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 Post subject: LARGE arc flash boundary
PostPosted: Thu Apr 28, 2016 2:23 pm 

Joined: Mon Aug 24, 2015 10:24 am
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So my labels will wind up with some very large arc flash boundary distances. For example, a main 4160V substation bus will have 24 Cal IE calculation and an estimated flash boundary (where 1.2 Cals is measured) of 394 inches. The substation is, like many utility substations, essentially a skin around the switchgear with only the minimum distances in front of and behind the switchgear to meet code clearances.

Do I simply put the 394" onto the label and that is that? I ask because I have about 48 inches to the wall across from the switchgear so 394" is unrealistic in any practical terms. I didn't want to slap a label on it and then get cross eyed looks from the electricians and such down the road. BTW - we do not intend to EVER work on the main substation breakers live.

Thanks.


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 Post subject: Re: LARGE arc flash boundary
PostPosted: Thu Apr 28, 2016 2:36 pm 
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I would label it as it is since that is what the calculation showed. You state that you would not work the breakers live but what about racking out, racking in a ground truck, testing and grounding?

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 Post subject: Re: LARGE arc flash boundary
PostPosted: Thu Apr 28, 2016 3:13 pm 

Joined: Mon Aug 24, 2015 10:24 am
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WBD - but does a 394" AFB imply that someone not suited up walking outside the substation trailer building within 394" would be in violation of the label? Not practical to safe guard the boundary that far outside of the substation building but I'm fine putting that distance on the label as that is what the calcs return for a result.


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 Post subject: Re: LARGE arc flash boundary
PostPosted: Fri Apr 29, 2016 5:19 am 
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DMB5mil wrote:
WBD - but does a 394" AFB imply that someone not suited up walking outside the substation trailer building within 394" would be in violation of the label? Not practical to safe guard the boundary that far outside of the substation building but I'm fine putting that distance on the label as that is what the calcs return for a result.


What type of construction is around the breakers? Steel siding, cinder blocks, chain link fence, etc?


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 Post subject: Re: LARGE arc flash boundary
PostPosted: Fri Apr 29, 2016 6:04 am 
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DMB5mil wrote:
WBD - but does a 394" AFB imply that someone not suited up walking outside the substation trailer building within 394" would be in violation of the label? Not practical to safe guard the boundary that far outside of the substation building but I'm fine putting that distance on the label as that is what the calcs return for a result.


No that is not what the AFB distance implies. This is a mis-conception that has been around for awhile. Walking around is the same type of work as reading meters on MCCs or Switchgear. You are most likely inside the AFB for those activities but you are not doing any work that would cause an arc flash. This is part of the risk analysis. Same thing applies to operating a breaker or disconnect switch. If certain conditions are met no AR PPE is needed as you are using the eqpt as designed.

If you refer to the definitions in NFPA 70E-2015, the Arc Flash Boundary is when an arc flash hazard exists it is the approach distance from a prospective arc source within which a person could receive a 2nd degree burn if an arc flash were to occur.

Arc flash hazard is defined with an Informational Note after it that states basically one has to be interacting with equipment in a manner that may cause an electrical arc. Under normal operating condition, enclosed energized equipment that has been properly installed and maintained is not likely to pose an arc flash hazard.

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 Post subject: Re: LARGE arc flash boundary
PostPosted: Fri Apr 29, 2016 10:05 am 
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wbd wrote:
DMB5mil wrote:
No that is not what the AFB distance implies. This is a mis-conception that has been around for awhile. Walking around is the same type of work as reading meters on MCCs or Switchgear. You are most likely inside the AFB for those activities but you are not doing any work that would cause an arc flash. This is part of the risk analysis. Same thing applies to operating a breaker or disconnect switch. If certain conditions are met no AR PPE is needed as you are using the eqpt as designed.


Close. An arc flash boundary does NOT EXIST if the activity is not likely to cause an arc flash in the first place. It's the same with equipment that is not exposed...this means that the shock protection boundaries simply don't exist. And as in both cases you list them but the practical effect is that as work occurs these boundaries may exist or disappear depending on the work.

Second frequently with "large" arc flash boundaries, once you reach a substantial "wall", it's not going to go beyond that point. So in a typical indoor substation the "boundary" does not apply outside of the walls of the substation. The concern is thermal radiation and essentially any wall that has a fire rating at all is going to be more than adequate to stop the thermal radiation component. There have been some concerns raised about arc blast, but see the article I posted in the articles section as to the realistic hazard imposed by arc blast.


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 Post subject: Re: LARGE arc flash boundary
PostPosted: Fri Apr 29, 2016 2:38 pm 

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Voltrael - the construction is a steel 'trailer' that is permanently installed on concrete peers. It has metal stairs and all metal construction.

I understand the discussion above about the AFB 'disappearing' due to the substantial wall. That being the case, would I just leave the AFB at 394" or perhaps something else? If I'm standing 20 feet away from the point of the arc label inside the trailer at the far end of the trailer (assuming the arc occurs at the other end of the trailer) I'd technically still be within 394" and should have PPE on if the work in question provides the potential for an arcing event. Outside the trailer is no PPE regardless of what is occurring inside the trailer due to the wall.

The label seems a bit silly to include 394" but better safe than sorry. Right?


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 Post subject: Re: LARGE arc flash boundary
PostPosted: Fri Apr 29, 2016 7:20 pm 
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You can put 394" on the label but it's not really very informative...think about the audience. It also seems silly to list it in inches instead of feet.

As to steel box...in thermal industries (kilns, boilers, furnaces) we use usually 1 or 2 sheets of tin siding as a heat shield. I can stand behind a couple layers of this stuff while working within a couple feet of an 800+ degree heat source without my hard hat even getting soft or getting red in the face from the heat.


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 Post subject: Re: LARGE arc flash boundary
PostPosted: Mon May 02, 2016 4:47 am 
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PaulEngr wrote:
You can put 394" on the label but it's not really very informative...think about the audience. It also seems silly to list it in inches instead of feet.

As to steel box...in thermal industries (kilns, boilers, furnaces) we use usually 1 or 2 sheets of tin siding as a heat shield. I can stand behind a couple layers of this stuff while working within a couple feet of an 800+ degree heat source without my hard hat even getting soft or getting red in the face from the heat.


I know SKM defaults to showing inches on the arc flash boundary, not sure about other software packages.


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 Post subject: Re: LARGE arc flash boundary
PostPosted: Mon May 02, 2016 6:24 am 
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You can set SKM to use feet instead but it does indeed default to inches. It might be easier to understand what "1.5 feet" means vs. "18 inches". Similarly "33 feet" might be more clear than "394 inches". Just saying...


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 Post subject: Re: LARGE arc flash boundary
PostPosted: Mon May 02, 2016 10:47 am 
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PaulEngr wrote:
You can set SKM to use feet instead but it does indeed default to inches. It might be easier to understand what "1.5 feet" means vs. "18 inches". Similarly "33 feet" might be more clear than "394 inches". Just saying...


Yes, thank you. After I hit post it occurred to me to look and I found the setting I needed to change. For our workers 12 feet is much easier to understand than 144 inches.


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