utility fault current interpretations

[mjmike]

Recently, we have been getting available fault current from the utility in multiple forms where in the past we had just been told the worst case. Now, we are getting data as follows on the primary bushings of a 208 3-phase pole mounted banks:

3-phase fault current: 3466A
phase to ground fault current: 3200A
positive sequence (X/R) = 2.71
zero sequence (X/R) = 2.9


They also indicated the bank will be 3 25KVA xfmrs with 12.47kV primary. The impedance for 1 transformer falls within 1.75% to 2.02% with an average of 1.88% for each transformer in the bank.

Based on all this data, I am trying to determine the secondary available fault current for the panel's AIC rating but not sure what data to use to figure the primary available fault. The program we use only has data entry for the primary available fault current, transformer KVA rating, % impedance.

 

[wbdvt]

Recently, we have been getting available fault current from the utility in multiple forms where in the past we had just been told the worst case. Now, we are getting data as follows on the primary bushings of a 208 3-phase pole mounted banks:

3-phase fault current: 3466A
phase to ground fault current: 3200A
positive sequence (X/R) = 2.71
zero sequence (X/R) = 2.9


They also indicated the bank will be 3 25KVA xfmrs with 12.47kV primary. The impedance for 1 transformer falls within 1.75% to 2.02% with an average of 1.88% for each transformer in the bank.

Based on all this data, I am trying to determine the secondary available fault current for the panel's AIC rating but not sure what data to use to figure the primary available fault. The program we use only has data entry for the primary available fault current, transformer KVA rating, % impedance.

Not sure what you mean there as the primary available fault current has been provided by the utility. What program are you using? I think you would enter the 3 phase values and use the average %Z.

 

[philly]

Recently, we have been getting available fault current from the utility in multiple forms where in the past we had just been told the worst case. Now, we are getting data as follows on the primary bushings of a 208 3-phase pole mounted banks:

3-phase fault current: 3466A
phase to ground fault current: 3200A
positive sequence (X/R) = 2.71
zero sequence (X/R) = 2.9


They also indicated the bank will be 3 25KVA xfmrs with 12.47kV primary. The impedance for 1 transformer falls within 1.75% to 2.02% with an average of 1.88% for each transformer in the bank.

Based on all this data, I am trying to determine the secondary available fault current for the panel's AIC rating but not sure what data to use to figure the primary available fault. The program we use only has data entry for the primary available fault current, transformer KVA rating, % impedance.

The 3,466A appears to be the available primary fault current at the 12.47kVA transformer primary. The total transformer kVA is 75kVA (3 x 24kVA single phase units) and I would use the lower of the impedance values listed to provide worst case results. So you can directly enter the primary available fault current, kVA rating and % impedance into your software.

 

[steve66]

If you are just doing a short circuit study, just assume infinite current on the xformer primary. Why make it harder than it needs to be?

Besides, the available fault current on the primary could change if the utility upgraded a substation or some of their lines.

Figure the transformer secondary current: 75KVA/(208V * sqrt(3)) = 208 amps.

Then divide by the worst case impedance: 208A / .0175 = 12,000 amps on the secondary.

 

[mjmike]

Thanks. I was not sure if the ratios needed to be considered.

 

[steve66]

Thanks. I was not sure if the ratios needed to be considered.

For an arc flash study, you should consider the utility info given, since a lower fault current can mean a longer trip time, and higher arc flash energy.

But if you are just trying to determine the needed interrupting ratings for panels and breakers, I would just assume infinite primary current on the transformer.

 

[mjmike]

makes sense

 

[Julius Right]

If transformer short circuit impedance will be 1.75% -considering the system impedance
and X/R=0.84 for 75 kVA transformer as per ANSI C37.010.- the short-circuit current-initial symmetrical short-circuit current I"k[three phase bolted]-will be 11.3kA. For 2.02% it will be 9.9 kA.