MCCB or ACB 1

[alivip]

In substation we usually consider 400v AC for internal feeding of S/S.
consultants suggest us to use ACB (Air circuit Breacker) for incoming feeder and MCCB (molded case circuit breaker ) for outgoing feeder.
Does anybody know why?
why we dont use mccb for incoming?
(if it is needed i can attach related drawing)

 

[rbulsara]

That is a common setup. Coordination is the main reason. ACBs have short time rating, meaning it can be used without instaneous element in the trip units, making them much more palatable to coordinate with the downstream breakers with INST. MCCBs have no short time rating and need INST function to meet UL SCCR rating. They are difficult to coordinate in series with other MCCBs.

ACB is more rugged and maintainable. Also all loads on that switchboard depend on the main, it is nice to make it more reliable and needing to replace it so often.

ICCB (insulated case breakers) may be other choice, but they are merely glorified MCCB as they also do not have short time ratings.

Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

Amp rating is another factor. In the USA, MCCBs are seldom used beyond 1200A, although they are available up to 2500A, I believe. There are no hard and fast rules, however.

Rafiq Bulsara

http://www.srengineersct.com

 

[Zogzog]

One factor is LOTO, simply opening a MCCB provides no visual verfication that the contacts are indeed open, a draw out ACB is LOT in the disconnect position provising a visual verification that meets the requirements of most LOTO plans.

 

[alivip]

whats LOTO?

 

[rbulsara]

LOTO= Lock Out & Tag Out.

A procedure of locking out and placing identifying tags on the devices for safety. A critical part of maintenance and repair procedures.

Rafiq Bulsara

http://www.srengineersct.com