Greetings,

It seems logical to use transformer ratio to refer fault current from secondary to primary. Now, in the case of a utility service transformer, would the same approach be valid? For example, if I use infinite bus method to estimate max. fault current on the secondary of the utility transformer, can I use the ratio to estimate utility fault contribution? I don't think this would be a valid approach, since the utility contribution upstream of the said transformer would be independent of the transformer rating and impedance. What gives?

I am a little confused here. Any advice would be appreciated.

Thanks,
EE

Using an infinite bus on the primary would give you a theoretical "maximum" secondary fault current. Specially if you are using the specified transformer impedance and you are assuming the tolerance is toward a lower impedance. This is an acceptable method to select equipment short circuit and withstand ratings. HOWEVER, if your intent is to estimate arc flash incident energy, this would identify one end of the possible range. Theoretical maximum arcing current and maximum incident energy per unit time. If you want to estimate the full range of potential incident energy you should take into account lower fault current and ensure that the corresponding lower arcing currents do not result in "much" slower protection and consequently higher incident energy.
In other words, to conservatively estimate incident energy you need to consider a range of potential fault currents as well as how protection will respond to them to properly tease out the potential maximum incident energy.