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: MOULDED CASE CIRCUIT BREAKER THERMAL OVERLOAD RELEASE USING SNAP ACTION DISC

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 particularly relates to the field of providing snap action disc for thermal overload release of moulded case circuit breaker (MCCB) using different spring forces to trip at different current settings.

BACKGROUND OF THE INVENTION

A thermal release is a device which senses the temperature rise of the switchgear device. It acts mechanically and delivers the force on the trip lever to break the current flowing through the device. Circuit Breaker is a mechanical switching device, capable of switching ON, carrying and breaking (i.e. switching OFF) current under normal circuit conditions and more specifically breaking current (Tripping) during abnormal circuit conditions such as short-circuit. An overload release is used to sense the overload current and hence trip the breaker on that current.

Conventionally, thermal release consists of bimetal and heater which acts as cantilever system and hit the trip plate to cause tripping. Present invention discloses snap action bimetal loaded with the spring which causes tripping. The trip time characteristics depend on the bimetal characteristics as well as loading of the spring.

US 3852697 discloses a bimetal snap disc having a positive deflection curve both above and below a predetermined temperature. The disc is formed to provide one or more operating temperatures both above and below the predetermined temperature. The disc is being operated at one temperature while being incapable of resetting in response to thermal stresses for use in manually resetting devices.

US 7024940 discloses apparatus and method for measuring the threshold temperature of the bimetallic actuator during transit between bi-stable states. The method includes forming a bimetallic disc having a mobile center portion surrounded by immobile peripheral portion.

However, above mentioned patent applications do not disclose the feature of utilizing different spring forces to trip at different current settings such as but not limited to 0.5In, 0.8In and 1In. Further, the present invention is very simple to assemble compared to the mentioned applications. It provides high actuating force compared to conventional cantilever design and it is easy to replace on the site. There is no need of heater because the terminal itself acts as heater which is a true indication of overload. Also additional component and further process such as spot welding is avoided. Due to all these features, the present invention helps in reducing total overall temperature rise of the breaker and accordingly adding huge thermal margins in the overall product.

OBJECTS OF THE INVENTION

The main object of the invention is to provide thermal release using snap action disc which works for variable set points such as but not limited to 0.5In, 0.8In and 1In of the Breakers rated current.

It is further an object of the invention to provide very simple assembling and high actuating force on the trip lever / plate which further trips the breaker compared to conventional cantilever design.

It is yet another object of the invention to remove the need of heater, additional components and spot welding which helps in reducing total overall temperature rise of the breaker and adding huge thermal margins in the overall product.

It is another object of the invention to provide precise tripping timing, easy replacing on site and repeat accuracy compared to conventional cantilever design.

SUMMARY OF THE INVENTION

The present invention is to get the variable temperature for tripping at various multiple of rated current. As the load in the snap action bimetal disc increases more via spring, the snap action bimetal disc trips on the higher temperature. This characteristic can be used for the thermal release of MCCB either for single phase or for three phase (per pole combination). The snap action bimetal disc is placed on the current carrying terminal. The spring is located by pin and held by spring retainer. When MCCB is loaded with 0.5In, 0.8In and 1In current settings, the force on the spring will be higher accordingly. As the overload condition occurs, the terminal temperature increase which in turn increases the temperature of the bimetal through conduction and convection of heat. The total travel for the tripping action is governed by the flipping distance of the bimetal. The difference in the temperature rise is directly proportional to the loading by the spring. If the force is more, the bimetal will take longer time to trip and vice versa.

DETAILED DESCRIPTION OF DRAWINGS

Figure 1 shows the position and shape of the disc in ON position of MCCB.
Figure 2 shows the position and shape of the disc in OFF position of MCCB.
Figure 3 shows an exploded view of the components used in the invention.
Figure 4 shows the loading of the bimetal with the spring.
Figure 5 shows tripping temperature of bimetal Vs Overload setting Characteristics.

DETAILED DESCRIPTION OF THE INVENTION

The main principal of the present invention is to get tripping at the various setting using snap action disc. As the bimetal is loaded more via spring, it trips on the higher temperature. This characteristic can be used for the thermal release of MCCB either for single phase or for three phase per pole combination.

Figure 1 shows the position and shape of the disc in ON position of MCCB which is of convex shape and Figure 2 shows the position and shape of the disc in OFF position of MCCB which is of concave shape.

Figure 3 shows the exploded view of the components being used in the present invention. Terminal (1) is the current carrying conductor on which the snap action disc bimetal (2) is placed. The snap action disc bimetal toggles on overload which is in turn acts as sensing unit for overload condition. Pin (3) guides the spring to load the bimetal disc. Spring (4) is a compression spring and Spring Retainer (5) avoids the buckling of spring.

When the current is flowing through the device, the release is in ON condition. The temperature of the bimetal is not sufficient to flip the bimetal. As the overload condition occurs, the terminal temperature increase which in turn increases the temperature of the bimetal through conduction and convection of heat. The total travel for the tripping action is governed by the flipping distance of the bimetal. The difference in the temperature rise is directly proportional to the loading by the spring (4). If the force is more, the bimetal (2) takes longer time to trip and vice versa.

As shown in Figure 4, bimetal (2) is loaded with the spring (4) to work for the different current settings. Spring (4) is located by pin (3) and held by spring retainer (5). When MCCB is loaded at 0.5In setting, spring is loaded with force, X. When MCCB is loaded at 0.8In setting, bimetal is loaded with force, Y and further when MCCB is at 1In setting, bimetal is loaded with force, Z, where X < Y< Z.

Figure 5 shows the characteristic of tripping temperature Vs overload setting in connection to the loading of bimetal with the spring for different current settings. Tripping temperature of B setting is more than tripping temperature of A setting. Tripping temperature of C setting is more than tripping temperature of B setting.

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. MCCB THERMAL OVERLOAD RELEASE USING SNAP ACTION DISC comprises of terminal (1), snap action bimetal disc (2), pin (3), spring (4) and spring retainer (5);
wherein the snap action bimetal disc (2) is placed on the current carrying terminal (1);
wherein the spring (4) is loaded to the snap action bimetal disc (2) through pin (3);
wherein buckling of spring is avoided through spring retainer (5);
wherein the snap action bimetal disc (2) trips on the higher temperature as per the load upon it via the said spring (4);

2. MCCB THERMAL OVERLOAD RELEASE USING SNAP ACTION DISC as claimed in claim 1, wherein the said snap action bimetal disc (2) is of convex shape, when MCCB is in ON condition and snap action bimetal disc (2) is of concave shape, when MCCB is in OFF condition.

3. MCCB THERMAL OVERLOAD RELEASE USING SNAP ACTION DISC as claimed in claim 1, wherein the spring force is different for different current settings of MCCB.

4. MCCB THERMAL OVERLOAD RELEASE USING SNAP ACTION DISC as claimed in claim 1, wherein the temperature of the terminal (1) increases at the overload condition which in turn increases the temperature of the snap action bimetal disc (2) causing it to trip.

5. MCCB THERMAL OVERLOAD RELEASE USING SNAP ACTION DISC as herein substantially described with foregoing drawings and description.