AIC Rating and Coordination of Circuit Breakers
AIC Rating and Coordination of Circuit Breakers
(OP)
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.
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.





RE: AIC Rating and Coordination of Circuit Breakers
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
RE: AIC Rating and Coordination of Circuit Breakers
2. No.
3. Every breaker manufacturer has one.
RE: AIC Rating and Coordination of Circuit Breakers
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!"
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RE: AIC Rating and Coordination of Circuit Breakers
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
RE: AIC Rating and Coordination of Circuit Breakers