Claims:1. A calibration screw adapted to be used in a circuit breaker, said screw comprising:
a threaded cylindrical body;
a first end and second end located at opposite ends of the threaded cylindrical body;
wherein the first end is a tip; and
wherein the second end is a polygonal slot.

2. The calibration screw as claimed in claim 1, wherein the tip is a convex tip.

3. The calibration screw as claimed in claim 1, wherein the polygonal slot is preferably hexagonal.

4. The calibration screw as claimed in claim 2, wherein a gear head is located in near the convex tip.

5. The calibration screw as claimed in claim 4, wherein the gear head comprises a plurality of teeth.

6. The calibration screw as claimed in claim 5, wherein the plurality of teeth is a screw driver engaging surface.

7. The calibration screw as claimed in claim 1, wherein the cylindrical body is a single point of contact with a bimetal of a circuit breaker.

8. The calibration screw as claimed in claim 1, wherein the screw is threaded in the bimetal until the gear head emerges on the other end of bimetal.

9. The calibration screw as claimed in claim 1, wherein final calibration and position of the screw is arrested by applying sealant in a cavity formed between the threads of screw and bimetal.

10. The calibration screw as claimed in claim 1, wherein the calibration screw is preferably manufactured by insert molding process.

11. The calibration screw as claimed in claim 1, wherein the calibration screw is optionally manufactured by casting process.

12. The calibration screw as claimed in claim 1, wherein the calibration screw is rotated throughout 360 degrees of screw rotation.

13. The calibration screw as claimed in claim 2, wherein the convex tip is preferably spherical in shape.

14. The calibration screw as claimed in claim 13, wherein the diameter of the convex tip is about 3mm.

15. The calibration screw as claimed in claim 5, wherein the number of gear teeth are about 12.

16. The calibration screw as claimed in claim 1, wherein trip time is set by calibrating the screw for multiple iterations.

17. The calibration screw as claimed in claim 9, wherein there is colour coding at every quarter turn of the screw for discerning the rotated calibration turns to determine the final calibration.

18. A circuit breaker comprising:
a molded plastic case;
a means for connecting the said circuit breaker to an external electrical power source and an external electrical load;
a thermal release comprising of a bimetal element welded to a current carrying conductor;
a calibration screw threaded into the bimetal that exerts force on a tripping element to trip the circuit breaker;
wherein the calibration screw further comprises a threaded cylindrical body, a first end accessible from outside of said molded plastic case and a second end located opposite to the first end;
wherein the first end is convex tip and a gear head located near the convex tip with a plurality of teeth to engage with a screw driver; and
wherein the second end is a hexagonal slot.

19. The circuit breaker as claimed in claim 18, wherein rotating the gear head pushes the tripping element.

20. The circuit breaker as claimed in claim 18, wherein the threaded portion of the calibration screw engages with the bimetal for robust coupling.

21. The circuit breaker as claimed in claim 18, wherein the gear head is for adjusting the number of turns of the calibration screw.

22. The circuit breaker as claimed in claim 18, wherein the calibration screw is rotated throughout 360 degrees of screw rotation.

23. The circuit breaker as claimed in claim 18, wherein the number of gear teeth are about 12.

24. The circuit breaker as claimed in claim 18, wherein the convex tip is preferably spherical in shape.

25. The circuit breaker as claimed in claim 24, wherein the diameter of the convex tip is about 3mm.

26. The circuit breaker as claimed in claim 18, wherein the trip time is set by calibrating the screw for multiple iterations.

27. The circuit breaker as claimed in claim 18, wherein final calibration and position of calibration screw is arrested by applying sealant in the cavity formed between the threads of calibration screw and bimetal.

28. The circuit breaker as claimed in claim 27, wherein the final calibration is determined by a colour coding at every quarter turn of the screw for discerning the rotated calibration turns.
, Description:FIELD OF THE INVENTION

[001] The subject matter of the present invention, in general, relates to thermal calibration of circuit breakers and particularly, pertains to thermal calibration of circuit breakers by a calibration screw for overcurrent protection.

BACKGROUND OF INVENTION

[002] A circuit breaker is an automatically operated electrical switching device that protects electrical circuits from damage owing to excess flow of current typically resulting from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected. Unlike a fuse that operates only once and needs to be replaced when blown, a circuit breaker is a resettable device (either manually or automatically) to resume normal operation.

[003] Molded Case Circuit Breakers (MCCBs) provide overcurrent 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.

