Amp Frame / Amp Trip

[edavis2293]

Industrial one-line diagrams illustrate circuit breakers in terms of "Amp Frame / Amp Trip: AF/AT. Any one have a definition of what the terms mean?

Thanks Folks

 

[kingpb]

There are multiple meanings, but the most common is that the breaker can handle for example a maximum of 400A, but it has a built-in trip setting that allows you to adjust the long-time overcurrent protection down to say 320A, or within some other range. Some devices come with what they called plugs, that allowed the user to change the range by taking one out and replacing with another. Usually there is a limit to the lower setting before you have to change to a smaller frame.

 

[infinity]

It similar to a fused disconnect. The frame of the CB will allow a certain maximum size, the trip setting is changeable by inserting different fixed sized or adjustable trip devices.

 

[zog]

Um, no, no and no.

AF is the frame size rating of the breaker
AT is the CT size on the breaker.

Neither of these tell you anything about when the breaker will trip, nothing to do with trip settings, or rating plugs.

 

[infinity]

Um, no, no and no.

AF is the frame size rating of the breaker
AT is the CT size on the breaker.

Neither of these tell you anything about when the breaker will trip, nothing to do with trip settings, or rating plugs.

Ok so the print shows a 400 amp frame with a 250 amp trip how do I size the wire if the 250 amp number is meaningless?

 

[zog]

Ok so the print shows a 400 amp frame with a 250 amp trip how do I size the wire if the 250 amp number is meaningless?

The print dosent show the trip at all, thats my point. You size the wire to supply the necessary loads and set the trip settings (LSIG) on the breaker to properly protect the equipment it is there to protect.

 

[infinity]

The print dosent show the trip at all, thats my point. You size the wire to supply the necessary loads and set the trip settings (LSIG) on the breaker to properly protect the equipment it is there to protect.

Sounds like semantics to me. The OP asked a simple question, the amp frame size pertains to the breaker frame size and the amp trip pertains to the trip setting. Why does it have to be more complicated than that?

 

[zog]

Sounds like semantics to me. The OP asked a simple question,

And I gave a simple correct answer, it is not semantics at all the CT size is not the same in any way shape or form as the trip setting.

the amp trip pertains to the trip setting.

It most certianly does not.

 

[jim dungar]

Um, no, no and no.

AF is the frame size rating of the breaker
AT is the CT size on the breaker.

Only if the breaker has CTs.

A Square D LA36350 is a thermal magnetic trip breaker in a 400AF (this is the physical size of the breaker) with a 350A long time trip element.

I have always used:

AF = maximum long time current available in that physical package.
AS = sensor. CT, or plug, size of the breaker (the LT setting is always a multiple of the AS)
AT = Long time trip setting of the breaker.

 

[brian john]

[QUOTE

AF = maximum long time current available in that physical package.
AS = sensor. CT, or plug, size of the breaker (the LT setting is always a multiple of the AS)
AT = Long time trip setting of the breaker.[/QUOTE]

And then hopefully you receive a coordination study to adjust the trip settings
(LSIG).

L=Long Time
S=Short Time
I=Instantaneous
G=Ground Fault

 

[kingpb]

Um, no, no and no.

AF is the frame size rating of the breaker
AT is the CT size on the breaker.

Neither of these tell you anything about when the breaker will trip, nothing to do with trip settings, or rating plugs.

Sorry, this is not correct.

CT's indicated on a drawing will provide the ratio of the CT installed, e.g. 500/5, 100/5 12,000/5, or for the IEC folks 500/1, 250/1 etc.

The CT ratio as shown on a drawing, is for an independent piece of equipment of which is separate from the trip setting of the protective relay; although it is certainly used in doing the calculations to develop relay trip settings.

 

[Jraef]

Um, no, no and no.

AF is the frame size rating of the breaker
AT is the CT size on the breaker.

Neither of these tell you anything about when the breaker will trip, nothing to do with trip settings, or rating plugs.

Yep, have to agree with the others, you are wrong on this. "AT" does in fact refer specifically to the Amp Trip setting. And by the way, it can be used in referring to either an ELCB or a TMCB, there is no specific delineation in that regard.

