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

A couple of things here:

1. Why are you using the infinite bus fault current on the secondary to determine the primary fault current? What purpose?
2. Why not request the available fault current on the primary side as well as the riser fuse, riser cable and transformer data to model the system?

Reflecting the fault current on the secondary of a transformer to the primary side using an infinite bus calculation results in the current flowing in the primary for a SECONDARY fault. The current for a fault on the PRIMARY side would still be INFINITE. As WBD states, if you can obtain the actual data on the primary it would be better. Good Luck!

Both of your explanations make sense now.Thank you gentlemen!

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.

Hi Marcelo,
You mean short circuit and withstand ratings of the protective device on the secondary of the transformer? If the said transformer is an utility transformer that is protected by overhead fuse, than in order to properly size the fuse, utility fault contribution upstream of the fuse is required.

Thank you,
EE

Yes, I meant the ratings on the secondary protective components and equipment. And, of course, for any similar components and equipment on the primary the primary fault current characterizations is the relevant on.

And, to re-emphasize, for equipment ratings selection using theoretical maximums is appropriate, but for arc flash calculations you want to look at a range of possibilities. Sometimes, lower fault currents result in a higher incident energy depending on what protective devices are selected and how they are set.