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 Post subject: NFPA 70E Tables
PostPosted: Tue Jul 27, 2010 1:29 pm 
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Reading several articles on use of the tables and how they compare to actual results. Under "When to Use the Tables" the author states that "the tables may be utilized as part of a simplified safety program for facilities with small number of buses (simple radial systems with less than 20 buses.)"

We have been hired to do studies on systems this small for companies that want to comply with 70E. The field surveys spot many other issues that have happened over time such as out of date single lines, a device without the proper IR that snuck in, coordination issues, etc. so there is definitely a benefit in other areas besides incident energy.

The articles are several years old so any thought on using tables only on small systems for hazard category and PPE? Does this "small system" come from IEEE?


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PostPosted: Wed Jul 28, 2010 4:49 pm 
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Whether or not you can use the tables in lieu of an incident energy analysis depends on the available fault current and interrupting time, not on the size of the system. See the table notes.


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PostPosted: Thu Jul 29, 2010 5:13 am 
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The article was written by a very knowledgeable source at one of the leading power analysis software companies. I didn't know if the statement was based on seeing thousands of actual studies or if it came from IEEE. My concern being a newbie was that it wasn't obvious to me and that I hadn't done enough studies to see what the author was seeing.


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PostPosted: Thu Jul 29, 2010 6:30 am 
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mountaineer wrote:
The article was written by a very knowledgeable source at one of the leading power analysis software companies.


Of course he doesn't have a vested interest in diminishing the usefulness of the tables and promoting the use of software to do an incident energy analysis...


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PostPosted: Thu Jul 29, 2010 11:47 am 
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Tables vs. Studies

The "small" systems is kind of an opinion. The table footnotes have tons of limitations. There are many benefits in studies as you pointed out but if a company is just looking at protecting workers and they meet the table footnotes (which takes some engineering work) they can almost always protect workers with a table approach.

It has been pointed out to look at what the article writer is selling. That's a key to seeing his/her biases.

I'm not against either method but IEEE nor NFPA has any definition of "small" in the author's use. It is common to say small systems. We find that if they have >150kVA transformers they usually can't meet the tables. If they have more than one utility feed, they usually can't meet the tables for the whole building.

If you don't oversell studies but point out the benefits, you will find more business than you care to do right now.

We recommend companies get the digital files so they can keep things up even if their engineering firm or "arc flash expert" company goes under. Arc flash is hitting mainstream now and the "arc flash expert" companies are starting to struggle in the recession. One we used to compete against has dropped 60%. I always give folks their digital files so they can have a local company keep them up. I also recommend larger facilities get an internal specialist who can keep the onelines up to date and get outside engineering help when needed and to provide an audit from time to time.

Hope this helps.


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PostPosted: Thu Jul 29, 2010 4:29 pm 
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elihuiv wrote:
If you don't oversell studies but point out the benefits, you will find more business than you care to do right now.

We recommend companies get the digital files so they can keep things up even if their engineering firm or "arc flash expert" company goes under. Arc flash is hitting mainstream now and the "arc flash expert" companies are starting to struggle in the recession. One we used to compete against has dropped 60%. I always give folks their digital files so they can have a local company keep them up. I also recommend larger facilities get an internal specialist who can keep the onelines up to date and get outside engineering help when needed and to provide an audit from time to time.

Hope this helps.


Sound advice.

Now that the study is required to be updated every 5 years at the most, if your company won't supply the digital files......find a different firm. And take that "arc flash expert" title lightly, that term gets thrown around pretty easy, get references and ask questions.

I am the guy that comes in after the study for mitigation solutions and see reports and recommendations every day. I have seen some ugly stuff out there.


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PostPosted: Thu Jul 29, 2010 6:41 pm 
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Industry

We are seeing the exact opposite, more people jumping in and claiming they are competent in the process.


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PostPosted: Fri Jul 30, 2010 6:32 am 
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elihuiv wrote:
We find that if they have >150kVA transformers they usually can't meet the tables. If they have more than one utility feed, they usually can't meet the tables for the whole building.




So if a customer wants a "study" on system with a 100kva transformer and one utility feed, should the full software study be done or should it be suggested to use the tables? Is there a liability difference to the firm with recommending the tables as opposed to doing the detailed analysis? What is the balance? As an engineer you want to be as accurate as possible but at the same time you don't want to be overengineering and running up the costs for the customer if a full detailed analysis isn't required.

Thanks for all your feedback and suggestions.


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PostPosted: Fri Jul 30, 2010 7:30 am 
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mountaineer wrote:
So if a customer wants a "study" on system with a 100kva transformer and one utility feed, should the full software study be done or should it be suggested to use the tables? Is there a liability difference to the firm with recommending the tables as opposed to doing the detailed analysis? What is the balance? As an engineer you want to be as accurate as possible but at the same time you don't want to be overengineering and running up the costs for the customer if a full detailed analysis isn't required.

Thanks for all your feedback and suggestions.


It is really more a matter of short circuit current and complying with the table's footnotes. The 150 kVA transformer that was mentioned, often means the short circuit current will be higher above this size. Same with multiple feeds. If the footnotes (short circuit current and clearing time) are met, the tables can be used in lieu of the calculation study.

Personally, I prefer the calculations because that gives you an absolute number. i.e. if you calculate 6 cal/cm^2 you wear Arc Rated FR clothing and PPE greater than this value - period.

With the tables, the selection depends on the task (risk). You could have several categories of PPE listed for the same equipment depending on the task. As an example, a 480 V MCC would require category 4 PPE for inserting a bucket but category 2* PPE for voltage testing. If there is an arc flash, it is the same energy regardless of what you were doing at the time of the incident.

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PostPosted: Fri Jul 30, 2010 7:41 am 
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brainfiller wrote:
As an example, a 480 V MCC would require category 4 PPE for inserting a bucket but category 2* PPE for voltage testing. If there is an arc flash, it is the same energy regardless of what you were doing at the time of the incident.


So what label would be placed on the MCC - Cat 4 or Cat 2?

Sorry, the more I learn, the more questions I ask. :o


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PostPosted: Fri Jul 30, 2010 10:12 am 
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mountaineer wrote:
So what label would be placed on the MCC - Cat 4 or Cat 2?

Sorry, the more I learn, the more questions I ask. :o


No problem, that happens to everyone. Even the IEEE 1584 people. The more we discover from new tests and research, the more new questions we have. Guess that means it is time to stop the research and testing so we quit coming up with more questions :D

Your question is a great one. When the tables were originally developed, incident energy and/or categories were not required on the labels. Now that this information is required on the labels (2009 Edition of NFPA 70E) it creates the dilema that you just discovered. Which category gets listed?

Many would say use the worst case on the label which is Cat 4 for this case. Then the electrical worker sees this, puts on the moon suit and grumbles because all they want to do is check the voltage.

Before long it is decided to proceed with the full arc flash calculation study to see if Cat 4 is really necessary or if the actual calculated incident energy is something much lower. The full study often reaches conclusions that allow the use of lighter weight / lower arc rated protection.

However in the absense of calculations, the tables are still the best game in town.

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