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:
LOAD ADJUSTMENT SYSTEM FOR MAGNETIC CALIBRATION IN MOLDED CASE MINIATURE CIRCUIT BREAKER
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 circuit breakers and particularly to magnetic calibration systems in circuit breakers. The present invention more particularly relates to variable load adjustment systems for magnetic calibration in circuit breakers such as molded case miniature circuit breakers (MCCB).
B) BACKGROUND OF THE INVENTION
[0002] Molded case circuit breakers provide over current protection for residential
and some commercial and industrial electrical circuits. These circuit breakers are generally installed in lighting or distribution load centers to supply electrical load at lower voltages and currents. Molded case circuit breakers usually include a thermal element in the form of a bimetal that initiates a trip in circuit breaker under low/over current conditions. Most molded case circuit breakers also include a magnet and armature that combine to initiate a trip in circuit breaker for over current values with higher magnitude.
[0003] Electrical circuit breakers may employ a variety of trip units for sensing an electrical current and for initiating a tripping action at the circuit breaker, including bimetallic, magnetic, and thermal/magnetic trip units. Magnetic trip units may include c-shaped magnets, oil-filled dashpots, coil-type solenoids and the like. Circuit breaker manufacturing processes employing such trip units may include a calibration routine to properly coordinate the responsiveness of the trip unit to an electrical current and to properly adjust for dimensional variations and tolerances among and between the circuit breaker components. One such calibration routine involves the setting of different parameters, such as a magnetic air gap and a mechanical air gap for example. However, the adjustment of one parameter may affect the adjustment of another parameter which may then need to be readjusted.

[0004] In the conventional circuit breaker, the breaker has the means for adjusting the spring bias and the gap for each pole of the breaker. The spring bias is adjusted individually for each pole by a rotatable cam which pivots a lever to adjust the bias applied to a rotatable armature by a tension spring. A threaded screw provides individual adjustment of the gap between the armature and the stationary magnetic structure.
[0005] In the conventional remote controlled circuit breakers, calibration arrangement is provided for calibrating the circuit breaker so that it interrupts the current path when the current flowing through the circuit breaker reaches the pre-determined level. Some circuit breakers utilize a calibration plate secured at one end to the load terminal and secured to the bimetal at its other end. A calibration screw is supported by circuit breaker housing and passes through the calibration plate. The pre-determined current level, at which the circuit breaker interrupts the current path, is obtained by tightening the calibration screw and hence by bending the calibration plate and repositioning the trip mechanism. However, the calibration plate is not sufficiently supported at the end that is coupled to the bimetal thereby allowing the end to float, thereby causing the circuit breaker not to hold its calibration. Additionally, when a wire is fixed into the load terminal, the turning of a load terminal screw causes the calibration plate to bend or shift thereby causing the circuit breaker to go out of calibration.
[0006] Moreover the currently available circuit breakers have a single spring requiring a heavy load to compensate for the magnetic force of attraction to provide a desired current
setting.
[0007] Hence there is a need to provide a spring based load adjustment mechanism for magnetic calibration in molded case circuit breaker to provide a desired tripping threshold of circuit breakers.
[0008] 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
[0009] The primary object of the present invention is to develop a load adjustment mechanism using double springs for magnetic calibration in circuit breakers.
[0010] Another object of the present invention is to develop a load adjustment mechanism provided with double spring arrangement to adjust the thereshold trip level to a desired level by adjusting the load of the spring.
[0011]Yet another object of the present invention is to develop a load adjustment
mechanism with double spring arrangement for magnetic calibration in molded case circuit breaker so that the initial current threshold is set based on the load provided by both the springs thereby reducing the load burden on each spring.
[0012] Yet another object of the present invention is to develop an a load adjustment mechanism with double spring arrangement for magnetic calibration in molded case circuit breaker to provide a soft movement of magnetic setting knob in the circuit breaker with consistent calibration readings.
[0013] Yet another object of the present invention is to develop a load adjustment mechanism with double spring arrangement for magnetic calibration in molded case circuit breaker to improve the consistency of the release calibration operation, to enhance the reliability and to improve the calibration time of the circuit breaker.
[0014] 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
[0015] The various embodiments of the present invention provide a load adjustment system with a double spring arrangement for magnetic calibration in a molded case circuit breaker. According to one embodiment of the present invention, a bracket is mounted on the wall of the release housing in the circuit breaker. A single torsion spring is secured to the bracket. A calibration screw is attached to the single torsion spring. A double torsion spring is secured to the bracket. A moving magnet is attached to the bracket. A fixed magnet is attached to a current carrying conductor in the circuit breaker.
[0016] The calibration screw is moved in forward direction or in reverse direction to achieve a desired combined force value of both the single torsion spring and the double torsion spring to set a trip threshold value of the circuit breaker. The calibration screw is moved in forward direction to increase the deflection of the single torsion spring to increase the opposing force provided by the single torsion spring to a magnetic attractive force between the fixed magnet and the moving magnet. The calibration screw is moved in reverse direction to decrease the deflection of the single torsion spring to decrease the . opposing force provided by the single torsion spring to a magnetic attractive force between the fixed magnet and the moving magnet.
[0017] The double torsion spring is a fixed spring which is designed to provide a specified current setting. The single torsion spring is a variable spring which is designed to adjust a current setting from the specified current setting to a desired current setting.
[0018] The bracket has a base frame provided with two side arms. The two side arms in the bracket are formed integrally with the base frame and protruded outwardly from the base frame. The bracket has two protrusions extended respectively from the two side arms along an orthogonal direction.

