More Arc Flash

[CEDEng]

Did a quick search to no avail so will post here - feel free to redirect me!

Meantime - thanks for any advice.

AF calculations/formulas re: NFPA and the www have given me few concrete answers, so here are some hypothetical situations:

a) I am resetting a circuit breaker in my basement, 15A single pole - what is the PPE requirement?
b) I have a solid-state power supply - 5000W, three phase output - what is the Arc Flash possibility on the OUTPUT side of that device?
c) I have a standard, home use UPS - 120V in/battery/120V out...the "usual" - on the output side, what are my Arc Flash possibilities?

Now - I know (I...think...) that the required energy to sustain a destructive arc or cause a painful explosion is just...not there..in these situations. But how can I prove it? Where is the math? All the numbers in play are far below the usual formulas. What is the Arc Flash potential at my kitchen toaster, butterknife installed?

It just seems like someone, somewhere, would start their article with: "Listen, if your situation is less than such and such...don't bother reading this..."

But so far - no luck.

Any ideas?

Thanks again!

 

[jeremy.zinkofsky]

The equations detailed in NFPA Annex D and IEEE 1584 are probably not accurate for lower levels of short circuit current that may occur in the scenario's you mentioned. Besides, incident energy is highly dependent on how fast the overcurrent device will open the circuit and respond to the fault. You may find some useful info in the form of kAIC rating and TCC curves if you look up the tech data spec of each breaker that feeds the branch circuit in question. If you could find the clearing times and use the kAIC rating of the breaker as a conservative minimum available fault current, the equations in NFPA and IEEE may work out for you.

 

[petersonra]

The equations detailed in NFPA Annex D and IEEE 1584 are probably not accurate for lower levels of short circuit current that may occur in the scenario's you mentioned. Besides, incident energy is highly dependent on how fast the overcurrent device will open the circuit and respond to the fault. You may find some useful info in the form of kAIC rating and TCC curves if you look up the tech data spec of each breaker that feeds the branch circuit in question. If you could find the clearing times and use the kAIC rating of the breaker as a conservative minimum available fault current, the equations in NFPA and IEEE may work out for you.

that is just not the way it is done. In the first place you cannot assume a CB will clear a fault at all when the available SCC exceeds the rating of the breaker. Secondly, you also cannot assume that any special value of SCC will produce the most IE. It is not unusual for a higher SCC to trip the CB fast enough that less IE is available than if a lower level of SCC is available, which ironically you more or less sated in the previous sentence.

 

[CEDEng]

Bob - right - that's exactly my issue.

I am capable of calculating AF hazards and energy in the realm of their likely occurrence. But now I have folks who want to see the math proving that their sweeper isn't an arc flash hazard. Maybe it is...but if I had some simple math - or an outright NFPA statement - that it is NOT - then everyone would be happy. (I'm using "sweeper" here as another example - I hope y'all caught on that it's for lesser energy, low voltage, low (available) SCCR devices.)

 

[petersonra]

Bob - right - that's exactly my issue.

I am capable of calculating AF hazards and energy in the realm of their likely occurrence. But now I have folks who want to see the math proving that their sweeper isn't an arc flash hazard. Maybe it is...but if I had some simple math - or an outright NFPA statement - that it is NOT - then everyone would be happy. (I'm using "sweeper" here as another example - I hope y'all caught on that it's for lesser energy, low voltage, low (available) SCCR devices.)

What "math" are you showing them to prove it for other situations? Most people use some kind of software because it is hard to manually do enough calculations along the SCC line to come up with the worst case, but the software does it with ease.

Does your software not do calculations for single phase?

At one time there was an IEEE interpretation that suggested it was impossible for an arc to be sustained at lower voltages on smaller transformers so there was no need to do the calcs. They seem to have done away with that suggestion for some reason.

 

[iceworm]

CED -
Since you are familiar with IEEE 1584, you already know:

Limits of the model are 208V, 700A, 3ph
Single phase faults and line to ground faults have conservative results
For PPE cat <1, the incident energy no more than 1.2 cal/cm^2​

And you are likely aware of the Bussman paper on point-to-point calcs for SCC.

I currently like Jim Phillips' book for one-off calcs:
http://brainfiller.com/product/complete-guide-to-arc-flash-hazard-calculation-studies/

No, you don't need software. A calculator and pen will do fine.

Recommend: Do a calc for 240V, 700A. Calculate the distance required to get the incident energy down to 1.2cal/cm^2.

For the 5000W power supply and the household UPS, you will have to get an estimate for the available SCC.

So, go be an engineer. (you can't type body language - so interpret this as encouragement. No demeaning intent). The first one might take an hour. But from there on out, you got it for ever.

It just seems like someone, somewhere, would start their article with: "Listen, if your situation is less than such and such...don't bother reading this..."

There you go, your first IEEE paper:thumbsup:

PS: For 240V, 700A SCC, 0.02 sec, 3", I got .7cal/cm^2. Which would not apply to a single phase house panel, cause, the SCC is likely closer to 2000A.

just some thoughts

ice

 

[lielec11]

a) I am resetting a circuit breaker in my basement, 15A single pole - what is the PPE requirement?
Thanks again!

