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: VARIABLE OVERLOAD PROTECTION FOR MCCB

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 generally to the overload protection for Circuit Breaker with variable setting. More particularly, the invention is concerned about a system for variable overload protection in MCCBs.

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.

The above mentioned 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 bimetal got deflected and trip the breaker, side view of the conventional method is shown in Fig. 2, 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.

With the conventional method the variable tripping property depends upon the trip plate profile only and it is not possible to give different variable overload protection to different pole if it is required for certain application, profile remains same for all the three pole in trip plate which will not allow variable overload protection to give above feature.

Here are some Patents on magnetic tripping system for short circuit protection. All uses conventional method for such arrangement.

US 4930038 discloses a current overload tripping device having a time-delayed leading tripping action during uneven loading of the phases of a multiphase circuit. The tripping device includes a current pickup for each phase wherein the current pickup members are connected in series with one another and with an electromagnetic converter. Each of the current pickup members comprises a winding surrounding a magnet yoke of an electromagnetic tripping device of each phase and the electromagnetic converter comprises a temperature/time-dependent device operating to provide a time-delayed tripping signal.

US 4347494 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 4990882 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 actuated by the push button and a bimetal tripping device for releasing the switch latch with the bimetal tripping device including a self-heated approximately U-shaped punched bimetal which is connected electrically in series in the current path through the switch. The one free arm end of the bimetal is fastened to the end of a connecting lug fixed in the interior of the switch housing. Its second free arm end is fastened to a connecting piece leading to a mating contact that is fixed to the housing. The base is formed by the deflectable end of the bimetal which is kinematically connected with the switch latch. Between the connecting flap of the connecting piece for the second arm end of the bimetal and the region of the connecting piece adjacent the mating contact that is fixed to the housing, the connecting piece is provided with a constriction formed by a slit passing through the plate-shaped central section of the connecting piece transversely to the plane of the plate. The connecting flap is rotatable, in order to adjust the response value of the bimetal around the constriction in that an adjusting screw charges the central section from the bottom face of the switch housing.
US 5184269 discloses an overload protective device 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 though 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.

These and other limitations and needs of the conventional bimetallic arrangement necessitate an improved bimetal arrangement system. Such an arrangement has been disclosed here which overcomes the limitations over and above the improvements and flexibility provided additionally.

The present inventors have found that a unique sliding arrangement can be introduced in a conventional bimetal arrangement of a circuit breaker which could provide variable overload protection.

OBJECTS OF THE INVENTION

A basic object of the present invention is to overcome the disadvantages of the prior art.

One object of the present invention is to provide a system for variable overload protection in MCCBs.

Another object of the present invention is to provide variable overload protection which will be more reliable.

Another object of the present invention is to provide variable overload protection with which variable setting from pole to pole can be provided.

These and other advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.

SUMMARY OF THE INVENTION

One aspect of the present invention is to provide a system for variable overload protection in moulded case circuit breakers, said system comprising:

a current carrying part having two arms of unequal length having one longer arm and other shorter arm uniting with each other to form a substantially u-shaped;
a bimetallic strip means having a predetermined active length allowed for deflection being fixed to said longer arm of said current carrying part at its one end while other end of said strip means comprising actuator means for hitting trip plate means of the circuit breaker;
a slider means wherein said slider means being engaged with said strip means such that sliding motion of said slider means varies said active length of said strip means;
a carrier element;
wherein said element being adapted to hold said slider means such that the slider means movable in a substantially vertical direction;
wherein said longer arm of said current carrying part comprising a slot means having length substantially equal to said strip means so as to accommodate said slider means and said carrier element in a manner that vertical movement of said carrier element being restricted, rotation of said carrier element allows said slider means to be vertically adjustable at various length position thereby providing various active lengths for said bimetallic strip.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to protection of circuit breakers at variable overload conditions. In the present description system has been described which could protect circuit breakers at various overload conditions.

One embodiment of the present invention describes the system comprising a current carrying part having two arms of unequal length. The arm having greater length is engaged with a bimetallic strip and units with the other shorter arm to form a substantially u-shaped profile. The bimetallic strip means has a predetermined active length allowed for deflection is fixed to said longer arm at its one end while other end of said strip means comprising actuator means for hitting trip plate means of the circuit breaker. The actuator means comprises a screw.

A slider means comprising a L-shaped collar is engaged with the bimetallic strip and the sliding motion of said slider means effectively varies the active length of said strip means. One end of the slider means which is accommodated in a slot provided in the longer arm is designed so as to move vertically upward and downward. The end is provided with hole having thread which helps the slider means to move on a carrier element which is also threaded. The carrier element comprises a screw which is equal in length to the length of the slot and therefore during rotation of the carrier element its vertical movement is restricted which allows the slider to only move vertically in upward and downward direction and position itself at certain lengths of the screw to provide variable active length of the bimetallic strip to deflect from its actual position.

