Moulded Case Circuit Breakers (MCCRs) :
The moulded case circuit breaker is a simple, compact, economical air break circuit breaker which can be conveniently mass produced because of its moulded construction (see IS: 13947-2), is cost lies somewhere between the cost of a switch fuse combination and conventional circuit breaker of an equally reputable.
However, for more complete comparison, it is necessary to consider the wholesome cost in terms of the size, weight, maintenance, assembly effort, speed of breaking, securing costs and other factors, such as down times and accuracy of protection
The moulded housing is made of heat resistant phenolasbestos or plastics which are easy to install. Only low voltage types are available in ratings up to 660 volts ac and current rating up to 2500 A and breaking capacities up to 100 kA ms symmetrical at 0.20 power factor
Apart from their modest breaking capacity having the same advantages and disadvantages as large air break circuit breakers, modern MCCBS afford reliable protection and are also available with a trip-free mechanism.
They are usually fitted with a built-in tripping system which can either be of a thermal-magnetic or fully magnetic type with inverse time-current characteristics respectively, for overload protection and instantaneous protection against short circuits
The typical inverse time-current characteristics for both the tripping releases . These characteristics are specified for certain ambient temperatures 20. 40°C and a suitable correction factor is specified for other ambient temperature conditions.
The tripping mechanism characteristics can be designed for specific requirements of the distribution circuit. Current transformers are not used with MCCB.
The thermal setting is usually adjustable between 50 and 130 percent of the rated current for overload protection as defined by IS 382 and magnetic setting between 300 and 1200 per cent of the rated current for short circuit protection
In case of ally magnetic tripping mechanism them current characteristic overload setting is usually been the range of 75 and 200 per cent and instantaneous setting bets and 1200 per cent Other protection provisions, such as shunt or under voltage release and one or two auxiliary switches are available as options.
The breaking capacity is a limitation only where the source impedance is very low and highly inductive. Since the minimum source impedance can be readily calculated in the design of many installations for a given location, the suitability of moulded case circuit breakers in such a location can be readily assessed
On the other hand, in improving the power supply which has the effect of reducing the source impedance of a system already fitted with moulded case circuit breakers, due notice should be taken of any limitations of the breaking capacity and provision made in the modified design. Modern current limiting MCCB afford very low let-through energy and belter protection coordination.
Three important applications of MCCBs are main incoming circuit breaker for secondary side of transformers, distribution feeder protection and motor control. In rural areas where there is a problem of getting the right fuse to replace blown fuses, these breakers can be very useful of providing protection to the distribution transformers.