And to elaborate a bit more, there is a valid reason to list both values. In a lot of cases there are overlaps in the trip ratings available. For instance, you may need a 250A breaker for something. That could be a 250AF/250AT, or it could be a 400AF/250AT. An engineer may specify a 400AF because he knows that someday, the load may increase to 300A. If a 400AF breaker was specified, only the trip unit or rating plug needs to be changed, or in the case of a basic TM CB, at least the space requirements stay the same and the bus connections don't need changing in the switchboard. Had he specified a 250AF/250AT, upgrading might be a lot more involved if not impossible on a full switchboard.

 

[zog]

Anyone have a reference on this, IEEE or ANSI abbrevations or something? Every print I have ever seen (Granted I only deal with substations) designate the CT rating. You may have a 1600A feeder (Frame size) and only 600A loads so a 800A CT would be used (1600AF/800AT) so the trip units can be adjusted to properly protect the 600A load, all settings are multiples of the CT's.

Maybe it is different for MCCB's that dont use CT's or breakers with series trip devices but thats not the kind of systems I am used to dealing with. I catually lost sleep over this last night thinking about different EE's using different meansing for AT.

If I am wrong, then great I learned something today but no one (Including myself) has referenced any standard at all, we are all just talking about what we do or have seen. Also, if I am wrong, there are hundreds of wrong one lines out there.

 

[zog]

If the AT is the trip setting do you go back and change the one line each time a trip setting is changed?

 

[kingpb]

If I am wrong, then great I learned something today but no one (Including myself) has referenced any standard at all, we are all just talking about what we do or have seen. Also, if I am wrong, there are hundreds of wrong one lines out there.

There is not a standard that specifically lists AT definition.

I would be interested to see your point of reference, so could you post a portion of a one-line you are referring too, so we can make sure we are on the same page.

Like I said in my first post, AT could mean different things, and the drawing it appears on, or equipment it is associated with can make a big difference.

It sounded like from the OP that it was in reference to a low voltage one-line that was related to was assumed to be a switchboard or panelboard. If it is a metering and protection, substation, or distribution one-line then it could mean something entirely different.

 

[zog]

There is not a standard that specifically lists AT definition.

Thats what I thought, so it is all a matter of personal preference

I would be interested to see your point of reference, so could you post a portion of a one-line you are referring too, so we can make sure we are on the same page.

Cant, all are propriatary info of big companies

Like I said in my first post, AT could mean different things, and the drawing it appears on, or equipment it is associated with can make a big difference.

I can accept that, my mind gies right to MV and LV switchgear, I need to take my blinders off sometimes.

It sounded like from the OP that it was in reference to a low voltage one-line that was related to was assumed to be a switchboard or panelboard. If it is a metering and protection, substation, or distribution one-line then it could mean something entirely different.

Again with the blinders

 

[Cold Fusion]

If the AT is the trip setting do you go back and change the one line each time a trip setting is changed?

zog -
I've seen two "standards".
For molded case stuff that will take rating plugs, but not adjustable beyond that, then the rating plug is on the one-line. And if the rating plug is changed, the one-line is reved.

For programmable protective relays, the sensor/plug rating is on the one-line, and the programmable trips are in a configuration file somewhere.

cf

 

[zog]

zog -
I've seen two "standards".
For molded case stuff that will take rating plugs, but not adjustable beyond that, then the rating plug is on the one-line. And if the rating plug is changed, the one-line is reved.

It SHOULD be but I bet it actually is less than 10% of the time

or programmable protective relays, the sensor/plug rating is on the one-line, and the programmable trips are in a configuration file somewhere.

cf

Yep, thats what I am used to seeing, but neither of these are "Standards", ANSI/IEEE or other, thay are just the way it is done by most.

 

[mull982]

So for a molded case circuit breaker the AF or frame size determines the maximum continuous current that can flow through that size breaker, and then the AT or trip rating is the plug that is installed in a particular frame type that can never exceed the AF rating?

I'm assuming that the larger the frame type is the larger the breaker is physically.

I guess it can get confusing if you are talking to someone and they say that they have a 400A breaker, because you dont know if they are refering to the trip rating or the frame rating?

When sizing breakers per the various tables in the NEC do you use the AT or trip rating of a particular breaker to select the correct size? I would not think that you would use the frame rating.

 

[weressl]

Industrial one-line diagrams illustrate circuit breakers in terms of "Amp Frame / Amp Trip: AF/AT. Any one have a definition of what the terms mean?

Thanks Folks

Amp Frame - Maximum continuous current carrying capacity for the thermal design of the circuit breaker.

Amp Trip - either fixed or adjustablle device that limits the continuous rating of the device.

Therefore the Amp Trip never exceeds the Amp Frame.