OT: Arc Flash Study

agarb

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May 2006
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The small OEM that I work for is a bit behind the times. The “powers that be” have a consultant coming in that is supposed to be training us on arc flash safety. I am always thankful for increased safety measures and awareness!

I have an older copy of NFPA 70E and been doing some reading and research on my own so I can ask intelligent questions.

As I understand it, an arc flash hazard analysis determines the protection boundary and therefore the required PPE. But if an analysis is not done, it appears that one can simply refer to table 130.7(C)(9)to determine the appropriate Hazard/Risk Category (HRC) and then choose the proper PPE from tables 130.7(C)(10) and 130.7(C)(12) based on the previously determined HRC.

For reference, most of our equipment has a mixture of 480 VAC (for drives, motors, resistance heating) down to 24vdc (for inputs, outputs, instrumentation etc.). On small projects, it is impossible to provide separate enclosures for high and low voltage due to space requirements. As I read the tables, it appears that we would generally end up with hazard risk category 2. I say this because of the voltage and the need to perform various tests (typically with multimeter) on energized components.

My concern in all this is that the consultant and “powers that be” may somehow try to implement more protection than necessary ("just to be on the safe side") which would adversely affect efforts by engineers and technicians as they debug new equipment.

My GUESS is that the tables are VERY conservative and likely require more and/or better PPE than an actual arc flash hazard analysis would support for our facility. Does anybody have any experience with this? For instance, is it possible that a proper analysis could reduce the HRC from 2 to 1 or perhaps a 2 down to a 0?

My reason for asking is that I think the company is balking at doing the actual arc flash hazard analysis for the facility due to what is perceived as too high of cost.
 
we have are flash categories here. A, B, C, D or 0, 1, 2, 3, 4
we only use B, D or 2 and 4.

arc flash analysis as I understand it determines the amount of calories of energy that can be discharged in the event of an arc flash. this calorie rating determines how much ppe is requited and what type.

the boundaries are covered in NEC 70 and also NEC70E

to keep things simple, we only use category B and D or 2 and 4.

yes, the rules for arc flash can be a pain, but I WILL NEVER let that get in the way of safety! at my current location, violation of this can get you fired.

may I suggest that you google are flash incidents.

Our training had us watch videos and the instructor stopped one video, the person died on camera at that plant.
i'm going to be as graphic as I can and yes, this did happen here.
I cannot go into details, but picture yourself in front of a panel and you turn
the disconnect switch on. the next thing you see, is a black face shield.
What the @##$$%$, your ears are ringing and you pull off your ppe hood.
the face shield is totally burnt, bubbles and melted metal on the shield. its destroyed. you look at your hood cloth and its outer cover is burnt up as well as your ppe coat. BUT! you are safe !!

what do you think that person would look like if they had no are flash and ppe equipment on?

if I am incorrect on the arc flash statement, someone please correct me.

james
 
To be clear, my intent was not to debate safety… I am all for that!

I want safety but not artificially inflated HRC categories that impose unnecessary PPE requirements.

I'm thinking that paying for a study may be the best way to accomplish this vs just reading the HRC from a table and choosing PPE based on that.
 
It can't hurt to have someone come in and do a study. Plus, you only have to pay the consultant 1 time, once they come in and do the study they never have to come back again.

Arc flash accidents are really terrifying and I saw one happen to one of my coworkers at my first job fresh out of school. This was at a small OEM. He was not wearing any PPE, just a pair of reading glasses. Well, I will just say that if he was not wearing reading glasses then he would probably be blind now.

You can also tell your managers that if they don't pay for a study, then you will end up guessing at what level of PPE is needed, and either underestimate it (unsafe) or overestimate it (too safe, in which case employees are more likely to fail to comply with it because they see it as unnecessary, which again creates unsafe situations.)
 
you turn
the disconnect switch on.

Excuse my ignorance but I thought arc flash was more likely during turning disconnect off?

Different topic, arc flash and vfds outside of panels. I've been seeing mixed results in the field, some places vfd in panel/MCC and others vfd just on the wall.
 
Where I work we started off using the table method for determining the PPE required, and by the table, almost everything in our plant was Cat. II or higher. Later on when we were finally able to get an actual study done, 90% of our cabinets became Cat. 0. It was expensive and time consuming to get the study done but well worth it in the long run.

The table system assumes the worst case scenario and dictates PPE based on that assumption. Having the study done lets you clearly understand each cabinet and apply the proper PPE for each one. Having the study done also gives you the information on how to reduce the hazard category for each cabinet. During the first part of our study several cabinets were identified as Cat II, but it also told us the things that would need to change in order to get the category reduced. All we had to do was change a few fuses to a different style and change some circuit breaker settings and those Cat II's became Cat 0's.
 
Where I work we started off using the table method for determining the PPE required, and by the table, almost everything in our plant was Cat. II or higher. Later on when we were finally able to get an actual study done, 90% of our cabinets became Cat. 0. It was expensive and time consuming to get the study done but well worth it in the long run.

Lynx777 - What you describe is exactly what I suspected with the table method! This was the type of feedback I was hoping to receive.

I don't really care about machine shop and facilities side of the plant because it doesn't affect me. But maybe I can use this info to push hard for an actual study in the side of the plant (different building) where most of the debug and testing takes place.
 
A lot of the calorie potential comes from the type of current limiting device used, and the distance involved. The integrity of your ground network will also have a large impact.
For example, a QO360 250' away will give you a much higher calorie potential than a fused disconnect with 60A current limiting fuses that is mounted right next to the machine. One company I worked for had local fused disconnects installed on the power/control cabinets following an arc flash study for this very reason. It took us from a CAT 2 down to a zero.
In the case of service panels and switch gear, there is not a lot that can be done to limit the calorie potential, but even so there are steps that can be taken to get you from say, 40 Cal down to 10 or 15 Cal. Given a choice of putting on a 40 Cal suit or 10 Cal overalls, the choice is simple for me.



Bubba.
 
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