I thought NFPA 70E had a section that stated something about not requiring PPE for at-home residential use, but I can't seem to find the specific reference at the moment... You may refer to tables 130.7(C)(15)(A)(a) and 130.7(C)(15)(A)(b) to get an idea of the PPE that would be required as well as the PPE category and arc flash boundary in a commercial facility.

 

[CEDEng]

Thanks, everyone, appreciate the feedback.

No offense taken on the "engineer" thing - I do plenty of (actual) engineering all day long, but finding this info has stumped me!

For the usual "math" - you can use Emb = 1038.7..... on and on, but this formula specifies a short circuit current greater than 16kA.

There are also several handy "graphs" and so on...but they bottom out around 5-10kA.

My question, I think, is probably one of physics as much as anything. Is there not a minimum energy required to produce an arc flash? This is a catastrophic situation, generally, for the hardware of course and for the unfortunate bystander, if present. But it's not generated from The Ether. Some minimum amount of voltage must be present at some air gap, and then some great amount of available current must be there to flow through the low-impedance plasma "arc" you just created. That "great amount" of current must be great enough to destroy copper, vaporize it really, and generally give you a bad day. (Apologies to the physics experts, I'm not going to pretend to truly understand the process - but I think you can see where I'm coming from.)

I don't think I can cause this situation with a 9V battery. I also don't think I can get it with 24VAC control power out of a 1kVA transformer that has 2A supplemental breakers above and below...or can I?

Somewhere between a AAA battery and the transformer outside on the pole is a number below which this kind of thing cannot happen. I'm just trying to find that number, in print, with a reference.

 

[junkhound]

just trying to find that number, in print, with a reference

fwiw

quote below From Littlefuse:
https://www.iwu.edu/physicalplant/safety-documents/section04/arcflashhazard-pamphlet.pdf


Misconception 3 :
Per IEEE 1584­­ — Guide for Performing Arc-Flash Hazard Calculations, there is no need to assess equipment under 240 volts from a transformer rated less than 125 kVA.

All equipment operating at 50 volts and higher must
be assessed for electrical shock and potential Arc-
Flash hazards per OSHA regulations and NFPA 70E
standards. IEEE 1584, published by the Institute of
Electrical and Electronic Engineers (IEEE), provides a
method of calculating the amount of Arc-Flash incident
energy possible at certain working distances from
exposed live conductors. IEEE 1584 helps to determine
the Hazard Risk Category and PPE requirements of
NFPA 70E. NFPA 70E references the IEEE 1584
calculation methods in NFPA 70E Annex D.
IEEE 1584 states, “Equipment below 240 V need not
be considered unless it involves at least one 125 kVA
or larger low-impedance transformer in its immediate
power supply.” This statement refers to the incident
energy exposure possible under these conditions as
observed during testing. The IEEE statement means
that it is not necessary to calculate incident energy on
equipment under 240V fed from a transformer less
than 125 kVA, because the available fault current is
not high enough to sustain an Arc-Flash and release
significant energy. It is important to remember that this
statement only refers to incident energy calculations,
and does not release employers from the responsibility
to assess all equipment operating at 50 volts and higher
in the workplace for other electrical hazards, such
as shock and overload conditions which may lead to
electrocutions, fires, or other hazards.

 

[petersonra]

just trying to find that number, in print, with a reference

fwiw

quote below From Littlefuse:
https://www.iwu.edu/physicalplant/safety-documents/section04/arcflashhazard-pamphlet.pdf


Misconception 3 :
Per IEEE 1584** — Guide for Performing Arc-Flash Hazard Calculations, there is no need to assess equipment under 240 volts from a transformer rated less than 125 kVA.

All equipment operating at 50 volts and higher must
be assessed for electrical shock and potential Arc-
Flash hazards per OSHA regulations and NFPA 70E
standards. IEEE 1584, published by the Institute of
Electrical and Electronic Engineers (IEEE), provides a
method of calculating the amount of Arc-Flash incident
energy possible at certain working distances from
exposed live conductors. IEEE 1584 helps to determine
the Hazard Risk Category and PPE requirements of
NFPA 70E. NFPA 70E references the IEEE 1584
calculation methods in NFPA 70E Annex D.
IEEE 1584 states, “Equipment below 240 V need not
be considered unless it involves at least one 125 kVA
or larger low-impedance transformer in its immediate
power supply.” This statement refers to the incident
energy exposure possible under these conditions as
observed during testing. The IEEE statement means
that it is not necessary to calculate incident energy on
equipment under 240V fed from a transformer less
than 125 kVA, because the available fault current is
not high enough to sustain an Arc-Flash and release
significant energy. It is important to remember that this
statement only refers to incident energy calculations,
and does not release employers from the responsibility
to assess all equipment operating at 50 volts and higher
in the workplace for other electrical hazards, such
as shock and overload conditions which may lead to
electrocutions, fires, or other hazards.

I seem to recall IEEE has recanted this statement though.