[0019] The single torsion spring is secured to the bracket through a fastener such as a
pin. The single torsion spring is fastened to the two protrusions in the bracket. One end of the single torsion spring is inserted into a cavity in the calibration screw and other end of the single torsion spring is made to rest on a slider mounted on the release housing wall in the circuit breaker.
[0020] The double torsion spring is secured to the bracket through a fastener, wherein the fastener is a pin. The double torsion spring is fastened to the two side arms in the bracket. The open ends of the double torsion spring are made to rest on the bracket. The arm of the double torsion spring is made to rest on the moving magnet. The moving magnet is rotated along the axis of the bracket.
[0021] According to one embodiment of the present invention, a load adjustment mechanism with doubte spring arrangement is provided for magnetic calibration in molded case circuit breaker. In double spring arrangement load value of one spring is divided into two springs. Therefore load value requirement of each spring is reduced. Out of the two springs, one is with fixed load and the load of other spring is variable by a calibration screw. The arrangement is such that the combination after assembly takes care for a certain value of current threshold. This threshold is varied by varying the load value of variable spring by calibration screw. The combination of fixed & variable spring takes care for initial current threshold required which otherwise had to be adjusted by pretension or by varying load of only one spring. Lower load requirement of individual springs compared to that required for a single spring arrangement. Movement of magnetic setting knob in the breaker is softer due to less load requirement of variable spring.
[0022] In the present design, two springs are used for achieving equilibrium point of current threshold instead of one spring. Single spring load value is divided into a

combination of two. One is adjustable and the other is fixed. Fixed spring is designed to compensate for a specified current setting and adjustable spring is being used for adjustment from initial specified setting to desired current setting using a calibration screw. This brings down each individual spring torque requirement and also reduces the adverse effect of high torque springs.
[0023] When a current is passed through the current carrying conductor, the fixed magnet assembled with it tries to attract the moving magnet. The moving magnet touches the trip plate while moving and the movement of trip plate is used for tripping the breaker. The opposing force is provided by combination of two springs with one leg of variable spring resting on the adjustment screw of moving magnet for adjustment.
E) BRIEF DESCRIPTION OF THE DRAWINGS
[0024] 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:
[0025] F1G.1 illustrates a cross sectional view of a molded case circuit breaker
provided with a load adjustment mechanism, according to one embodiment of the present
invention.
[0026] F1G.2 illustrates a cross sectional view of a release assembly provided with a load adjustment mechanism, according to the embodiments of the present invention.
[0027] FIG.3 illustrates a perspective view of a load adjustment mechanism with a double spring arrangement for magnetic calibration in molded case circuit breaker, according to one embodiment of the present invention,

