FORM 2 The Patents Act 1970 (39 of 1970) & The Patent Rules 2003 COMPLETE SPECIFICATION (See Section 10 and rule 13) TITLE OF THE INVENTION: A CONTACT MECHANISM FOR A MOULDED CASE CIRCUIT BREAKER APPLICANT: LARSEN & TOUBRO LIMITED L&T House, Ballard Estate, P.O. Box No. 278, Mumbai, 400 001, Maharashtra INDIA. PREAMBLE OF THE DESCRIPTION: THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED A) TECHNICAL FIELD [0001] The present invention generally relates to low voltage Moulded Case Circuit Breakers (MCCB) and particularly to a rotary contact system in the MCCB. The present invention more particularly relates to a rotary contact system with a locking arrangement for locking a moving contact arm in open and closed position during an over current condition to provide a discrimination with the downstream devices. B) BACKGROUND OF THE INVENTION [0002] Trre CIRCUIT BRAKER AND WIDELY USEDE, commerce/ ancf light industrial installations. The circuit breakers provide a protection for the electrical systems from the electrical fault conditions such as current overloads, short circuits and low level voltage conditions. A moldec] case circuit breaker generally comprises a terminal module which connects a po\ver source or an electrical load of the circuit, a switching mechanism which switches the circuit by contacting a fixed contact arm with a moving contact arm or Separate them from each other within the molded case circuit breaker and a trip mechanism which senses a fault current to trip the switching mechanism to a circuit-broken position. A Moulded case circuit breaker (MCCB) is a circuit breaker assembled as an integral unit in a supportive and enclosed housing of insulative material. [0003] In order to provide comprehensive coordination with downstream protection devices, the circuit breaker is required to withstand maximum through fault current. Simultaneously, the circuit breaker should respond instantaneously, when the fault current is higher than a preset threshold force value designated to withstand the electro-dynamic forces. The preset threshold force value is a mechanical compensation force required to withstand the electro-dynamic forces exerted on the moving contact arm due to a high fault current. Moreover, a significant amount of compensation to offset the ensuing mechanical stresses is required for withstanding the high electro-dynamic forces without repulsion requires. The compensation of the electro-dynamic forces are done through one of special current path configurations or mechanical locking arrangements. However, such compensation requires a higher space to accommodate the circuit breaker and require an external actuation or impetus for the circuit breaker operation. [0004] In one of a scheme, the MCCB comprising a rotary contact assembly utilizes a magnetic repulsion force that is generated between the fixed contact arm and the moving contact arm during a short-circuit condition to quickly open the circuit breaker contact arms and separate the fixed and the moving contact arms. The separation of the contact arms interrupts the current flow through the circuit. However, the contact arms develop a high velocity due to the large magnetic forces, thus causing the moving contact arm to rebound off the contact arm stop surface. Further, the contact arm recloses allowing high levels of current to flow once again into the system. This reclosing action also releases a large amount of energy which often damages the circuit breaker. [0005] Hence, there exists a need to provide an efficient contact mechanism for a MCCB, to facilitate locking of the moving contact arm with the fixed contact arm below a preset threshold force value and to unlock the moving contact arm from the fixed contact arm above the preset threshold force value. Also there exists a need to provide a rapid rotation of the moving contact arm to ensure a fast arc quenching process and clearance of the fault. Further, there exists a need to provide a self de-latching and a positive locking arrangement in the MCCB to prevent a re-closure of the moving contact during a fault current. [0006] The above mentioned shortcomings, disadvantages and problems are addressed herein and which will be understood by reading and ssVudying the following specification. C) OBJECT OF THE INVENTION [0007] These and other objects and advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings. [0008] The primary object of the present invention is to provide a contact mechanism for a MCCB to facilitate locking of the moving contact arm with the fixed contact arm below a preset threshold force value and to unlock the moving contact arm from the fixed contact arm above the preset threshold force value. [0009] Another object of the present invention is to provide a contact mechanism for a MCCB to ensure a fast arc quenching process and clearance of the fault. [0010] Yet another object of the present invention is to provide a contact mechanism for a MCCB to prevent a re-closure of the moving contact in presence of the fault current. [0011] Yet another object of the present invention is to provide a compact contact mechanism for a MCCB to ensure a contact' stability up to a preset threshold force value. [0012] Yet another object of the present invention is to provide a contact mechanism for a MCCB with less number of components and thereby reduce assembling time. [0013] Yet another object of the present invention is to provide a contact mechanism for a MCCB which is easily adapted to be one of a single break, double break, and multiple finger type contact system. D) SUMMARY OF THE INVENTION [0014] The various embodiments of present invention provide a contact mechanism for a Moulded Case Circuit Breaker (MCCB). The mechanism includes at least one moving contact arm movable between a closed position and an open position, a rotor coupled to the at least one moving contact arm, at least one fixed contact arm and a housing to support the at least one fixed contact arm and the rotor. Also, the mechanism includes a locking link assembly coupled to the rotor and an operating mechanism of the circuit breaker coupled to the rotor. Each of lateral faces of the moving contact arm includes a cam profile engaged with the locking link assembly to facilitate a locking of the moving contact arm with the fixed contact arm until a preset threshold force value is achieved and to unlock the moving contact arm from the fixed contact arm above the preset threshold force value. The preset threshold force value is a mechanical compensation force required to withstand the electro-dynamic forces exerted on the at least one moving contact arm due to a high fault current. [0015] In various embodiments of the present invention, the preset threshold force value is a mechanical compensation force required to withstand the electro-dynamic forces exerted on the at least one moving contact arm due to a high fault current. The contact mechanism is tuned to achieve a desired threshold force value by changing the constructional parameters of the MCCB. [0016] According to one embodiment of the present invention, the preset threshold force value is defined by a cam profile. The moving contact arm is coupled to the rotor using a centre pin. The operating mechanism of the circuit breaker is adapted to provide an energy to the rotor to achieve a rotary motion of the moving contact arm. The operating mechanism is adapted to operate the MCCB in one of three operations, namely an ON to OFF operation, an OFF to ON operation and an ON to TRIP operation. The rotor further comprises a centre slot to define a closed position and an open position of the moving contact arm and at least one contact spring is provided in the Centre slot of the rotor. Further. one end of the contact spring is assembled to the rotor using a contact spring pin and another end of the contact spring is assembled to the rotor using a rotor contact spring pin. The centre slot defines a rotary locus of the moving contact arm from one position to another position. [0017] The contact spring is an energy storing element which stores energy until the moving contact arm reaches a dead centre position and releases the energy once the moving contact arm reaches post dead centre position. The dead centre position is a position of the moving contact arm when the centre pin, the contact spring pin and the rotor contact spring pin all align on a single line. The locking link assembly further comprises at least one locking spring, a locking spring pin provided in the locking link assembly to facilitate the assembling of the locking spring with the locking link assembly. A locking link pivot pin is provided to facilitate the assembling of the locking link assembly with the rotor. The locking spring is adapted in the contact mechanism to apply an anticlockwise torque on the locking link assembly at a locked position and to restore the locking link assembly to an unlocked position. [0018] In various embodiments of the present invention, the examples of the constructional parameters of the MCCB include but are not limited to a perpendicular distance between a Contact Force line of action and a Contact pivot, a perpendicular distance between a Normal Cam force line of action and the Contact pivot, a perpendicular distance between Normal Cam force line of action and locking link pivot, a perpendicular distance between Locking Spring force line of action and the locking link pivot, a perpendicular distance between Contact Spring force line of action and the contact pivot, a perpendicular distance between Tangential Cam force line of action and the Contact Pivot, a perpendicular distance between Normal Support C