MOLDED CASE CIRCUIT BREAKER (MCCB)
Definition – MCCB is a switching device which is used in LT electrical system. It provides protection against overload & short circuit. Fault sensing arrangements are installed inside MCCBs&shunt release is provided for remote tripping of MCCB. It is available between 100A & 630Acurrent.
Working Principle – MCCB provides protection against overloadthrough thermal mechanism.It has bimetallic contacts which expand & contract on temperature changes. Under normal condition these contacts allow normal current to flow but when current exceeds its trip limit, the bimetallic contacts start heating up & expand until the circuit is isolated/tripped. When MCCB is tripped, faulty circuit is isolated from circuit & the temperature of bimetallic contacts starts getting normal & MCCB again is ready for next operation.
Secondly,protection against short circuit – if current is very high, fault current should be interrupted immediately. This is achieved by electromagnetic induction. Whenever fault occurs, the high current induces a magnetic field in a solenoid coil located inside the breaker, this magnetic induction trips a contact & current is interrupted. In the tripping process, arc is produced & that is dissipated by taking suitable measures inside the breaker.These breakers can be manually switched off or on also which is required during maintenance or other purpose.
MCCBs are available with protection range from 50% to 100% of rated MCCB.
Ex –200A, TP MCCB can be used from 100A load to 200A in steps of 10% e.g. 110A, 120A, 130A, 140A, 150A, 160A, 170A, 180A, 190A & 200A. On every stage MCCB will provide overload & short circuit protection.
Selection of MCCB –
- It should be capable to carry the rated current of load, (Note: Rating of MCCB should not be much higher than rated/full load current otherwise MCCB will not trip on fault).
- Check number of poles – TP, TPN & 4P where
TP (for 3 phase load) = TP stands for Triple pole which means three phases (R,Y,B) & these phases will be part of MCCB & neutral is not required for load. These 3 phases are opened & closed with the operation of MCCB.
TPN (for 3-phase load) = TPN stands for Triple pole with neutral where three phases will be part of MCCB & will be closed or opened with the operation of MCCB but neutral will be connected directly with load & neutral will remain connected with load ,
4P (for 3-phase load) = 4P means Fore pole which means 3 phases & a neutral and all will be part of MCCB. In this case, 3-phases &neutral will be part of MCCB &opened & closed with MCCB operation.
- Type of protection–According to Time Curve protection.
- Fault current rating which should be in kA (thousand amperes) which means that a MCCB can withstand the fault current of short circuit,
- With operating Handle or without handle
Ex – 200A TP MCCB with 35kA fault current& operating handle.
CURRENT RATING OF MCCBs –
A typical range of Three phase MCCB – 100A, 125A, 150A, 200A, 250A, 315A, 400A, 630A,
(Note: MCCBs are available with 25-50KA fault level.)