10 c] With a neat circuit diagram explain the operation of MCB and RCCB.
Solution :
Miniature circuit breaker (MCB) :
An MCB – miniature circuit breaker is an electromagnetic device that embodies complete enclosure in a molded insulating material.
The main function of an MCB is to open the circuit automatically when the current passing through MCB exceeds the value for which it is set. It can be manually switched ON and OFF as similar to normal switch if necessary. An MCB is a simple, easily operable device and is maintenance-free too. It can be easily replaced.
The trip unit is the key part of the MCB on which the unit operates. The bi-metal present in the MCB circuit protects against overload current and the electromagnet in the circuit protects against short-circuit current.
Working :
When the overflow of current takes place through MCB , the bimetallic strip gets heated and it deflects by bending. The deflection of the bi-metallic strip or trip bar releases a latch. The latch causes the MCB to turn off by stopping the flow of the current in the circuit. This process helps to safeguard the appliances or devices from the hazards happening due to overload or overcurrent. To restart the flow of current, MCB must be turned ON manually.
In the case of short circuit conditions, the current rises suddenly in an unpredictable way, leading to the electromechanical displacement of the plunger associated with a solenoid. The plunger hits the trip lever, it causes the automatic release of the latch mechanism by opening the circuit breaker contacts.
Residual current circuit breaker (RCCB):
RCCB consist of a small current transformer surrounding live and neutral wire. The sensing coil on the current transformer is connected to a trip coil of a circuit breaker.
Under a normal condition the current in a line conductor(IL) is same as the current in Neutral conductor (IN). Hence IL- IN is Zero, therefore two fluxes produced by IL and IN cancel each other and sensing coil does not sense any imbalance.
If there is fault the fault current IF flows through the earth conductor hence there is difference between the current IL and IN. The difference IL –IN is called residual Current .The fluxes produced by IL and IN are no longer same under a fault condition, producing flux in the core. Due to the residual flux EMF get induced in the sensing coil, which circulates the current through the tripping coil of the circuit breaker. This operates the circuit breaker and disconnects the supply from the device. As the action of the trip coil depends on the residual current the device is called residual current circuit breaker.