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Old 12-26-2012, 12:23 PM   #1
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Default Fault Current Rating, IC Rating...

Dictionary sez...Fault Close Rating - The ability, in amps, of a switching device to "close" into a fault of specific magnitude, without excessive arcing

But my question is...Is the Interrupting-Capacity (IC) rating of a breaker and the Fault Close Rating considered the SAME THING ? ( two different terms for the same thing ? Or are they different ? ). If not the same thing, then anybody know where I can find some more verbage on Fault Close Rating (how it differs from IC) ? Thanks.


Last edited by Zaped; 12-26-2012 at 07:36 PM. Reason: minor typo
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Old 12-26-2012, 12:54 PM   #2
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Quote:
Originally Posted by Zaped View Post
Dictionary sez...Fault Close Rating - The ability, in amps, of a switching device to "close" into a fault of specific magnitude, without excessive arcing

But my question is...Is the Interrupting-Capacity (IC) rating of a breaker and the Fault Close Rating considered the SAME THING ? ( two different terms for the same thing ? Or are they different ? ). If not the same thing, then anybody know where I can find some more verbage on Fault Close Rating (how it differs from IC ? Thanks.
"Fault close rating" is used for load break switches, ATS, etc and is not a circuit breaker term.

Also it is AIC, not IC.

What are you trying to figure out exactly? Also, be advised ANSI and IEC equipment uses different terms, as do different classes of circuit breakers. Specifics would be helpful in answering your question.

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Old 12-26-2012, 05:50 PM   #3
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Quote:
Originally Posted by Zog View Post
"Fault close rating" is used for load break switches, ATS, etc and is not a circuit breaker term.

Also it is AIC, not IC.

What are you trying to figure out exactly? Also, be advised ANSI and IEC equipment uses different terms, as do different classes of circuit breakers. Specifics would be helpful in answering your question.


Selective coordination of OCP devices?
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Old 12-26-2012, 07:44 PM   #4
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Default Clarification...

Thanks for responses. To clarify, please consider the below
NEC excerpt, quote:

225.53 Type. Each building or structure disconnect shall simultaneously disconnect all ungrounded supply conductors it controls and shall have a fault-closing rating not less than the maximum available short-circuit current available at its supply terminals. END QUOTE


As much as possible I am trying to gain an accurate understanding of this passage (from 225.53 Code section ). And so I would like to find out what 'fault-closing rating' means. Is the term, 'fault-closing rating' defined somewhere?

Thanks.
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Old 12-26-2012, 09:20 PM   #5
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That is poorly worded, fault close is a term used for disconnect switches, not breakers. It refers to the maximum fault a disconnect can be closed into.
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Old 12-26-2012, 10:51 PM   #6
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I think he may be asking what the aic ( ambient inrush ratting) rating of circuit breakers is all about.
For the most part utility supply transformers are impedance rated. A good portion of them can be four percent. So say you have a 300 kva pad mount transformer 120/208 Y. When selecting the service gears ct/main section must have an aic rating not less than the fault current of the transformers output aic ratting. To calculate the trans output we take 300 x 1000 / 208x1.732 (360) = 833 amps. Four percent (multiplier 100/4= 25) 833amps x 25= A.I.C Ratting of 20,825 amps
If the transformer is a real pig, they can be two percent rated. (100/2=50) 833 amps x 50= A.I.C rating of 41, 650 amps. Most larger open bottom gear has a 65k A.I.C Ratting.
We don't want a couple of smaller load centers (22k aic ratted) troffed off the service laterals because the transformer may be two percent impedance ratted. The utility company will (normally) provide you the ratting in impedance of your particular transformer on the site.
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Old 12-27-2012, 12:12 PM   #7
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That is poorly worded, fault close is a term used for disconnect switches, not breakers. It refers to the maximum fault a disconnect can be closed into.


Thanks for the helpful response.

Regarding your distinction between 'disconnect switches' and 'breakers', if 225.38(A) establishes that a disconnecting means (i.e., disconnect switch?) "shall" among other things consist of, quoting from 225.38(A), "a circuit breaker", then I am examining the meaning of 225.53 (including the phrase 'fault-closing rating' therein) as it would conceivably apply to a breaker (if, again, 225.38 establishes that a circuit breaker can be a 'disconnecting means' ).

Next, your explanation, quote, "[Fault close] refers to the maximum fault a disconnect can be closed into", has got me thinking. And I would like to ask, does "closed into" (as in "maximum fault a disconnect can be CLOSED INTO" ) mean...a short circuit condition that is "energized" when the containing circuit is completed by switching the circuit's disconnect switch into the closed/on position? And, if so, does a switch (whether it be disconnect or breaker) that is a switch with a rating, that indicates an X size fault is for that switch within the "maximum fault a disconnect can be closed into", mean that that switch/disconnect, when closed upon that fault, will survive the arc that will probably occur inside the switch/disconnect (and without melting down or otherwise sustaining catastrophic damage) ? [ that admittedly is an interesting sounding scenario but I am just following the ideas wherever they go, in pursuit of information and understanding ] ( and, just as a flashback, all my inquiry in this thread stems from my quest to undertand what is the meaning of the phrase "fault-closing rating" as used in 225.53 ).

( Or, if it means something else, other than what was just asked above, I am trying to find out what. )

Thanks for any response, comment, etc.

Last edited by Zaped; 12-27-2012 at 02:03 PM. Reason: small typo
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Old 12-27-2012, 12:36 PM   #8
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Since the section you're quoting is in the "Over 600 Volt" part of the article, try posting the question in the linework forum. One of the Hi voltage guys might have a a good explanation.
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Old 12-27-2012, 01:10 PM   #9
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Quote:
Originally Posted by Zog View Post
That is poorly worded, fault close is a term used for disconnect switches, not breakers. It refers to the maximum fault a disconnect can be closed into.
Is there the same type of rating for a circuit breaker, or is a circuit breaker "fault closing" rating the same as its AIC.
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Old 12-27-2012, 01:20 PM   #10
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Is there the same type of rating for a circuit breaker, or is a circuit breaker "fault closing" rating the same as its AIC.
I could write a book on that (There are many IEEE papers on the topic). There are a ton of different terms used for breakers, fault closing is not one of them. The terms differ for UL, IEC, and ANSI breakers, and many of them were changed around 2000 to confuse us even more. "Close and latch" would be the closest rating.

Last edited by Zog; 12-27-2012 at 01:24 PM.
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Old 12-27-2012, 02:49 PM   #11
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I guess my question is "what happens when you close into a bolted fault" What rating applies? I know that that shouldn't really happen, but I also know that after a trip, the first thing that happens in many cases, is that the breaker is closed into the fault. I assume that if the current is high enough, it can't "close and latch".
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Old 12-27-2012, 03:48 PM   #12
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Old 12-27-2012, 03:48 PM   #13
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Old 12-27-2012, 03:50 PM   #14
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