The MCCBs are typically installed in a distribution load center with other like breakers. Usually multiple MCCBs have a single load center which consists of a sheet metal enclosure with a hinged door that allows access to the face of the enclosed MCCBs. The circuit breakers are secured within an inner sheet metal panel. Electrical busses and conductor raceways are located beneath this inner panel. The MCCBs generally include a molded case main breaker supplying at least one common bus located within the load center.

[004] An MCCB provides protection by combining a temperature sensitive device with a current sensitive electromagnetic device. Both these devices act mechanically on the trip mechanism. The MCCBs involve thermal and magnetic release that detect the overcurrent in fault condition and provides a trip command to the operating mechanism to interrupt the circuit. The thermal release consists of a bimetal element that is heated directly and indirectly when the current passes through the circuit breaker. When a current whose value is over the rated value passes through the circuit breaker, the bimetal gets deflected. The thermal release also consists of a tripping mechanism that provides the mechanical trip command to the circuit breaker. In overcurrent condition, the bimetal deflects and pushes the tripping mechanism to trip the breaker. The tripping is achieved when the calibration screw coupled with the bimetal exerts the force on the tripping mechanism.

[005] The circuit breakers are usually calibrated at a current higher than the rated current to not down the trip times for achieving the desired trip times. The calibration of the circuit breaker is facilitated by the adjusting the calibration screw that in turns changes the gap between the bimetal and the tripping mechanism. The calibration process is usually iterative and requires continuous calibration and recalibration to achieve the desired trip times. The circuit breakers are tested and calibrated in a specific test setup that passes the calibration current through the phases of the circuit breaker for a particular period of time.

[006] Reference is made to US 6486759 B2 wherein a molded case circuit breaker that includes double headed calibration screw is disclosed. The calibration screw has a first and second end with identical convex tip surfaces and screw driver engagement surfaces. The convex tip surface is suitable for engaging a bimetal thermal overload element. The screw driver engagement surface is suitable for applying screw driver to adjust the deflection of the bimetal and calibrate the thermal overload protection.

[007] Reference is also made to EP 0175976 A2 wherein a circuit breaker with thermal trip means including an elongate bimetallic element and adjusting means therefor are disclosed. The adjusting means comprises a lever that is rigidly connected to the bimetallic element adjacent the supported end thereof, and an adjusting screw which cooperates with the lever and is manually operable to adjust the lever and through its rigid connection with the bimetallic element. The stress introduced in the bimetallic element during adjustment is confined to the supported and inactive end portion of the element, whereas the active portion remains unstressed.

[008] In the conventional MCCBs, after every iteration of calibration, the circuit breaker needs to be re-calibrated by rotating the calibration screw to a particular angle. The calibration screw has two ends, namely, the convex tip that pushes the tripping mechanism and a hexagonal slot. Presently, the calibration screw is adjusted by inserting a specific tool in the hexagonal slot and is rotated by an angle. In case of modular circuit breakers, the said process becomes tedious and time consuming as after every calibration iteration, the thermal release needs to be dissembled and the screws must be rotated with the help of dedicated tools. There also remains an issue that the thread present on the calibration screw does not provide enough grip for proper rotation by a screw driver. Moreover, the said method of calibration does not provide an access to calibrate the thermal release, when it is integrated with the breaker, especially in case of modular circuit breakers.

[009] Therefore, there is a need for calibration screw that provides consistent calibration adjustment and can be calibration in different orientations in an MCCB to do away with drawbacks of the conventional calibration screws and methods for calibrating modular circuit breakers.

SUMMARY OF THE INVENTION

[0010] The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the present invention. It is not intended to identify the key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concept of the invention in a simplified form as a prelude to a more detailed description of the invention presented later.

[0011] An object of the present invention is to provide a calibration screw for consistent calibration adjustment.

[0012] Another object of the present invention is to provide a calibration screw that calibrates in a plurality of orientations.

[0013] Another object of the present invention is to provide a calibration screw that will reduce the calibration time, improve manufacturing speed and improve calibration adjustment of molded case circuit breakers.

[0014] Yet another object of the present invention is to provide a calibration screw comprising of a threaded cylindrical body and a gear head wherein the gear is an integral part of screw.

[0015] Yet another object of the present invention is to provide a calibration screw comprising of two ends that are located on the opposite sides of the threaded body.

[0016] Yet another object of the present invention is to provide a calibration wherein the gear head is located at the one end near the tip of the screw while the other end comprises of a hexagonal slot.

[0017] Yet another object of the present invention is to provide a calibration screw comprising of a plurality of teeth for rotating the screw by a screw driver.

[0018] Yet another object of the present invention is to provide a calibration screw that can be rotated to 360° by engaging a screw driver with the teeth of the gear.

