AF/AT

AF – Ampere Frame (Frame size of the breaker)

AT – Ampere Trip (Breaker size)

AF should be greater or equal to AT. If AF greater than AT, the breaker can be easily replaced with a higher size for future expansion.

See also

http://cr4.globalspec.com/thread/44398

http://cr4.globalspec.com/thread/36609

- MS

Just because I am a semantics stickler...

MCB is now usually used to describe a Miniature Circuit Breaker, which would not have any designation of AF or AT. Are you referring to a Main Circuit Breaker? Most people have stopped using the term "MCB" for Main Circuit Breaker because of the confusion it can now create with the introduction of Mini-CBs in North America.

Jraef, I am more inclined to think midharpeer means "Molded-Case Circuit Breaker" in his question. Then it would make sense, especially at this frame size.

I am puzzled, however, by a trip rating higher than the frame rating. As others have said, trip rating is standardly less than or equal to frame rating, because the trip rating is only the minimum pickup value - the breaker will take a relatively long time to trip at that point. Please, midharpeer, can you verify that you really did mean those values?