[0028] FIG.4 illustrates an exploded perspective view of a load adjustment mechanism with a double spring arrangement for magnetic calibration in molded case circuit breaker, according to one embodiment of the present invention.
[0029] FIG.5 illustrates a graph indicating the relationship between a deflection of the spring and the restraining force of a spring.
[0030] 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
[0031] 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.
[0032] The various embodiments of the present invention provide a load adjustment
system with a double spring arrangement for magnetic calibration in a molded case circuit breaker. According to one embodiment of the present invention, a bracket is mounted on a wall of release housing in a circuit breaker. A single torsion spring is secured to the bracket. A calibration screw is attached to the single torsion spring. A double torsion spring is secured to the bracket. A moving magnet is attached to the bracket. A fixed magnet is attached to a current carrying conductor in the circuit breaker.

[0033] The calibration screw is moved in forward direction or in reverse direction to achieve a desired combined force value of both the single torsion spring and the double torsion spring to set a trip threshold value of the circuit breaker. The calibration screw is moved in forward direction to increase the deflection of the single torsion spring to increase the opposing force provided by the single torsion spring to a magnetic attractive force between the fixed magnet and the moving magnet. The calibration screw is moved in reverse direction to decrease the deflection of the single torsion spring to decrease the opposing force provided by the single torsion spring to a magnetic attractive force between the fixed magnet and the moving magnet.
[0034] The double torsion spring is a fixed spring which is designed to provide a specified current setting. The single torsion spring is a variable spring, which, is designed, to adjust a current setting from the specified current setting to a desired current setting.
[0035] The bracket has a base frame provided with two side arms. The two side arms in the bracket are formed integrally with the base frame and protruded outwardly from the base frame. The bracket has two protrusions extended respectively from the two side arms along an orthogonal direction.
[0036] The single torsion spring is secured to the bracket through a fastener such as a
pin. The single torsion spring is fastened to the two protrusions in the bracket. One end of the single torsion spring is inserted into a cavity in the calibration screw and other end of the single torsion spring is made to rest on a slider mounted on the release housing of the circuit breaker.
[0037] The double torsion spring is secured to the bracket through a fastener, wherein the fastener is a pin. The double torsion spring is fastened to the two side arms in the bracket. The open ends of the double torsion spring are made to rest on the bracket. The arm

of the double torsion spring is made to rest on the moving magnet. The moving magnet is rotated along the axis of the bracket.
[0038] According to one embodiment of the present invention, a double spring
arrangement is provided fixed and variable loads for magnetic calibration in molded case circuit breaker. In double spring arrangement, the load value of one spring is divided into two springs. Therefore the load value requirement of each spring is reduced. Out of the two springs, one spring is provided with a fixed load while the load of other spring is varied by a calibration screw. The arrangement is such that the combination after assembly takes care for a certain value of current threshold. This threshold is varied by varying the load value of variable spring by calibration screw. The combination of fixed and variable spring takes care for initiai current threshold required which otherwise had to be adjusted by pretension or by varying load of only one spring. Lower load requirement of individual springs compared to that required for a single spring arrangement. The Movement of magnetic setting knob in the breaker is softer due to less load requirement of variable spring. In the double fix spring arrangement the current threshold for magnetic tripping is set by simultaneous pretension of 2 springs.
[0039] FIG. 1 illustrates a cross sectional view of a molded case circuit breaker provided with a load adjustment mechanism, according to one embodiment of the present invention. With respect to FIG.], the molded case circuit breaker includes a cable termination region 1, an arc quenching area including contacts region 2, a main mechanism region 3, a release assembly region 4 and a cable termination region 5. [0040] FIG.2 illustrates a cross sectional view of a release assembly provided with a load adjustment mechanism, according to the embodiments of the present invention while FIG.3 illustrates a perspective view of a load adjustment mechanism with a double spring arrangement for magnetic calibration in molded case circuit breaker, according to one embodiment of the present invention.

