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 Post subject: Normal operation of electrical equipment
PostPosted: Wed Oct 07, 2009 10:12 am 
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All our workers are presently permitted to operate breakers, low and medium voltage switchgear disconnects and field disconnects. We are struggling with "normal operation of equipment" for these workers. I would consider a mechanic going into a switchroom to lock out a 600 volt starter so he can do maintenance on a pump as normal operation of that circuit. My problem is that the MCC might be rated at 10 cal/cm┬▓ therefore the 8 cal clothing the mechanic is dressed in is inadequate. Looking at "normal operation" the mechanic is not exposed to the electrical hazard and doesn't need to observe the ppe level required. Looking at the incident energy level, the mechanic can't shut off the disconnect without gearing up to 10 cal level. Table 130.7(c)(9)(a) and CSA table 4 say level 0 hazard (based on experience) in lieu of the flash hazard analysis. I've done an analysis and have labeling so my mechanic needs to be suited up in 10 cal gear???


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PostPosted: Wed Oct 07, 2009 11:57 am 
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Canuck01 wrote:
I've done an analysis and have labeling so my mechanic needs to be suited up in 10 cal gear???


That is correct.
Or give him a stick. Although, in my experience you can't trust a mechanic with a stick, he'll end up beating something with it... :D


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PostPosted: Wed Oct 07, 2009 6:14 pm 
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Locking out continued...

Sorry, maybe I wasn't clear...
Do you consider the act of throwing a disconnect to apply a lockout for non-electrical work a hazard which requires Arc Flash protective gear? If so, why?
The tables would seem to allow the activity except if you did an Arc Flash study and had labeled your switchgear with the incident energy levels.
It is counter-intuitive.
What was the rationale for making the determination of risk?
Can anyone help me with this one?


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PostPosted: Wed Oct 07, 2009 6:32 pm 
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Sorry I calculated?

Canuck01 wrote:
Sorry, maybe I wasn't clear...
Do you consider the act of throwing a disconnect to apply a lockout for non-electrical work a hazard which requires Arc Flash protective gear? If so, why?
The tables would seem to allow the activity except if you did an Arc Flash study and had labeled your switchgear with the incident energy levels.
It is counter-intuitive.
What was the rationale for making the determination of risk?
Can anyone help me with this one?


We see this a lot. The risk part is in the back of both standards in Annex F but this isn't much help. The real issue here is that IF a disconnect fails and you recieve 10 cal/cm2 at 18 inches away you have gone from no issue to clothing ignition and a 50-80% probabality of killing a worker.

The task tables ONLY apply if you met ALL the footnote requirements. Most facilities which do calculations didn't meet those requirements but remember the tables are NOT perfect. They are an estimate based on the committee's experience along with their "risk" factor. This wasn't scientific. It was many compromises and will work pretty well. I would never go back when I have done a study. Better to see if you can do any engineering fixes to lower those energy levels.

Hope this helps,


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PostPosted: Thu Oct 08, 2009 7:04 am 
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Where in NFPA 70E does it require PPE for non-energized work, ie. operating a disconnect switch? Other than the table 130.7(C)(9) which is used only in liu of an arc flash study. I think it is a good idea to suit up according to the table, but I have not seen this requirement in the standard if an arc flash study has been performed. Also, what is 130.7(C)(9)(a)?


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PostPosted: Thu Oct 08, 2009 7:21 am 
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Table 130.7(C)(9) is referenced in Article 100 Definition of Arc Flah Hazard, FPN No. 2 as examples of activities that could pose an arc flash hazard. A reasonable interpretation would be that since "CB or fused switch or starter operation with enclosure doors closed" in 600V MCCs has an HRC of zero in Table 130.7(C)(9), then it is not likely to pose an arc flash hazard.

If a detailed analysis is made and the IE is 10 cal/cm┬▓, this is without a metal door between the arc and the worker. There is no IE calculation made with the metal door taken into account. True, a high level arc may have an explosive force that blows the door off, but will wearing a Nomex shirt rated for 10 cal/cm┬▓ provide any protection against flying doors?


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PostPosted: Thu Oct 08, 2009 7:29 am 
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Thanks Hugh
I finally see how the table was put together to manage the risk. 2 cycle clearing time is pretty quick. Even the SEL 751A can't consistently clear a fault that quickly. Was the table designed by Bussman??


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PostPosted: Thu Oct 08, 2009 10:54 am 
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Canuck01 wrote:
Sorry, maybe I wasn't clear...
Do you consider the act of throwing a disconnect to apply a lockout for non-electrical work a hazard which requires Arc Flash protective gear? If so, why?


Sorry I wasn't clear. Yes, I do consider this an act that requires AF gear. Largely because you have now calculated the energy potential, and you KNOW that there is a risk and what the protection should be against the Arc Flash.

elihuiv wrote:
They are an estimate based on the committee's experience along with their "risk" factor. This wasn't scientific.


Exactly as Hugh says. The tables take into the account the 70E Committees SUBJECTIVE analysis of what the risk of an event happening may be, and what the damage could be. You have performed the analysis. You don't have the luxury of saying it Probably Won't Happen to This Guy.

Besides, would you rather he get burned AND broken bones?

jghrist wrote:
True, a high level arc may have an explosive force that blows the door off, but will wearing a Nomex shirt rated for 10 cal/cm┬▓ provide any protection against flying doors?


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PostPosted: Thu Oct 08, 2009 3:29 pm 
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2 cycle clearing time and de-energized work

If work is truely de-energized (electrically safe work conditon) NO PPE is required but the definition of electrically safe work condition includes VERIFIED, LOTO, rarely is this done before disconnect operation so that is still energized work.

On the tables development, really no fuse mfg or switchgear guys drove this. It was from practical experience of the group. The 2 cycle clearing time is the based on fuses OR SSCB's. You have to have a fast upstreaming clearing device. It doesn't have to WORK that fast but should theoretically clear that fast.

Lots of assumptions in the tables. They are really only useful for small facilities, contractors where energies aren't easy to determine or short term preliminary plan until the arc flash studies can be budgeted and done.

Hope this is helpful,


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