit according to one embodiment of the present invention.
[0030] FIG. 8 illustrates the position of the screw member at minimum
setting with respect to variable thermal arrangement in a circuit breaker protection unit according to one embodiment of the present invention.
[0031] FIG. 9 illustrates the position of the screw member at intermediate
setting with respect to variable thermal arrangement in a circuit breaker protection unit according to one embodiment of the present invention.
[0032] FIG. 10 illustrates the position of the screw member at maximum
setting with respect to variable thermal arrangement in a circuit breaker protection unit according to one embodiment of the present invention.
[0033] 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
[0034] 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.
[0035] 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 using rivets. A screw member is suspended from top of the circuit breaker housing. The screw member is threaded in the bottom portion of its length and is typically made with an electric conductor with a particular resistivity. Further the threaded portion of screw member engages with the threaded hole provided in the heater. The screw member is connected on top to adjustment knob which is directly exposed to the outside of the circuit breaker and is used for thermal adjustment by the user. Additionally, a bimetallic element is provided with calibration screw at its tip. This screw is mainly for the purpose of calibration of the thermal protection. 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.
[0036] The principle behind invention is the change in electrical resistance of
the conductor based on the area of the conductor. The resistance is inversely proportional to the cross sectional area of the conductor. Therefore, a decrease in cross sectional area will lead to increase in resistance of the conductor. Also, it is well known that current carrying conductor generate heat. This generated heat is directly proportional to the resistance of the conductor. Thus, at some cross section of the conductor, if area decreases, heat generation in that area will increase. Also, if the current passes through two parallel paths with different resistances, the overall resistance of the path is the inverse of the sum of the inverse of resistances. This resistance is always less than the least of the resistances in the parallel path.

[0037] In the invention presented here, the current carrying conductor is
heating element. If the cross sectional area near to the bimetal is decreased, there is localized increase in the heat generation which gets transmitted to the bimetal. This is achieved in the present invention by providing a hole in the heater near to the region where bimetallic element is fixed.
[0038] The rate of heat generation by the heating element is varied thus at
minimum setting more heat is transferred to bimetallic element and at maximum setting, less heat is transferred to the bimetallic element. Thus, there is a consistency in the trip times at maximum and minimum settings of the thermal unit. Also, since the method of adjustment is vertical along the bimetallic element rather than in the direction of width of the breaker, the amount of space required for such arrangement of thermal variable setting will be minimum in comparison to the conventional methods.
[0039] The arrangement shown here is taken on the basis of ease of
understanding. This however does not restrict the construction to that shown here. Many variations can be produced. However, the basic principle of varying the cross sectional areas near the bimetallic element and use of screw members of different conductivity properties remains the same. One such variation could be use of a guiding member which moves in the hole of the heating element and which in turn is connected to the screw member and the adjustment knob. Another variation could be use of multiples such holes or use of different shaped slots in the heating element.
[0040] 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. According to one embodiment of the present invention the circuit breaker thermal protection comprises of a heating element 1 which is part of the conducting path of the circuit breaker, a bimetallic element 2 which is connected to the heating element using rivets 3. A screw member 4 is suspended from top of the circuit breaker housing. The screw member 4 is threaded in the bottom portion of its

length and is typically made with an electric conductor with a particular resistivity. Further the threaded portion of screw member 4 engages with the threaded hole provided in the heater. The screw member 4 is connected on top to adjustment knob 5 which is directly exposed to the outside of the circuit breaker and is used for thermal adjustment by the user. Additionally, a bimetallic element is provided with calibration screw 6 at its tip. This screw is mainly for the purpose of calibration of the thermal protection. Movement of screw 6 adjusts the distance between the bimetallic element 2 and the trip bar of the circuit breaker thereby adjusting the deflection of the bimetallic element required for tripping the circuit breaker.
[0041] 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.
[0042] 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.
[0043] FIG. 4 illustrates the rear view of the variable thermal arrangement in
a circuit breaker protection system according to one embodiment of the present invention.
[0044] FIG. 5 illustrates the side view of the variable thermal arrangement in
a circuit breaker protection system according to one embodiment of the present invention.
[0045] FIG. 6 illustrates the top view of the variable thermal arrangement in a
circuit breaker protection system according to one embodiment of the present invention.

