AIC Rating Cross Check from POCO

[wwhitney]

Dangerous in what way? Is the danger being overstated or understated?

Dangerous = danger understated. My understanding is that a lower AFC can lead to a higher total energy released during a fault, due to slower clearing times. So for Arc Flash, it is non-conservative to make assumptions that overestimate AFC.

Cheers, Wayne

 

[jim dungar]

Dangerous in what way? Is the danger being overstated or understated?

Understated.
The higher the current the faster a OCPD decides to open which gives a low incident energy.
The lower the current the longer it takes an OCPD to decide to open resulting in a long arcing time and higher incident energy.

 

[wwhitney]

The higher the current the faster a OCPD decides to open which gives a low incident energy.
The lower the current the longer it takes an OCPD to decide to open resulting in a long arcing time and higher incident energy.

So if the POCO only gives you an upper bound AFC for purposes of selecting the AIC of equipment, how do you determine a lower bound AFC for the service for the purposes of Arc Flash studies?

Also, if the POCO ever replaces equipment on their end and thereby reduces the actual AFC at the service, is there a mechanism for the customer to be notified so they can redo their Arc Flash study?

Thanks,
Wayne

 

[jim dungar]

So if the POCO only gives you an upper bound AFC for purposes of selecting the AIC of equipment, how do you determine a lower bound AFC for the service for the purposes of Arc Flash studies?

I have had studies "paused" for many many months while we waited for the utility to provide more realistic values.

In some cases we ran multiple analyses using various lower available fault currents with the intent of finding a worst case, highest incident energy, scenario. These processes were detailed in our final engineering report.

Typically worst case incident energy was found when the facilities were lightly loaded, say during planned maintenance when motors are off, and when they were fed from their backup/emergency enerators.
By using worst case incident energy worker PPE was more likely to be overstated during normal conditions.

 

[steve66]

Engineers rarely qualify, or train in PPE, to access equipment covers, much less meter energized parts, so infinate current may be assumed.

Which gives electricians, with access to equipment, opportunity to bid tighter equipment ratings.

A calibrated ASCC measurement from CATIII rated meter reading, can always subtract any supply-side equipment impedance using Bussmann series FC2 Available Fault Current Calculator

Until AI makes an App for that, engineering supervision still has a job.

Are you saying you ignore the specs wrote by engineers on bid projects?

 

[ramsy]

Are you saying you ignore the specs wrote by engineers on bid projects?

In my area General Contractor laborer shops bid the large projects, relying on developer architects.

Licensed electricians may subcontract, be Responsible Managing Officers, or small shops, flipping panels to existing AIC ratings, with plan check done by AHJ’s, if permits are pulled.

My service work typically abates remodel hazards, power failures, and verifies ASCC before one-for-one replacements.

POCO’s may allow impedance to increase with expanding demand, since most equipment is rated well above ASCC.

If my service outbids rivals, its the client ignoring all other factors, subject only to the AHJ involved.

 

[jim dungar]

POCO’s may allow impedance to increase with expanding demand, since most equipment is rated well above ASCC.

This makes no sense.
Utilities pay for losses due to impedance. As systems and load grows they want lower impedance. If they allowed impedance to increase the available Short Ciruit Amps (SCA) would actually fall.

 

[ramsy]

This makes no sense.
Utilities pay for losses due to impedance. As systems and load grows they want lower impedance. If they allowed impedance to increase the available Short Ciruit Amps (SCA) would actually fall.

If we just consider rising demand of existing dwellings, on existing distribution infrastructure, impedance must increase as fault current drops.

As panels are flipped from <=100A to 200A, there is new demand for additions, HVAC, car chargers, and tankless water heaters, which did not exist when built.

It makes perfect sense that increased current on the same wire raises impedance.

 

[jim dungar]

It makes perfect sense that increased current on the same wire raises impedance.

No it does not.

Unless you are stretching the point that conductor resistance increases with temperature as the conductors warm up. But this change in resistance is insignificant in regards to available fault current.

impedance must increase as fault current drops.

This is the wrong cause and effect.
Please check your instructional material. The impedance affects the current, the current does not affect the impedance.

I will stick with my engineering books and peer reviewed publications.

 

[ramsy]

The impedance affects the current, the current does not affect the impedance.

You may describe it better.

Perhaps I2R loss only increase impedance within the relatively small distribution xfmr, until eventually replaced with larger units, which are designed with higher impedance.

I will stick with my engineering books and peer reviewed publications.

You are better than book smart.

If transmission, and sub-station xfmrs have the lowest design impedances, and are directly suppling your industrial plants, then an infinite current calc would certainly be preferred at such services.

 

[jim dungar]

infinite current calc would certainly be preferred at such services.

Infinite fault current is never preferred when performing Arc Flash calculations.

You may describe it better.

Actually I describe it correctly.

