Hi All,

I'm fairly new to the Arc Flash scene and the board. Forgive me if I'm in the wrong section.

Looking for some help as I am quite stumped. Hopefully those ETAP experts can give some insight as I am reaching the end of my rope.

I am trying to recreate a study that was done previously. Fairly basic system. There are two operation modes, normal and on emergency generator. I have attached my file and the study. I am able to come close to normal operation results in the report. However, the emergency operation results are quite different. I have tried many things to make it work to no avail.



Let me know if you need further information. All the data is on the report.

You have to find where the difference is. Its fault current and clearing time. If something in the recreated model is different, and affects one of those things the results will be different. I would suggest comparing the arc flash spreadsheet to the original and see what is different. Do you have the same breaker model and type that was in the original study? Generator data? etc.

Yes. I've tried to compare it and put it in exactly same data (same breaker, same generator data) as what was there in the old report into the newer ETAP version. I've come close to normal operation (within 5-10%).

Emergency operation using the generator is different. I get the same Ibf. Incident energy is different.

I tried doing many different things..but when I force FCT to 2 sec max...I get quite close.

Is there any reason as to why the original report would have this "forced"?

There are many reasons for this change in results. Incident energy calculation is a time based process. It may be also due to variation in arc current if your faulted location is less than 1kV. You may plot both source PD arc currents in the TCC to check the time. It will show the solution. For more details, I would suggest to contact ETAP support department and you will find the answer quickly

Maybe there was a reason to believe that the OCPD isn't working/

I think I figured it out.

I believe the FCT was forced because this was the bus or point between the Generator and the Generator breaker (no PD - labelled BKR-GEN lineside on the SLD).
I don't know if this bus should even be faulted. But this is what the consultant did.


And the '2 second rule' was because of:

A 2 second maximum arc flash duration is a reasonable cut off based on IEEE 1584-2002. It's the average time for a person to react to arc flash and leave the harzard area. So it's meaningless to calculate the energy after 2 sec.

Is this generally acceptable?

The 2 second rule is something of a mention in passing and not so much a "rule" in IEEE 1584-2002 but became a practice. But in IEEE 1584-2018 came around it is now a prominent rule. It just one of many clauses that are more or less taken on blind faith simply because they seem to have withstood the test of time.