Industry standard for calculating fault currents

Tainted

Tainted

Senior Member
Location
New York
Occupation
Engineer (PE)
A lot of people in this industry tend to calculate fault currents based on 3-phase bolted symmetrical fault. Even products listed are based on L-L-L fault symmetrical.

Why do people only calculate L-L-L faults in this industry only? Isn't that not a good thing to do because there are instances where L-G or L-L-G faults are higher than L-L-L faults.

Even when I ask utility for their fault current at the service, they only gave me 3-phase symmetrical bolted fault. Should I even ask them for L-L-L-G, L-L-G, L-G? Should I even ask them for positive, negative, or zero sequence impedances? It just boggled my mind why nobody in this industry goes further beyond than calculating fault currents based on L-L-L only.
 
Tainted said:
A lot of people in this industry tend to calculate fault currents based on 3-phase bolted symmetrical fault. Even products listed are based on L-L-L fault symmetrical.

Why do people only calculate L-L-L faults in this industry only? Isn't that not a good thing to do because there are instances where L-G or L-L-G faults are higher than L-L-L faults.

Even when I ask utility for their fault current at the service, they only gave me 3-phase symmetrical bolted fault. Should I even ask them for L-L-L-G, L-L-G, L-G? Should I even ask them for positive, negative, or zero sequence impedances? It just boggled my mind why nobody in this industry goes further beyond than calculating fault currents based on L-L-L only.
Click to expand...
Systems fed from single phase center tapped transformers is one instance where the L-G AFC is often higher than the L-L AFC. Many people seem to not be aware of this or just ignore it, but for these systems it is often prudent to use a 1.5 multiplier for the L-L available fault current.

see starting at page 239 here

 
electrofelon said:
Systems fed from single phase center tapped transformers is one instance where the L-G AFC is often higher than the L-L AFC. Many people seem to not be aware of this or just ignore it, but for these systems it is often prudent to use a 1.5 multiplier for the L-L available fault current.

see starting at page 239 here

Click to expand...

I rarely ever work on single phase transformer systems. 99% what I work with is 3-phase. What multiplying factor would be safe to use for a 3-phase system?

I'm just afraid if I calculate L-L-L that the fault will be lower than L-L-G or L-G or L-L-L-G
 
Tainted said:
I rarely ever work on single phase transformer systems. 99% what I work with is 3-phase. What multiplying factor would be safe to use for a 3-phase system?

I'm just afraid if I calculate L-L-L that the fault will be lower than L-L-G or L-G or L-L-L-G
Click to expand...
My understanding is that L-L-L is the worst case, but perhaps others will chime in with a better explanation and/or exceptions
 
electrofelon said:
My understanding is that L-L-L is the worst case, but perhaps others will chime in with a better explanation and/or exceptions
Click to expand...
I think it depends on value of zero sequence impedance from utility. I believe every 3-phase transformer has positive, negative and zero sequence impedances. Anything that is faulted to ground can only go through the zero sequence impedances.
 
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3-phase faults are used for equipment testing and ratings and are generally the worst-case. On the secondary side of delta-grounded wye transformer, the L-G fault can be slightly higher than the 3-phase at the transformer terminals but as the distance from the transformer increases, this effect goes away pretty quickly. You should ask the utility for the 3-phase and L-G fault currents along with their respective X/R ratios. 3-phase fault data is by far more important in most cases than the L-G.
 
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