So what does an AIC / AFC failure look like in the field ? 10k breaker on 18k AFC drop.

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brycenesbitt

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I know what an AIC calculation is, and the basics of how to do them. My local jurisdiction is increasingly demanding that even upgrade projects recalculate AIC.

But the question is now what does an AIC / AFCI failure look like in the field, for 120/208Y or 240V Split Phase service?
I get that a breaker might physically explode inside a box, in theory, on a hard ground fault.
What does that look like in the real world? Anyone have pictures? Anyone seen it happen in their practice?
How common is it?

If I see a 10K AIC breaker exposed to 18K or 35K of AFC in an existing building,
should I be worried and calling that out for correction?

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I did find THIS video which seems related, sort of. The short in question was not cleared by the main, and the found an arc at the weatherhaed
 
I'm not saying it can't happen, but I've never seen a serious 208/120 Y or 240V serious arc event. Sure I've seen lots of equipment get damaged by arcs at those kind of voltages, but nothing crazy like breaching it's enclosure in any kind of serious way.

I have seen the aftermath of 480 and higher arcs that have you looking at the pieces racking your brain wondering how all that metal got bent like that
 
Most cases a 10kA branch breaker will be series rated with the 22-25kA rated main ahead of it.

I will add that about 35 years ago I got to go through a QO breaker manufacturing plant. Was still a student then and didn't know what questions to ask that I may ask today. closer to end of the tour we went into the testing lab. The demonstrated a fault current test procedure (again did not fully understand all of this at the time but still got an impression from it). From recollection - they took a three pole probably in the 35-70 amp range and tied all three output lines together with probably 6 or 8 awg. Plugged it onto some small panelboard bus and closed some rather heavy doors. The test current applied was supposedly 10kA. It made a fairly loud boom when they gave it power, no idea what kind of controller was applying power or how many tests it was good for one of those questions that comes up years later, but when they opened the heavy door smoke was still lingering in the enclosure. They did successfully reset the breaker handle. All the carbon deposits left kind of made you wonder if you should even attempt to use the breaker again even though it supposedly passed the testing process.
 
It all depends on the amount of fault current flowing through the breaker. But the probable worst case is a violent explosion resulting in shrapnel and collateral damage as well as as large arc flash. It is easy to find videos on line from equipment manufacturers.

In reality worse case situations do not exist very often and so are fairly rare. In most real world events bolted faults do not occur on operating systems, they usually show up after installation or maintenance.

The video in the OP is an arcing event not a bolted fault so it is not likely a worst case event. It is also upstream of any protective device so AIC is not an issue. These events have to do with the SCA, available short circuit fault current.
 
in my experience in the QO plant, I can't recall if they placed a cover on the small load center they plugged the breaker on or not. I would think if they did not have a cover on it would have more chance of blowing the breaker off the bus bar (was plug on type breaker). Also would think the more tests they did on that bus the more it would weaken the bus to breaker connection, but maybe they have limited number of tests or even only use the bus for one test? I know after seeing carbon deposits everywhere it made you question whether you should use any of those items again.

Again not experienced enough back then to know what questions to ask. And years later thinking about it, with carbon deposits everywhere - where exactly did that come from? Something had to give it up in the blast. Another reason to question use of anything again. Would been interesting to see them disassemble that breaker and see what the contacts looked like afterwards. Or to see it any other components looked compromised.
 
Here is a video that has circulated for decades now, put out by Bussman (before they were owned by Eaton, who now has breakers too, so they no longer promote this). The examples are a little on the extreme side, i.e. subjecting a breaker rated for 14kAIC to a fault of 50kA, but in the 2nd half, they show a 14kAIC breaker and a 14kA fault, which is a little more accurate (although you typically would not have 12ga wires on the line side of the breaker too...).
So you can see from that later test, even if the breaker survives at it's rated IC, there is still damage. In the AFC is higher, all bets are off in terms of collateral damage.
 
