ACB vs MCCB 6

[144x]

can anybody tell me what is the advantages of ACB(Air Circuit Breaker)
over MCCB(Mouldede Case Circuit Breakers)?and would it be possible to
use MCCB instead of ACB for the incoming feeders of LVAC distribution
panels of a substation with current rating of 630 A and a short circuit capacity of 25KA.is there any standard for their usage and differences?
Thanks.

 

[dpc]

There are a lot of differences. From a coordination standpoint, the biggest difference is that Low Voltage Power Circuit Breakers have a withstand rating for through-faults, so they can be used *without* any instantaneous trip. A MCCB is required to have instantaneous trip. If you use a MCCB as a main breaker it will trip for any downstream fault exceeding its instantaneous setting. This makes it nearly impossible to achieve selectivity for high magnitude faults.

MCCB are not maintainable and LVPCB are field-maintainable. The flip side of this is that MCCB (supposedly) never require maintenance, while LVPCB do require it.

MCCB are normally 80% rated, while all LVPCB are 100% rated.

MCCB are a lot cheaper of course.

Hope that helps.

dpc

 

[144x]

Thanks dpc,is there any difference regarding safety between
them.are there withdrawable MCCB's?

 

[dpc]

You can buy "insulated case" circuit breakers in drawout construction. These are built under UL requirements for molded case breakers but typically have higher interrupting ratings (and cost more).

I don't think safety would be an issue, although use of LVPCB would probably imply a little higher level of skill for maintenance electrician to do necessary maintenance, but this can be sub-contracted.

Both types can carry UL label, but they are built to different UL standards.

The LVPCB can be easily electrically-operated for remote operation. This is possible for MCCB as well, but the motor operator is pretty clunky.

dpc

 

[coyney]

consider the fault-level at the transformer terminals by assessment of the HV system and the t/f reactance.

Then consider the MCCB and ACB fault-making/breaking capacity.

The down stream breakers/fuses need to checked to discriminate with the ACB/MCCB.

 

[electrifire]

You are asking a question that goes to the heart of electrical design. DCP is correct in the information provided in the two post.

Low voltage power circuit breakers (LVPCB) are the older workhorse of the industry. They are fixed or drawout mounting, have an interrupting rating and a withstand rating that were missing from olded molded case circuit breakers (MCCB)and were generally preferred for industrial and large commercial applications because their trip times were adjustible to allow coordination with upstream and down stream devices.

In todays market MCCBs with electronic trips do cover most of the coordination problems, but most still need an instantenous trip even if a short time delay is included. MCCBs can also be purchased with higher interrupting ratings without added fuses for the higher ratings to a point.

Insulated case breakers are similiar to the MCCB except with more withstand ability and a higher price.

To answer your question maintenance is the question. A drawout LVPCB is easier to maintain and clean and test to assure operation within initial settings. MCCBs are typically fixed mounted and are therefore harder to test trip without removing the entire switchboard from service and removing the circuit breaker from its mounting. This seldom happens.

Newer electronic trip devices on all three classes of breakers allow trip device testig with a low current and low voltage testset to verify the settings in lieu of the high current testset required for testing at full current.

A rating of 800 amps with an AIC of 25,000 amps set for 630 amps is within the ability of most MCCBs until you get to 600 volts. You did not state voltage and to get 25,000 AIC at 600 volts requires a high interrupting capacity breaker.

I would not recomment a fixed trip MCCB for a main application because of the limited coordination with down stream devices which would have to be rated about 50% of the main trip rating. Also with a fixed trip device you will have to choose 600 amps or 700 amps to come closer to the stated 630 amps.

If either type MCCB is selected it should be operated at 80% of its continuous current application. An 800 amp frame breaker is required for a continuous 630 amp load. With the LVPCB it can be operated at 100% load, but at the 630 amp level it becomes a mute point.

All the information provided herein and that provided by DP assumes a low voltage system, 600 volts or below. This information was not provided in the original post.

I am not aware of any standard for the application of one type vs. the other. Aorrect application of either is approved by UL and the NEC. The preferred application is a question of maintenance preference, downtime cost affect and length of life vs. initial cost.

Hopes this helps.

 

[czping]

Hi,
Just a small note. If you require remote switching, it might not be possible with MCCB.

I have a battery charger which is connected to a DB through one MCCB. Thus this MCCB is always "ON" because the DC output is used as control supply for all our switchboards. I need to do a reliability study on this DC system. Has anyone come across any article on failure rates of MCCB? If really need to, is there any maintenance required? By the way, this MCCB is only carrying 30% loaded most of the time.

Thank you!

 

[dpc]

to czping:

I would be more concerned about the battery charger than the MCCB. There is no maintenance on a MCCB other than periodically cycling the breaker OFF and ON. In fact, per UL, there is no maintenance permitted - they are not considered repairable devices.

 

[coyney]

Look at the IEC 947-2 definitions for Iuc, Icw, Isc & Ibc.

ACB's:
Larger and heavier
More expensive
Maintainable components
Longer times fault for withstand capacity.
Can be used with IDMT protection relays


MCCBs:
Smaller and lighter
Please be carefull about the interpretation of fault ratings
Lower withstand time (secs.) fault capacity
Need to be replaced following a limited number of faults

Regards,

 

[Foofoo]

Good responses but why use and MCCB or ACB as an incomer when an isolator would do ? Presumably there will be upstream devices providing cable protection. Introducing an MCCB or ACB as an incomer only messes up discrimination within the power system.

 

[jbartos]

Suggestion:
ACB (or low voltage switchgear) traditionally tend to be applied to higher current ratings, top is now well exceeding 6000A. e.g. Masterpack Merlin Gerin is rated up to 6300A.

http://www.buildingdesign.co.uk/elec/siemens/siemens-7.htm

Siemens ACB rating is up to 6400A.
MCCB are used up to about 2000A.
Therefore, comparisons are only partially possible in the current rating ranges that are overlapping and available.
MCCBs tend to be preferred in the overlapped range. In cases, where a special protection that does not come with MCCB is required the ACBs are selected.