[0019] Yet another object of the present invention is to provide a plurality of teeth on the geared screw head wherein the depth of each teeth enables effective engagement and rotation.

[0020] Yet another object of the present invention is to provide a calibration screw coupled with a bimetal element at the threading portion of the screw, ahead of the bimetal in direction of bimetal deflection.

[0021] Yet another object of the present invention is to provide a calibration screw-bimetal assembly to be calibrated by a screw driver from the top, enabling easier access during calibration of a circuit breaker.

[0022] Yet another object of the present invention is to provide a calibration screw wherein the diameter of the spherical tip is about 3 mm and number of teeth in the gear are 12.

[0023] Yet another object of the present invention is to provide colour coding at every quarter turn for discerning the rotated calibration turns to achieve final calibration.

[0024] Yet another object of the present invention is to provide a calibration screw wherein the gear is offset from the bimetal at a distance of about 1.2 mm and is located in the direction of bimetal deflection.

[0025] Yet another object of the present invention is to provide a gear head calibration screw that is threaded with the threaded portion of the bimetal element of the molded case circuit breaker, wherein the gear head provides engagement surfaces with the tool interaction feature.

[0026] Briefly, a calibration screw for improved calibration of circuit breakers is disclosed. The calibration screw has a gear head near the convex tip of the cylindrical body that is used to engage the screw driver onto the calibration screw in order to adjust the position of bimetal during calibration process. The calibration of the circuit breaker by adjusting the said calibration screw provides an increase in manufacturing speed and reduction in the time required for calibration thereby providing improved calibration of molded case circuit breakers.

[0027] The present invention discloses a calibration screw for thermal calibration of molded case circuit breakers. The circuit breaker consists of a thermal release that consists of a bimetal element that is welded with the current carrying conductor in a manner that as a current over the rated current value of the circuit breaker passes through the current carrying conductor, the bimetal gets heated indirectly and starts deflecting. The bimetal has a calibration screw coupled to it that exerts a force on the tripping element to trip the circuit breaker. The thermal release is calibrated at a particular calibration current to obtain the desired trip time. The calibration of the thermal release is achieved by adjusting the gap between the calibration screw and the tripping mechanism. To adjust the gap between them, the calibration screw is moved towards the tripping element or away from the tripping element depending on the trip time. The calibration screw is adjusted with the help of a screw driver, by engaging it with the gear head of the calibration screw. The gear head calibration screw is rotated with the help of screw driver, for adjusting its distance with respect to the tripping element during calibration. The calibration screw contains an elongated cylindrical body that is threaded, and a first end and a second end, located at the opposite ends of the threaded cylinder. The first end has a spherical convex shaped end and a gear head while the second end has a hexagonal slot.

[0028] Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0029] The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which:

[0030] Figure 1 illustrates the side view of a thermal magnetic release of a molded case circuit breaker according to one implementation of the present invention.

[0031] Figure 2 illustrates the calibration screw with bimetal of a molded case circuit breaker according to one implementation of the present invention.

[0032] Figure 3 illustrates the various views of calibration screw of a molded case circuit breaker according to one implementation of the present invention.

[0033] Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure. Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0034] The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary.

[0035] Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.

[0036] The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

[0037] It is to be understood that the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise.

[0038] By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide.

[0039] Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments.

[0040] It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or component but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0041] The subject invention lies in providing a calibration screw for improved thermal calibration of circuit breakers.

[0042] In the present invention pertains to a calibration screw for improved calibration of molded case circuit breakers. The calibration screw has a gear head near the convex tip of the cylindrical body that is used to engage a screw driver onto the calibration screw thereby enabling an operator to adjust the position of the bimetal of a molded case circuit breaker during the calibration process. The calibration screw provides consistent calibration adjustment in different orientations. It reduces the calibration time thereby improving the manufacturing speed and calibration adjustment process. Following the are realized by optimizing the number of teeth/cogs of gear, depth of each cog for better engagement and colour coding at every quarter turn for discerning the rotated calibration turns while setting the final calibration.

[0043] In one implementation, a calibration screw for consistent calibration adjustment is provided for.

[0044] In one implementation, a calibration screw that calibrates in a plurality of orientations is provided for.

[0045] In one implementation, a calibration screw that reduces the calibration time, improves the manufacturing speed and improves the calibration adjustment of molded case circuit breakers is provided for.

[0046] In another implementation, a calibration screw comprising of a threaded cylindrical body and a gear head wherein the gear is an integral part of calibration screw is provided for.

[0047] In another implementation, a calibration screw comprising of two ends that are located on the opposite sides of the threaded body is provided for.

