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 Post subject: Lighting Panels, small motors- Arc Flash
PostPosted: Mon Feb 03, 2014 7:52 pm 

Joined: Sat Feb 01, 2014 6:41 am
Posts: 3
Hi,

I just start working as a manufacturing engineer at a chemical plant. One of my assignment is to learn how to model electrical distribution in this plant for the purpose of conducting short circuit, coordination and arc flash. My immediate assignment is to develop the single line diagram in ETAP (we just purchase it a couple weeks back).

We have several lighting panels (240VAC panels, with main CBs and several feeder CBs). I understand from other engineers that lighting load does not contribute to short circuit current, but we still need to model them for arc flash labeling and incident energy analysis.

ETAP does not have an element to model the lighting load (except lumped load or static load). Typical wiring is that we have a fused bus plug feeding a lighting panel, inside the panel is a main CB and several smaller CBs for feeders to lights and other small loads). What would be the better way to model lighting panel for IE analysis?

We also have several (at least 15 per line) small motors (15HP or less) that are also hanging off of the bus. Typical wiring would be Federal Pacific fused bus plug, load side use SO cord (20'-25') to twist lock receptacle. The motor pigtail would have twist lock plug which then is plugged into the receptacle. These are repeated throughout 200' of Federal Pacific bus. On this same bus, there are also several 150HP motors (these are hardwired from the bus to control panels (there is a main in each panel). The load side of the motor contactor is then hardwired to the motors. Each panel would have 2 or more motor contactors inside. My question, is there a better way to model this type of setup for short circuit and IE? Do I include the main CB in the panel or the fused disconnect at the bus be enough for IE analysis? For the small motors, I read in this forum that the can be grouped together (ie. add together HP), how would that work to calculate IE at each motor terminals?

I am new to this work and just trying to understand. My mentor simply say to figure it out. Hope you can help me.

Regards,

DP.


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PostPosted: Tue Feb 04, 2014 9:25 am 
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Joined: Wed May 07, 2008 5:00 pm
Posts: 823
Location: Rutland, VT
My first concern is that this work is not being done by a qualified electrical engineer who has experience in this. As a manufacturing engineer, what electrical engineering courses did you take in college? Do have some experience in modeling or analyzing electrical systems?

Another item that jumps out at me is that ETAP was recently purchased. As with any of the analytical software packages, knowing how to set up the software to give valid results is important to know and understand. Have you gone to ETAP training? Many of your questions would probably be answered there.

Another concern, to be blunt, is the attitude of your mentor. I would hope that he doesn't really mean go figure it out but rather go spend some time with it and come back with specific questions.

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Barry Donovan, P.E.
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PostPosted: Wed Feb 05, 2014 6:21 am 
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Location: Rutland, VT
I thought of something else to add and this all based on experience. Last year a company contacted us to provide NFPA 70E training. When we asked for their arc flash study, they said it was partially complete and gave it to us. I found numerous errors that affected the results which were erroneously high. I spoke to the individual who was performing the study and here is what I found out:
  • they purchased ETAP 3 years ago and were still working on the study. They had believed it would be less expensive to do in house
  • he had never had any ETAP training
  • he was not an electrical engineer
  • he had not read IEEE 1584
  • he had not read NFPA 70E
  • he had not contacted the utility to obtain fault current data. He chose to use a percentage of the transformer max fault current based on the loading of the transformer.
FWIW, that is one of my experiences with companies trying to do this work in house. On the flip side, I know there are larger companies that have successfully done this in house.

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Barry Donovan, P.E.
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PostPosted: Wed Feb 05, 2014 10:39 am 

Joined: Fri Aug 10, 2012 11:47 am
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David,

While WBD makes very important points I'm going to make the assumption that your company has competent engineers who understand electrical systems that will be able to oversee the project when it comes to the arc flash analysis part. Since you only asked about how to model the one-line, which I believe is relatively straightforward, I don't see any issues with a manufacturing engineer, electrician, or any person who understands power systems and electrical safety performing.

Unfortunately, I don't have much experience with ETAP however when I evaluating the demo version a while back I did run across the tutorial videos on their website http://etap.com/m/videos/. The 5th video listed is specifically for creating one-lines.

From your description of the system, the hardest part will probably be figuring out how to model the 200' long bus in ETAP. If you don't have any documentation on the existing system, the real work is when it comes to determining (or verifying) all of the system parameters (fuse/CB sizes, manufacturers, model numbers, wire sizes, conduit types, conductor length, etc.). This is potentially the most dangerous since you don't know what the arc flash hazard is yet and will need to open enclosures and panel doors (that have probably not been maintained/cleaned in several years). Per NFPA 70E (flow chart/figure J.2) you are allowed to open live panels for inspection however you still need wear appropriate protective equipment per Section 130.7. The whole thing is a bit of a catch 22.

Good luck and be safe. At a minimum, get a copy of NFPA 70E (its cheap) and check out a few of the youtube videos on NFPA 70E before you dive in.


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PostPosted: Tue Feb 11, 2014 5:32 am 
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Location: Swanton, Vermont
I'm sure many of us have stories of clients who decide to do projects 'in house' with no notion of the scope of work. One particular pulp mill engineer worked for years on their arc flash using a demonstration utility on a fuse manufacturer's web site. This project was not successful. David P.'s situation seems to be likely to have success while he learns everything he needs to know to accomplish the task. However it may not be as inexpensive as his managers assumed.


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PostPosted: Tue Feb 11, 2014 7:37 am 

Joined: Fri Sep 21, 2012 8:23 am
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I would suggest that you sit down with one of your on-site electricians if there is one. Someone who has been there for a long time will be one of yor biggest assets. He may be able to help you with conduits that are run in concrete or underground since estimating the size and lengths are difficult--he may have been there to see them installed. He should also be able to help you hand draw a rough one-line digram of the system. Often OCP in older installations are mislabeled or not even in use, yet the cable is capped off in a jbox hidden somewhere.

I hope this helps.
Mike


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