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How do you label single phase equipment (for arc flash)?
no label
Category 00
Category 0
I use a software program to analyze the specific incident energy
I don't believe in arc flash
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ekstra   ara
 Post subject: Category 00
PostPosted: Tue May 01, 2012 2:10 pm 
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1. Has anyone considered adopting a "Category 00" or something similar for equipment outside of the scope of IEEE Std 1584-2002?
2. How many of you put a "Category 0" label on this equipment?
3. Jim, do you want to give us any indication where 1584 may be going with regard to the scope statements below?

1584 states the following;
  • Single-phase ac systems and dc systems are not included in this guide.
  • Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply.
  • The arc-flash hazard need only be considered for large 208 V systems: systems fed by transformers smaller than 125 kVA should not be a concern.


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PostPosted: Tue May 01, 2012 4:19 pm 
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MattB wrote:
3. Jim, do you want to give us any indication where 1584 may be going with regard to the scope statements below?

1584 states the following;
  • Single-phase ac systems and dc systems are not included in this guide.
  • Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply.
  • The arc-flash hazard need only be considered for large 208 V systems: systems fed by transformers smaller than 125 kVA should not be a concern.


Great question. I've provided info along the way at a few other locations on the forum as well about this topic since I've been deep in the middle since the middle of it all.

This is the latest. The majority of the group believes some form of exception is necessary. That is about all we can agree on right now. :confused:

At low levels of short circuit current, the incident energy as a function of time is not all that great. The issue is how long can an arc sustain which would result in a greater incident energy.

I still feel strongly about a current and kVA cut off something like 2500 A / 30 kVA This seems to be a good cut off for most. I also kept pushing for a default incident energy such as 4 cal/cm2 if the exception is used since the present language doesn't say what to do if you do not include a particular circuit in the study.

The problem is that under certain conditions, electrode gap, electrode orientation, open tip/barrier electrode termination, it is possible to sustain an arc at lower currents albiet not very common. Since it is "possible" regardless of the liklihood, a few are saying we should not have any exception and leave it up to the person performing the study to determine their own cut off. - Really?! :eek:

This is about where it ended up when I was leading the charge and now another person picked up the reins and we are just about back in the same place.

So in all honesty, I am no longer certain what the final language will look like. But I'm hopeful!

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PostPosted: Tue May 01, 2012 4:20 pm 
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MattB wrote:
3. Jim, do you want to give us any indication where 1584 may be going with regard to the scope statements below?

1584 states the following;
  • Single-phase ac systems and dc systems are not included in this guide.
  • Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply.
  • The arc-flash hazard need only be considered for large 208 V systems: systems fed by transformers smaller than 125 kVA should not be a concern.


Great question. I've provided info along the way at a few other locations on the forum as well about this topic since I've been deep in the middle since the discussion began.

This is the latest. The majority of the group believes some form of exception is necessary. That is about all we can agree on right now. :confused:

At low levels of short circuit current, the incident energy as a function of time is not all that great. The issue is how long can an arc sustain which would result in a greater incident energy.

I still feel strongly about a current and kVA cut off something like 2500 A / 30 kVA This seems to be a good cut off for most. I also kept pushing for a default incident energy such as 4 cal/cm2 if the exception is used since the present language doesn't say what to do if you do not include a particular circuit in the study.

The problem is that under certain conditions, electrode gap, electrode orientation, open tip/barrier electrode termination, it is possible to sustain an arc at lower currents albiet not very common. Since it is "possible" regardless of the liklihood, a few are saying we should not have any exception and leave it up to the person performing the study to determine their own cut off. - Really?! :eek:

This is about where it ended up when I was leading the charge and now another person picked up the reins and we are about back in the same place.

So in all honesty, I am no longer certain what the final language will look like. But I'm hopeful!

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Brainfiller.com


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PostPosted: Wed May 02, 2012 5:10 am 
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Thanks Jim


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PostPosted: Thu May 03, 2012 3:34 am 
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Quote:
1. Has anyone considered adopting a "Category 00" or something similar for equipment outside of the scope of IEEE Std 1584-2002


No. No reason to do so. 0 is as low as it gets. Below that, you'd have to consider whether or not meltable fabrics could melt and catch fire. So you'd label it "no hazard". A tricky thing to prove.

Quote:
2. How many of you put a "Category 0" label on this equipment?


Based on some of the data out there I for one am beginning to second guess even doing that and whether "1" should be the default, suck it up, and wear hot, nasty fire retardant shirts and pants as standard work wear.

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3. Jim, do you want to give us any indication where 1584 may be going with regard to the scope statements below?


This is a tough issue. Any time you force electrical and safety engineers into a corner where a decision has to be made and it's a grey area, they tend to either go 100% conservative or 100% liberal for you, or go into analysis paralysis where you get nowhere. The best answer is what the 70E Technical Committee tends to do...go away from the engineering data and look at real world data. Unfortunately as I said, you get the 0/100/"scared to death" crowd which makes it impossible to get to consensus.


