F O R M 2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10; rule 13)

1. Title of the invention: THERMAL OVERLOAD PROTECTION FOR
CIRCUIT BREAKERS

2. Applicant(s):

(a) NAME : LARSEN & TOUBRO LIMITED

(b) NATIONALITY : An Indian Company

(c) ADDRESS : L & T House, Ballard Estate, Mumbai 400 001,
State of Maharashtra, India

3. PREAMBLE TO THE DESCRIPTION

The following specification particularly describes the invention and the manner in which it is to be performed:

FIELD OF THE INVENTION
The present invention relates to the field of the thermal overload protection for Circuit Breaker with variable setting. The present invention particularly relates to the field of providing variable thermal overload protection through indirect heating of the bimetal.
BACKGROUND OF THE INVENTION
The electric switching devices and circuit interrupters such as circuit breakers, 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.
These switching devices typically have two positions corresponding to the status of the separable contacts. These positions would include an ON position, in which the separable contacts are closed, an OFF position in which the contacts are open. The third state would be tripped position in which the contacts are tripped open.
When circuit breaker is in ON condition and an overload fault condition arises, large amount of current flows through the circuit and circuit breaker should trip in certain time by sensing the fault condition through overload sensing mechanism such as bimetallic tripping arrangement. The time taken from a fault condition appearing and the current being limited by circuit breaker determines the let through energy of the circuit interrupter which should remain constant for all rating of a particular breaker which can be attained by keeping the trip time same. Variable overload protecting system is required to keep the same trip time.
For overload protection of circuit, the bimetal is deployed. Heat is produced by the current (I^2R loss). This heat is transferred either directly or indirectly on the bimetal. When the bimetal carries current directly, it is called as directly heated system. When the heat is produced by the conductor due to flow of current and this heat is transferred to bimetal by placing the bimetal on the conductor, the heat is transferred to the conductor due to conduction, convection and radiation. This system is called indirectly heated system.
The conventional thermal system as shown in Figure – 1 (Prior art) is indirect heating in which current is carried by the conductor. A bimetal is attached to the current carrying part which can transfer heat to the bimetal. Thus the bimetal deflects due to the heat. Here bimetal (2) is fixed with heater (1), the screw (3) which is connected to the bimetal hits the trip bar when bimetal deflects beyond threshold level and trips the breaker. In most of the application where high current flows indirect heating of bimetal is used.
US 4347494 discloses an overload protection switch including a narrow housing, a bimetal strip means mounted in housing, a push button mounted in and projecting out of the housing and made of insulating housing, the push button being provided with a partition and being moveable relative to the housing between an on position in which the conductive path is established via the strip means using the push button into its off position. The spring pressure exerted on the push button is dimensioned such that in “ON” position of the switch, the pressure is not sufficient to deflect the springs so as to press the contact piece completely through the wedge formed between the contact edges on the second part of the bimetallic strips.
US 4990882 discloses a push button actuated excess current protection switch with manual actuation and bimetal controlled automatic tripping. It has a self heated approximately U – shaped punched bimetal which is connected in series in the current path through the switch. The disadvantage with the said patent is that the U – shaped bimetal is having higher volume compared to the strip shape which is not desirable. Further, the number of components used in the said patent is more compared to the present invention. The direct heating is also not desirable for high current range.
In the conventional methods the variable thermal arrangement is given by creating a profile on the trip plate, where trip plate has to be moved along the axis to give different settings.
In conventional circuit breakers, thermal arrangement is given by heater (conductor) having some special profile generally U bend. For very high current rating circuit breakers, the conductor section required is thick. Getting particular profile of heater is difficult as the conductor section is thicker. Also the heater requires some bending operation which ultimately increases the length of the heater (conductor) thereby increasing the overall usage of conducting material generally copper.
These limitations and needs of the conventional bimetallic arrangement necessitate an improved bimetal arrangement system which is disclosed by the present invention providing variable thermal overload protection. It is more reliable and is easy in adjustment to provide variable rating. Further, it minimizes the use of conductor as the total conductor section is minimized.
OBJECT OF THE INVENTION
The main objective of the invention is to provide thermal overload protection to higher rated circuit breakers.
It is further an objective of the invention to provide easier factory calibration.
It is yet another object of the invention to provide variable thermal overload protection which is more reliable.
It is further an objective of the invention to provide capability of converting any higher range circuit breaker in to thermally protected circuit breaker with ease.
SUMMARY OF THE INVENTION
The present invention discloses the thermal overload protection system for circuit breaker. The bimetal is placed on the conductor which acts as a heater in such a way that heat produced by the conductor is transferred to the bimetal. The bimetal deflects due to the heat. The screw placed on C clamp which is placed on the bimetal moves upwards towards the direction of trip bar and hits the trip bar. After a threshold deflection of bimetal, the screw on C clamp displaces the trip bar and the circuit breaker mechanism trips the circuit breaker. The actuator is the part of trip bar which is actuated by the screw. The distance between the screw and the actuator decides the tripping threshold of the circuit breaker. The distance between the mating point of actuator and screw is varied by moving the trip bar in left or right direction. When the trip bar is moved towards right the distance between the actuator and screw is reduced and the bimetal deflection required for tripping the circuit breaker is reduced accordingly. Therefore, the same circuit breaker leads to variation in tripping current by changing the position of trip bar.
DETAILED DESCRIPTION OF DRAWINGS
Figure -1 shows the conventional thermal overload protection system.
Figure -2 shows the constructional features of thermal overload protection system.
Figure -3 shows the exploded view of the present invention.
Figure 4 shows the side view of the invention particularly displaying the trip bar (8) free to be rated about axis (9).
Figure 5 shows the variable thermal setting arrangement of the circuit breaker.
DETAILED DESCRIPTION OF INVENTION
In a thermal protected circuit breaker generally bimetal is used. The bimetal deflection is proportional to the heat produced by the conductor (heater). Under normal operating condition, the bimetal deflection is below threshold limit of the circuit breaker tripping. When higher than rated current flows through the circuit breaker, higher amount of heat is generated and therefore higher amount of heat is transferred to the bimetal. Bimetal deflection which is proportional to the heat transferred to the bimetal is also higher. The said higher than threshold deflection of bimetal trips the circuit breaker.
Thermal overload tripping system disclosed in the present invention is shown in Figure – 2. It consist of a current carrying part (4) which is used as heater, a bimetal (5) attached to the heater at the end to receive heat from heater (conductor). C shaped clamp (6) is connected to the other end of the bimetal (5). A screw (7) is connected on the other end of the C clamp (6). When the current flows through the conductor (4), by virtue of flow of current heat is produced in the conductor. The bimetal (5) is placed on the conductor (4) in such a way that heat produced by the conductor is transferred to the bimetal. Thus the bimetal (5) deflects due to the heat. The screw (7) placed on C clamp placed (6) on the bimetal also moves upwards towards the direction of trip bar (8) and hits the trip bar. After a threshold deflection of bimetal (5), the trip bar screw (7) displaces the trip bar (8) and the circuit breaker mechanism trips the circuit breaker.
In the said invention, the conductor (4) is a strip in which no bending operation is involved and no additional flexible element acting as heater is connected. The bimetal is placed directly on the strip and the arrangement is attached to bimetal so that the bimetal deflection can be transferred to the trip bar. Thus the said invention is capable of converting any higher range circuit breaker in to thermally protected circuit breaker.
The C clamp (6) acting as interface between bimetal and trip bar. The screw (7) is placed on the other side of the C clamp is used in establishing the factory setting of the thermal band. The screw (7) can be loosen to increase the distance between the trip bar (8) and screw tip, therefore higher bending of bimetal is required to trip the circuit breaker. When the screw is tightened, the gap between the screw (7) tip and trip bar (8) therefore lesser bimetal deflection is required to trip the circuit breaker. Thus the factory setting (calibration) of the tripping threshold is possible. Figure 3 shows the exploded view of the invention.
Figure 4 shows the side view of the invention, where the trip bar (8) is free to be rated about axis (9). Figure 5 shows the variable thermal setting arrangement of the circuit breaker. The actuator (10) is the part of trip bar (8) which is actuated by the screw (7). The distance between the screw (7) and the actuator (10) decides the tripping threshold of the circuit breaker. The distance between the mating point of actuator (10) and screw (7) can be varied by moving the trip bar either left of right in the direction of arrow as shown in Figure 5.
This arrangement can be achieved by various means. When the trip bar (8) is moved towards left, the actuator being integral part of trip bar also moves along with the trip bar. Thus the distance between the actuation point of actuator and screw is increased. So the deflection required to trip the circuit breaker is higher and accordingly more current is required to trip the circuit breaker. When the trip bar is moved towards right the distance between the actuating point of actuator (10) and screw (7) is reduced and thus the bimetal deflection required tripping the circuit breaker is reduced. Therefore, the same circuit breaker leads to variation in tripping current by changing the position of trip bar towards either left or right. Thus variable current setting is possible in this arrangement.
Many modifications and variations of the present invention are possible in the light of the above teaching. Thus, it is to be understood that, within the scope of the appended claims the invention may be practiced otherwise than as specifically described above.

