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 Post subject: DC Arc flash study
PostPosted: Wed Feb 13, 2019 4:52 pm 

Joined: Fri Nov 21, 2014 10:14 am
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Location: Edmonton, Alberta, Canada
We have a DC panel fed from a 25kVA, 575/240Vac transformer.

Do we need to perform a DC Arc Flash study on the DC panel?


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 Post subject: Re: DC Arc flash study
PostPosted: Thu Feb 14, 2019 7:53 am 
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I don't see where you are getting DC voltage from if it is an AC transformer. Is it a panel that is rated for DC being used in an AC system?

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 Post subject: Re: DC Arc flash study
PostPosted: Thu Feb 14, 2019 12:26 pm 
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Yes, at least to verify that you don't need to go any further. Note that out of the dozens of times I've done a DC calculation, it only get above 1.2 cal/cm2 once or twice.

First off, realize that unlike AC incident energy the way that DC is currently modelled is using theoretical calculations. If we look at the maximum power transfer to the arc it is at a maximum when the arc resistance equals the system resistance. Thus the arc power is equal to V x I(short circuit) x 0.5. Then we just multiply by the (arcing time) so that we get energy, convert to calories, and then divide by the area of a sphere at the working distance to calculate incident energy in cal/cm2. That's the formula you see in 70E, Annex D.8. Ammerman has a more refined iterative nonlinear calculation that improves on this somewhat but it's not a huge change. So with such a simple calculation, we can calculate it by hand...ALMOST!

Realize that the rectifier is a semiconductor...that means it has a considerable amount of resistance. In all likelihood this alone is the major resistance so we can ignore everything else. Chances are the rectifier won't have a "short circuit current" number published but if we look at the overload formula (some perentage overrated for so many seconds) we can plug those into the arc flash calculation as a prospective fault current. Greater short circuit currents are of course possible but this just causes the device to fail faster (I^2*t) so consider this a conservative number.

This is rough and maybe not "SKM grade" but its the kind of thing you can work out on a sheet of paper using data that you already have. As a first pass it will be conservative (high) and allow us to ignore DC arc flashes that are not severe.


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 Post subject: Re: DC Arc flash study
PostPosted: Sat Feb 16, 2019 8:02 am 

Joined: Fri Nov 21, 2014 10:14 am
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Location: Edmonton, Alberta, Canada
Thank you for answering my question and sorry because I really did not give a lot of details on this post.

We have 2 panels side by side, one is an AC panel fed from a 25kVA transformer, 575/240VAC. In this panel there are 2 power supplies that convert Ac to 24VDC.

This 24VDC feeds the DC panel.

The 2 panels came without arc flash labels but the question we have is that if the arc flash and shock study must be performed or not.

thx


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 Post subject: Re: DC Arc flash study
PostPosted: Sun Feb 17, 2019 8:30 am 

Joined: Fri Nov 21, 2014 10:14 am
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Location: Edmonton, Alberta, Canada
Also, I have another question, when you buy new equipment, like in this case, 2 additional panels to the electrical system, who is responsible to provide the arc flash assessment? the Employer or the Manufacturer or Supplier of the new equipment.

thx

Jorge


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 Post subject: Re: DC Arc flash study
PostPosted: Sun Feb 17, 2019 10:35 am 
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jorgepaz wrote:
Thank you for answering my question and sorry because I really did not give a lot of details on this post.

We have 2 panels side by side, one is an AC panel fed from a 25kVA transformer, 575/240VAC. In this panel there are 2 power supplies that convert Ac to 24VDC.

This 24VDC feeds the DC panel.

The 2 panels came without arc flash labels but the question we have is that if the arc flash and shock study must be performed or not.

thx


No arc flash at 24 VDC. Need at least 50 V before it should even be considered and even then the testing that has been done at 125 VDC so far shows that even at some extreme cases they can't get enough of an arc to matter.


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 Post subject: Re: DC Arc flash study
PostPosted: Sun Feb 17, 2019 10:51 am 
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jorgepaz wrote:
Also, I have another question, when you buy new equipment, like in this case, 2 additional panels to the electrical system, who is responsible to provide the arc flash assessment? the Employer or the Manufacturer or Supplier of the new equipment.

thx

Jorge


The employer is on the hook for risk assessments for hazards of any kind in the worst place including for instance slips, trips, and falls. The employer is ultimately responsible for addressing electrical hazards, too. That doesn't mean that they can't go for outside services to address this need.

Part of what determines electrical hazards too is the condition of the equipment...whether or not it is unsafe. Just because it is installed properly and electrical equipment often requires very little maintenance doesn't mean it is zero maintenance. The employer is responsible for making sure that preventative maintenance is being done. For example even though you may not do the work yourself, regularly changing the oil and checking/adjusting tire pressure is required maintenance. If you elect not to do it at all, the vehicle will become unsafe to operate.

The manufacturer/supplier in most cases cannot tell you what the incident energy is for something that they supply. The available power to an arcing fault is determined by the supply which is in most cases external to the supplied equipment...it depends on the transformer and wiring feeding it, not what's inside the panel. So unless the manufacturer does an engineering study of the equipment feeding the panel, they can't tell you a thing about the incident energy. At best some manufacturers use a very generic sticker that says something like "Warning! Arc flash hazard may be present" which meets the requirements of the NEC but hardly provides any kind of guidance at all. A lot of manufacturers that sell switchgear also offer arc flash study engineering services simply because customers may not shop around and pay a premium price for the service. This is sort of like getting all your auto maintenance done at the dealer, often with very high markups that you may not realize if you don't shop around a little.