[0041] With respect to F1G.2-3, the load adjustment mechanism is provided with two springs 6, 8. One is torsion spring 6and other one is double torsion spring 8. These two springs 6, 8 are assembled on the bracket 10 using the pin 11 and pin 12 respectively. As shown in FIG.3, one leg of the spring 6 rests on the cavity of the calibration screw 7 and the other leg rests on the slider 16 mounted on release housing 17 as shown in FIG. 2. Moving magnet 9 is assembled with the bracket 10 using a pin 12 and is rotated along the axis of the bracket 10. Fixed magnet 14 as shown in FJG.2 is assembled with current carrying conductor 13 with the help of mounting holes and screws. Open legs of the spring 8 rests on the bracket and the other arm rests on the moving magnet 9. Spring 6 and spring 8 provides the necessary restraining force to the moving magnet 9 to overcome the magnetic force created by the fixed magnet 14 on actuation by current flowing through current carrying conductor 13.
[0042] To calibrate the magnetic release, certain value of current is passed through the heater 13 as shown in F1G.2. The fixed magnet 14 behaves like an electro magnet and tries to attract the moving magnet 9. If the force of attraction is greater than the opposing force provided by the spring 6 and spring 8, then the magnet 9 gets attracted towards a fix magnet 14 and touches the trip plate 15 and subsequently molded case circuit breaker (MCCB) goes into trip condition. Calibration screw 7 is adjusted in forward as well as reverse direction. When the screw is adjusted in the forward direction, the deflection of the spring 6 is increased thereby increasing the opposing spring 6 force values. If the screw 7 is adjusted in the reverse direction, the deflection of spring 6 is decreased thereby decreasing the opposing spring 6 force value. The adjustment of the screw 7 is done in the forward and reverse directions to achieve the desired combined force value of spring 6 and spring 8 thereby setting the tripping threshold of MCCB. All the three poles are subjected to the calibration separately.
[0043] FIG.4 illustrates an exploded perspective view of a load adjustment mechanism with a double spring arrangement for magnetic calibration in molded case circuit breaker,

according to one embodiment of the present invention. With respect to FIG.4, a single torsion spring 6 is attached to the bracket 10 with the help of pin 12. Double torsion spring 8 is assembled with the bracket 10 with the pin ] 1. Moving magnet 9 is assembled with the bracket 10 using the pin 12 and is rotates along the axis of the bracket 10. A calibration screw is coupled to the torsion spring 6.
[0044] FIG.5 illustrates a graph indicating the relationship between a deflection of the spring and the restraining force of a spring for both the single spring arrangement and double spring arrangement. Consider a single spring arrangement and a double spring arrangement equivalent to each other. Single spring arrangement consists of spring X having load value x, the load of which can be varied, which contributes 100% of the load requirement required for adjusting the magnetic threshold of the MCCB. Double spring arrangement consists spring XI & X2 having load values xl & x2 respectively. Load value of the spring are such that x = xl+x2. Also in the double spring arrangement the load is such that the XI contributes the maximum percentage of the load value say 80% of the required load which cannot be varied & is fixed. Then the load value of the other spring X2 will contribute for the remaining 20% of the total load required which can be varied to adjust the tripping threshold of the MCCB.
[0045] While doing the screw adjustment in the case of double spring arrangement, greater percentage of the load is shared by the fixed spring having higher load than variable one and adjustment is done using the other spring which is having comparatively lesser load value. This serves very precise adjustment in magnetic threshold due to very less spring rate as compared to the single spring arrangement having higher spring rate due to the 100% contribution by single spring. The change in restraining force verses change in deflection of spring is shown in FIG.5. The restraining force provided by spring arrangement is directly proportional to the current tripping threshold of MCCB.

