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What is the difference between a MCCB and ACB / MCB ?


Suggestion: Reference:
Donald G. Fink, H. Wayne Beaty "Standard Handbook for Electrical Engineers," 14th Ed., McGraw-Hill, 2000,
page 10-84
Air Circuit Breaker
Briefly, ACBs resemble switchgear. They are applied as switchgear in switchgear housing or switchboards.
Molded-Case Circuit Breaker
MCCBs are completely enclosed within a ruggedly constructed molded case of insulating material.
Often available from 15A to 4000A, 1-, 2-, 3- and 4-poles, 5kA to 45kA Isc and high interrupting ratings up to 200kA.
Also, refer to manufacturers' catalogs.


By "ACB", I'm going to assume you're referring to what ANSI calls Low Voltage Power Circuit Breakers(LVPCB).  You might try a search of this forum - I think this question has come before.

Basically the LVPCB is a more heavy-duty breaker than a molded-case breaker.  They are built to different standards (ANSI C37) and are more flexible in application.  LVPCB are field-maintainable, MCCBs are not.  

LVPCBs have a short-time withstand rating that allows them to be applied without any instantaneous trip.  Molded case circuit breakers must have instantaneous trips.  This means that MCCBs are impossible to coordinate for high levels of fault current.  

Probably the most informative thing to do is to go to a manufacturer's website and look at some pictures.  You'll see a big difference in size and configuration.  (and cost, of course.)


See also


Per UL, MCCB's and ICCB's must have instantaneous trip functions to protect the circuit breaker itself. All UL listed MCCG's and ICCB's have an 'instantaneous override" function, meaning when the current reaches some pre-set level the breaker trips instantaenously, regardless of trip settings. ANSI power circuit breakers do not have instantaneous override.

This gets a little fuzzy when discussing products such as the Square D Masterpact and Cutler-Hammer MagnumDS, which can be ordered as either an ICCB (UL489) or ANSI power breaker (UL1066). With these you must specify which version you want.


The basic difference between MCCB (Moulded case circuit breaker) and ACB(Air circuit breaker) is that the former can not be closed remotely because the operating mechanism of MCCB is different from the ACB.Secondly,MCCb is not repairable and complete unit is required replacement while ACB can be repaired.AS far as tripping system is concerned both can have instantaneous as well as time graded tripping device.It can be magnetic,thermal and electronic also.Construction wise ACB is more robust.
MCB(Miniature Circuit Breaker)is used for handling small load.For example lighting load.It is available for a maximum short circuit capacity of 10Kilo Amp.It also has instantaneous and overload tripping devises,but these setting are fixed i.e. factory set and can not be changed.
More information you can have from the manufacturer's catalogue.


Further clarification: insulated case circuit breakers (ICCB) are constructed and tested to the same standards as MCCBs, but include two-step stored energy mechanism, remote control capability and draw-out mounting capability, similar to ACBs.


A major difference is that most MCCB are designed with a "Blow Open" contact arrangement.  The magnetic forces caused by rapid rise of current actually aid in the opening of the contacts under fault conditions, the higher the current the stronger the repulsion.  This is why there is no short time with-stand rating for these devices.  Even those that have a short time adjustment are difficult to coordinate. If you were to disassemble one of the inexpensive residential style molded case breaker you will not find a magnetic coil for instantaneous trip.  They rely solely on the blow open design.   LVPCB (ACB) has the opposite contact design, "Blow Closed".  As current rises rapidly the magnetic forces pull the contact surfaces together.  This allows the LVPCB to hang in there on a fault for a short period of time before tripping to allow a down stream device to clear and give superior system coordination.


Suggestion: Square D Masterpact has got the instantaneous withstand rating.
The short circuit coordination is actually performed via the short-time delay band since the instantaneous band doed not have any delay available.  


I agree with the above postings. The Masterpact does have withstand ratings required per ANSI C37.13. That allows the Masterpact frame to be used for either UL489 or ANSI C37.13 applications. Notice the ratings on the last page of each of these links however. The UL489 version shows an "Instantaneous override" pickup value and the ANSI version does not.

Notice the UL489 version is called "insulated case circuit breaker", and the ANSI version is called "low voltage power circuit breaker".

UL489 circuit breakers (MCCB/ICCB) are for use in UL891 rated "switchboards", whereas ANSI rated circuit breakers (ACB) are for use in ANSI C37 "low voltage power switchgear"
This discussion applies mainly to U.S. standards.


As described above, MCCBs are very fast. Therefore they limit the (peak) short circuit current (as also fuses do).
In opposite to this, ACBs do not limit the (peak) short circuit current. That means, that a switchgear that is connect to a transformer via an ACB has to be rated for the full short circuit current (e.g. 50kA for 1600kVA 400V transformer, regarding also motor load) while it can be designed to a lower short circuit current (e.g. 35 kA in this case) when it is connected via an MCCB.


Suggestion: MCCBs have the interrupting rating and withstand rating combined, e.g. 200kA, 100kA, at particular voltage rating.
UL Component Recognized Series-Connected Ratings and CSA Certified Series-Rated Combinations, DET008B by GE Industrial Systems, available on:


Try this link
It is an insert from IEEE Std 1015-1997 - IEEE Recommended Practice fo Applying Low Voltage Circuit Breakers Used in Industrial and Commercial Power Systems. You should find you answers there.

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