Engineering Intern tasked with Arc Flash study help

[tigim101]

Hello everyone,

I am currently a Facilities Engineering Student (BE) that is set to graduate in 2016. I am interning at another universities facilities department. I work with both the senior stationary engineer and the electrical supervisor doing various projects. The main project I am working on with the electrical supervisor is an Arc Flash Study. A lot of the buildings and equipment are old (more than 30 years or so) and do not have arc flash stickers on them. The electrical supervisor wants to calculate incident energy and boundaries so he can get stickers placed on the older equipment.

At first he tasked me with getting some nameplate data from panels in one of the machine rooms, and try my hand at a calculation myself, so I could know where the numbers were coming from instead of just using software. The scope of doing the arc flash was something that I did not expect, I quickly found myself reading pages and pages from NFPA and IEEE to even try and get an estimate, and I felt that the plate data was not enough. I realized the university had access to IEEE's online database so I read through IEEE 1584-2002 and tried to get a number for a 100 amp 250 volt switch. I got a number but I really have no idea if it's right at all, and it seems like a tremendous amount of material to learn.

I showed him what information I found, which was basically that most everything is a constant based on the equipment type (open air or box, grounded or not, working distance, gap b/w conductors etc) and the hardest things to get were bolted fault current, arc current, and arc duration. He said to read through IEEE-1584 some more and then we are going to look for some kind of software to purchase for the university to do the calculations.

The electrical supervisor has over 30 years of electrical experience and two master electrician licenses, but I'm not sure if he has really done anything with arc flash analysis. I have about a month left in my internship and I want to try to help as much as I can, but finding the numbers by hand just seems impossible. I also have limited electrical engineering knowledge (my facilities program is basically a mechanical engineering degree with a few electrical theory courses with project management) so I wouldn't trust the numbers I put out (I doubt he'd just let me paste stickers on panels with what I found by hand). What should I suggest be the best use of my time? I've read several places that say data collection is the biggest component to arc flash, so I'm guessing I should contribute to that. Is there anything regarding data collection except plate data from mechanical rooms? I believe he has line diagrams for all the buildings, but i'm not sure if he has things like trip curves for fuses, especially on the older equipment.

 

[charlie b]

First of all, you need to understand what an arc flash event is. You might already know this, but I will say it for the benefit of some of the other readers. If something causes a short circuit between two phases within an electrical enclosure (or between a phase and ground), it will cause high currents to flow, and it may cause sparks to fly. The sparks can heat the surrounding air, causing it to ionize, thereby turning the air itself into a fairly good conductor of electricity. That will further increase the amount of fault current flowing, and this is what creates the heat and the pressure wave that can damage the human body standing in front of the equipment. This event will continue until some upstream overcurrent device opens. The amount of energy that will be released during the event will depend on such things as,

• The amount of fault energy that could be supplied by the utility,
• The sizes and lengths of all conductors from the service point to the fault point,
• The amount of fault current that would be contributed by any large motors in the facility, and most importantly,
• The make and model of each overcurrent device upstream of the fault point, as well as any settings for any overcurrent device that is adjustable.

Collecting all that information is, as you have already said, the hardest part of the arc flash analysis. The trip curves would not be useful. All you need is to know that the breaker is made by XXX and is model YYY and is set to ZZZ. Getting accurate information on the conductor sizes and lengths takes a bit of work, especially in older facilities that do not have current and reliable as-built drawings.

As to performing the analysis, I have never tried to do it by hand. I use the SKM PowerTools software. Once I create a model of the distribution system and input the information I listed above, I only have to click one button, and I get the arc flash results.

Welcome to the forum, and I wish you the best of fortune in your career.

 

[GoldDigger]

The reason that you do not need the trip curves is that the software has them built into its database. Since the arc energy is the arc power times the time it takes for upstream protection to open, the trip curve plays a large part in the calculation.
It is also one of the reasons (low fault current relative to nominal trip point because of voltage drop across the arc) that sometimes available energy is a lot higher as a multiple of the norma power level for low voltages like 480 than for medium voltages.

