Busbar Rating - Incomer Selection

[NickParker]

Is it mandatory to have the bus bar rating equal to the Ampere Frame (AF) of the Incoming Breaker (MCCB) in the MCC, sub distribution panels. Is it okay to have the bus bar rating equal to the AT ratings?
In general, what is the criteria/

 

[RRaghunath]

Ampere Frame rating can be higher depending on the temperature derating factor applied. So, what is important is the design (after derating as required) rating of Incomer and Busbar have to be same so that there is no bottle neck either in the incomer or in the busbar.

 

[cuky2000]

Yes, I believe that it is okay to have the bus bar rating equal to the AT ratings.

See the edited example below

 

[che12345]

Dear Mr NickParker,
"Is it okay to have the bus bar rating equal to the AT ratings?"
1. It is okay to have the bus bar rating equal to the AT rating. It is NOT necessary to have the bus bar rating equal to the AF rating.
2. Most IEC breakers are manufactured with say five to six AF (i.e. physical size) current ratings. Each AF size is possible to installed with different AT unit (i.e. current rating). Each AT unit is further adjustable within say 0.5 to 1.0 of the AT value.
3. As an example, the busbar is rated 250A, suitable for ambient temperature of 40DegC.
The MCCB AT should be at least 250A. But some manufacturer's rating may not have a AT of 250A. The rating is covered by:
a) A 320A AF breaker installed with a 320A AT unit. The trip unit is adjusted down to 250A/320A=0.78. The tripping characteristic will be that of a 320A x 0.78 = 250A breaker.
b) A 630A AF breaker installed with a 320A AT unit. The trip unit is adjusted down to 250A/320A=0.78. The tripping characteristic will be that of a 320A x 0.78 = 250A breaker.
Note: alternative b) would be of bigger in physical size and at a higher cost but of same rated 250A.
4. It is extremely important that the [trip unit and the adjustable dial setting] MUST be sealed with a one-time seal/sticker over it. This is to prevent the tested setting being tampered with to a higher current rating.
Che Kuan Yau (Singapore)

 

[Sazirul]

Trying to learn from this discussion and went to google for finding the meaning of "AT". But nothing found. Is it Ambient Temperature? If not then, what does it means by "AT" in this discussion and cuky2000, could you please upload the image with better resolution.

Thanks

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[cuky2000]

For molded Case Circuit Breakers (MCCB) or Power Circuit breakers, the following apply
1) Ampere trip [AT] it is the current set to trip the circuit [from 100% of the AF to less than 100% - say ~50%].
Ampere Frame [AF] it is the rating breaker current [maximum current which the breaker will withstand for a long time]

For better resolution, copy the image and pasted in MS Word, Power Point or similar software. If that do not work, I see if I can do differntly.

Hope this help

 

[che12345]

Dear Mr Sazirul

1. The terminology "ampere trip (AT)" and "ampere frame (AF)" are not official terms as defined in the IEC or NEC publications. BTWo you know what is IEC and NEC ? Inform me, if not.
1.1 Following terms are widely used by the breaker manufactures: for "Molded Case Circuit Breakers (MCCB)", and "Power Circuit Breakers (PCB)in NEC = Air Circuit Breakers (ACB) in IEC "
2. All breaker manufactures irrespective of whether in IEC or NEC land, covers from very low (say 100A up to 6300A) etc.
2.1 It is not sensible to manufacture thousands of different (frame=physical)sizes and different (trip=current) ratings.
3. "Ampere frame (AF)" is the [physical] size.
a) e.g. for MCCB: F1 is rated up to 100A, F2 (up to 200A)...F6 (up to 630A).
b) e.g. for ACB: F1 is rated up to 1kA, F2 (up to 2kA)....F6 (up to 6.3kA).
In this way, a small number of frame=physical sizes cover a wide current range.
4. "Ampere trip (AT)" is the [trip] current rating.
a) e.g. for MCCB: T1 trip current (adjustable from 50-100A), T2 (from 100-200A)...etc.
b) likewise for ACB.
5. Selection of breaker AF and AT
a)e.g. for MCCB: to select a breaker with [trip] current of 450A may select a [AF] size F5 with a [AT] size T5 trip unit adjusted down to 450/500=0.9. It is NOT possible to select a [AF] size F4 as it is rated up to maximum current of 400A. However, it is possible but with [higher cost and physical size] to select a [AF] size F6 installed with a [AT] size T5 unit adjusted from 250-500A.

Che Kuan Yau (Singapore)

 

[Sazirul]

Dear Mr. Che Kuan Yau,

I don't know very well about those IEC or NEC. But, by your discussion, I got some idea. I have some ACB and all are following the IEC 60947-2. Could you please give me the full documents that I can read and learn.

Thanks

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[che12345]

Dear Mr Sazirul

1. You can Google for more details about IEC and NEC.
2. The breaker you had posted on the top LHS reads SACE E6.2H and on the RHS top reads ABB SACE-Italy. The breaker is manufactured in Italy by [SACE] (which is now under ABB).
3. Google "ABB Electrical installation handbook" and many more ABB publications/catalogues etc., to have more details.
3.1 All IEC breaker manufactures classify "Ampere frame (AF)" and "Ampere trip (AT)" similarly, except NEC land under UL standards defers.
4. Under guidelines: Promoting, selling, recruiting, ...is forbidden.

Che Kuan Yau (Singapore)

 

[Sazirul]

Thank you, Mr. Che Kuan Yau.

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