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 Post subject: Minimum voltage and mva requirements for arc flash study
PostPosted: Mon Jun 21, 2010 9:17 am 

Joined: Mon Jun 21, 2010 9:00 am
Posts: 3
Hi,

I have a a fairly simple 120/240V split phase system with a 50kVA transformer. Typically I do not recommend conducting an arc flash for a system such as this as the fault currents involved are very small.

However, a client of mine has asked me to back this up with a clause from a relevant publication because he is worried about liablity. I am unable to find anything in the NFPA 70E or CSA Z462 which recommends a minimum system for which an arc flash study needs to be conducted. Let me know if you have found something related to this.

Your help will be highly appreciated.

Thanks,

Smit


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PostPosted: Mon Jun 21, 2010 11:21 am 
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Joined: Mon Mar 29, 2010 11:52 am
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Location: Yankton SD/ Lead SD
NFPA 70E, 130.3 Exception #1. This exempts systems less than, or equal to 240 volts, one transformer rated less than 125 kVA


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PostPosted: Mon Jun 21, 2010 1:26 pm 

Joined: Mon Jun 21, 2010 9:00 am
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Thanks for that info cbauer. I had an older version of NFPA 70E so did not know that the new NFPA 70E has an explicit exemption now.


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PostPosted: Mon Jun 21, 2010 2:19 pm 
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Joined: Mon Jan 18, 2010 11:35 am
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Location: Wisconsin
smitb wrote:
Thanks for that info cbauer. I had an older version of NFPA 70E so did not know that the new NFPA 70E has an explicit exemption now.


There were some pretty significant changes from NFPA 70E-2004 to 2009, you really should get a new edition.

In particular:
In formal rounding of Cat 0 up to 2.0cal/cm² is prohibited.
Automatic maximum of Cat 0 for 240V circuits less than 10kA is gone.
Face shields are now required for Cat 1.
Cat -1 no longer exists.
Maintenance of equipment is now explicitly required.


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PostPosted: Mon Jun 21, 2010 5:19 pm 

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You are right JBD. In fact I am going to get my hand on the 2009 edition as soon as possible. But the thing is we normally follow the CSA Z462 which has not been updated since 2008. For example this exception clause is not there in the CSA standard also face masks are not there in cat 1 for CSA (will need to double check on that one).


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PostPosted: Mon Jun 21, 2010 5:55 pm 
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smitb wrote:
You are right JBD. In fact I am going to get my hand on the 2009 edition as soon as possible. But the thing is we normally follow the CSA Z462 which has not been updated since 2008. For example this exception clause is not there in the CSA standard also face masks are not there in cat 1 for CSA (will need to double check on that one).


See article 4.3.3.1 in CSA Z462-08.

NFPA 70E 2009 and CSA Z462-08 articles are in the same order, even if the numbering is different.


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PostPosted: Tue Jun 22, 2010 6:21 am 
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Joined: Wed Oct 29, 2008 6:06 am
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Location: Michigan
Does no hazard analysis required mean no labeling required?

cbauer wrote:
NFPA 70E, 130.3 Exception #1. This exempts systems less than, or equal to 240 volts, one transformer rated less than 125 kVA


This is something I have been wrestling with and perhaps I'm over complicating it. Does no hazard analysis mean no labeling required?

As I understand it, just because a hazard analysis is not required, does not mean there is no hazard. Underneath this exception the 70E handbook notes say, "Workers should wear heavy leather gloves to protect their hands from any arc that might occur due to the fact that a worker's hands are much closer to the potential arc than the torso." :confused:

NEC 110.16 specifies what equipment is required to be field marked to warn qualified persons of potential arc flash hazards. NFPA-70E 130.3(C) specifies what information the label should contain.

NFPA-70E 130.3(A) states in those cases where detailed arc flash hazard analysis calculations are not performed for systems between 50-600V, the arc flash boundary shall be 4 ft. …

NFPA-70E 130.3(B) says that where it has been determined that work will be performed in the Arc Flash Protection Boundary as identified by 130.3(A), one of the following methods shall be used for the selection of PPE:
(1) Incident Energy Analysis.
(2) Hazard/Risk Categories.

So now I’m back to doing the analysis or following the tables. According to the tables, HRC 0 is the lowest category and it requires non-melting long sleeved shirt and pants, safety glasses, hearing protection and (if you're not already wearing them) leather gloves.

