AIC rating formula???

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steve066

Senior Member
A more dangerous scenario is that given a utility transformer failure, the utility company replaces the transformer with one they have pulled out from another location and is sitting in their yard, the impedance happens to be Z=4.2%. PCO doesn't care, i.e. 2500KVA is 2500KVA.

Using the conventional worst case scenario calculation of infinite bus, the available fault current on the LV side (480V) of the transformer now goes from approx. 63kA to around 67kA. The breaker originally installed rated for 65kA is not rated properly now.

The moral of the story is: if the PCO replaces a transformer to your service, don't think there isn't anything to be checked on the Owner's side.

Food for thought: How many facilities do you think exist that have inappropriately rated breakers, simply because the original designer tried to save a couple bucks and go with the lower kA rated device instead of conservative choice, or they should have used fuses instead of a breaker. When it comes to AIC ratings always error on the conservative side, safety should never be compromised.

Our POCO doesn't ever give us the actual Z of the transformer, but instead they give us their spec. for the minimum Z any of their transformers of a certain size will have.

But then, there isn't anything to stop them from replacing a 300 KVA with a 500 KVA, so the same problem is still there, just less frequent.

But if you do calcs with infinite primary, that usually gives a pretty good buffer between the calculated and actual available fault current.

Steve
 

steve066

Senior Member
OMG, as soon as I saw that my head started to hurt remembering Statistics classes... and it's been 30 years since i took one. Still had a textbook though, don't ask me why. I'm putting them in the donation bin today.

AIC = Akaike?s Information Criterion, a method used in evaluating statistical data models for non-linear regressions.

Don't EVER make me think of that again...

Statistics was the 2nd worst class I had in college. All the Communications-Engineering majors had to pass that class (everyone that wanted to work for Motorola) and I think it was their weed-out class.

The teacher didn't use the book for homework or classroom lectures. For homework problems, even after the teacher handed out solutions, and discussed them in class, I still didn't even understand the questions, much less the solutions.

Steve
 

Karl H

Senior Member
Location
San Diego,CA
My poco (SDG&E) puts out a PDF that says @208Y 3ph,
201- to 3000I the avalable fault current will be less than 42k

3001-4000I will be less than 65K

480v @ 2k or less 30k.

Does that sound right to you guys?
 

Last Leg

Member
Location
Houston, TX
It is stamped on the nameplate of the transformer

Duh ! If the transformer was already there, I could call the POCO for the info. But, just in case, where is the nameplate located on those pole mounted oil filled transformers - I've never really looked.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
My poco (SDG&E) puts out a PDF that says @208Y 3ph,
201- to 3000I the avalable fault current will be less than 42k

3001-4000I will be less than 65K

480v @ 2k or less 30k.

Does that sound right to you guys?

Each utility is different
Wisconsin POCO,
208V = 57.3kA 600-800A service
208V = 71.6kA 1200-2000A
480V = 34.5kA 600-1600A padmount
480V = 46.2kA 2000A padmount

Minnesota POCO,
%Z = 1.6 75-300kVA padmount
%Z = 1.8 500kVA padmount
%Z = 5.25 750-2500kVA padmount
%Z = 1.4 all sizes pole mount, wye connection
 
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