New to Short Circuit / Arc Flash Study
New to Short Circuit / Arc Flash Study
(OP)
Currently I am a completing a EE co-op. My project that I have been assigned is to determine the Arc Flash boundries and PPE requirements for their equipment. I'm a bit thrown to the wolves in this b/c my background has been more of circuit design up until this point.
Some company power info: ( this is for multiple buildings)
Pad-mounted Transformers range from 1000-2500 kVA (12.47kV supply)
34 buslines
operating voltages inside the plant 480v-120v
So far I have done a one line diagram of the electrical for all the buildings. With the researching that I have done to this point, It's my understanding that using the MVA method would be my next step.
Correct me if I am wrong but it's just converting the kVA into MVA 10^3 vs. 10^6
If this is right, would there be an easier method for calculating Short Circuit current in my situation where the vast majority of the voltage is at 480v?
Thanks for any of your help.
Some company power info: ( this is for multiple buildings)
Pad-mounted Transformers range from 1000-2500 kVA (12.47kV supply)
34 buslines
operating voltages inside the plant 480v-120v
So far I have done a one line diagram of the electrical for all the buildings. With the researching that I have done to this point, It's my understanding that using the MVA method would be my next step.
Correct me if I am wrong but it's just converting the kVA into MVA 10^3 vs. 10^6
If this is right, would there be an easier method for calculating Short Circuit current in my situation where the vast majority of the voltage is at 480v?
Thanks for any of your help.






RE: New to Short Circuit / Arc Flash Study
You need to get proper training on the subject of short circuit calculations and arc flash. Attend some seminars, classes, read good books and learn in company with some experienced engineers.
If your firm hired you just for this task, or they think they can assign such responsible task to a new comers or inexperienced person, they are playing with lives of other people. You have a choice to be part of that or not.
RE: New to Short Circuit / Arc Flash Study
RE: New to Short Circuit / Arc Flash Study
Now the context of your original post is clearer, I can offer my two cents.
To answer your basic question, you can use either KVA or MVA as your base ratinng (this assumes you are using per unit method). Just be careful of proper conversions when using formulas. There are formulas written for using kVAs too.
KVA=MVA*10^3.
If you are using a software, it will not matter much. You can use any rating as a base in a software as it does the rest. If doing by hand, it saves you a few steps of conversion if you choose rating of the transfromer most often used in the project as the base, but it does not have to be.
For the system you are describinig, you are better off using a software.
But you should get proper understanfing of SCC calcultions. Refer to IEEE Buff book (Std. 242) and Red book. Your company should have those books.
RE: New to Short Circuit / Arc Flash Study
other than my wages
So i get to formulate everything in excel to make it easier on myself. So far I have found the IEEE Arc Flash Hazard Calculator for Simple CB Applications spreadsheet to assist.
Transformers at this plant pretty much stay in the 1-45 KVA range. (excluding the pad-mounted) Could you point me in the direction of some websites that I might find some more formulas for my situation? The Boss doesn't get back until Wed. so I won't be able to find out about the books until then.
Thank you for all of your assistance.
RE: New to Short Circuit / Arc Flash Study
Per IEEE 1584, you can ignore xfmrs below 125kVA (maybe that will cut down some of your work)! Watch our for 112.5kVA though, as they can sometimes contribute more than 10kA at the secondary, depending on impedance. Get a copy of NFPA as well.
Mike
RE: New to Short Circuit / Arc Flash Study
Per NFPA-70E, anything over 50 volts is a possible arc-flash hazard, so even the small 208V panels probably still require labeling.
RE: New to Short Circuit / Arc Flash Study
Mike
RE: New to Short Circuit / Arc Flash Study
RE: New to Short Circuit / Arc Flash Study
RE: New to Short Circuit / Arc Flash Study
NFPA70E implies (look at their task tables) that 240V max systems with available fault current of 10kA maximum will never exceed a hazard category 0.
Notice how these two conditions conflict with each other. It is up to your company to decide which "standard" they want to follow.
Another conflict exists with single phase circuits. NFPA70E does not specifically address these circuits. IEEE 1584 notes that all of its methodology is for three phase AC circuits only.
RE: New to Short Circuit / Arc Flash Study
For single-phase 240V panels, IEEE 1584 recommends analyzing as a three-phase system. Again, I don't think they had much test data on single-phase systems.
RE: New to Short Circuit / Arc Flash Study
I have never seen any test data to support the 125kVA size. In fact, on this issue IEEE 1584-2002 does not use definitive language. On page 25 it says "systems fed by transformers smaller than 125 kVA should not be a concern".
What is magic about the size of <125kVA. In all other calculations we are concerned with the impedance and the resultant available fault current. But for some reason at 208V all IEEE 1584-2002 cares about is full load amps.
