Thursday 5 January 2012

Miniature circuit breaker


Miniature circuit breaker



            In earth leakage circuit breaker system, the parts of an installation which are sought to be protected are connected to an earth electrode through the coil of an earth leakage circuit breaker, which controls the supply to that part of the installation. In addition to the equipment earth. It provides the means of disconnecting the faulty circuit at a current level (about 30 mille amperes) which is only a fraction of the normal full load current, and is the most effective method of protection
            The trip circuit breaker may be provided with time-lag feature to allow the operation of the protective devices protecting individual circuits.
            MCB body is made of flame retardant high strength plastic with high melting point and high dielectric strength. The contacts are of silver, silver alloy, or of silver graphite with wiping action to ensure maintenance-free long life. The arc produced on separation of the contacts is rapidly moved under the influence of a magnetic field, under the arc chute stack where it is broken down in to partial arcs, resulting in rapid extinction of arc. Most MCBs are equipped with thermal and/or magnetic and current limiting protection for normal overload inverse time current characteristic protection. Thermal operation is achieved with a bimetal strip which deflects when heated by the overload current flowing through it. In doing so, it moves the trip lever releasing the latch mechanism and causing the contacts to open under spring action, when short circuit fault occurs, the rising fault current energizes the solenoid of hammer trip mechanism and the trip lever causes immediate opening of the contacts.
            “L”series circuit breakers are recommended for installations with low inductive loads like heaters,ovens,geysers,GLS lamp etc.They have pickup current five times more than the normal current.
            “G”series or motor duty MCBs are recommended for installations with high inrush current peaks and which, at the same time require closer overload protection such as motors, air conditioners, fluorescent lamps, sodium vapor lamps, machine tools etc.These MCBs have eight times the normal rated current as magnetic pickup current.
            Back up protection-If the prospective short circuit current is expected to be more than 9000Amps, MCBs will require suitable back up protection for example HRC fuse or higher capacity circuit breakers.