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).