[0046] FIG. 7 illustrates the exploded view of the variable thermal
arrangement in a circuit breaker protection system according to one embodiment of the present invention.
[0047] FIG. 8 illustrates the position of the screw member 4 at maximum
condition of the thermal setting with respect to variable thermal arrangement in a circuit breaker protection system according to one embodiment of the present invention. In this case, the screw member 4 will be downward in a way to completely fill the hole. The current flow will be higher in this case in comparison to that in the minimum condition. Hence, the heat generation will be sufficient enough to deflect the bimetal 2 and trip the breaker. In this condition, the cross section for current path at the hole area is the sum of the individual cross sections of the heating element 1 and the screw member 4. The resistance at the cross section also depends on the individual resistances of the heating element 1 and the screw member 4.
[0048] FIG. 9 illustrates the position of the screw member 4 at intermediate
setting with respect to variable thermal arrangement in a circuit breaker protection system according to one embodiment of the present invention. This position can be any intermediate value of the thermal setting required for the application.
[0049] FIG. 10 illustrates the position of the screw member 4 at minimum
setting of the thermal unit with respect to variable thermal arrangement in a circuit breaker protection system according to one embodiment of the present invention. Here the screw member 4 is completely outside of the heating element 1 and is suspended on the housing at the top. Since the cross section at the hole is unfilled, there is a reduction in cross sectional area at that section thereby increasing the heat generated. This increased heat is transferred through the heating element 1 to the bimetallic element 2. This results in an increased deflection of the bimetal element 2 even for less current flow in the heating element 1.

G) ADVANTAGES OF THE INVENTION
[0050] The various embodiments of the present invention provide a variable
thermal arrangement for a circuit breaker protection unit. The variable thermal adjustment is by varying the heat generation in the heating element. As the adjustment of typical thermal setting is provided by moving the trip bar, the problems associated with it such as unintentional release of latch member of circuit breaker is avoided. Such arrangement also increases the flexibility of achievable range of thermal adjustment.
[0051] The present invention provides variable thermal arrangement
mechanism where the rate of heat generation by the heating element is varied, thus at minimum setting more heat is transferred to bimetallic element and at maximum setting, less heat is transferred to the bimetallic element. Thus, there is a consistency in the trip times at maximum and minimum settings of the thermal unit.
[0052] 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.
[0053] 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 of:
a heating element;
a bimetallic element connected to the heating element;
an adjustment knob; and
a screw member;
Wherein the movement of the adjustment knob varies the heat generated in the
heating element, thereby heat transferred to the bimetallic element resulting in
deflection of the bimetallic element required for tripping the circuit breaker.
2. The arrangement according to claim 1, further comprising a calibration screw
provided in the bimetallic element,
3. The arrangement according to claim 2, wherein the movement of the calibration 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.
4. The arrangement according to claim 1, wherein the screw member is connected to the adjustment knob for thermal adjustment.
5. The arrangement according to claim 1, wherein the screw member is threaded on a bottom portion when placed in along its length.
6. The arrangement according to claim 5, wherein the threaded portion of screw member engages with a threaded hole provided in the heater.
7. The arrangement according to claim 1, wherein the screw member is made of an electric conductor with a preset resistivity.
8. The arrangement according to claim 1, wherein the movement of the adjustment knob is translational.
9. The arrangement according to claim 1, wherein the movement of the adjustment knob is rotational.
10. The arrangement according to claim 1, further comprises a connecting means to connect the bimetallic element to the heating element.

11. The arrangement according to claim 8, wherein the connecting means is a
rivet.
12. The arrangement according to claim 1, wherein the screw member is
inserted completely in the heating element such that sufficient heat is generated to
deflect the bimetal element and to trip the circuit breaker.
13. The arrangement according to claim 1, wherein a reduction in the cross sectional area when the screw is not completely inserted in the heating element increases the heat transferred from the heating element to the bimetallic element thereby increasing the deflection of the bimetal.
14. The arrangement according to claim 1, wherein a deflection of the bimetallic element is varied to provide for a predetermined thermal setting.