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 Post subject: Re: DC Arc flash study
PostPosted: Sun Feb 17, 2019 11:13 am 

Joined: Fri Nov 21, 2014 10:14 am
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Location: Edmonton, Alberta, Canada
Hello Paul

Thanks a lot for answering our questions.

Jorge


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 Post subject: Re: DC Arc flash study
PostPosted: Mon Feb 18, 2019 12:24 pm 
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"...who is responsible to provide the arc flash assessment? the Employer or the Manufacturer or Supplier of the new equipment...."

Per NFPA 70E 130.5(H), the owner of the equipment is responsible for labeling the electrical equipment. It is NOT the manufacturer or electrictrical contractor's responsibility. However, check the specifications created by the electrical designer. They may state that the electrical contractor must hire a specialist to determine the electrical energy and label the electrical equipment.

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 Post subject: Re: DC Arc flash study
PostPosted: Mon Feb 18, 2019 8:59 pm 

Joined: Fri Nov 21, 2014 10:14 am
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Location: Edmonton, Alberta, Canada
Hello Robert

That's a good point. i will check the Manufacturer specifications.

thank you very much for sharing that information.

Jorge


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 Post subject: Re: DC Arc flash study
PostPosted: Fri Feb 22, 2019 12:18 am 

Joined: Mon Jun 05, 2017 8:07 pm
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PaulEngr wrote:
jorgepaz wrote:
Thank you for answering my question and sorry because I really did not give a lot of details on this post.

We have 2 panels side by side, one is an AC panel fed from a 25kVA transformer, 575/240VAC. In this panel there are 2 power supplies that convert Ac to 24VDC.

This 24VDC feeds the DC panel.

The 2 panels came without arc flash labels but the question we have is that if the arc flash and shock study must be performed or not.

thx


No arc flash at 24 VDC. Need at least 50 V before it should even be considered and even then the testing that has been done at 125 VDC so far shows that even at some extreme cases they can't get enough of an arc to matter.


Paul,

Is there a reference /guideline to show the minimum voltage to perform DC arc flash is at 50 V? What about Gap length?


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 Post subject: Re: DC Arc flash study
PostPosted: Sun Feb 24, 2019 3:17 pm 
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Raghu wrote:
PaulEngr wrote:
jorgepaz wrote:
Thank you for answering my question and sorry because I really did not give a lot of details on this post.

We have 2 panels side by side, one is an AC panel fed from a 25kVA transformer, 575/240VAC. In this panel there are 2 power supplies that convert Ac to 24VDC.

This 24VDC feeds the DC panel.

The 2 panels came without arc flash labels but the question we have is that if the arc flash and shock study must be performed or not.

thx


No arc flash at 24 VDC. Need at least 50 V before it should even be considered and even then the testing that has been done at 125 VDC so far shows that even at some extreme cases they can't get enough of an arc to matter.




Paul,

Is there a reference /guideline to show the minimum voltage to perform DC arc flash is at 50 V? What about Gap length?


Hertha Ayrton in The Electric Arc has a formula that gives an absolute minimum voltage of about 28 VDC. This is a very old book and tough going but the formula is in there. Above that point you need a minimum current as well so it becomes dependent on fault current but reasonable short circuit currents such as 200% of the power supply maximum output quickly show why there are practical limits. Ammerman covers this and more in detail here: DC Arc Models and Incident Energy Calculations. R. Ammerman, T. Gammon, P.K. Sen, J. Nelson. IEEE Transactions on Industry Applications, Vol. 46, No. 5, September/October 2010

See also this free source:
http://www.neiengineering.com/wp-conten ... ations.pdf

Essentially the minimum theoretical arc voltage is around 25-30 VDC but this is with nearly infinite available current and working at a 1 mm gap. One of the more recent papers (referenced above) is the Stokes and Oppelander (1991) work that reviewed a lot of the others and gives a formula for arc voltage as Varc=(20+0.534*L)*I^0.12 where L is the arc gap in millimeters. So if we set the gap to 1 mm then to get to a 50 V arcing fault we'd need 2.435 = I^0.12. Taking the log of both sides we get 0.3865 = 0.12 * log(I) or 3.221 = log(I). Solving finally for I we get 1662 A which is quite a bit more current than most DC power supplies can put out. Don't forget that this is with a ZERO system resistance. The real system would need at least twice that current based on maximum power transfer arguments so we need at least 3324 A to sustain an arc at 50 VDC with no gap at all across air. Welders do not need this much current because the arc is within a metal vapor/liquid. Note that pretty much all the models in the above paper show an absolute theoretical minimum of around 25-30 VDC for arcing no matter how much current is applied even as the arc gap drops. You can run through Ammerman's full theoretical model working it to see where the cutoff is but it's there and there's no way to worry about anything under around 50 V. So the limit in 70E stands.

Practically for instance work at Bruce Hydro on substation battery systems was able to just barely reach the 1.2 cal/cm2 threshold at 18" working distance with a 1/4" arc gap and a 20 kA available fault current with 130 VDC system voltage. As voltage goes lower the necessary voltage to sustain an arc drops as we saw above but then the incident energy is also going down rapidly because the arc current is dropping so even if we can sustain an arc, the incident energy isn't high enough to matter.

There was more recent data published in Battcon as well as far as arc flash goes. See the latest proposals to 70E which discusses this.


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 Post subject: Re: DC Arc flash study
PostPosted: Mon Feb 25, 2019 2:16 pm 

Joined: Fri Nov 21, 2014 10:14 am
Posts: 23
Location: Edmonton, Alberta, Canada
Hello Raghu

thank you for sending detailed information about it. I will check out the link you sent as well.

Jorge


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