FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10; rule 13)
1. Title of the Invention. - VARIABLE OVERLOAD TRIPPING SYSTEM FOR MOULDED CASE CIRCUIT BREAKER HAVING BIMETALLIC ARRANGEMENT
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:

TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to a moulded case circuit breaker (MCCB) and more particularly to a variable overload tripping system for moulded case circuit breaker having bimetallic arrangement.
BACKGROUND AND THE PRIOR ART
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. For example, these positions would include an ON position, in which the separable contacts are closed, an OFF position in which the contacts are open. In a few switching device there could be a third state which would include tripped position in which the contacts are tripped open. One such switching device is a moulded case circuit breaker (MCCB) which is generally used to provide over current protection for various types of electrical equipment. MCCBs have three stable positions - ON, OFF and TRIP in which conditions of contacts as described above.
When circuit breaker is in ON condition and a overload fault condition arises, large amount of current flows through the circuit and in which case, 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. So to keep the same trip time variable overload protecting system is required.
US 4,763,768 discloses a bi-directionally effective overload protection device for a rotation machine part includes two coaxially arranged, mutually juxtaposed ring discs frictionally connected to a machine; a gear disc with outer gearing and inwardly directed flange means circumscribes the ring discs; a plurality of springs hold said gear disc normally in particular position in relation to the ring discs, permitting the gear disc to rotate relative to the ring discs upon occurrence of an overload in either directional rotation; a dual switching ring having an inner and an outer ring portion is connected to the second ring disc through resilient bar or leaf spring portions of the switching ring; actuator ramps on the outer and inner ring portions cooperate with axially extending cams on the gear disc so that either the inner or the outer ring is axially shifted on overload in one or the other direction; switches are arranged in relation to the inner and outer ring portion for being actuated by the particular one that is axially shifted on occurrence of overload.
US 4,347,494 discloses an overload protection switch including a narrow housing, bimetal strips mounted in the housing, a pushbutton mounted in, and projecting out of, the housing and made of insulating material, the pushbutton being provided with a partition and being movable relative to the housing between an on position in which a conductive path is established via the strips and an off position in which such conductive path is broken, and a

spring device urging the pushbutton into its off position, the housing and bimetal strips are physically symmetrical to the longitudinal center axis of the housing, the path of movement of the pushbutton is along that center axis, there are two identical bimetal strips disposed symmetrically relative to the center axis, each strip having a first part fastened to the housing and a second part which is movable relative to the housing in response to temperature changes, with the second parts converging toward one another while remaining out of contact, the pushbutton carries a conductive contact piece at a location such that in the on position of the pushbutton the contact piece is wedged between the strips and is held there against by the force of the spring device and in the off position of the pushbutton the partition is disposed between the strips.
US 4,990,882 discloses a push button actuated excess current protection switch, particularly an on-board electrical system protection switch with manual actuation and bimetal controlled automatic tripping includes a switch latch (7) actuated by the push button (41) and a bimetal tripping device for releasing the switch latch (7) with the bimetal tripping device including a self-heated approximately U-shaped punched bimetal (101) which is connected electrically in series in the current path through the switch.
US 5,184,269 discloses an overload protective device to the disposed in an electric circuit serving to supply current to a load has a pair of fixed contacts provided inside of a case and an inversible disk-like bimetal of a curved shape having a pair of movable contacts capable of coming in contact with the fixed contacts, respectively. A shaft is fixed to the case at one end thereof and formed with a head portion at the free end portion. The shaft extends through a hole formed in the central portion of the bimetal. When the

bimetal breaks, a circuit breaker breaks the electric circuit permanently to thereby prevent the load and the overload protective device from being burnt out.
The above mentioned prior art discloses conventional Bimetallic tripping arrangement for overload protection. The drawbacks of the above mentioned prior art is that overload protecting mechanism i.e. bimetallic arrangement requires either direct heating or indirect heating, for that it require either to carry current or to get assembled with current carrying part which can transfer heat to the bimetal to deflect it, the conventional method as shown in Fig. 1, where bimetal 2 is brazed with heater 1 and is example of indirect heating, the screw 3 hits the trip plate when bimeta! got deflected and trip the breaker, in most of the application where high current flows indirect heating of bimetal is used. 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.
Thus there is a need to provide an improved bimetal arrangement. Such an arrangement has been disclosed in the present invention which overcomes the limitations over and above the improvements and flexibility provided additionally. Present invention provides an improved bimetal arrangement system in MCCB adapted for easy adjustment to provide variable rating.
OBJECTS OF THE INVENTION
A basic object of the present invention is to overcome the disadvantages/drawbacks of the known art.

Another object of the present invention is to provide an improved bimetal arrangement in MCCB adapted for variable thermal overload protection.
Yet another object of the present invention is to provide an improved bimetal arrangement system in MCCB adapted for easy adjustment to provide variable rating.
These and other advantages of the present invention will become readily apparent from the following detailed description read in conjunction with the accompanying drawings.
SUMMARY OF THE INVENTION
There is provided a variable thermal overload protection for Circuit Breaker having variable setting.
One embodiment of the present invention provides for an improved bimetal arrangement in MCCB adapted for variable thermal overload protection.
Other embodiment of the present invention provides an improved bimetal arrangement system in MCCB adapted for easy adjustment to provide variable rating.
Additionally the present invention provides for a reliable protection. It provides easy adjustment for variable rating.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 illustrates the Variable overload tripping system (conventional).
Figure 2 illustrates the side view of Variable overload tripping system (present).
Figure 3 illustrates the exploded view of the bimetal arrangement in Variable overload tripping system.
Figure 4 illustrates the side view with both the screws tight.
Figure 5 illustrates the arrangement with one of the two the screws tight.
Figure 6 illustrates the arrangement with none of the two screws tight.
DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The following drawings are illustrative of particular examples for enabling methods of the present invention, are descriptive of some of the methods, and are not intended to limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description.
Reference is first invited to Fig 1 where the conventional variable overload tripping system is shown. The screw (3) hits the trip plate when bimetal (2) get deflected and trip the breaker.