RE: New to Short Circuit / Arc Flash Study
There were discussions on this issue in other posts too. Do a search.
The fact is arc flash calculation method is still evolving and it is in embryonic stage. You do the best you can with available techniques.
The IEEE 1584 formulas are all empirical and are curve fit formulas. Meaning curves ere plotted based on several test results and then mathematician were called in to find the equation that fits the curves. So there are infinite numbers of scenario that are not yet covered.
IEEE recommends ignoring below 125kVA and 240V because the tests proved so, I believe or historically they have not been found as dangerous to a person. Remember arc flash analysis is for personnel protection and not for equipment or fire safety.
IEEE also concentrated on more prevailing and pressing issues. Historically arc faults below 240V have not caused serious burn injuries compared to higher voltage or bigger systems. The same is the case with single-phase systems. IEEE also performed tests on 3 phase systems, as they believe that single-phase arc fault quickly develops into a 3-phase fault and cause the damage/injuries.
DC systems are also not yet investigated probably because they form a very small part of existing electrical systems. This does not mean that they do not pose hazard. It will take time to validate risk of those systems by tests and translate them into a standard. I am sure arc flash analysis methods for single-phase AC and/or DC systems will be included in IEEE standard in due course of time.
Also see my post in thread237-171087 regarding the fact that PPE only protects against burn injuries and not other hazards. That too only to limit the injuries to a second-degree burn.
To get some background of IEEE 1584 and NFPS 70E from engineering point of view, I recommend attending the seminar offered by University of Wisconsin, MD on the subject. They had the right presenters, when I attended in June 2006.
RE: New to Short Circuit / Arc Flash Study
My point is that IEEE based their limit of 125kVA and 208V primarily on anecdotal rather than on actual evidence (in one set of data, I saw 1 test at 208V, 0 at 240V, and 5 at 250V). The IEEE standard does not clearly address 240V systems at all, in fact they only refer to 240V once.
And, on page 25 the standard says "While the accuracy of the model at 208 V is not in the same class with the accuracy at 250 V and higher, it will work and will yield conservative results." So why chose 208V and not 240V as the cut off point? Was it because the authors were not aware of the amount of 240V or was it other factors like like propblems with grounded-B phase or high-leg 240/120V systems.
IEEE 1584-2002 picked 208V without regard to the system impedance and NFPA 70E chose 240V max with a 10kA max fault current. A choice must be made as to which method the customer wants to follow when creating their Electrical Safe Work Practices program.
I agree with the reasoning that OSHA is enforcing NFPA 70E not the IEEE standard, therefore conflicts should be resolved in favor of NFPA 70E. I do wish that NFPA 70E would severly edit their task based PPE selection, it ignores more questions than it answers. For example, what PPE is required when operating a 600V heavy duty "safety switch" when voltage testing is not being performed (i.e turning off a machine at the end of a shift)?
RE: New to Short Circuit / Arc Flash Study
The point you are missing is that IEEE arc flash standard is work in progress and they are addressing (testing) more dangerous and more prevelant systems first. Rest will follow.
NFPA 70E is based on IEEE 1584 and it refers to it. Tables in NFPA 70E are conservative compared to calculated results of IEEE 1584. NFPA tables are created for simplicity and to address most common smaller (light commercial) installtions without need for too much calcs.
NFPA 70E recognizes use of IEEE 1584.
RE: New to Short Circuit / Arc Flash Study
You are missing my point. I fully support NFPA 70E and IEEE 1584-2002. Although, I don't believe the values in the NFPA are always more conservative.
I am trying to point out the 125kVA and 208V max limits in IEEE are in conflict with the 240V and 10kA limits in NFPA 70E. Someone needs to make a choice on which method will be adopted and used in their facility.
RE: New to Short Circuit / Arc Flash Study
NFPA tables only apply with listed limitations. Beyond that you use IEEE. Even there not all scenarios may be covered. You cannnot extrapolate IEEE equations for conditions beyond those stated in the standard. So there will be instances you may not have any guidelines for calculating arc flash.
It is akin to SCC calculations where results may depend on how many assumptions you make and how much detailed data you use and still arrive at slightly different numbers.
Less calculations, less accuracy, more conservative results. This of course assumes that assumptions are on coservative side.
RE: New to Short Circuit / Arc Flash Study
Ignore
208v and below
125kVA transformers and below. For this, are they saying that I can ignore them at a 480v level, or only if the secondary is below 208v?
RE: New to Short Circuit / Arc Flash Study
NO. You can assume hazard category 0 only for equipment rated 240V or below "AND" fed by a transformer rated 125kVA or less. This is per IEEE 1584.
