Miniature Circuit Breaker or MCB has a contact system. That means of arc quenching a mechanism and tripping and protection system to open the circuit breaker under fault conditions during operation like the MCCB. In this post, we will learn about the MCB, MCB’s History, Construction, and Working Principle.
What is a Miniature Circuit Breaker?
Present days, we use a Miniature Circuit Breaker or MCB for a low voltage electrical network instead of a fuse. One of the best reasons to use this MCB instant of the fuse it that, we can see MCB is tripe when there is any fault on the system and we don’t need to change it after a fault. MCBs are designed and tested according to the requirements of IEC 898.
Related: Low Voltage Switchgear Standards and Testing
History of Miniature Circuit Breaker:
MCB has advanced numerously in the past 30 years. Early devices were generally of the ‘zero-cutting’ type. During a short circuit time, the current had to pass through a zero before the arc was extinguished and the provided a short circuit breaking current capacity was about 3kA. These early MCB’s were housed or covered in a bakelite molding.
But now modern MCB is a much smaller and more sophisticated device than before. Recent developments associated with Moulded Case Circuit Breakers have been incorporated into modern MCB’s to improve their performance. Modern MCB’s come with breaking capacities of 10 kA to 16 kA which now are available in the market. Now a day’s MCB’s are used in all areas of home and industry as a reliable means of protection instant of fuse protection.
Construction of Miniature Circuit Breaker:
Most of the MCBs are single-pole construction for using in single-phase electrical circuits. Modern day’s MCBs complete working system is housed within a plastic molding. The contact system of MCB comprises a fixed and a moving contact and attached to each is a contact tip that provides a low resilience contact junction to resist welding. Now a day’s modern MCBs are fitted with arc chutes consisting of metal plates which are held in position by insulating material. The arc chute does not necessarily surround the contact. In some designs, arc runners are provided to pull the arc into the arc chute.
Construction of Miniature Circuit Breaker. Source – Wikimedia
Working of Miniature Circuit Breaker:
The tripping mechanism for an MCB usually consists of a thermal-magnetic arrangement. The thermal action is provided by a bimetal with in some cases a heater. For ratings between the ranges of 6 to 63A the part of the bimetal forms of the current path. The heat generated within the bimetal itself being sufficient to cause deflection. The deflection is then used to activate the tripping mechanism inside the MCB.
The characteristics of the bimetal are chosen to provide particular delays time under specific overload or fault currents according to the required time-current characteristic. Generally, a high resistance bimetal is used for low current devices and a lower resistance bimetal for high current devices. For very low current MCBs a heater may be incorporated around the bimetal in order to generate sufficient heat to deflect it.
Now the magnetic tripping element usually consists of a coil that is wrapped around a tube and there is a spring-loaded slug within the tube. Movement of the slug operates the tripping mechanism to open the MCB. It can also be used to assist in opening the contacts by locating the coil close to the moving contact. When a fault current flows, then a high magnetic field generated by the coil overcomes the spring force holding the slug in position. Then the slug moves to actuate the tripping mechanism and forces the contacts apart by striking the moving contact arm.
For low MCB ratings, the coil is formed from the thin wire with many turns. For higher ratings, the wire is thicker with fewer turns. The magnetic trip is set by the manufacturer according to the required characteristics where MCB will install.
Related: Features of MCCB and ACB with Advantage
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