[0046] When the change in restraining force and the change/variation in calibration readings in case of double spring arrangement are compared to that of single spring arrangement using the graph in F1G.5, it is well understood that the calibration readings in case of the later will be close to each other and consistent with less variation.
G) ADVANTAGES OF THE INVENTION
[0047] Thus the various embodiments of the present invention provide a load adjustment system with a double spring arrangement to providing fixed and variable loads for magnetic calibration in molded case circuit breaker. The adjustment of calibration screw from initial specified setting to desired current setting is easy. Variable as well as fixed current threshold can be achieved in same arrangement with the help of fixed and variable springs. The system of the present invention reduces sensitivity of calibration screw, provides consistent calibration readings and also reduces the calibration time. The system softens the operation of magnetic setting knob due to less load requirement of variable spring.
[0048] 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.
[0049] 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 load adjustment system for magnetic calibration in a molded case circuit breaker
system comprising:
A bracket mounted on a wall in a circuit breaker;
A single torsion spring secured to the bracket;
A calibration screw attached to the single torsion spring;
A double torsion spring secured to the bracket;
A moving magnet attached to the bracket; and
A fixed magnet attached to a current carrying conductor in the circuit breaker;
Wherein the calibration screw is moved in a forward direction or in a reverse direction to
achieve a desired combined force value of both the single torsion spring and the double
torsion spring to set a trip threshold value of the circuit breaker.
2. The system according to claim 1, wherein calibration screw is moved in the forward direction to increase the deflection of the single torsion spring to increase the opposing force provided by the single torsion spring to a magnetic attractive force between the fixed magnet and the moving magnet.
3. The system according to claim ls wherein calibration screw is moved in the reverse direction to decrease the deflection of the single torsion spring to decrease the opposing force provided by the single torsion spring to a magnetic attractive force between the fixed magnet and the moving magnet.
4. The system according to claim 1, wherein the double torsion spring is a fixed spring which is designed to provide a specified current setting.

5. The system according to claim 1, wherein the single torsion spring is a variable spring which is designed to adjust a current setting from the specified current setting to a desired current setting.
6. The system according to claim 1, wherein the single torsion spring is secured to the bracket through a fastener.
7. The system according to claim 6, wherein the fastener is a pin.
8. The system according to claim 1, wherein one end of the single torsion spring is inserted into a cavity in the calibration screw and other end of the single torsion spring is made to rest on a slider which is mounted on a release housing wall in the circuit breaker.
9. The system according to claim 1, wherein the double torsion spring is secured to the
bracket through a fastener.
10. The system according to claim 9, wherein the fastener is a pin.
11. The system according to claim 1, wherein the bracket has a base frame provided with two side arms.
12. The system according to claim 1, wherein the two side arms in the bracket are formed integrally with the base frame and protruded outwardly from the base frame.
13. The system according to claim 1, wherein the bracket has two protrusions extended respectively from the two side arms along an orthogonal direction.
14. The system according to claim 1, wherein single torsion spring is fastened to the two
protrusions in the bracket.

15. The system according to claim 1, wherein the double torsion spring is fastened to the two
side arms in the bracket.
16. The system according to claim 1, wherein the open ends of the double torsion spring are made to rest on the bracket.
17. The system according to claim 1, wherein the arm of the double torsion spring is made to rest on the moving magnet.
18. The system according to claim 1, wherein the moving magnet is rotated along the axis of the bracket.
19. The system according to claim 1, wherein the bracket is mounted on a wall in a housing which stores a release unit.