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 Post subject: 120 Volt Single Phase Arc Flash
PostPosted: Wed Jun 13, 2012 7:27 am 
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I work in an industrial manufaturing plant. We have many machines with interlocking disconnects that run on 120V circuits. They are powered from an HRC 0 transformer. From what i am reading in 70E 130.5(C), I need to have these labeled for ARC flash. If not and I open the panel to troubleshoot I would need HRC 1 protection {using table 130.7(C)(15)(a), 240 v and below} to measure the voltages. Am I reading this correctly? Am I missing something? Our contractor that provided our survey and labels did not go beyond 480 3 phase.

In addition i noticed 130.5(C) states "The method of calculating and data to support the information for the label shall be documented". Is there any prefered location for this documentation?


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PostPosted: Thu Jun 14, 2012 5:41 am 
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Rookie Hoyer wrote:
I work in an industrial manufaturing plant. We have many machines with interlocking disconnects that run on 120V circuits. They are powered from an HRC 0 transformer.

I assume that when you say an HRC 0 transformer, you are referring to the primary because your analysis only covered 480 V. The low side may not be HRC 0 when calculated on the 120 V side. IEEE 1584 doesn't cover single phase, but you would be conservative to assume a three-phase fault of the same magnitude.

If the transformer is less than 125 kVA IEE 1584 says that it "need not be considered" or "should not be a concern." Since this is dealing with 3Ø, I would consider the limit for 1Ø to be 125/3. The equipment still needs to be labeled, and what we have done is label 120/208 V equipment that falls below the limit as PPE category 0 (HRC implies that a hazard/risk analysis has been done).


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PostPosted: Thu Jun 14, 2012 9:04 am 
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My boss is referencing IEEE 1584a-2004. I believe this exempts us from calculations by does not exempt us from labeling the equipment. Am I correct in assuming this? Does 1584b-2011 include anything? I don't have access to 1584 at all!


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PostPosted: Fri Jun 15, 2012 4:03 am 
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Rookie Hoyer wrote:
My boss is referencing IEEE 1584a-2004. I believe this exempts us from calculations by does not exempt us from labeling the equipment. Am I correct in assuming this? Does 1584b-2011 include anything? I don't have access to 1584 at all!


The part about being documented is pretty clear. You have to document somewhere how you came up with your rating. Where/how this is done is not specified.

As to what MUST be labeled, NEC indicates what equipment must be have a label for arc flash hazards (Article 110).

As a general rule for low voltage equipment such as 120 V/single phase, most plants adapt a rule somewhere along the lines of if certain types of equipment aren't labeled, then use an assumption. Two of the most popular are:
1. H/RC 0
2. H/RC 4

A third popular one is to read the nearest upstream equipment label. There are two problems with this. First, as mentioned earlier generally you label the "upstream" side and not the downstream side. Second especially if cable lengths get long, even if the label was for the "downstream" side, it is likely lower (increasing resistance in cable leads to longer trip times and larger cal/cm^2). There is no valid version of this.

I would recommend that if your transformer is less than 125 kVA and 208 V or less and the device is fed from a single source, then use the IEEE 1584 exception while it is still there. Real soon now, IEEE 1584 will get revised. They will have a new calculation method which takes more factors into account like bus bar configuration and enclosure size, but also the "125 kVA rule" is probably going to get deleted.


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PostPosted: Fri Jun 15, 2012 6:22 am 
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Rookie Hoyer wrote:
My boss is referencing IEEE 1584a-2004. I believe this exempts us from calculations by does not exempt us from labeling the equipment. Am I correct in assuming this? Does 1584b-2011 include anything? I don't have access to 1584 at all!

1584a is an amendment that doesn't address the 125 kVA exemption.

You should (diplomatically) suggest to your boss that if he wants you to answer his questions on IEEE 1584, he should spring for the expense of buying it. :eek:


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PostPosted: Fri Jun 15, 2012 7:24 am 
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Rookie Hoyer wrote:
I believe this exempts us from calculations by does not exempt us from labeling the equipment. Am I correct in assuming this?


You are correct.
1584 says not to wory about making calculations.

OSHA says you need to advise your employees of known hazards and provide them the tools to select the appropriate PPE.

The trend I am seeing is for facilities to provide arc flash information labels on all equipment down to the maintenance LOTO points or the process machines themselves.

