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ekstra   ara
PostPosted: Tue Jun 05, 2018 1:12 pm 

Joined: Sat Sep 23, 2017 3:15 pm
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Hello everyone,

Hope you are all doing well. I have a question pertaining to an arc flash study that I'm doing for a University building. To put things into perspective, I have a 800 amp, 120/208V, 3 phase, main circuit breaker panel on the secondary side of a 225 KVA transformer. The incident energy is understandably high on the secondary side, around 23 cal/cm^2 to be exact. The university engineer is trying to get me to drop the instantaneous on the primary over current protective device in order to drop the incident energy down, but this causes another issue. The transformer inrush is now tripping the breaker if I try to do this. I have tried arguing the point with the university engineer that they should not be working on the secondary panel live anyways because we are not dealing with a hospital or critical operations center here. He argued back to me saying that they don't want to have to "suit up" every time they have to flip a breaker on the secondary panel. He also talked about taking the secondary panels main and placing it outside of the panel at a different location in order to reduce arc flash incident energy. I would rather not go a route that involves making changes at this point because most of the equipment is on site. Am I crazy or is telling them to shut down the power not the right thing to say? I want to be careful because this is my client and I don't want to tell them I can't help them.

Best Regards,

Engineer in training

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PostPosted: Tue Jun 05, 2018 3:37 pm 
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Joined: Tue Jan 13, 2009 5:00 pm
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Shutting down the power is an incomplete answer. After the power is shut off, one must test for potential to ensure the the circuit is dead. Testing for potential on a circuit in an un-proven state is considered energized work.

Have you considered using both settings? The lower maintenance pickup could be switched in when desired.

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PostPosted: Wed Jun 06, 2018 8:54 pm 
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Three items to note. 70E is very clear that operating equipment does not require PPE. Your engineer needs to do a risk assessment, not just plug numbers into a software program.

Second, IEEE 1584 notes state that they only got a stable arc at 208 V at 1/4" bus gap at the highest available fault current in the tests. None of the other tests at more typical 1/2" or larger bus gaps produced stable enough arcs. Basically IEEE 1584 results at 208 V are all but invalid and your 23 cal/cm2 value is dubious at best. You may want to consider using tested results from the tables in IEEE C2 (NESC) which is intended for distribution equipment. In this case the highest incident energy measured is 3-4 cal/cm2 based on up to I believe 20 ka from testing performed by EPRI which is publicly available for free. If the value is 4 xal/cm2 there might not be much point in playing with breaker settings, just ignoring erroneous software numbers. The software just does estimates based on curve fitting data that gives questionable results at the "edges". For instance in some cases it predicts higher arcing fault currents than the bolted fault current which is obviously completely invalid.

Third this problem is quite common. You need to work out a way to have 2 or maybe 3 settings. During energizing you need high settings to avoid that and the control signal is readily available. It is trivial to set up for instance an SEL 751A to handle close/open either from an input signal amperage, or better yet from using it to do close/open commands. Then after the inrush passes, switch to tighter settings that address secondary side protection.

Finally you may need even tighter settings that further lower potential. Incident energy such as using an instantaneous setting that isn't normally used on feeder breakers that again lowers incident energy at the expense of loss of coordination, that is typically manually triggered.

The idea that you MUST test for absence of voltage at the work site loses track of the fact that in many practical instances especially in line-of-sight instances testing at higher voltages on the primary side of a transformer is MUCH safer and ignores obvious things like cord-and-plug equipment which obviously doesn't need testing. Again...risk assessments are key, not software.

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PostPosted: Mon Jun 11, 2018 4:33 pm 
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Joined: Tue Jan 19, 2010 2:35 pm
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You said the breaker trips on the primary side of the transformer due to inrush. I assume it's a 480 volt system? If so, that's pretty common with a cold transformer. How about adjusting the instantaneous trip down after you've got the transformer energized? I assume the university does not regularly turn this transformer on and off? If that's correct, why not explain that it's possible (however not likely) that this transformer could trip the input breaker following a power outage. And that they should weigh the possibility of the breaker tripping verses the increased safety of lowered arc flash energy at the secondary and at the line side of the main circuit breaker.

Alternately, you could offer them the option of adding low peak fuses between the secondary of the transformer and the 800 amp main breaker. Of course you'd need to specify which low peak fuses and the cost of adding these fuses is not inexpensive, but if you weighed that cost against the possible burn injuries, it's easy to justify.

We can argue about the voltage required to maintain an arc, about buss clearances or available fault currents, but in the end I believe you'll find that safety trumps everything else.

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