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Moulded Case Circuit Breakers (MCCB) - Types and Working

Moulded Case Circuit Breakers are electromechanical devices which protect a circuit from Overcurrent and Short Circuit. They provide Overcurrent and Short Circuit Protection for circuits ranging from 63 Amps up to 3000 Amps.

Their primary functions are to provide a means to manually open a circuit and automatically open a circuit under overload or short circuit conditions. The overcurrent, in an electrical circuit, may result from short circuit, overload or faulty design. 

MCCB is an alternative to a fuse since it does not require replacement once an overload is detected. Unlike fuse, an MCCB can be easily reset after a fault and offers improved operational safety and convenience without incurring operating cost. 

Moulded case circuit breakers generally have a 
  • Thermal element for overcurrent and 
  • Magnetic element for short circuit release which has to operate faster. 

MCCBs are manufactured such that end user will not have access to internal workings of the over-current protection device. Generally constructed of two pieces of heavy-duty electrically insulated plastic, these two halves are riveted together to form the whole. Inside the plastic shell is a series of thermal elements and a Spring-loaded trigger. When the thermal element gets too warm, from an overcurrent situation, the spring trips, which in turn will shut off the electrical circuit. 

Types of MCCBs

Larger moulded case circuit breakers have adjustable range setting on the face of the device. Moulded case circuit breakers can range in size from 32amperes up to 3000 amperes. 

Moulded Case Circuit Breakers have the following Specifications

Current Rating - Amperes
Current Setting Range - Amperes
Short Circuit Rating - Kilo Amperes (KA)
operating Characteristics - Normal / Current Limiting Type

MCCBs are now available with a variety of Releases or Operating Mechanisms these are given below 

  1. Thermal Magnetic Release 
  2. Electronic Release 
  3. Microprocessor Release 

Thermal Magnetic Release MCCB

Thermal-magnetic circuit breakers use bimetals and electromagnetic assemblies to provide overcurrent protection. Their characteristic inverse time tripping under overload conditions is ideally suited for many applications varying from residential to heavy industrial loads. For higher level (short circuit) over currents, instantaneous trip characteristics allow moulded case circuit breakers to interrupt with no intentional delay. 

The adjustable overload protection is from 70% to 100% of the nominal current and short circuit setting from 5 to 10 times of the rated current is possible. 

The minor disadvantage of the release is that operating characteristics of the breaker may vary depending on the ambient temperature. 

Electronic Release MCCB

Electronic or Static Release Moulded Case circuit breakers use power electronic circuitry to provide overcurrent protection. The Continuous adjustable overload protection from 60% to 100% of the nominal current and short circuit setting from 2 to 10 times of the rated current is possible. 

The advantage Of the release is that operating characteristics of the breaker is independent of the ambient temperature. 

This wide flexibility takes care of future increases in load capacity of an installation and ensures better planning at an optimum cost.

Microprocessor release MCCB

Microprocessor release Moulded Case circuit breakers use microprocessors to provide overcurrent protection. The Microprocessor release works on monitoring of current True R.M.S value. It is simulated and calculated from peak values, which installed microprocessor, can detect. 

There is high Flexibility through multiple adjustments of protection settings, High repeat accuracy and High reliability.

Time delays can be provided for Short Circuit Release better discrimination and co-ordination using LCD display. System Diagnosis is possible as it stores the Trip history within the internal memory. Trip current indication is also available for understanding of type of fault and set-up programming at site.