Fig 2 shows the side view of Variable overload tripping system of the present system. It consist of a current carrying part (1) i.e. heater, a bimetal (2) brazed to the heater at the end to receive heat through conduction, a screw (3) to hit trip plate for tripping the breaker.
Fig 3 shows the exploded view of the bimetal arrangement in Variable overload tripping system. Different elements Heater (1), Bimetal (2), Screw (3,4,5) and metal insert (6).
Fig 4, 5 & 6 shows the side view of the arrangement when both the screws are tight, one of the two the screws tight, none of the two screws tight respectively.
The invented system is thus a variable overload tripping system for moulded case circuit breaker having bimetallic arrangement.
DETAILED DESCRIPTION OF THE INVENTION
Accordingly the present invention provide for overload tripping. Variable overload tripping system disclosed herein is shown in Fig 2. It consist of a current carrying element/part 1 i.e. heater, a bimetal 2 brazed to the heater at the end to receive heat through conduction, a screw 3 to hit trip plate for tripping the breaker.
Two more adjusting means being screws 4 and 5 are configured in the present invention to provide variable thermal overload protection. In present invention there is no need to give a special profile to the trip plate because in this invention variable tripping is nothing to do with trip plate here it is

achieved by changing the active length of bimetal with the help screw 4 and 5. Metal insert 6 with internal threads is shown in Fig. 3 against which the screw 4 and 5 are tighten.
Fig. 2 shows the 3D view of the arrangement, Fig. 3 shows the exploded view of the arrangement with 1 as current carrying part or heating part, Fig. 4 shows side view of the present invention.
The working of the invention is shown in fig. 4, 5 and 6. The same is explained below.
As the current flows through the heater 1, heat gets generated, this generated heat passes to the bimetal through conduction results in increase in temperature of bimetal and as per the bimetal property it will get deflected in the particular direction. It is known that deflection of bimetal depends mainly upon the active length of bimetal which is also shown in the formula below which has been used for bimetal deflection calculation.

Where, A - Deflection at tip a - Specific Deflection L - Active length b - Width s - Thickness E - Young's Modulus T - Bimetal Temperature

To- Ambient Temperature
It can be seen in Fig. 4 that both the screw 4 and 5 are tight i.e. holding the bimetal in vertical position and not allowing that particular portion of bimetal to deflect, so the overall active length is only above the upper screw, as the current flow through the heater 1 it gets heated and passes a portion of heat to the bimetal because of which it gets deflected, so in Fig 4, the active length is less so the bimetal require more heat to get deflected to a particular value, let's give this setting as 100 percent of rated current, so in this case the bimetal tip travel specific distance, know to provide variable rating it has to be kept in mind that the travel of tip of bimetal will remain same, so let's say it has to be given a setting of 90 percent of the rated current in which deflection has to be same.
As the current value decreases the heat generated also gets down and heat reaching the bimetal reduces, to maintain the same deflection with less heat, bimetal active length has to be increased as it is clear from the formula above, so to achieve this the bimetal is relieved from screw 4 by loosening it as shown in Fig. 5, by doing so the active length got increased and it will compensate for the reduction of heat and give same deflection so tripping time will remain same, similarly if the rating has to reduce further to say 80 percent of rated current the same can be achieved by loosening the screw 5 which increases the active length further. This condition is shown in Fig. 6.
The additional feature added to achieve this is two collinear holes on heater and bimetal added with screw and metal insert, by doing so with just rotating the screw the variable thermal overload protection is achieved. The advantage of metal insert is to give proper threading to engage the screw

which incase of copper heater and bimetal is not feasible, because the copper being the soft material the thread will wash out. So the deflection required is adjusted by location of screw only and can be used for wide range of application.
Although the embodiments herein are described with various specific embodiments, it will be obvious for a person skilled in the art to practice the embodiments herein with modifications. However, all such modifications are deemed to be within the scope of the claims.
It is also to be understood that the following claims are intended to cover all of the generic and specific features of the embodiments described herein and all the statements of the scope of the embodiments which as a matter of language might be said to fall there between.

WE CLAIM
1. A variable overload tripping system for moulded case circuit breaker, said system comprising:
a current carrying element (1);
a bimetallic strip means (2) joined at its one end to said current carrying element (1) in a manner to receive heat conducted from said current carrying element when current flows through said current carrying element (1);
plurality of adjusting elements engaged with said current carrying element (1) and strip means (2) wherein said adjusting elements is adjusted such that active length of said bimetallic strip means (2) increased or decreased corresponding to the increment or decrement in the rated current flowing through said current carry element (1).
2. System as claimed in claim 1 wherein said adjusting elements comprising screw means (4, 5).
3. System as claimed in claim 1 further comprising plural metal insert means (6) adapted to tighten said adjusting elements (4, 5).
4. System as claimed in claim 3 wherein said metal insert means (6) are threaded.
5. System as claimed in claim 1 further comprising screw means (3) for tripping.
6. System as claimed in claim 1 wherein said bimetallic strip means (2) joined to said current carrying element (1) by means of brazing.

7. A variable overload tripping system for moulded case circuit breaker as herein substantially described and illustrated with reference to the accompanying drawings.