Increased loading only changes impedance based on the insignificant resistance change due to conductor temperature increase.
When performing available Short Circuit Amp calculations loading is never considered.

 

[brycenesbitt]

"Jiminy Christmas"
All I wanted to do was replace some old FPE breakers.
I had no idea I needed to get a graduate degree, buy expensive tools, and an Arc flash suit along with a J-Hook and helper to pull me off.

No wonder housing costs are so high.

 

[jim dungar]

All I wanted to do was replace some old FPE breakers.

What breakers did you want to replace?
The ones directly below each meter should be available with an AIC that equals or exceeds the ratings provided by the POCO.
The problem would be any breakers downstream of those outdoor breakers.
You would need to calculate the available Short Circuit Amps at the line side of these breaker, on line tools are available for these calcs. 110.09 has been in the NEC for at least 50 years.

The area you would have problems with is if the downstream breaker location exceeds 5kA, because you said your breakers are unlabeled. In this case you will need to replaced.
Series Ratings, per NEC 240.86, have been in the code since the 80s which is probably not as old as your FPE equipment. I have never met an engineer that would certify small branch breaker because these breakers can not be guaranteed to always remain closed during the time it takes an upstream OCPD to operate, which is a requirement to use 240.86(A).

 

[brycenesbitt]

What breakers did you want to replace?

In this case there are no internal subpanels or breakers : each apartment has a single #12 MWBC fed FPE breakers just under the meter. This all works fine, there's no capacity issue.

I'd love to take a course on AIC calculation through the chain of historic equipment: fuses, meter bases, wire, breakers.
But that said AIC is a theoretical hazard, and FPE are a proven hazard. This all seems like focusing on the tree needles not the forest fire.

 

[jim dungar]

But that said AIC is a theoretical hazard....

AIC is not really a theoretical issue. Do not confuse this with Arc Flash Incident Energy (AFIE) and its PPE requirements.

AIC is a problem that is most likely to appear after installation or maintenance, primarily when wires have been landed on incorrect terminals causing short circuits. AFIE is most likely to occur when miswiring or tool contact creates an arcing fault.

All of the major OCPD manufacturers have some pretty good literature on AIC calculations. We used to teach this during 'lunch and learns'. My preference would be something from the breaker manufacturers because they will cover typical series ratings issues. Decades ago fuse manufacturers it seemed to want you to simply use fuses everywhere, even for branch circuits.

 

[brycenesbitt]

AIC is not really a theoretical issue.

Nobody here is doubting this is a serious issue in big power systems.
The question is how appliable is that big power learning to 120/240V split phase residential systems. Are the older 5K breakers blowing up in the field, and to what level should the AIC calculations become a barrier to replacement and upgrade of older systems.

 

[jim dungar]

Are the older 5K breakers blowing up in the field, and to what level should the AIC calculations become a barrier to replacement and upgrade of older systems.

This is the problem: What is the actual available fault current at the equipment line side terminals. And how do we get utilities to provide meaningful data rather than being afraid of being sued years later after their system has been upgraded.

Luckily most wiring mistakes are located at some distance away, from the OCPD at the end of relatively small branch circuit conductors. Which is why we do not see breakers regularly exploding when a 15A receptacle fails or is miswired. But the internet has many photos of spectacular breaker failures when the fault occurs at the breaker loadside terminals.

I think the cut off for 5kA using #12AWG is less than 50', regardless of the starting current value, however the NEC does not allow us to select OCPD AIC based on the through fault amount (this is part of the problem of the plug in tester results).

 

[JoeStillman]

In this case there are no internal subpanels or breakers : each apartment has a single #12 MWBC fed FPE breakers just under the meter. This all works fine, there's no capacity issue.

I'd love to take a course on AIC calculation through the chain of historic equipment: fuses, meter bases, wire, breakers.
But that said AIC is a theoretical hazard, and FPE are a proven hazard. This all seems like focusing on the tree needles not the forest fire.

Sign up for Fuse Fridays with Eaton. Or check this out...

https://www.eaton.com/us/en-us/products/electrical-circuit-protection/fuses/training-presentations.html

 

[jaggedben]

AIC is not really a theoretical issue. ...

Serious questions: Can you point to any statistics on AIC related failure of equipment in single phase residential installations of 200A or less? Are there incidents to justify AHJs adding red tape where an installer proposes to incease the AIC rating of equipment, especially load-side equipment where they are not changing the service on the supply side of the first OCPD?

 

[jim dungar]

Are there incidents to justify AHJs adding red tape where an installer proposes to incease the AIC rating of equipment, especially load-side equipment where they are not changing the service on the supply side of the first OCPD?

I have know idea about the statistics. I just follow the requirements that have been in the NEC for some 50 years.
Those requirements require line side calculations and series ratings that have been tested.

This sounds like something that could be handled at the local level as part of rehab codes.