Breaker AF/AT Ratings

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DLCOX

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Needing some help understanding why some breakers have AF/AT ratings and others don't. On my one-line drawing it appears that all breakers above 400A have this rating but lower amperage breakers don't? Is that just the engineers choice or do most add it?

Thx D
 

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Its not a rating, its a specification of the part. 800 amp frame with a 800 amp trip unit. The smaller breakers likely don't have interchangeable trip units so there's nothing to specify.
 
Just an engineering choice.
Many breakers in the 250A and smaller size have fixed continuous ratings. Nowadays most larger breaker have adjustable long time rating.
 
Old thread that may help explain this

AF is physical frame size of the breaker.

AT is trip setting of the breaker.

You can possibly find a breaker with a 600 amp frame size but with a trip setting of lesser rating, like 400 amps.

You likely never see such specifications on less than 400 amps frame size because it is pretty rare to see such units under a 400 amp frame

This coming from what I get out of that other thread. I haven't been around adjustable trip settings (other than adjustable magnetic trip level) so it may also be associated with adjustable trip breakers as well.
 
kwired said:
You can possibly find a breaker with a 600 amp frame size but with a trip setting of lesser rating, like 400 amps.

You likely never see such specifications on less than 400 amps frame size because it is pretty rare to see such units under a 400 amp frame
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It is common to have different trip settings available than the frame size. It is unusual to see adjustable trip settings in smaller frame size breakers though.

Small frame MCCB are typically 125 A frame but the trip points you can buy them with start as low as 10 or 15 Amps and go up to 125 Amps. I have even seen a few where the trip setting can be larger than the frame size, but I think those are only for use in IEC areas.
 
petersonra said:
I have even seen a few where the trip setting can be larger than the frame size, but I think those are only for use in IEC areas.
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Trip ratings above 100% continuous were fairly common on the old "iron frame" ANSI rated drawout switchgear breakers, particularly those with dashpot relays. They pretty much fell out of favor once solid state trip units became popular back in the 70s.
 
petersonra said:
It is common to have different trip settings available than the frame size. It is unusual to see adjustable trip settings in smaller frame size breakers though.

Small frame MCCB are typically 125 A frame but the trip points you can buy them with start as low as 10 or 15 Amps and go up to 125 Amps. I have even seen a few where the trip setting can be larger than the frame size, but I think those are only for use in IEC areas.
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Breaker that is more of a definite purpose breaker but sort of fits in the discussion is Square D QOM series. You only see them in 100 and 125 amps factory installed in that frame size, but they do have them in as low as 60 amp in that same frame size. Not certain but possibly can get a 100 or 125 amp unit in a 225 amp frame size. I have use the 60 amp version a time or two.
 
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