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 Miniature Circuit Breaker?
Present days, we use Miniature Circuit Breaker or MCB for low voltage electrical network instead of 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 system and we don’t need to change it after fault. MCBs are designed and tested according to the requirements of IEC 898.
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 moulding.
But now the modern MCB is a much smaller and more sophisticated device than before. Recent developments that 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 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 moulding. The contact system of MCB comprises a fixed and a moving contact and attached to each is a contact tip which 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.
Working of Miniature Circuit Breaker:
The tripping mechanism for a 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 bimetal forms part 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 which is wrapped around a tube and there being 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 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 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.
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