calculating arc fault current

[Ken 6789]

How do you calculate arc fault current of a transformer? I believe KVA and transformer impedance are factors in calculation, but not sure how it's done. Would someone give an example? Thanks.

 

[don_resqcapt19]

If you are asking about the short circuit current, it is the full load amps divided by the impedance (in decimal form, 5% =0.05). This will give you a "infinite bus" value which is the worst case.

 

[Ken 6789]

Thanks, I meant to say short circuit. I don't know why I was thinking arc.

So is the following correct? 75 KVA = 75,000/240 volts = 312.5 amps. 312.5/.05 = 6,250 short circuit current

 

[jim dungar]

Do not use the infinite bus method to determine the only short circuit current which will be used when performing an arc flash calculation.

 

[steve66]

Thanks, I meant to say short circuit. I don't know why I was thinking arc.

So is the following correct? 75 KVA = 75,000/240 volts = 312.5 amps. 312.5/.05 = 6,250 short circuit current

Yes. That's a conservative estimate, assuming there aren't any large motors that would contribute a lot.

 

[tkb]

Is that for single phase or three phase.

I get 4,009 on a three phase transformer using the Bussman point to point calculations.

 

[jim dungar]

Is that for single phase or three phase.

I get 4,009 on a three phase transformer using the Bussman point to point calculations.

FWIW,
With the current generation of energy efficient transformers, 5% is a relatively high impedance for a 75kVA 3-ph dry type, and it is extremely high for a utility supplied liquid filled unit (Xcel Energy recommends using 1.6%Z)

 

[tkb]

Is the result correct in my formula with the parameters that I used?

I am trying to see if I interpreted it correctly and that I wrote the excel formula right.

 

[Ken 6789]

Is that for single phase or three phase.

I get 4,009 on a three phase transformer using the Bussman point to point calculations.

single phase

 

[don_resqcapt19]

Tim,
Is that a single phase transformer?

 

[tkb]

Tim,
Is that a single phase transformer?

My calculation is based on three phase.

 

[steve66]

Is the result correct in my formula with the parameters that I used?

I am trying to see if I interpreted it correctly and that I wrote the excel formula right.

If we assume its a 240V, three phase system, with 5% Z: I get 3608 Amps.

75,000/(240*1.73)= 180 amps.

180/.05 = 3608

 

[don_resqcapt19]

The original posters calculation left out the 1.73 so I assume we are talking about a single phase transformer.