For 120/240V single phase circuits it seems my customers fall into three major groups.
Most simply use an HRC=0 tag, they feel the risk of injury is minimal based on their historical experience.
Many use HRC=1 as this is the 'worst case' based on the task tables in 70E
<10% have us use the procedures in the IEEE Violet book to 'convert' single phase faults into equivalent three phase ones and then compute an AFIE.


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PostPosted: Fri Jun 15, 2012 8:31 am 
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THANKS, I think we are going to go HRC 0 based on the proximity and rating of the upstream devices. I have advised management that we need a training document and safety program. We'll include a section on determining hazards and evaluate as we go.


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PostPosted: Fri Jun 15, 2012 3:58 pm 
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JBD wrote:
<10% have us use the procedures in the IEEE Violet book to 'convert' single phase faults into equivalent three phase ones and then compute an AFIE.


Not a good idea. Violet book is for bolted faults, not arcing faults. I don't think you can make this logical leap.


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PostPosted: Sun Jun 17, 2012 7:19 am 
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Violet book?


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PostPosted: Sun Jun 17, 2012 5:32 pm 
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John P. Leseganich CPEA wrote:
Violet book?

http://www.angelfire.com/al/CommissioneElettrica/ieee/atti/overview.pdf

There is a series of IEEE standards called the "Color Book" series that are considered something of a "foundation". The Violet cover book is IEEE Std. 551, the standard for short circuit calculations. To handle single phase, it has a procedure for converting single phase into an equivalent 3 phase fault.


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PostPosted: Mon Jun 18, 2012 5:13 am 
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WOW another std. This is the first i've heard of 551. I'll have to go find more info on that one.


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PostPosted: Mon Jun 18, 2012 6:45 am 
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PaulEngr wrote:
Not a good idea. Violet book is for bolted faults, not arcing faults. I don't think you can make this logical leap.

The IEEE1584 calculations are based on bolted fault values, not arcing fault.

My whole point,really, was that any calculations of a single phase arc fault incident energy value are just past being subjective and are hardly better than 'feel good' exercises. There are no industry accepted, fully peer reviewed, methods for single phase AFIE particularly at =<240V. Therefore it is very easy to justify a non-calculation methodology.


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PostPosted: Mon Jun 18, 2012 4:19 pm 
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JBD wrote:
The IEEE1584 calculations are based on bolted fault values, not arcing fault.


Not quite true. IEEE 1584 research developed a formula that relates arcing fault currents back to bolted fault currents. Part of the calculation includes calculating the arcing fault current. This is particular important at lower voltages and currents. At higher voltages especially, there's hardly any difference between bolted and arcing fault currents for 3 phase (L-L-L) faults. Of course as stated other types (L-G, L-L, L-G-L) have not been studied and/or tested.


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PostPosted: Tue Jun 19, 2012 7:35 am 
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PaulEngr wrote:
Not quite true. IEEE 1584 research developed a formula that relates arcing fault currents back to bolted fault currents. Part of the calculation includes calculating the arcing fault current. This is particular important at lower voltages and currents. At higher voltages especially, there's hardly any difference between bolted and arcing fault currents for 3 phase (L-L-L) faults. Of course as stated other types (L-G, L-L, L-G-L) have not been studied and/or tested.

To calculate the arcing fault you start with the bolted fault.

Regardless, developing 1-phase, <=240V, arc flash incident energy values using 3-phase methodologies is not a defined 'worst case' scenario, it is simply a guess. It may make people feel good because, number crunching is involved, but isn't proven to be more conservative than the 'task tables'.


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PostPosted: Tue Jun 19, 2012 12:24 pm 
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jghrist wrote:
I assume that when you say an HRC 0 transformer, you are referring to the primary because your analysis only covered 480 V. The low side may not be HRC 0 when calculated on the 120 V side. IEEE 1584 doesn't cover single phase, but you would be conservative to assume a three-phase fault of the same magnitude.

If the transformer is less than 125 kVA IEE 1584 says that it "need not be considered" or "should not be a concern." Since this is dealing with 3Ø, I would consider the limit for 1Ø to be 125/3. The equipment still needs to be labeled, and what we have done is label 120/208 V equipment that falls below the limit as PPE category 0 (HRC implies that a hazard/risk analysis has been done).


I would like to add a very humble opinion:

1. 70E eliminated the 125 kVA rule almost three years ago, it was done via a TIA.

