I have a local utility that gave me maximum and minimum fault current values which is rare but nice. The problem is the maximum on 12,470 is around 5000 amps and the minimum is only 177 amps. The minimum values give terrible arc flash ratings for the equipment in the plant I am working in. Does this value seem right? I would think they have more than that value on the feeder in any given day under normal operating conditions. Thanks

177 is pretty low. Is the plant miles away from the substation on a radial feed under the minimum scenario?

The lowest I've run into was maybe around 1,500 amps.

The engineer who gave me the numbers came out to the site and we decided that he gave me the minimum service on that line. He said he never gets asked for the minimum fault current values. Does everybody else ask for these numbers or just the maximum?

I just generically ask for the available fault current and x/r. If there is a transformer and I don't have that info I ask for it too.

If they supply the max and min they usually also supply the normal condition which is what I use. I often find the max and min values aren't that much different and don't have a large impact on results if the values are at a transformer primary.

If I only had the max and min I would probably run both scenarios and use worst case. If I had a min as low as yours I'd ask for how likely or how often that condition existed. If is was a possible scenario that had never actually been used and probably wouldn't be I probably wouldn't use it if it was making a significant difference in the study results for several busses as the results wouldn't be realistic. If the very low min was a common operating mode I would absolutely be using it.

Really just need to run the max and min scenarios, compare the results and use engineering judgment as to the likelihood of the min scenario.

Oh, one more thing, all you can do is your best with the information available. Who's to say 6 months after the study the POCO doesn’t makes a significant change to their system? You think they will tell you? Not a chance.

Also, whatever you do I would document it your report and inform the client. Even better to get their opinion on it before the study is final.

This comes from Eaton (Step 2, Note 2):

UL 1561 listed transformers 25kVA and larger have a ± 10%
impedance tolerance. Short circuit amps can be affected by this tolerance. Therefore, for
high end worst case, multiply %Z by .9. For low end of worst case, multiply %Z by 1.1.
Transformers constructed to ANSI standards have a ±7.5% impedance tolerance
(two-winding construction)

Source:
https://www.eaton.com/content/dam/eaton ... rmulas.pdf

I think all the repliers are missing something. 7200V Line-to-Ground system has a minimum fault current ASSUMING a high fault impedance value. The utility used the old-school outdated 40ohm RUS criteria from back in the day. 7.2kV/40ohms =180 amps. Add some system impedance in there in the order of a few ohms and that brings you down to 177 amps minimum line to ground. He/she may have pulled it from his/her system fault model. My company's software does this minimum fault current based on a ground fault impedance you set. Sounds like he/she used 40 ohms. I would not use that for arc flash.

Key is to ask for fault current values that do not involve a high fault impedance path to ground. The utility in the case went to the fault current report and grabbed MAX and MIN and MIN used a high fault impedance in the calculation. A no-no for arc flash studies.
I say this because the utility probably uses our software and we provide MAX and MIN, and that MIN value is always a small line to ground fault current, many still use the terrible 40 ohm Rural Utility Services recommended value from 50 years ag0.

I agree. Minimum fault values are for setting medium voltage protection to trip for faults at the end of the line with some fault resistance. They are not for IE calculations on the secondary side of the transformer. Ask for the bolted fault current plus transformer impedance at the location in question..

I believe you are misunderstanding the purpose of the line-to-ground plus 40-ohm fault current (or other fault impedance like 10 or 20 ohms). There is always the possibility that a utility system fault will not result in a bolted line-to-line, line-to-ground, or three-phase fault, and can be observed when phase wires land on the ground and remain energized [1]. Since there is always the possibility of an energized line on the ground, distribution utilities establish a minimum sensitivity for their protection studies.

Yes, at one point in time the RUS standard for this minimum sensitivity was line-to-ground plus 40 ohms (based on [2]?). However, today this standard would require fault sensitivity at that feeder source and along the main-line section of the line that would not allow for the magnitudes of load those feeders carry. Today the RUS design guide calls for engineering judgment [3].

[1] IEEE PES TR2 Downed Power Line: Why They Can't Always be Detected, IEEE Power Engineering Society, February 22, 1989.

[2] Lincks, Edge, McKinley, Overcurrent Investigation on a Rural Distribution System, AIEE, 1946.

[3] Bulletin 1724E-102 Design Guide for Sectionalizing Distribution Lines, US Department of Agriculture, Rural Utilities Service, p. 39., 2012.