It is currently Fri Aug 14, 2020 4:09 am



Post new topic Reply to topic
Author Message
ekstra   ara
 Post subject: Maximum Fault Durations
PostPosted: Wed Aug 04, 2010 8:00 am 

Joined: Wed Aug 04, 2010 7:49 am
Posts: 1
I can understand the reasoning IEEE 1584 is using for the 2 second rule of arc duration for personal arc flash protection, but is this not ignoring other issues such as equipment damage and even possible fire hazards for the entire switchgear, transformer, or building. Also considering down time for equipment replacement versus a possible quick clean-up and re-energizing. Just wondering what everyones view is on this and what the latest industry norms are for the max duration. I don't care for the 2 second rule for the reasons above but at the same time think a 1/2 mile arc boundary is ridiculous.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Aug 04, 2010 9:00 am 
Plasma Level
User avatar

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1503
Location: Scottsdale, Arizona
Nperry wrote:
I can understand the reasoning IEEE 1584 is using for the 2 second rule of arc duration for personal arc flash protection, but is this not ignoring other issues such as equipment damage and even possible fire hazards for the entire switchgear, transformer, or building. Also considering down time for equipment replacement versus a possible quick clean-up and re-energizing. Just wondering what everyones view is on this and what the latest industry norms are for the max duration. I don't care for the 2 second rule for the reasons above but at the same time think a 1/2 mile arc boundary is ridiculous.


The 2 second rule is more of a temporary fix IMHO. When IEEE 1584 was released in 2002, it was an amazing effort but it could not cover everything.

One of the open issues was low fault currents leading to long clearing times that can produce some "unique" arc flash protection boundaries. A 2 second cut off was added as "something is better than nothing" for a cut off that can be used with good judgement when excessive clearing times are an issue. This is based on human reaction time.

New research is slowly coming and it will perhaps provide a more realistic way to look at this. For now, 2 seconds is as good as gets if you want something that can be tracked back to a standard. At 208V and low short circuit current, the arc often can not even sustain.

_________________
Jim Phillips, P.E.
Brainfiller.com


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Aug 04, 2010 11:32 am 
Sparks Level
User avatar

Joined: Mon Nov 23, 2009 6:37 am
Posts: 51
Location: Tampa, FL
brainfiller wrote:
The 2 second rule is more of a temporary fix IMHO. When IEEE 1584 was released in 2002, it was an amazing effort but it could not cover everything.

One of the open issues was low fault currents leading to long clearing times that can produce some "unique" arc flash protection boundaries. A 2 second cut off was added as "something is better than nothing" for a cut off that can be used with good judgement when excessive clearing times are an issue. This is based on human reaction time.

New research is slowly coming and it will perhaps provide a more realistic way to look at this. For now, 2 seconds is as good as gets if you want something that can be tracked back to a standard. At 208V and low short circuit current, the arc often can not even sustain.


The two second rule also implied that the person subjected to the arc flash/blast will be able to get away or be thrown away from the immediate area. If the enclosure or area surrounding the work is small or confining or the worker is in an awkward position, then sound engineering judgement must be used in order to justify the use of the two second rule.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Aug 04, 2010 11:40 am 
Plasma Level
User avatar

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1503
Location: Scottsdale, Arizona
richxtlc wrote:
The two second rule also implied that the person subjected to the arc flash/blast will be able to get away or be thrown away from the immediate area. If the enclosure or area surrounding the work is small or confining or the worker is in an awkward position, then sound engineering judgement must be used in order to justify the use of the two second rule.


Absolutely! That is why I mentioned judgement in my second paragraph. Not having space to get blown clear or jump back would present a major issue with the 2 second rule. Also, I have been studying this a bit with a few others and the typical first reaction is not so much to jump back but to put your arms up and turn sideways. Then you jumping back. Picture someone throwing a bad pitch at you.

- Thanks for your comments.

_________________
Jim Phillips, P.E.
Brainfiller.com


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Aug 05, 2010 6:15 am 
Arc Level

Joined: Mon Jan 18, 2010 11:35 am
Posts: 523
Location: Wisconsin
A problem with most (all?) formulas, is they assume the arc will maintain constant characteristics throughout the entire clearing time. There is no accommodation made for the increased arcing resistance that will occur as the arc is lengthened due to material being 'consumed', likewise the effect of plasma is not considered.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Aug 06, 2010 7:37 am 
Arc Level

Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
Nperry wrote:
I can understand the reasoning IEEE 1584 is using for the 2 second rule of arc duration for personal arc flash protection, but is this not ignoring other issues such as equipment damage and even possible fire hazards for the entire switchgear, transformer, or building. Also considering down time for equipment replacement versus a possible quick clean-up and re-energizing. Just wondering what everyones view is on this and what the latest industry norms are for the max duration. I don't care for the 2 second rule for the reasons above but at the same time think a 1/2 mile arc boundary is ridiculous.

Arc hazard analyis is for personnel safety only and is not intended to provide any protection for equipment.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Aug 06, 2010 8:24 am 
Arc Level

Joined: Tue Jan 13, 2009 5:00 pm
Posts: 532
jghrist wrote:
Arc hazard analyis is for personnel safety only and is not intended to provide any protection for equipment.


Agreed. And the 2 second rule is an assumed exposure time for someone exposed to a longer duration arc who begins to move away. It has nothing to do with the actual arcing time the equipment might be exposed to.

If you are concerned with equipment damage at the arc location, suggest you look into arc resistant switchgear. 1584 is out of scope.

If you are concerned with fire protection, 1584 is again out of scope. Try some of the NFPA standards or building codes.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed May 04, 2011 11:17 am 

Joined: Tue Apr 12, 2011 6:11 am
Posts: 17
Location: Oregon, USA
Regarding Jim's previous comment :

"New research is slowly coming and it will perhaps provide a more realistic way to look at this. For now, 2 seconds is as good as gets if you want something that can be tracked back to a standard. At 208V and low short circuit current, the arc often can not even sustain."

I understand then that, pending further arc duration research, where the arcing current is low enough to result in prolonged OCPD clearance times, the actual arc duration is difficult to impossible to predict. Therefore we should assume the arc may sustain until cleared by the OCPDs, even when it seems that probably would unlikely be the case. Which will give us high, but conservative Ei results.

But then, as Jim pointed out, at some lower voltage and current levels it is known that an arc cannot sustain. By this, I take it that the arc will always extinquish almost immediately - say, no longer than 1 cycle.

Have any certain voltage, amperage, and/or gap parameter thresholds been established for which one is justified in assuming an arc duration of less than 1 cycle ?

For example, a case at 480V, but where the arc current is quite small, or the gap quite wide ?


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu May 05, 2011 9:21 am 
Sparks Level

Joined: Wed May 13, 2009 3:19 pm
Posts: 56
There are a few recent papers out there discussing fault durations, at LV levels, based on actual testing conducted by EPRI and other Utilities.


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 9 posts ] 

All times are UTC - 7 hours


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:  
© 2019 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883