It is currently Fri Nov 17, 2017 11:21 pm



Post new topic Reply to topic Go to page Previous  1, 2
Author Message
ekstra   ara
 Post subject:
PostPosted: Tue May 28, 2013 11:22 am 
Offline
Sparks Level
User avatar

Joined: Mon Nov 10, 2008 1:08 pm
Posts: 60
Location: Cincinnat, OH
Larry Stutts wrote:
Are you looking for validation for working on incident energies greater than 40 cal/cm^2?
Or just asking if it is safe to do so as long as you have PPE rated greater than the calculated incident energy?
Or is your question based on acedemic curiosity? I ask a lot of those


All three if possible, but mostly, the third question. Academic Curiosity. I just want to know what is right, not what is the answer I should give to my client.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed May 29, 2013 11:21 am 
Offline
Sparks Level
User avatar

Joined: Tue Apr 17, 2012 8:19 am
Posts: 240
Location: Charlotte, NC
PaulEngr wrote:
ESFI reports arc flash fatalities based on BLS statistics at 1 in 100, 000/year. Shock is twice that rate. Equipment arcing fault rates are generally around 1 in 10k to 1000k depending on type. Not sure your data source but it seems extremely high from comparative data. I think using electrical workers is skewing your data high. Many arc flash incidents happen to nonelectrical workers so you'd have to look at the occupational data. For instance most electrical injuries occur in construction.


While I was looking for current data on arc flash incident injuries, I found a current source with the same statistics I was using. I am still modifying my presentation based on the ESFI reports you refered me to though.
[SIZE=3]FROM http://arcflash.com.au/arc_flash_statistics.php[/size]
Arc Flash Statistics
·[font=Times New Roman][/font][SIZE=3]30,000 Arc Flash Incidents per year[/size]
·[font=Times New Roman][/font][SIZE=3]7,000 Burn Injuries per year[/size]
·[font=Times New Roman][/font][SIZE=3]2,000 Hospitalisations per year[/size]
·[font=Times New Roman][/font][SIZE=3]400 Fatalities per year[/size]
·[font=Times New Roman][/font][SIZE=3]80% of Electrical Worker fatalities are due to burns, not shock[/size]


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed May 29, 2013 12:45 pm 
Offline
Sparks Level
User avatar

Joined: Tue Apr 17, 2012 8:19 am
Posts: 240
Location: Charlotte, NC
As I was looking for arc flash statistics, I came across something that caught my attention at http://exiscan.com/electrical-safety-arc-flash-statistics

Arc flash fatalities are not counted in electrocution statistics, they are logged under burn injuries.

This will certainly makes it more difficult to assess the number of arc flash injuries, since I was focusing on electrical data, not burn or explosion data.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed May 29, 2013 7:41 pm 
Offline
Plasma Level
User avatar

Joined: Tue Oct 26, 2010 9:08 am
Posts: 1796
Location: North Carolina
Keep in mind that for instance the phase angle plays a big part in the incident energy and can often almost double the incident energy in the event of an "instantandous" trip with a typical vaccuum breaker (3 cycles in a new one), lockout relay (1 cycle), sometimes an auxiliary relay (1 cycle), and the 50/51 relay itself (1 cycle), for a total of 6 cycles unless you aggressively redesign it to get under 4 cycles, and perhaps use a magnetic actuator with a 1 cycle trip time, and adapt DC relaying to drop relay trip times to 1/4 cycle to get under 50 ms. Out over 10 cycles or so, assymetrical fault current is a minor issue.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Jun 03, 2013 4:40 am 
Offline
Sparks Level

Joined: Wed Oct 29, 2008 6:06 am
Posts: 136
Location: Michigan
Larry Stutts wrote:
As I was looking for arc flash statistics, I came across something that caught my attention at http://exiscan.com/electrical-safety-arc-flash-statistics

Arc flash fatalities are not counted in electrocution statistics, they are logged under burn injuries.

This will certainly makes it more difficult to assess the number of arc flash injuries, since I was focusing on electrical data, not burn or explosion data.



I think it's possible they often get logged into either category... :(

http://www.esfi.org/index.cfm/pid/11506


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Jun 04, 2013 9:28 am 
Offline
Sparks Level

Joined: Thu Jul 19, 2007 7:54 am
Posts: 201
Location: St. Louis, MO
McFlash wrote:
Article 130.7 (A) Note 3. Same note that has been in previous editions. That is why the concern for the levels above 40. I do know what you are saying in regards to operating the disconnects and that is more of a risk assessment matter to understand if significant risk is present.

My question was more from the angle of is there ever a time that it is acceptable to put on PPE higher than 40 and work as long as the PPE exceeds the calculated Incident Energy. That seems to be a question with no apparent answer.


The question does not have an easy answer, as you already know.
The only reference to anything over 40 cal/cm2 is in the Informational note you mentioned already, and Informational notes are not enforceable.
Many of the locations I have seen are rated over 40 calories due to the long clearing time, and not due to high arcing currents, always that is not always the case. The question of long clearing times has been addressed by Jim above pretty well.
The danger above 40 calories has always been said to be the pressure wave, but that conventional wisdom is being put to question as well.
I still would not advise anyone working on equipment over 40 calories for this reason, however.