 

[iceworm]

I can not find it in the current IEEE 1584 - 2002.

However, the limits on the model puts the lower end (208V, 700A) to where you damn near couldn't get a match to lite.

ice

 

[lielec11]

I can not find it in the current IEEE 1584 - 2002.

However, the limits on the model puts the lower end (208V, 700A) to where you damn near couldn't get a match to lite.

ice

My company is too cheap to purchase the 1584 standard. I just looked it up to see what it would cost if I purchased it on my own... IEEE wants $805 for this standard?! It is barely 100 pages long, including appendices, is it actually worth it? What is in this document that I don't already know from 70E and from using SKM all these years?

 

[junkhound]

Request it thru your local library, they can access thru interlibrary load if nothing else. Yearly subscription to IEEExpress is probably less than the $$ you saw.

Then simply (for you own use only per copyright laws) make your own copy with a digital camera.

 

[iceworm]

My company is too cheap to purchase the 1584 standard. I just looked it up to see what it would cost if I purchased it on my own... IEEE wants $805 for this standard?! It is barely 100 pages long, including appendices, is it actually worth it? What is in this document that I don't already know from 70E and from using SKM all these years?

The company can afford SKM, but can't afford an IEEE document and they are selling services based on that document. Interesting management concept.

As for what is in the document that you don't know from using SKM and 70E - I truly wouldn't know.

One thing I hear a lot on here is that one must have software to calculate arcflash. My inclination is that if one can't calculate arcflash using a calculator and a pen, then it is likely that one does not understand the physics or the algolrythm. I've never been a fan of blind acceptance because, "the computer says so." And, I'm not saying that is where you are - cause ........... I don't know.

That being said, NFPA 70E, Informative Annex D, Incident Energy and Arc Flash Boundry Calculation Methods, covers most of the IEEE 1584 - but not all..

My current favorate for understanding the physics and simple calculation methods is Jim Phillips' book, Arc Flash Hazard Calculation Studies. $70 - $80 as I recall. The other one I am using is Arc Flash Hazard Analysis and Mitigation, JC Das. I don't recall how much that one was.

The only other thing that I could say is that in a conversation with a regulatory agency, I can quote a post on MH forum or, I can say, "IEEE 1584, paragraph x.xxx, says .........." Which one do you think will get me more traction?

Important Note:The guys and girls on here are pretty good and have helped me close in on required documentation. However, I still have to have the documentation, not conjecture.

Free philosophy from:

The Worm
And worth every penny you paid for it:roll:

 

[iceworm]

...Then simply (for you own use only per copyright laws) make your own copy with a digital camera.

I certainly would never suggest you fradulently run it through a copy machine

 

[JoeStillman]

...One thing I hear a lot on here is that one must have software to calculate arcflash. My inclination is that if one can't calculate arcflash using a calculator and a pen, then it is likely that one does not understand the physics or the algorithm. I've never been a fan of blind acceptance because, "the computer says so." And, I'm not saying that is where you are - cause ........... I don't know.

SKM sure makes the Short Circuit calculation easier than pen and paper. And MVA_bf is a factor in the D_c calculation.

...My current favorate for understanding the physics and simple calculation methods is Jim Phillips' book, Arc Flash Hazard Calculation Studies. $70 - $80 as I recall.

Thanks for the reference. I took Jim Phillips's protective relaying class during the last century and he was brilliant. I just ordered the book.

 

[CEDEng]

Thanks, all, again for the input and the levity.

I have no problem doing the math by pencil and calculator. The problem is that the results are non-sensible in the domain I am investigating!

I also have no problem using the software - providing it has some stout backing, which I believe many sites do, but in each case, the lower-limits of this math is not available.

The exception for <125kVA is no longer valid.

Exploring other IEEE and NFPA70e white papers and such, many sample analysis (plural...whatever the word is...) are shown, some with charted results - but the charts always start with a bolted-current available in the kA region.

Believe me - I know it's a seemingly simple question - but I did not post to this forum until I exhausted my searching-patience, and that took some time!

I'm more educated today than I ever have been about arc flash, but I don't feel like I'm any closer to knowing the answer to a minimum VA combo below which the calculation is not necessary, and the arc hazard is just physically not possible.

But this limit MUST exist...right?

 

[vicdog]

Somewhere between a AAA battery and the transformer outside on the pole is a number below which this kind of thing cannot happen. I'm just trying to find that number, in print, with a reference.

You and the entire 1584 team are trying to figure that out!

 

[wbdvt]

The exception for <125kVA is no longer valid.

Please explain this as I have not found anything from IEEE which removed this from the current standard. There is discussion on revising that in the new standard based on testing results but the new standard is probably about 1.5 yrs away from being issued, so under the current standard IEEE 1584-2002 that exception is still valid.

 

[zog]

Please explain this as I have not found anything from IEEE which removed this from the current standard. There is discussion on revising that in the new standard based on testing results but the new standard is probably about 1.5 yrs away from being issued, so under the current standard IEEE 1584-2002 that exception is still valid.

I believe they just removed the reference to it from the 70E