The utility refuses to provide fault values to the service transformer primary. They tell me to assume an infinite bus. That's fine for sizing the AIC of equipment, but with arc fault I find that sometimes lower fault current values can generate higher incident energy values as some breakers never reach their instantaneous trip threshold.

Does anyone have a typical estimate on the reduction in fault current when calculating a finite bus. I remember seeing a reference that a 10% reduction is common. This is a typical street feed 13.8KV overhead line supplying a 300KVA 208/120 transformer.

Before anyone starts, I realize this is not ideal and if I ran the utility I would make it available. But that fight is not winnable at this time. I'm thinking its in the 10 to 20 percent range, but would like your opinion.

thanks

Rather than a single estimate you could run, say, 5 scenarios. I usually work in amps so say, 2,000, 5,000, 10,000, 20,000 amps and infinite or something like that based on what your experience is in that area.

Run the scenarios to automatically select worse case. Not ideal, I agree, but it would probably get you there.

Don't know what software you are using but in SKM that would be easy to do.

Good luck.

Oh, if you choose to try this, please report back with which scenario was typically the worst case. It would be good to know.

I had nothing better to do this morning so I briefly played around with what I described above. Anyone have any better ideas or find fault with this approach?

Here's the vid:

https://youtu.be/LAktCt3RjA0

There is an IEEE publication that could help solve this issue.

Impact of available fault current variations on arc-flash calculations

There, you will find an excellent method, with two scenarios, to estimate the arc flash incident energy at the line side of the service equipment.

I teach this method in the seminars I deliver for the SKM arc flash studies.

This: http://www.eatonfuller.com/ecm/groups/p ... 372889.pdf

Conclusion: Run an infinite bus scenario and a 50% scenario. This is indeed better than infinite bus alone but probably not as conservative as running more scenarios than just infinite and 50%:

See https://brainfiller.com/arcflashforum/viewtopic.php?f=23&t=4665&p=21385&hilit=Impact+of+available+fault+current+variations+on+arc+flash+calculations#p21385 for a discussion of the process proposed by the IEEE paper RECs spoke of.

Thanks!