short circuit current recommendation

[steve66]

Somewhere I found a recommendation about how much extra AIC rating a breaker should have above the available fault current rating. It had to do with how many times a breaker was rated to interrupt a certain percentage of its rating. But I can't remember the exact recommendation. Does anyone know where I can find that?

For example, the recommendation said something like a 65KAIC breaker was only rated to interrupt 80 or 90% of that current 3 or 4 times. So if the available fault current was more than 80 or 80%, and the breaker had already tripped 4 times, the breaker basically needed replaced.

 

[petersonra]

You are right about a limited number of trips at full short circuit interrupting rating. Breakers have a limited number of times they can open at max conditions. But since such things are really pretty rare it should not be much of an issue.

The AIC rating is what it is. No need to derate to 80%.

 

[jim dungar]

For example, the recommendation said something like a 65KAIC breaker was only rated to interrupt 80 or 90% of that current 3 or 4 times. So if the available fault current was more than 80 or 80%, and the breaker had already tripped 4 times, the breaker basically needed replaced.

Breakers are rated for one cycle of open-close-open at full rated fault current. This is their AIC rating. At this point they need to be replaced.
Breakers are tested for many, sometimes hundreds, of operations at their full load handle rating and similar amounts up to about 15X.

Between 15X and their AIC, I don't believe there are any standard number of operations that can be estimated.

Now as far as how close you should chose a breakers AIC rating versus the available short circuit current (SCA)is different matter. Many people don't like to over size their AIC to about 20% more than what is SCA is available. This is not an uncommon practice, partly due the short cuts and rules of thumb that are employed in determining the SCA. However, if the available fault current was obtained through a detailed analysis that included the X/R ratio of the circuit and the breaker, then the 20% additional buffer is not needed.

 

[Jraef]

At Rated Short Circuit Interrupt Capacity levels of current, the breaker is only required to operate (go from Closed to Open) one time, after that it is toast.

What you MAY have heard along those likes is that ANSI C37 standards for Power Circuit Breakers (PCBs, not MCCBs) used in switchgear has an additional per-pole testing requirement at their SHORT TIME withstand rating of holding in for 0.5 seconds (to allow a fault to clear at a lower level). That level is 87% of the Maximum Interrupt Current rating. That has NO BEARING on anything having to do with standard MCCBs (UL489)

 

[steve66]

I know I read something like that recently in a magazine article or something similar recently. But I sure can't find it again.

Anyhow, I'm not pushing the rating as much as I thought I was.

 

[Jraef]

I wrote a White paper once and my company thought it would make for a good article. After 6 months of back and forth editing where they kept trying to "appeal to the average reader by simplifying" what should have remained as complex issues (because they ARE complex, which was exactly my point), I finally gave up and let them publish it. What it says bears only a passing resemblance to what I actually wrote, but they wore me down. Following that experience, I'm left with little regard for most magazine articles.

 

[PaulMmn]

I wrote a White paper once and my company thought it would make for a good article. After 6 months of back and forth editing where they kept trying to "appeal to the average reader by simplifying" what should have remained as complex issues (because they ARE complex, which was exactly my point), I finally gave up and let them publish it. What it says bears only a passing resemblance to what I actually wrote, but they wore me down. Following that experience, I'm left with little regard for most magazine articles.

Unfortunately, most trade magazine articles are designed to appeal to the lowest common denominator reader, and not interrupt the flow of full-page ads.