The frame is the max. continous current that the CB can be seted to open (not under fault conditions, but overloading for example) in the long-time characteristic curve. The trip is the actual current that the CB is seted.
AF is a rating based on heat dissapation capabilities of the breaker and is the max continous duty rating. AT is interpreted differently by many people, it can be the CT size, the Rating plus, the CT tap, or the actual LTPU setting.
AT has been discussed here before and the consenus was there is not standard for this, IMO it should be the CT size or CT tap if applicable, because when settings get changed the prints are rarly updated.
AT stands for Ampere Trip setting, in the lingo of consulting engineers/vendors in the USA.
There can be a 600AF breaker with a 450A rating plug, it will be designated as 600AF/450AT breaker on one-lines, specs etc. For all practical purposes it will be a 450A breaker. The frame size also relates to the physical dimensions so you know how much panel space it would take up.
Look up MCCB info on websites of leading manufacturers like SQD, GE, Eaton etc.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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From a Switchboard/gear perspective: If the Breaker has a Non Interchangeable Trip....We size our cable/bus to the trip (AT).
If the Breaker has an Interchangeable Trip, we size our cable/bus to the frame (AF).
Also for a Fuse Holder or a Fused Switch, we'll size to the frame (or the maximum fuse that the holder/switch will accept).
These are UL requirements...See 13.7.9, 10, 11.
From a Breaker manufacturer's perspective: Breakers are described by a Model/Type (usually a couple of letters), and a Frame Size.
The Model/Type will determine the short circuit rating.
The frame size is the maximum size trip the frame will accept (or can be adjusted to.
