Short Circuit Current Calcs - Z from source?

[Murloc]

I've look at different calculations for short circuit analysis. Some mention adding the Z from source and some don't.

I've done a hand calculation one that includes the Z from source appears to be closer to ETAP values.




Here is an example using an online calculator.
Isc = FLA/%Z * 100

Zs = 6.5%
Ztr = 4.85%

Zt = 11.35% which is significantly larger than 4.85%

SCA is lower in that scenario, some calculations do things with infinite bus, so I guess they set the Z from source to 0.

 

[steve66]

Yes, infinite bus means there is an infinite amount of fault current available. That's basically the same as setting the Z to 0.

You never know when the power company will run new lines, increase wire sizes, or change out a transformer that is upstream of you project.

Any of that could change the Z value of your source, and make you calculations invalid.

So you have to be careful about using a value of Z that's too high for the source. Hence, most engineers just assume infinite source bus current. It usually doesn't have a huge impact on the project budget.

 

[Julius Right]

In my opinion the difference is not so large. The actually short-circuit if the System is limited at 247 MVA has to be 17.14 kA.
It is something wrong in the source impedance calculation.[it has to be 0.24^2/247=0.000233 ohm] and transformer 0.24^2/0.367*5/100=0.007847 ohm. Then Isc∞=17.6573 kA and Isc247=17.148 kA

 

[kingpb]

You can utilize infinite bus on the utility side because the fault current is going to be limited to whatever the transformer capability is to pass it through which is limited by the transformer impedance.

However, using the actual values shows that the available at the utility is significantly less than infinite.

For equipment rating purposes, I would use infinite bus, for settings protection use actual.

 

[ron]

Keep in mind that if you use infinite bus (Z=0) for short circuit calculations, that you re-run it with the actual utility impedance for arc flash calculations, as the arc flash incident energies will be artificially lowered by infinite bus.