However, upon learning the definition of the AFP Boundary changed in '09 edition (it is not listed in the summary of changes at the beginning of the chapter either) I’ve run into some difficulty. It used to define it as a distance from exposed live parts; however, now it says a distance from a perspective arc source. :mad:

While I understand the enclosure is not rated to contain an arc, I am struggling with the fact that (as I understand it if this panel must be labeled) turning on the lights in the morning at the breaker box now requires a minimum HRC 0 PPE. :eek:


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PostPosted: Tue Jun 22, 2010 6:40 am 
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In my opinion, no analysis does not mean no labelling. Not sure where I got the following article, but it may help to clarify the issue for you.


This question comes up quite frequently.[/color] It is normal and typical in industrial plants, at least historically, for operators without electrical qualifications to perform 'lock out/tag out' on field equipment by transition of a breaker, MCC disconnect, or safety switch.

Art 130 specifically deals with PPE protection and qualifications for performing 'live' work. Obviously, making a transition of any of the above devices, with doors closed, and panels on, is NOT live work.

The 'change in opinion' about operation with doors closed, comes from 130.7(c)9 FPN No. 1:

"...In several cases where the risk of an arc flash incident is considered low, very low, or extremely low by the task group, the hazard/risk category number has been reduced by 1,2, or 3 numbers, respectively. The collective experience of the task group is that in most cases closed doors do not provide enough protection to eliminate the need for PPE for instances where the state of the equipment is known to readily change (e.g., doors open or closed, rack in or rack out). The premise used by the Task Group is considered to be reasonable, based on the consensus judgement of the full NFPA 70E Technical Committee."

Almost all agree that the above statement would include Switchgear, because of the high change in energy. This would include, mere operation of switching in or out a switchgear breaker even with doors closed. The point of disagreement is how far do we take this. An office 120V wall switch is changing the state of 800 watts of fluorescent fixtures, but do we require the office worker to don PPE to turn on the lights in the office? The janitor, at the end of the day, walks to the 208/120 lighting panel and turns off all the breakers for the hall lights before he leaves, and the office manager turns them back on again when she arrives in the morning. Do they need PPE, while operating these breakers, which are designed and listed for this service, with panels on? This is usually where the point of disagreement begins.

It is my opinion, at least until the next edition or technical advisory appears, that Art 130 is designed to reduce the risk of injury by protecting trained personnel while performing 'live' work. Live work means exposure to live conductors. The intent of the code was NOT to stop utilization of electrical equipment by non-qualified persons when that equipment was specifically designed and rated for such service. This permits the secretary in her poly/nylon pants suit to turn on her light switch as she enters the office. From my prospectus, which others disagree, it also permits operation of basic and customary equipment like circuit breakers, safety switches, and MCC bucket handles (with doors closed). When such equipment, begins to resemble the energy levels of switching similar to Switchgear, then I believe the limit appears. What constitutes this 'level of energy' is not specified and up to the judgment of the property owner. For an industrial site I would not limit operators from continuing to do lock outs on MCCs up to size 3 starters, maybe 4; 480 and 208 breakers to 225A.

I would mention that I do not agree that any operator should touch electrical equipment without some basic and repeated training. They should be taught not to stand directly in front of the door, to stand off to the side with back to equipment and to turn away while transitioning. Lock out/tag out training usually includes this as well as means to verify that the correct disconnect was operated for the piece of equipment in question.

These are my thoughts of interpretation. I have these because NFPA did not do a very good job defining the limit with a fault current value, or IE value.

The argument also comes up that most MCC's, safety switches, and panel boards are NOT arc flash rated. Meaning they might 'NOT' contain an arc flash and result in moving sheet metal. This is a risk. In an industrial site I would rate this risk remote in comparison to the daily risk associated with any manufacturing site. It comes down to what is acceptable. While I have seen electrical equipment 'short' because of age or water intrusion, and arc internally, I have not seen any that blew the doors off, especially not on the low energy equipment. I am sure it happened but what is the likelihood. Do we reach the point that an office worker can not even 'walk' by the lighting panel in the hallway without PPE?

I think a discussion and agreement on where that permissible limit exists would be welcomed. [/color]


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