Utility Fault Current

[clausb]

When performing short circuit calculations, typically the utility will provide the available three phase fault current at the secondary of their transformer or at the point of their delivery. Often they will provide both a three phase and single phase short circuit current. When we are directed by the utility to assume an infinite short circuit current we can easily calculate the available short circuit current on the secondary of the transformer.

Is there a rule of thumb for coming up with the single phase fault current or is this something that is specific to an installation?

 

[bob]

When performing short circuit calculations, typically the utility will provide the available three phase fault current at the secondary of their transformer or at the point of their delivery. Often they will provide both a three phase and single phase short circuit current. When we are directed by the utility to assume an infinite short circuit current we can easily calculate the available short circuit current on the secondary of the transformer.

Is there a rule of thumb for coming up with the single phase fault current or is this something that is specific to an installation?

There is no rule of thumb except that at the transformer the single phase and three phase fault are generally about the same magnitude. The equipment is usually based on the three phase fault since it will be the larger value at the main disconnect. The single phase fault falls off faster than the three phase fault. The calculations are some what complex when calculating the single phase fault therefore when its done, its done using computer software to make the calculation. Saves time.

 

[jtester]

Bobs guidelines are applicable to smaller, less than about 1000 kva installations, but can be a problem with bigger installations, particularly if the transformer connection is wye-wye.

Three phase fault currents are limited by only positive sequence impedances, whereas L-G faults are limited by positive, negative, and zero sequence impedances.

If you're doing a typical building, negative and zero sequence impedances aren't very applicable, but if you work with utility systems, they are very important. L-G faults need to be designed for, as much as 3 phase.

Jim T