 

[don_resqcapt19]

One of the hardest parts, as far as the information gathering, is getting the size and length of the feeder conductors, conductor material, and the type of raceway that they are installed in. This information is needed to find the available fault current at the load end of the feeder.

 

[tigim101]

Thank you for the replies. On an older facility how would I get size and length of conductors? The plan is to get software to input all of this, but I'm not sure if we are going to be able to get the software before I leave. It is a state/city funded community college on a large campus and from what I've seen going to the budget meetings it's a pain to even get approved to order cleaning supplies. I'm not sure if the IT department can operate any quicker. So I would need to know what information the software asks for so I can start to gather it before it is ordered.

I need to help find a cost effective software package for a project like this. I looked at SKM's price list and they have DAPPER 300 buses at $4500 plus $2000 for the arc flash evaluation module. I guess by buses they mean how many nodes you can input in your circuit? Would I have to be able to input every node for an entire building or can I do it for each machine room for example? I will see him again probably on Monday so we can discuss this further.

 

[bark]

arc flash ID plaque

arc flash ID plaque

The code now requires arc flash and voltage ratings plaques on service switchgear. Isn't there are simple way to calculate this with having to have an engineering company help. I am going to help my son out with a 400 amp single phase service for his new modular office building. It's only about 2800 sq feet. This is something new as far as the code and I'm starting to do my research on it now before I need this. Thanks for your help.

 

[mgookin]

Thank you for the replies. On an older facility how would I get size and length of conductors? The plan is to get software to input all of this, but I'm not sure if we are going to be able to get the software before I leave. It is a state/city funded community college on a large campus and from what I've seen going to the budget meetings it's a pain to even get approved to order cleaning supplies. I'm not sure if the IT department can operate any quicker. So I would need to know what information the software asks for so I can start to gather it before it is ordered.

I need to help find a cost effective software package for a project like this. I looked at SKM's price list and they have DAPPER 300 buses at $4500 plus $2000 for the arc flash evaluation module. I guess by buses they mean how many nodes you can input in your circuit? Would I have to be able to input every node for an entire building or can I do it for each machine room for example? I will see him again probably on Monday so we can discuss this further.

I spent a couple decades in government so I know where you're coming from in trying to get approval. If your top level person presents this matter as a legal compliance issue with which you are presently not in compliance with, that will help get funding approval, possibly even on an emergency (immediate) basis. If you just say you want it and they tell you to ask for it in next budget cycle, you may or may not see it in coming years.

Welcome to the forums. I wish you the highest level of success in your education & career. :thumbsup::thumbsup:

 

[don_resqcapt19]

... On an older facility how would I get size and length of conductors? ...

Someone has to physically look and measure. The length does not have to be exact, but has to be reasonably close as the impedance of the conductors can provide a significant reduction of the available fault current at the load end of the feeder.

Often difficult to do with out taking power outages because of the electrical safe work rules. In many cases an electrical contractor is hired to do this part of the work.

 

[don_resqcapt19]

The code now requires arc flash and voltage ratings plaques on service switchgear. Isn't there are simple way to calculate this with having to have an engineering help. I am going to help my son out with a 400 amp single phase service for his new modular office building. It's only about 2800 sq feet. This is something new as far as the code and I'm starting to do my research on it now before I need this. Thanks for your help.

The current code only requires a generic arc flash warning label and a label that shows the available fault current at the service equipment. You should be able to get the fault current from the utility. The will normally give you the maximum available fault current at the secondary side of their transformer. You may have to a calculation based on the size and length of the conductors that run from the transformer to the service equipment. Bussmann has an app that works great for that.

Based on the first draft report for the 2017, an actual arc flash calculation will be required for service equipment when that code is adopted.