AIC Rating and Coordination of Circuit Breakers

[a10jp]

In the past when I did the short circuit study on the one line diagram and try to determine the AIC rating for devices, I was taught to make sure the AIC of the device can handle the fault current available at the point of my calculation. Lately, I wonder, as long as I know what the SCCR available at any point in the circuit diagram, does it matter how large the AIC rating of the SPD can be as long as it is larger than the fault current available. I understand the larger the AIC rating (for breakers), the more expensive it will be, but let's put cost aside. Say, if I know the fault would be 25KA, I would use SPD with 28KAIC rating, but how about 42KAIC, or 65KAIC...I realised there are other consideration, such the voltage class of the system, and whether it is even feasible to install a breaker that is rated 20A trip with 65KAIC rating...but I want to ask is:

1. Having a higher KAIC rating, say 65KAIC and 42KAIC above, would that necessarily means they might have differernt let-thru characteristic such that, let say you do have a 25KA fault, the 65KAIC will let-thru more cycles of the 25KA than a 42KAIC breaker would? Meaning are we changing the clearing time if one were to use a higher KAIC rating than necessary?

2. Is there an empiricial relationship between fault current available and the optimum KAIC rating? I recall seeing literature in the past where they used K-factor for breakers used in substation, where they might specify K=1.0, K=1.3, etc...any relationship to what I discussed here?

3. I am also looking for a table that shows the KAIC listing availale for each voltage class...If anyone knows of a handy table in existence...

Thanks.

 

[rbulsara]

1. There is no limit on higher AIC rating of OCPD. For thermal damage, you need to compare that separately on the TCC. Device clearing time depends on the device settings, the fault current and the shape of the TCC. AIC rating itself has no direct bearing on it.

2. Not that I know of.
3. All leading mfr's websites/catalogs have those on their product data sheets.





Rafiq Bulsara

http://www.srengineersct.com

 

[davidbeach]

1. Forget let-through, look at the time-current characteristics (TCC) for the breaker. Just make sure that the breaker can interrupt the available fault current.

2. No.

3. Every breaker manufacturer has one.

 

[jraef]

The Interrupt Capacity (kAIC as you put it) is a rating based upon the maximum fault current the breaker can withstand and safely interrupt. Safely is essentially meaning "without becoming shrapnel". It has nothing whatsoever to do with current limitation. Coincidentally though, "Current Limiting" breakers, those designed to open FASTER the higher the fault current it sees, are generally ALSO going to have higher interrupt ratings.

There s nothing, other than economics, preventing you from using breakers with higher IC ratings than your available fault current, in fact breaker mfrs will love you for it. A dirty little secret in the breaker industry is that there is often (but not always) no difference between a 25kAIC and a 65kAIC rated breaker, other than the label saying the IC is higher. On small breakers, it costs the mfr too much to maintain two separate production lines. They make one breaker, if a batch tests at a higher IC, it gets the higher label, if not, it gets the lower one.

This of course begs the question "Why then do they charge more for the higher AIC rating?" The answer, as is true of all "why" questions involving cost, is "Because they can!"

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[MacGyverS2000]

jr,

No different than CPU manufacturers. They manufacture to the tightest tolerance they can afford and test every chip for the highest speed rating they shot for. If it meets spec, it's labeled as the fastest... if not, it goes into the next bin down. Once they've met their quota for a specific speed bin, all further chips off of the line get downgraded to the next bin. That's how PC overclocking got it's start... people started figuring out that many CPUs could handle the higher speeds but were binned for a lower speed... purchase a cheap low-speed bin, overclock it and get the benefit of a high-speed bin part.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[a10jp]

Thank you all. I think these answer my ingling confusions I had all these times.