Yeah AIC has become one of those "hot button" issues for inspectors and plan reviewers in the last few decades. I think the concern is far overblown and not based on any history of failures or issues. The stars would really have to align to have an OCPD have a catastrophic failure.
 
electrofelon said:
Yeah AIC has become one of those "hot button" issues for inspectors and plan reviewers in the last few decades. I think the concern is far overblown and not based on any history of failures or issues. The stars would really have to align to have an OCPD have a catastrophic failure.
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I was recently (last week) denied a permit to change a 30 amp Federal Pacific Electric breaker,
like for like, for a modern breaker until meeting the following conditions:

2. Revise the drawings to provide a complete panel schedule(s) which clearly indicates the
panel size, bus rating, AIC rating, voltage, phases, location, circuit layout, loads, load
type, overcurrent protection, and load balancing. Clearly identify those portions of the
system which are within the proposed scope of work and those which are existing to
remain. [CEC 110.1]

3. Provide series rated or engineer rated electrical design system. When modifications to
the electrical installation occur that affect the maximum available fault current at the
service, the maximum available fault current shall be verified or recalculated as
necessary to ensure the service equipment ratings are sufficient for the maximum
available fault current at the line terminals of the equipment [CEC 110.24.B]
Click to expand...

This is an older building, so no AIC ratings were produced at the time for the equipment, and none are available now.
Our local utility then chimed in and said they have a new policy of not writing an AIC letter for the incoming service without
a full panel replacement project (policy as of September 2025, Pacific Gas and Electric Company as relayed by the field service rep).

So we sh*t canned the project and left the FPE breakers in place.
 
brycenesbitt said:
I was recently (last week) denied a permit to change a 30 amp Federal Pacific Electric breaker,
like for like, for a modern breaker until meeting the following conditions:



This is an older building, so no AIC ratings were produced at the time for the equipment, and none are available now.
Our local utility then chimed in and said they have a new policy of not writing an AIC letter for the incoming service without
a full panel replacement project (policy as of September 2025, Pacific Gas and Electric Company as relayed by the field service rep).

So we sh*t canned the project and left the FPE breakers in place.
Click to expand...
Love it! 🙄
 
brycenesbitt said:
But how will that help if the AHJ demands to see the AIC rating for a 70 year old whatever box that's existing?
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Whatever box equipment listings are published, if labels are missing.

AHJ gets AIC for all models, similar to unit in question, and has no more excuses with worse-case scenario in hand.
 
brycenesbitt said:
Cool, did not know we could measure it. But how will that help if the AHJ demands to see the AIC rating for a 70 year old whatever box that's existing? I put my new 50 KAIC subpanel after it, but what about the other crap in the line of current?
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You don’t. You tell the end user that their only option is for you to rip and replace the entire panel. Nobody likes hearing that, but it is what it is.
 
Residential style breaker AIC values have been available since at least the 50's. Back then 5kA was standard. UL raised the minimum to 10kA some time in the 60s.
 
I wonder what the available fault current was with a 60 amp REA service installed in the 40's. Single customer transformer, 3 conductors on an insulator rack aerial drop. Bet it was low. 1,500 amp maybe?

What was used on the pole, 5KVA, 7.5KVA?
 
Joethemechanic said:
I wonder what the available fault current was with a 60 amp REA service installed in the 40's. Single customer transformer, 3 conductors on an insulator rack aerial drop. Bet it was low. 1,500 amp maybe?

What was used on the pole, 5KVA, 7.5KVA?
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This ... many times the AFC isn't all that high to begin with, particularly on 200 amp single phase service or less. Not always true but if you take a look at what is in the supply items you can have a pretty good idea whether you even need to do much calculating.

and in the past 40-50 years main breakers are usually rated at least 22kA and are series rated with the 10kA branch breakers that are listed for that panel. On most single phase applications of 200 amps or less if you have at least 25 feet or conductor from the source it often will be enough impedance in that conductor to keep AFC low enough. Maybe need to start paying closer attention if the source is over 50 kVA

I've encountered temporary service for construction that is right next to a large transformer and end up needing fused disconnect for the service disconnect and use series rating for 10 kA breakers in the temp panel. Permanent service equipment either had lesser AFC after some service conductor length, higher AIC ratings, little of both.
 
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