[0048] In another implementation, a calibration wherein the gear head is located at the one end near the tip of the screw while the other end comprises of a hexagonal slot is provided for.

[0049] In another implementation, a calibration screw comprising of a plurality of teeth for rotating the screw by a screw driver is provided for.

[0050] In another implementation, a calibration screw that can be rotated to 360° by engaging a screw driver with the teeth of the gear is provided for.

[0051] In another implementation, a plurality of teeth on the geared screw head wherein the depth of each teeth enables effective engagement and rotation of the calibration screw is provided for.

[0052] In another implementation, a calibration screw coupled with a bimetal element at the threading portion of the screw, ahead of the bimetal in direction of bimetal deflection is provided for.

[0053] In another implementation, a calibration screw-bimetal assembly to be calibrated by a screw driver from the top that enables easier access during calibration of a circuit breaker is provided for.

[0054] In another implementation, a calibration screw wherein the diameter of the spherical tip is about 3 mm and number of teeth in the gear are 12 is provided for.

[0055] In another implementation, colour coding at every quarter turn for discerning the rotated calibration turns to achieve final calibration is provided for.

[0056] In another implementation, a calibration screw wherein the gear is offset from the bimetal at a distance of about 1.2 mm and is located in the direction of bimetal deflection is provided for.

[0057] In another implementation, a gear head calibration screw that is threaded with the threaded portion of the bimetal element of the molded case circuit breaker, wherein the gear head provides engagement surfaces with the tool interaction feature is provided for.

[0058] Molded case circuit breakers usually include a thermal and magnetic release that provides the interruption of the circuit in fault condition thereby providing protection during overcurrent and short circuit conditions. Figure 1 illustrates the side view of a thermal magnetic release of a molded case circuit breaker. The thermal magnetic release comprises of a circuit for detecting the overcurrent fault and providing protection for overcurrent and short circuit faults. The overcurrent protection is provided by the thermal circuit that consists of a bimetal element and a current carrying conductor. The bimetal element can be heated directly or indirectly, depending upon its connection with the current carrying conductor (e.g., welded, screwed, and riveted).

[0059] The thermal release (1) consists of thermal element in form of bimetal element (3). When the current is above the rated current of the thermal release, the bimetal element (3) gets heated and is deflected to trip the breaker with the help of tripping circuit provided in the thermal release. The molded case circuit breaker also includes a calibration screw (4), illustrated in Figure 2, that is used to adjust the bimetal (3) element and calibrate the overcurrent protection. The calibration screw (4) is a cylindrical body, having a tip at one end and hexagonal slot (42) on the other end. The calibration screw (4) is provided with the thread on the cylindrical body, for robust coupling with the bimetal (3).

[0060] Figure 3 illustrates the various views of the calibration screw (4) that comprises of a gear head (41) as an ensemble, at the one end of the body near the tip of the screw. The gear component (42) can be attached on the calibration screw (4) with different processes or can be an integral part of the calibration screw. The calibration screw (4) assembly consists of gear head (41); comprising of a plurality of teeth geometries for effective engagement of screw driver. The number of teeth and the depth of each teeth depends upon the kind of engagement required. A screw driver can be used to move the calibration screw (4) for adjusting the bimetal element and calibrate the thermal release.

[0061] The gear head (42) is shaped to allow a screw driver to engage with the teeth and apply rotational force to drive the calibration screw (4) in or opposite to the direction of bimetal deflection. The gear headed (42) component is fixed to the calibration screw (4) ahead in the direction of the bimetal deflection. The circuit breaker trips when the bimetal (3) deflects and the calibration screw touches the tripping element (5). The distance between the bimetal (3) and tripping element (5) can be adjusted by moving the calibration screw (4), thereby calibrating the circuit breaker for overcurrent protection.

[0062] The circuit breaker for overcurrent protection was calibrated in the test setup by passing a particular calibration current until the circuit breaker tripped. To obtain the desired trip time, the bimetal (3) is adjusted with the help of calibration screw (4); the calibration screw (4) can be adjusted by rotating it from the top thereby providing better access for adjustment and calibration. The desired trip time is achieved by calibrating the screw for multiple iterations. The final iteration, getting the desired trip time, can be changed to permanent setting by locking the calibration screw (4) with bimetal (3), with the help of an adhesive.

[0063] The gear head (42) provides 360° rotation of the calibration screw (4) and can be rotated from the top with the help of screw driver, at the integration level when the breaker is in the thermal calibration test set up or otherwise, providing an effective and accessible way of calibration especially suited for high speed manufacturing process. The gear headed calibration screw (4) can be rotated by quarter, half or full turn during calibration as per the requirement of calibration trip times. In the present invention, identification of the rotated turns post subsequent calibrations is not provided for. However, colour coding may be provided at every quarter turn for discerning the rotation turns of the calibration screw. Similarly, reduction in the number of teeth to four or eight may also be explored, as per the requirements of smooth and error free calibration.

