I was just reading a study done by another company for a client. The local utility provides only "actual" fault current data. The data provided was 1.7ka three phase at 35kv, X/R 2.28. The engineer calculated a second scenario using 5ka and X/R 15. The study states the scenario was not provided by the utility but selected by the engineer.

My question is - "How do you pick a second scenario?" What do you look for to know that it is relevant or possible?

Thanks for your help.

The real goal is to select a number that might indicate a potential problem off in the future just as a "what if". Back in the 1980's when I headed up the short circuit studies group of a large public utility, we would provide a "future" short circuit value for customers by looking at conditions such as:

Future planned substation additions (which no one else would have acces to)

The maximum interrupting rating of substation breakers (assuming the short circuit current would not exceed it)

Judgement based on what we have seen at the same voltage level in other places.

The goal for alternate scenarios like this in an arc flash study is to see if any PPE levels take a significant jump and go beyond what you plan to use.

It is also a good idea to take the number lower although 1.7 kA at 35 kA is already down there.

The X/R ratio impacts how impedances are added in the study. Typically a higher X/R ratio is more of a worst case for short circuit studies since it results in a steeper impedance angle which creates a "smaller" total impedance resulting in a greater short circuit current.

I have seen articles that use an X/R of 15 in the calcs for worst case conditions. But if you don't have access to any utility info, what would be a way to choose the increased fault current? 1.5x? 2x? 20%?

Utility SC Scenarios

IEEE 1584 recommends calculations at minimum and maximum available Utility Short Circuit values. I have so far had very good luck asking utilities for this type of data in the South Eastern US. I have been given multiple scenarios of their existing switching configurations with accompanying data as well as projected values/schedules for future changes. I usually get good data on their relay settings as well, along with one-line diagrams of the pertainent portion of their electrical system. They will usually give you what you ask for if you get in touch with the right engineer or manager and you are specific and persistent (as required) with your request(s).