DESC:TECHNICAL FIELD OF THE INVENTION The present invention generally relates to low voltage switchgear. More particularly, the present invention relates to molded case circuit breakers using bimetal for overload protection. BACKGROUND AND THE PRIOR ART Circuit breaker is used to protect the device from overload & short circuit current. Circuit breakers like molded case circuit breaker (MCCB) can be functionally divided into 3 sub systems namely, Mechanism, Contact system & Release. Release part in MCCB is responsible to give the circuit break command to the MCCB. There are 2 kinds of releases available in market i.e. Thermo magnetic (TM) release and Electronic release. Both the release has to do the basic function of giving the circuit break command during overload & short circuit condition. Usually TM releases are used when cost is the concern. Electronic release delivers superior performance at a relatively higher cost. TM release can be further sub divided into 2 sub parts, namely, Thermal part and Magnetic part. Overload condition in simple terms can be explained as high current (though not as high as short circuit current) that can hamper the wellbeing of the system in use. The system will be designed to carry a particular rated current for a period of time based on its duty cycle. If higher current (overload current) flows in the system for longer periods, then the system will not be able to dissipate the excess heat generated because of the overload and ultimately the system will break down. The thermal part of the TM release usually consists of a heater, a bimetal, a calibration screw, and a shaft. . The calibration screw will be placed at the top or bottom of the bimetal which will serve for two purposes, One point hitting and Calibration. However, calibration is actually a tedious and a cumbersome process. Much iteration is to be done to achieve the perfect operation. The iterations are intercepted with a cooling period during which the bimetal cools and comes back to the original condition and gets ready for the next iteration In the conventional calibration method, Bimetal has to deflect ‘X’ mm to trip the circuit breaker. The operator will pass current to breaker and the heater and/or bimetal heats up to deflect the bimetal by ‘X - ?x’ mm. The breaker does not trip within the given time limit. The operator allows the breaker to cool it for a period of time (varies from 20 to 45 minutes). The operator changes the position, in this case reduces the gap between the calibration screw (4) and shaft (5) so that the circuit breaks open within the desired time limit, of the calibration screw and puts it into test. The bimetal deflects ‘X - ?y’ mm. (where yX after mechanical pre calibration. This way it is taken care that, there is no early trip in the breaker. This means the hot calibration is done only to reduce trip gap (7). For a trip time band of say, T1 to T4 sec, the hot calibration is done as follows. 1. During 0 – T1 sec, the breaker may trip due to pre-calibration or assembly error. 2. During T1 – T2 sec, it trips due to pre-calibration only 3. During T2 - T3 sec, it may trip either due to pre-calibration or by adjustment of the hot calibration screw in this period 4. During T3 - T4 sec, breaker trips only due to the adjustment of the screw in T2 – T3 sec and during this period the hot calibration screw is not adjusted 5. During T4 - T5 sec, it trips due to any error in the assembly. In this case the breaker is sent back for mechanical pre calibration This way all the poles are calibrated such that the breaker obeys the trip characteristics. The present invention is for MCCB 3-Pole breaker and calibration screw used is standard dome head grub screw with fine pitch. The calibration screw is fixed in heater not in housing because due to heat produced in calibration screw during breaker ON duty condition it may damage the housing. To avoid it an extra component (nut) might be used which will be trapped in housing. The present invention provides provision for screw in heater which is a part of breaker itself and do not use extra component for the same. FEATURES AND BENEFITS OF THE INVENTION • It reduces the calibration iteration to one which is trial and error basis in the conventional method • It eliminates the need for verification of the calibrated circuit breaker • Due to reduction in iterations, it helps to save in terms of time and cost • By using automation process time consumed for one iteration can be further brought down • The loading of bimetal is avoided during hot calibration by providing a rigid support for hot calibration screw. • The breaker is calibrated pole wise and hence ensuring that all the poles behave similarly to clear the fault. • Reduces calibration time • Increases productivity • Reduces time & money wastage • Reduces labour force • Ensures fresher breakers in the market ,CLAIMS:1. A calibration method of a thermal magnetic trip mechanism for a circuit breaker, said method comprising steps of: deflection of a bimetallic element in response to a test current through the circuit breaker, adjusting the position of said bimetallic element using a hot calibration screw, when said circuit breaker is not tripped due to said test current, reducing a trip gap for tripping said circuit breaker thereby providing a calibrated circuit breaker. 2. The method as claimed in claim 1, wherein said hot calibration screw is placed on a rigid support of said circuit breaker for changing the position of said bimetallic element. 3. The method as claimed in claim 1, wherein said trip gap is pre adjusted by a mechanical pre-calibration through fixing the position of said bimetallic element. 4. The method as claimed in claim 1, wherein said rigid support preventing the loading of said bimetallic element during calibration.