WE CLAIM:

1. THERMAL OVERLOAD PROTECTION FOR CIRCUIT BREAKER comprises of conductor (4), bimetal (5), C shapes clam (6), screw (7), trip bar (8) and actuator means (10);
wherein the said bimetal (5) is placed on the said conductor (4) at the end such that the bimetal deflection is transferred to the trip bar (8);
wherein C shaped clamp (6) is connected to the other end of the bimetal (5);
wherein the said screw (7) is connected on the other end of the C clamp (6);
wherein the said screw (7) placed on said C clamp (6) moves upwards towards the direction of trip bar (8);
wherein screw (7) displaces the trip bar (8) and the circuit breaker mechanism trips the circuit breaker after a threshold deflection of bimetal (5);
wherein the trip bar (8) is free to be rated about the axis (9); and
wherein The actuator means (10) is the part of trip bar (8) actuated by the screw (7).

2. THERMAL OVERLOAD PROTECTION FOR CIRCUIT BREAKER as claimed in claim 1, wherein the said conductor (4) is a strip acting as a heater.

3. THERMAL OVERLOAD PROTECTION FOR CIRCUIT BREAKER as claimed in claim 1, wherein the said screw (7) is loosen to increase the distance between the trip bar (8) and screw (7) tip.

4. THERMAL OVERLOAD PROTECTION FOR CIRCUIT BREAKER as claimed in claim 1, wherein the said screw (7) is tightened to decrease the distance between the screw (7) tip and trip bar (8).

5. THERMAL OVERLOAD PROTECTION FOR CIRCUIT BREAKER as claimed in claim 1, wherein the distance between the mating point of actuator means (10) and screw (7) is varied by moving the trip bar (8) in left or right direction.

6. THERMAL OVERLOAD PROTECTION FOR CIRCUIT BREAKER as substantially described herewith the forgoing description and drawings.