IEC equivalent of AIC Rating

[EddyWirbelstrom]

I understand AIC Rating to be the interrupt rating of a breaker.
For low voltage breakers is AIC the initial symmetrical rms current ?
For medium/high voltage breakers is AIC the symmetrical rms current at the instant of contact parting ?
Does AIC include the d.c. offset ?
Is AIC asymmetrical current ?
What is the IEC equivalent of the AIC ?

 

[waross]

The ratings are based on the Available Short Circuit Current which is symmetrical.
These ratings are used to select breakers and other equipment.
The equipment is tested with asymmetrical current corresponding to the DC offset of a given X:R ratio.
A fault current may be greater than the ASCC.
The device testing takes this into account.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

For NEMA and UL low voltage breakers: The AIC is generally the maximum symmetrical current that occurs in the first half cycle. However, this rating is only valid up to X/R ratio that the breaker was tested at. The test X/R ratio (generally referenced as a power factor) varies depending on the type of breaker. If the actual system X/R is higher than the test value, the rating must be adjusted.

For ANSI-rated high voltage breakers, there are two separate short circuit ratings. The primary rating is an interrupting rating based on the fault current at the time the contacts open. It is generally expressed in rms symmetrical amps. There is also a first-cycle or close and latch rating that is based on the first cycle peak current. It is generally referenced as a "crest" or peak current. For new breakers, the close and latch rating is specified by the standard as 2.6 times the interrupting rating. The rating methods have been modified a few times in the past so there will be some differences depending on the vintage of the breaker. Also, calculation of short circuit duty for high voltage breakers is complicated when fault current gets close to the rating.

I can't help much with IEC standards.

Cheers,

Dave

 

[sky2020]

Hi EddyWirbelstrom,
Sorry to put here my question but the thread is closed

https://www.eng-tips.com/viewthread.cfm?qid=278589

and I am not sure how to contact you. Do you still have the PTW file? Can you please share it?

 

[cuky2000]

Echoing Waross, all circuit breakers are rated on symmetrical RMS value for a tested X/R ratio. If this value is exceeded, the AIC rated breaker required to be derated following the guideline in the standard.

The following is an attend to summarize the different application based on the rated voltage of the circuit breaker:

1) HV Circuit Breaker: Recent homogenization between IEC and the IEEE standard issued several amendments to the applicable IEEE Std C37’s. An important change was the %dc component based on a standard time constant of 45 ms (2.7 cycles) corresponding to X/R=17 for 60 Hz.

2) MV Circuit Breakers: application sees the following Link show the relation between the IEC Std IEC 62271-100 & IEEE Std C37.

3) LV circuit breakers see the enclosed Link

For reference see the related thread Multiplying Factor in Momentary Duty .

The illustration below shows different X/R tested values for several protective devices used in the power industry.

 

[EddyWirbelstrom]

Thank you Waross, dpc, and cuky2000 for comprehensively explaining how the IEEE Standards, and Standards IEC 62271.100 ( MV ) and IEC 61439.1 ( LV ) for circuit-breaker short-circuit withstand ratings have been homogenized.

I have attached an excel spreadsheet which summarizes the short-circuit duty – calculated in accordance with IEC60909-0 ( AS3851 ) - and the corresponding LV and MV switchgear short-circuit withstand ratings required in accordance with IEC 61439.1 and AS 62271 respectively.
( Australian Standards are almost carbon copies of IEC Standards )

Hello sky2020
I no longer have the PTW model.
I will revisit Eng-Tips thread238-278589 and re-create the PTW model and post on Eng-Tips.
I will also review odlanor’s postings.

 

[waross]

The Available Short Circuit Current (ASCC) of a transformer is easily determined by dividing the transformer rated current by the Percent Impedance Voltage.
When this first came into common usage, transformers and switchgear were often selected by electricians. and the AHJ was for the most part an inspector who had come up through the ranks as an electrician.
Safe installations could be designed and inspected for compliance with only high school arithmetic.
Calculating asymmetrical current peaks was beyond the educational level of the persons responsible for installations in days past.
Using ASCC values for equipment that was actually tested for safe operation at reasonable X:R ratios, shifted the responsibility to the design engineers who did have the educational ability to calculate asymmetrical peak currents and test accordingly.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[EddyWirbelstrom]

Hello Skye2020
The link below is to the PTW32 Ver 9006 project with unearthed star connected generator and zig-zag earthing transformer.

https://www.dropbox.com/s/nzgfih2hzlw48w4/Eng-Tips_Thread238-278589_RevA.zip?dl=0

Use the datablock Eng-Tips Thread238-278589
A folder Man Calc has manual calcs.