[0064] The circuit breaker can be calibrated at a particular calibration current by adjusting the distance between the bimetal element (3) and tripping element (5) by rotating the screw by quarter, half or full turn. This calibration process is iterated to achieve the desired trip time. The gear head screw (42) can be adjusted after a particular iteration from the top of circuit breaker, when in the test setup or otherwise, eliminating the pre-calibration or calibration at thermal release level alone, thus improving the thermal calibration method.

[0065] The circuit breaker for which the present invention discloses a calibration screw consists a thermal release (1) which consists of bimetal element (3) that is welded with the current carrying conductor (2) so that when current over the rated current value of the breaker is passed through the current carrying conductor (2), the bimetal (3) gets heated indirectly and starts deflecting. The bimetal (3) has the disclosed calibration screw (4) coupled to it, which exerts a force on the tripping element (5) and trips the breaker. The thermal release (1) is calibrated at a particular calibration current to obtain the desired trip time. The calibration of the thermal release can be achieved by adjusting the gap between the calibration screw (4) and the tripping mechanism (5). In order to adjust, the gap between them, the calibration screw (4) can be moved ahead (towards the tripping element) or back (away from the tripping element) depending upon the trip time required. The calibration screw (4) can be adjusted with the help of screw driver, by engaging it with the gear head calibration screw (4). The gear head calibration screw (4) is rotated with the help of screw driver, for adjusting its distance with respect to the tripping element during calibration.

[0066] The gear head is oriented towards the tip of the calibration screw, in the direction of the bimetal deflection, for the ease of access for calibration and to accommodate more calibration turns. The molded case circuit breaker is calibrated by adjusting a certain gap between calibration screw (4) and tripping element (5) initially. The calibration time is verified by passing calibration current through the three phases of circuit breaker. Depending on the time obtained in the first iteration, the gap between the calibration screw (4) and tripping element (5) can be adjusted by rotating the gear headed calibration screw (4) by specific angle of rotation. This adjustment of the calibration screw can be done by turning the gear with the use of circuit breaker from top on the same test setup without disassembling the trip mechanism release from the breaker (in case of modular arrangement). The calibration process can be repeated in the same process till the desired trip time is achieved and the final iteration can be converted to permanent setting by application of adhesive to the calibration screw (4). Thus, this mode of calibration reduces the calibration time, thereby reducing the manufacturing time of the breakers.

[0067] The calibration screw (4) consists of two ends as illustrated in Figure 3, the one end has a convex tip, which pushes the tripping element and the other end has a hexagonal slot (42). The center portion of the calibration screw consists of thread, which provides engagement of the screw with the bimetal for robust coupling. The tip end of the calibration screw (4) has a gear head (41) for adjusting the number of turns of calibration screw (4). The profile of the calibration screw (4) can be made as gear (41) as an integral part of the screw with the help of casting process or can be molded separately on the calibration screw with insert molding process, if the gear is of plastic material.

[0068] The number of teeth on the gear (41) and the depth of each teeth can be iterated depending on the requirement of the number of turns for calibration process. The calibration screw can be rotated by 90°, 180° or 360° or any angle in between to achieve the desired trip time. These features allow for an improved mode of calibrating a circuit breaker thermal overcurrent protection. The gear headed calibration screw (4) including convex tip at one end and hexagonal slot (42) at the other end is inserted into threaded slot on bimetal. The calibration screw (4) is threaded in the bimetal (3) until the gear head end of the calibration screw (4) comes on the other end of bimetal (3) towards the tripping element (5). The screw (4) is then rotated for predetermined number of degrees to finalize the calibration screw (4) setting. The screw driver is disengaged and the breaker is tested at calibration current to verify trip time. The fine tuning on the trip time can be achieved by adjusting the calibration screw (4) for number of degrees to a permanent setting. The final calibration and position of calibration screw (4) is arrested by applying sealant in the cavity formed between the threads of calibration screw (4) and bimetal (3).

[0069] Although a simple, economic and cost effective manner of thermal calibrating molded case circuit breakers by a gear headed calibration screw has been described in language specific to structural features indicated, it is to be understood that the embodiments disclosed in the above section are not necessarily limited to the specific features or components or devices described therein. Rather, the specific features are disclosed as examples of implementations of a calibration screw for thermal calibration of circuit breakers.