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PostPosted: Thu May 03, 2012 6:22 am 
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1. I agree with Paul. Category 0 is the minimum for energized electrical work.

2. If exempted from the analysis due to being "Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low impedance transformer in its immediate power supply", then I consider it to be Category 0. Special cases should be considered. However I would not consider single phase and dc systems to be an automatic 0. Just because the equations haven't been developed, doesn't mean that there is no hazard.

Wearing Category 1 PPE (AR clothing, hard hat, face shield, etc.) as default is overkill. Over 90% of our end use panels are calculated to be <1.2 cal/cm2.


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PostPosted: Thu May 03, 2012 9:58 am 
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PaulEngr wrote:
No. No reason to do so. 0 is as low as it gets. Below that, you'd have to consider whether or not meltable fabrics could melt and catch fire. So you'd label it "no hazard". A tricky thing to prove.


We can agree that I wouldn't label anything "no hazard" and there's no place for melting fabrics around electricity.

This question comes from the following thoughts. I don't like the idea of labeling something for arc flash (Category 0) which is clearly outside of the scope of the standard. I don't want to tell someone to suit up if it's not necessary. When your outside in the sun at a single phase control panel, there's a difference between a light cotton short sleeve shirt and a heavy long sleeve shirt. As an interpreter of the code, I don't want to tell someone to wear long sleeves if it's not in the code. For a safety consultant, I can see that it's a no-brainer; wear long sleeves and safety glasses and you're basically suited up to Category 0.

PaulEngr wrote:
Based on some of the data out there I for one am beginning to second guess even doing that and whether "1" should be the default, suck it up, and wear hot, nasty fire retardant shirts and pants as standard work wear.


I don't like the idea of fire retardant clothes as standard work wear because regular laundering destroys the rating of the clothing. How many of you share this opinion? That may be a good survey/poll.

PaulEngr wrote:
This is a tough issue. Any time you force electrical and safety engineers into a corner where a decision has to be made and it's a grey area, they tend to either go 100% conservative or 100% liberal for you, or go into analysis paralysis where you get nowhere. The best answer is what the 70E Technical Committee tends to do...go away from the engineering data and look at real world data. Unfortunately as I said, you get the 0/100/"scared to death" crowd which makes it impossible to get to consensus.


I hope they find the money and time to do some single phase testing.


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PostPosted: Thu May 03, 2012 10:14 am 
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JJH wrote:
...However I would not consider single phase and dc systems to be an automatic 0. Just because the equations haven't been developed, doesn't mean that there is no hazard...


I recognize that there are arc flash hazards at single phase locations, I'm just not convinced that they are long sleeve hazards per the standard.

I recognize that there are arc flash hazards for DC locations, and I wouldn't dream of putting a Cat 0 label on DC equipment. It's not in the scope of 1584 and I agree that doesn't mean it's Cat 0. I have a number of DC questions, but I'll save those for another thread.


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PostPosted: Thu May 03, 2012 4:26 pm 
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MattB wrote:
We can agree that I wouldn't label anything "no hazard" and there's no place for melting fabrics around electricity.


My point is that if you were ever to create a category below "0" it would have to be where it's not even possible to ignite melting fabrics. For example intrinsically safe equipment can't ignite anything by design.

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I don't like the idea of fire retardant clothes as standard work wear because regular laundering destroys the rating of the clothing. How many of you share this opinion? That may be a good survey/poll.


First, depends on the environment. In a steel mill, glass plant, welding shop, oil refinery, and others, fire retardant clothing is ALREADY standard work wear. It truly isn't as bad as it sounds but it is expensive and hot. If I can work in 110-115 F working conditions day in and day out without all the claims of heat exhaustion, etc., in a foundry, and all my co-workers could as well (after acclimatizing to it), including in Southern states, and when my current employer has guys wearing this stuff in Trinidad, I think the arguments about what you can and can't do in summer are vastly overrated. I didn't say comfortable...just not as impossible as some make it out to be.

Second, not all fire retardant fabrics are created equal. Nomex and Indura Ultrasoft in particular are inherently fire retardant regardless of the number of washings. The cheap welding "greens" on the other hand seem to actually be even more flammable than regular cotton work clothes once they lose their fire retardancy, from personal experience (foundry). The ultrasoft stuff has come way down in price and breathes much better than Nomex. As far as I know, ANY 12 oz. or heavier cotton fabric can be treated to be fire retardant permanently.

There was a test done on soiled and severely abused coveralls worn by several different crafts of underground miners in Saskatoon, Canada for 2 years. The coveralls were Indura Ultrasoft. After 2 years the fire retardancy was basically unaffected, even when soaked with saltwater ("sweat"). It appears to me from the tests I saw (I think Arcwear ran them) that except for rubbing grease on them, they are immune. Even the "grease" test appeared to be that the grease lit on fire and burned until it went out. The coveralls themselves didn't look like they were affected. The conclusion I got from it is that the "fragile" nature of fire retardant clothing is highly overrated.


