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 Post subject: How to weigh device coordination vs. arc flash energies.
PostPosted: Mon Oct 26, 2015 12:00 pm 

Joined: Mon Aug 24, 2015 10:24 am
Posts: 28
We all know there are trade offs between coordinating protection devices to prevent unnecessary system wide outages and reducing arc flash hazards at equipment expected to be serviced while energized. But how does one go about weighing the best blend of coordination and reduced arc flash energy combinations?

I'm not referring here to code required coordination that must be met for health care and life safety systems. Rather I was wondering what are the general steps to consider when dealing with these opposing forces for smaller scale commercial type buildings that have 480V systems but small occupancies and no critical systems.

Perhaps the answer is to present to the Owner the potential results if full coordination is met (no nuisance tripping but high arc flash potential) versus if arc flash energizes are reduced (potential nuisance tripping but much safer situation for live work) and let them decide??

Please shed some light on your experiences.

Thanks,
Brian


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 Post subject: Re: How to weigh device coordination vs. arc flash energies.
PostPosted: Mon Oct 26, 2015 7:32 pm 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
Why not both?

Seriously, that's the purpose of maintenance switches. A cheap way to choose safety when needed over coordination.


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 Post subject: Re: How to weigh device coordination vs. arc flash energies.
PostPosted: Mon Nov 02, 2015 7:39 am 

Joined: Mon Aug 24, 2015 10:24 am
Posts: 28
I completely agree PaulEngr - I haven't had the chance to pick the brain of the owner & their maintenance staff. I do not know their budget or if they have policies in place already to answer questions about safety vs. coordination. I want to be prepared for the question from the Owner "well, what do you recommend we do?" by having an understanding of what people typically do in such circumstances, pros & cons of the various solutions, etc. This is a general question to get a feel for how forum users deal with the tug of war between safety & coordination.


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 Post subject: Re: How to weigh device coordination vs. arc flash energies.
PostPosted: Mon Nov 02, 2015 8:42 am 
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Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
I also agree with PaulEngr. Do the coordination as best as possible. If this results in unacceptable arc flash energies, add maintenance switches to reduce IE during maintenance.


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 Post subject: Re: How to weigh device coordination vs. arc flash energies.
PostPosted: Mon Nov 02, 2015 9:26 am 
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Joined: Fri Apr 15, 2011 7:43 am
Posts: 177
Location: Colorado
Sometime the balance leans one way or the other. I have seen transformer protection set so low to get 8-10 cal that the transformer is current limited to 75% of full rating. I tend to look at the application (MCC is only 85% loaded and 50%utilized) then pick either arc flash reduction (current limit) or partial curren and partial arc flash reduction (assuming no maint switch)


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 Post subject: Re: How to weigh device coordination vs. arc flash energies.
PostPosted: Mon Nov 02, 2015 10:22 am 
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Joined: Tue Mar 08, 2011 11:23 am
Posts: 34
Location: Utah
What type of commercial facilities are these that they need to work on energized equipment? What maintenance are they doing that the equipment needs to be energized, racking out breakers, etc? OSHA requirements are only work on energized equipment if it presents a higher risk by de-energizing the system. Thee is the times where you will need the equipment energized for trouble shooting (again what type of facility is this that they need to do a lot of trouble shooting) and this is where the maintenance switches would be a good option.


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 Post subject: Re: How to weigh device coordination vs. arc flash energies.
PostPosted: Wed Nov 04, 2015 5:45 pm 
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You can't work on it de-energized until you de-energized it first and the act of de-energizing it (while following OSHA Subchapter S or 1910.269 under multiple headings) is energized work. So the whole idea of never working on equipment while energized can only be done by never, ever energizing it in the first place. Subchapter J lockouts are only for non-electrical work and Subchapter J states this pretty plainly.

And yes, the result of a power system study should be to present the owner with options. Frequently when fuses and breakers are mixed, you have to sacrifice coordination for protection to meet Code. In the past as long as 'short term' protection met short circuit requirements and long term settings provided over current protection, a secondary requirement was coordination. Often arcing faults would effectively never trip for minutes. With arcing faults taken into consideration this drives short term settings down as low as reasonably practical unlike past practice. And if we set 'absolute' incident energy goals while working only with protective devices, its nearly impossible to retrofit and meet all objectives simultaneously.

On new construction, arc flash considerations establish more stringent limits on currents and thus transformer sizing, especially as voltage decreases. You can't realistically get under 40 cal/cm2 with over a 1500 kva transformer at 480 V for instance with traditional designs.


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 Post subject: Re: How to weigh device coordination vs. arc flash energies.
PostPosted: Thu Nov 05, 2015 3:03 pm 

Joined: Mon Aug 24, 2015 10:24 am
Posts: 28
Thank you everyone for the detailed discussion. Much appreciated.

Sincerely,
Brian


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