2. The PGE testing has shown that it is very difficult to maintain an arc at 120/208 (three phase), in addition, there is a low end arc extinguishing point. If you keep that in mind and also realize that single phase arcs always must flow through voltage zero - with great engineering certainty we can say that it is difficult if not impossible to have a single phase (120V) arc flash that can cause second degree burns at 18". My reasons for the comment, I have trained several thousand electrical workers and I am very cautious about over dressing personnel for multiple reasons.


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PostPosted: Tue Jun 19, 2012 2:02 pm 
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Flash wrote:
1. 70E eliminated the 125 kVA rule almost three years ago, it was done via a TIA.

They tried to, however the TIA was never issued.


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PostPosted: Wed Jun 20, 2012 9:06 am 
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Flash wrote:
If you keep that in mind and also realize that single phase arcs always must flow through voltage zero - with great engineering certainty we can say that it is difficult if not impossible to have a single phase (120V) arc flash that can cause second degree burns at 18".

If arcs could not be sustained through a voltage zero, all circuit breakers would have a fault clearing time of one-half cycle over the contact opening time.

While I suspect that your conclusions are correct, I don't think you can say it with "great engineering certainty" and it would be nice to have some industry accepted documentation that 1Ø 120 V arcs are not a hazard.


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PostPosted: Thu Jun 21, 2012 4:05 am 
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Flash wrote:
I would like to add a very humble opinion:
2. The PGE testing has shown that it is very difficult to maintain an arc at 120/208 (three phase), in addition, there is a low end arc extinguishing point.


Unfortunately more recent data contradicts the PG&E test that you referred to. I just don't know where that data has been published that you can get a copy of it. 70E-2012 "pulled" the 125 kVA/208V (or is it 240V?) exception because that is specific to ONE modeling method (IEEE 1584). Others do not contain that cutoff. Further, the 1584 Committee is probably going to change the wording in the next edition. A recent "Forum" poll asked a question specifically about this issue for that reason. So for right now, the exception still "stands" if you are using IEEE 1584 as the basis for the calculation. Once 1584 updates, the "exception" is invalidated. Three problems occur regardless:
1. The arc DOES extinguish during every current transition through zero. Whether or not the arc can restrike is the question. This is primarily a function of the plasma/ions present in the area and air temperature since as air temperature increases, conductivity increases as well. As long as the path is sufficiently conductive, the arc will restrike. In a single phase/low voltage scenario, the rate of cooling is such that it puts a limit on how long an arc can be sustained.
2. The standard test used a fairly heavy gauge wire as a fuse to ensure reliability of the arc flash tests. More recent testing has found that the reason that many times low energy arcs would not ignite or would not self sustain was because of the "fuse". Going to a lower gauge wire allows reliable testing at lower energies. I believe this was documented by Mersenne.
3. The "125 kVA exception" is extremely weak in general. If you read the actual 1584 text, it is hardly a glowing endorsement of anything but the idea that a cutoff exists.

Other sources:

http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=4503412&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F28%2F4503404%2F04503412.pdf%3Farnumber%3D4503412

I recently came across an article covering single phase faults that suggested that it cannot be swept under the rug and that it is NOT conclusive that single phase fault incident energy is less than a three phase case.But I forgot to flag where I found it and can't seem to find it again.


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PostPosted: Mon Jun 25, 2012 7:07 am 
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Flash wrote:
I would like to add a very humble opinion:

1. 70E eliminated the 125 kVA rule almost three years ago, it was done via a TIA.

2. The PGE testing has shown that it is very difficult to maintain an arc at 120/208 (three phase), in addition, there is a low end arc extinguishing point. If you keep that in mind and also realize that single phase arcs always must flow through voltage zero - with great engineering certainty we can say that it is difficult if not impossible to have a single phase (120V) arc flash that can cause second degree burns at 18". My reasons for the comment, I have trained several thousand electrical workers and I am very cautious about over dressing personnel for multiple reasons.


Lets fo back to base one:

1. Has the 125 kVA rule been removed from 70E-2012?

2. Does anyone on this board have evidence beyond a doubt that there has been arc flash burns on a single phase circuit at 18"?


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PostPosted: Mon Jun 25, 2012 8:20 am 
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2. Does anyone on this board have evidence beyond a doubt that there has been arc flash burns on a single phase circuit at 18"?

yes, I've seen the burn scars on an electrician who faulted a small single phase residential panel. His chest is quite a sight, it looks like he swallowed a bomb.


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