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PostPosted: Fri May 04, 2012 4:38 am 
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So what is considered low enough of an impedance to warrant a dangerous level? Would a 300 kva 5.7 ohm imp. 240v transformer be dangerous, or would the impededance at 2.5% be low enough to consider? I am looking at a situation just like this with a s.c.c. under 10 ka at 240v with a xfmr over 5%the impedance. 70e's cutoff of "low impedance" is a little too ambitious in my opinion when dealing with 240v and 208v systems.


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PostPosted: Fri May 04, 2012 4:39 am 
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Kenneth Sellars wrote:
So what is considered low enough of an impedance to warrant a dangerous level? Would a 300 kva 5.7 (% not ohm...sorry.) imp. 240v transformer be dangerous, or would the impededance at 2.5% be low enough to consider? I am looking at a situation just like this with a s.c.c. under 10 ka at 240v with a xfmr over 5%the impedance. 70e's cutoff of "low impedance" is a little too ambitious in my opinion when dealing with 240v and 208v systems.


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PostPosted: Fri May 04, 2012 1:08 pm 
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PaulEngr,
Thanks for sharing the info about clothing, that's good information.

Kenneth,
I disregard the impedance and only look at the size of the transformer because the second mention of the "smaller than 125 kVA" issue in 1584 does not mention impedance. If the transformer is 125 kVA or greater, I evaluate the downstream equipment. If the transformer is less than 125 kVA and the the equipment is less than 240 V, I don't evaluate the downstream equipment.


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PostPosted: Sat May 05, 2012 3:46 am 
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Kenneth Sellars wrote:
So what is considered low enough of an impedance to warrant a dangerous level? Would a 300 kva 5.7 ohm imp. 240v transformer be dangerous, or would the impededance at 2.5% be low enough to consider? I am looking at a situation just like this with a s.c.c. under 10 ka at 240v with a xfmr over 5%the impedance. 70e's cutoff of "low impedance" is a little too ambitious in my opinion when dealing with 240v and 208v systems.


You are on the right track. There are three different issues to deal with here.

First, IEEE 1584 currently gives a cutoff of 125 kVA or less from a single transformer at 240 V (or 208 V depending on how you interpret the language) or less. This will likely be changing/going away in the next update of IEEE 1584 but it's anyone's guess at this point what the new cutoff will be (if there is one).

Second, there's the issue of whether or not there is enough fault energy in the first place. As a first cut, you could assume say 2 second cutoff and look at what bolted fault current is required for 1.2 cal/cm^2 at each system voltage under consideration. This gives you an immediate indication of whether or not a complete calculation is necessary. Note that motor contribution can matter but you can make some assumptions about this as well.

Third, and finally, we arrive at the very, very sticky issue of under what conditions arcs can develop in the first place. In a DC system, this is pretty easy to resolve. There have been formulas (with some refinements) involving current and voltage available for decades indicating when a self sustaining DC arc can form. This cutoff has been researched extensively due to its value in discharge lighting. Under AC however, the situation is much more difficult.

Arcing in an AC system, the arc extinguishes and reignites at twice the line frequency. The inherent difficulty is predicting if the arc will reignite. It is strongly influenced by not only voltage, current, and arc gap (actually, arc length which is even more complicated), but also by how well the air is cooling between arcs and even somewhat by the electrode material. Something as simple as a change in the location within an enclosure or the shape of the enclosure controls how well the arc cools and can directly influence whether or not the arc can form or arc over enough cycles to be an arc flash issue. The end result is that the threshold in which an arcing fault can start in the first place is not well quantified. Worse yet, the arc may spontaneously go out after a few cycles. In fact this even occurs at 480 V. Some early test data on 240 V suggested that the cutoff was easily predicted but later data makes it much more questionable.

In light off all this, we are left today with only two choices:

A. Model everything down to the minimum predicted by Hertha Ayrnton's equation. This takes care of both AC and DC (it's the DC equation). Note that this even covers some scenarios below 50 volts. This is the most conservative approach but the one no one is using because you'd even have to model stuff below the current/voltage cutoff of IEEE 1584 using say the Neal/Doughty or even Lee method since IEEE 1584 falls apart outside of its empirical boundaries.
B. Use the current IEEE 1584 cutoff as a consensus safety standard. Model everything else. This follows a RAGAGEP (recognized and generally accepted good engineering practices) approach...ie, what most everyone is doing. It is known that there is trouble with the formula at the low end but right now there is nothing else available.


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PostPosted: Tue May 08, 2012 6:30 pm 
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Great information to have, again! Thanks so much, Paul, for sharing this...and btw, the book I downloaded at your suggestion looks incredible! Lots of great info!


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