Following are the features and benefits of the invention

• Main objective/benefit of the present invention is to give variable overload protection which will be more reliable.
• Variable thermal tripping, by adjusting the screw only which give accurate output because the location of the collar depend upon the pitch of thread in screw and collar which will be fixed.
• Different variable thermal tripping for different pole because it not depends upon the trip plate profile and possible to give different setting to each pole.
• Simple trip plate profile, and many number of setting is possible.
• No two poles depends upon each other settings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Other features as well as the advantages of the invention will be clear from the following description.
In the appended drawing:

Figures 1 and 2 illustrate conventional bimetallic strip arrangement.
Figure 3 illustrates isometric view of the system of the present invention.
Figure 4 illustrates exploded view of the system of the present invention
Figures 5 to 7 illustrate different positions of the slider in the present invention.
Figure 8 illustrates current carrying part of the system in the present invention.

DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Variable overload tripping system disclosed herein is shown in Fig 3. 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. 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 of collar 4 and screw 5.

Fig. 3 shows the 3D view of the arrangement, Fig. 4 shows the exploded view of the arrangement with 1 as current carrying part or heating part , Fig. 5 shows side view of the present invention.

The working of the invention is shown in fig. 3, 6 and 7. The same is explained below. In Fig. 2 the arrow indicates direction of deflection of the bimetal. The deflection direction will remain same for rest of the figure. As the current flows through the heater 1, heat got generated, this generated heat will pass through 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 it i.e. which is free to move or deflect, as we decrease the length the deflection got reduced for same amount of heat passed to it 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
F - Force

It is clear from the formula that deflection is directly proportional to the active length, as active length increase deflection at the tip increase.

Fig. 3 shows the arrangement where collar is at the end of bimetal where it is brazed, in this case active length of the bimetal is maximum and the bimetal will deflect maximum which give one setting say of 80 percent of rated current, know to change the setting that is to keep the bimetal deflection constant for higher current i.e. say at 90 percent of rated current, the active length of the bimetal is reduced because higher current produce higher heat results in higher deflection so by reducing the active length the deflection can be maintained constant for that current, to achieve this an arrangement is made with the help of collar 4 and screw 5, as screw got rotated whose vertical movement is restricted, it will lift the collar up which is engaged with screw through internal thread, as the collar moves up it will restrict the certain portion of bimetal to deflect so that portion will act as fixed to the heater which results in reduction of active length of the bimetal and the deflection will remain same for high heat generated, this condition is shown in Fig. 6, similarly is case for another rating where at 100 percent the requirement of deflection is same, the active length is to be further reduced by making the collar to move up with a rotational motion of screw as shown in Fig. 7. Likewise it is possible to give multiple rating which solves the purpose of variable overload protection.

To accommodate the present invention a slot has been created on the heater which is shown in Fig. 8, in that slot the combination of collar and screw is inserted and then the active length is maintained by the rotation of screw which results in translation or vertical movement of collar.

In present invention the variable overload tripping feature is configured with ease by only tightening the screw and changing the position of collar or active length of the bimetal. The present invention adds the feature of giving different setting in different pole if it is required in certain application, like 80 percent in one pole 90 percent in other and 100 percent in third pole i.e. independency of different pole on each other.

Thus the above disadvantages of the prior methods can be avoided and the present overload arrangement can be used for wide range of application.

In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and illustrate the best mode presently contemplated for carrying out the invention. Further functioning of the mechanism has been discussed below to describe the way the mechanism operates. However, such description should not be considered as any limitation of scope of the present mechanism. The structure thus conceived is susceptible of numerous modifications and variations, all the details may furthermore be replaced with elements having technical equivalence. In practice the materials and dimensions may be any according to the requirements, which will still be comprised within its true spirit.

WE CLAIM

1. A system for variable overload protection in moulded case circuit breakers, said system comprising:

a current carrying part having two arms of unequal length having one longer arm and other shorter arm uniting with each other to form a substantially u-shaped;
a bimetallic strip means having a predetermined active length allowed for deflection being fixed to said longer arm of said current carrying part at its one end while other end of said strip means comprising actuator means for hitting trip plate means of the circuit breaker;
a slider means wherein said slider means being engaged with said strip means such that sliding motion of said slider means varies said active length of said strip means;
a carrier element;
wherein said element being adapted to hold said slider means such that the slider means movable in a substantially vertical direction;
wherein said longer arm of said current carrying part comprising a slot means having length substantially equal to said strip means so as to accommodate said slider means and said carrier element in a manner that vertical movement of said carrier element being restricted, rotation of said carrier element allows said slider means to be vertically adjustable at various length position thereby providing various active lengths for said bimetallic strip.

2. System as claimed in claim 1 wherein said carrier element is a screw having length substantially equal to the slot length.

3. System as claimed in claim 1 wherein said slider means comprising a collar.

4. System as claimed in claim 1 wherein said collar is substantially L-shaped.

5. System as claimed in claims 3 and 4 wherein said collar comprising hole having internal thread to co-operate with said carrier element so as to move in vertical directions.

6. System as claimed in claim 1 wherein said slider means being adapted to provide constant deflection at high rated currents.

7. A system for variable overload protection in moulded case circuit breakers as herein described and illustrated with reference to accompanying figures.