The best fallback arguement I can offer is OSHA - personnel should be provided with a safe workplace, and with appropriate PPE. The available PPE is not rated for pressure, only for fire. Since pressure is the question, I cannot say that personnel will be safe.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Jun 04, 2013 10:49 am 
Offline
Sparks Level
User avatar

Joined: Tue Apr 17, 2012 8:19 am
Posts: 240
Location: Charlotte, NC
WDeanN wrote:
The best fallback arguement I can offer is OSHA - personnel should be provided with a safe workplace, and with appropriate PPE. The available PPE is not rated for pressure, only for fire. Since pressure is the question, I cannot say that personnel will be safe.


Someone thinks it is safe to work in close proximity to incident energy over 40 cal/cm^2, and some would vocalize their 'right' to work in that enviroment at the top of their lungs (I am not talking about anyone in particular)

Right next to that person is another who is soundly convinced enterring the Arc Flash Boundary will result in certain death (Again, I am not talking about anyone in particular).

I would agree that enterring the Arc Flash Boundary presents a certain level of risk.

It seems to me that for too many people 'safe' is too relative a term. Someone thinks an action is 'safe', someone else thinks the same action in the same circumstances is 'unsafe'. They both can't be right all the time.

By the same token, sometimes it stands to reason that it might be relatively safe to perform certain tasks. Given the same Incident Energy:
[INDENT=1] When the tA is longer, the arc blast pressure is lower. [/INDENT]
[INDENT=2]The thermal rating of the suit would provide adequate protection.[/INDENT]
[INDENT=1] But when the tA is short and the arc blast pressure is high.[/INDENT]
[INDENT=2]The thermal rating of the suit would provide adequate protection against the heat - but it's not exactly body armor.[/INDENT]

I wonder if maybe tA needs to be included on the Arc Flash label so a worker can know which hypothetical situation he is stepping into?


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Jun 04, 2013 6:42 pm 
Offline
Plasma Level
User avatar

Joined: Tue Oct 26, 2010 9:08 am
Posts: 1796
Location: North Carolina
There is an article on this site proposing just that...effectively setting the exposure limits without PPE based on thermal flux instead of heat flux...effectively dropping integration over time, based on 2nd degree burn data at under 1 second. Data from Privett suggests this is incorrect and that appears to be the general consensus.

Safe is a relative term even with current models. IEEE 1584 talks about the "long tail" where some arcs just happen to be much higher thermal energy. Numerically 1584 estimates that ATPV ratings will be effective about 90% of the time. So even if you wear a 40 cal suit to do work below 40 cals and inspect it and wear it properly, about 10% of the time, the incident energy will exceed the cakculated value. Fortunately we only get about 1:100, 000 worker injuries per year (source ESFI) and for most equipment the likelihood of an arcing fault in the first place with just about anything except drawout mechanisms is around 1:100, 000 (source: IEEE 493 data) and then 90% of the time, if wearing proper PPE, one can walk away so wearing the PPE lowers your risk for most tasks to about 1 in a million, even the risky ones. That's assuming doing proper maintenance as well because if we cannot assume this then equipment reliability with certain types of equipment gets really bad while others are not appreciably affected (source: electrical power distribution reliability 2nd edition).


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Jun 05, 2013 5:19 am 
Offline
Sparks Level
User avatar

Joined: Tue Apr 17, 2012 8:19 am
Posts: 240
Location: Charlotte, NC
PaulEngr wrote:

Safe is a relative term even with current models. IEEE 1584 talks about the "long tail" where some arcs just happen to be much higher thermal energy. Numerically 1584 estimates that ATPV ratings will be effective about 90% of the time. So even if you wear a 40 cal suit to do work below 40 cals and inspect it and wear it properly, about 10% of the time, the incident energy will exceed the cakculated value. Fortunately we only get about 1:100, 000 worker injuries per year (source ESFI) and for most equipment the likelihood of an arcing fault in the first place with just about anything except drawout mechanisms is around 1:100, 000 (source: IEEE 493 data) and then 90% of the time, if wearing proper PPE, one can walk away so wearing the PPE lowers your risk for most tasks to about 1 in a million, even the risky ones. That's assuming doing proper maintenance as well because if we cannot assume this then equipment reliability with certain types of equipment gets really bad while others are not appreciably affected (source: electrical power distribution reliability 2nd edition).


That is very interesting. It leads me to wonder what proportion of injuries related to an arc flash event are due to the arc blast and not burns from the arc flash.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Jun 13, 2013 7:00 am 
Offline
Sparks Level
User avatar

Joined: Mon Nov 10, 2008 1:08 pm
Posts: 60
Location: Cincinnat, OH
Thank you to all for the discussion. As I can see this is a topic that many in the industry are dealing with and have not clearly found the answer that is clearly spelled out by a standard.

I think we will stick with what we have been doing in telling workers that above 40 Cal you should not work within the boundary. There are compelling arguments that I happen to agree with concerning tA being a factor that could lead to the pressure above 40 being reduced, but at the same time what is that time element, and where is the breaking point. For now it is exactly as WDeanN "The best fallback arguement I can offer is OSHA - personnel should be provided with a safe workplace, and with appropriate PPE. The available PPE is not rated for pressure, only for fire. Since pressure is the question, I cannot say that personnel will be safe. - See more at: http://arcflashforum.brainfiller.com/threads/2728/page-2#post-13274"

As far as I am concerned PPE use above 40 Cal is really not defined and that is what I am going to tell clients. The appropriate PPE above 40 Cal may exist based upon specific factors at the location (tA) but that is not defined by any standard at this time. Hopefully with the extensive testing being done we will have some greater information from the IEEE 1584 committee in the future. Thank you all for the great conversation on the topic.


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 30 posts ]  Go to page Previous  1, 2

All times are UTC - 7 hours


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
© 2017 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883