Thursday, October 3, 2013

MCCB, Moulded case circuit breaker Working Principle

MCCB name is derived from Moulded case Circuit breaker Breaker. MCCB is used for following purposes :-
(i) ON/off of electrical load connected across MCCB
(ii) Protection of  an electrical circuit from damage caused by overload ,short circuit and earth fault.

Difference between MCCB and Isolator:-
This means MCCB also provides protection against overload, Short circuit and earth fault which an Isolator can't do. Isolators are only used to isolate the power supply to circuit.

It is used to interrupt the circuit as soon as any fault occurred and they are used in every industry where connected load is having current rating above 100A .

Difference between MCCB and Fuses:-
MCCB are completely different from fuses. Fuses are used only for interruption  of the circuit and get blown off after it's operation. Although they serve the same purpose in case of any fault in system as these isolate the circuit after getting blown off. But in MCCB , MCCB get tripped in event of fault and same will be reset to normal operation after resetting the MCCB.

There are three positions for operation of MCCBs:-

(i) ON position
In this position power supply at input will be available at the output

(ii) OFF position
In this position power supply at Input will not achieve at output of MCCB. This position is known as isolated position

(iii) Trip position
This position is in-between ON and OFF position of MCCB. In this position power supply at input will not be available at output. This position in fault position which indicates that fault needed to be cleared while getting it ON.  While resetting the MCCB from trip position you have to first OFF the MCCB then only you get ON the MCCB, you can't switch ON the MCCB directly from trip position.

Air is used as ARC quenching Medium in Circuit breakers.


Circuit breakers are having breaking current specified on it's name plate. Breaking capacity is the current which a circuit breaker will interrupt during fault condition and usually in KA'S having range from 18KA to 50KA depending upon the connected load across MCCB.

Making capacity of a circuit breaker is the maximum current which the breaker can conduct at the instant of closing. The making capacity is considered to the peak value of the first cycle when there is an imaginary short circuit between the phases.

Breaking capacity of Circuit breaker is the current that breaker able to interrupt without being destroyed. The short circuit generated in case of fault should not be greater than breaking capacity otherwise MCCB get failed or you can say breaking of circuit can't be ensured.

Now question arises Out of Breaking and making capacity which will be higher??

The making capacity of the circuit breaker is usually greater than the breaking capacity of a circuit breaker as breaking an electric circuit is difficult due to arcing which occurs and which has to be quenched.

No. of Mechanical and electrical operations of MCCB

No. of Mechanical operation of MCCB are 20000 depending upon Temp. i.e. called Mechanical life of MCCB and

No. of Electrical operations i.e. Electrical life is 5000-7000 depending upon MCCB rating.



Standards Used in MCCB construction:-

IEC-60947-1& 2/IS-13947-1 & 2 are Electrical standards for MCCB. 
IS 10118-1 (1982): Code of Practice for Selection, Installation and Maintenance of Switchgear and Control gear.

MCCB Operating Cycle:-

O-0.3s-CO-3min-CO 

This is the Rated Operating Sequence of the circuit breaker. Which denotes the sequence of opening and closing operations which the circuit breaker . 

The operating mechanism experiences sever stress during the auto-re-closure duty; however, the circuit breaker should be able to perform the operating sequence as follows: 

O-t-CO-T-CO 

where,
O = opening operation
t = time required for circuit breaker to be ready to receive closing order from auto-reclosure relay (0.3 s to be used for rapid reclosure), (3 min not to be used for rapid reclosure). 
CO = close operation followed by open operations 
T = time required by the circuit breaker, insulating media for regeneration and operating mechanism (3 min) CO = close operation followed by open operations