NFPA 70E, Table 130.7 (C)9a, lists several scenarios for equipment of different voltages with listed limitation of available short circuit currents and opening time of overcurrent protection devices. Lowest voltage there is 240V and below. (Not 208V and below).
NFPA 70E, clearly mentions that arc flash analysis shall be performed per article 130.3 therein which has formulas based on IEEE publication. It further states for conditions not listed in the table or covered by formulas in article 130.3, refer to Annex D. Annex D contains guidelines, formulas and an example of a detailed arc flash analysis based on IEEE 1584. The Annex D further states that for more complete analysis refer to IEEE standard 1584.
So there you go. If you are seriously interested in arc flash analysis buy NFPA 70E Handbook and IEEE 1584. On top of that you will need a software program such as SKM /EDSA for proper arc flash analysis.
RE: New to Short Circuit / Arc Flash Study
NFPA 70E allows you to define 240V circuits with 10kA max fault current as hazard category 0.
rbulsara,
I am not an advocate of the NFPA70E task tables, especially as they are currently written. However, they are part of a standard enfoced by OSHA so they can not be dismissed simply.
Are you saying that because NFPA 70E does not use very many calculations it is less accurate and therefore more conservative than the IEEE standard?
IEEE says to ignore a 112.2kVA 2%Z 208V transformer but NFPA 70E requires calculations be performed.
RE: New to Short Circuit / Arc Flash Study
Firstly, to think that NFPA 70E is a document independent of IEEE 1584 is not correct. NFPA 70E "IS" based on IEEE 1584 and other papers that IEEE 1584 is based on.
and yes, in most cases NFPA tables will result in more conservative PPE requirement or hazard category. There may be exception, who knows. This is again where calculations have to be performed.
The tables are created for same reasons as other tables in NEC or other NFPA standards are created, that is not to burden small businesses with complex engineering calculations or supervision.
RE: New to Short Circuit / Arc Flash Study
If you think that the NFPA 70E tables are conservative, then how do you reconcile the different ways that IEEE and NFPA handle a 150kVA 240V 4%Z d-d transformer?
By the way, I make my living performing systems studies including arc flash. I am a advocate of IEEE 1584-2002 methodology and NFPA 70E. Right now NFPA 70E is law (due to OSHA) and IEEE 1584-2002 is a tool for complying with the law. I look forward to the resolution of these types of questions as more data is collected, analyzed, and debated.
RE: New to Short Circuit / Arc Flash Study
I agree that there are inconsistencies between 1584 and 70E. There are actually a lot of inconsistencies in 70E all by itself.
I do think you're overstating when you say that NFPA 70E "is law." OSHA requires a safety plan and requires hazard assessment, but it doesn't specifically mandate compliance with 70E, as far as I know. If things have changed recently, let me know.
Cheers,
Dave
RE: New to Short Circuit / Arc Flash Study
The document as they exist now are first cut so to speak, they will keep evolving endlessly. In fact there are already proposed changes for NFPA 70E table (included in the handbook) and IEEE 1584 have issued interim addendum. So there is nothing to get excitetd about things being not perfect yet. As mentioned before whatever guidelines we have today is better than what we had before.
It is not that arc flash hazard never existed before. People still took all precautions they could, some understood the risk and some underestimated it.
I will post my views in different words in the next post as to why you will not see NFPA standard exactly matching IEEE standards.
RE: New to Short Circuit / Arc Flash Study
Let me rephrase what I am trying to say:
NFPA standards are “Fire Safety” standards and often included as part of Codes. Codes are also intended solely to ensure “Safety” of the public. They couldn’t care less whether equipment really works to help a business for example.
IEEE standards are “engineering” standards and are there to guide engineering approach.
In order to make it easy for the public and small businesses, which make up large part of the public and not to burden them with complex engineering, NFPA type standards endeavors to create table for ready use. They mostly tend to be conservative. There may be a few exceptions.
For example, NEC’s ampacity tables, they are there so most electricians do not have to worry about calculations yet ensuring “safety”. You can show engineering calculations all day long that smaller size conductors (than those in NEC tables) are good enough, but it does not matter. Safety comes first.
However, NFPA standards and local Codes (in the USA) have always recognized and accepted the installations based on “engineering supervision” for situations not covered by the tables or the standard itself and including those covered by the standards as long as they do not violate minimum requirement set forth.
Conversely, codes and safety standards may permit installations, which may appear to defy engineering principles, but yet historically may have been found to cause no safety issues. One of such examples is tap rules of NEC.
So yes in the end, to meet the letter of the Code you can use more favorable approach, if there are more than one standard are applicable. In this case, you can choose either the pre-calculated tables or where permitted, use of engineering calculations.
So I do not see a reason to get excited about precieved inconsistancies between a safety standard and an engineering manual.
RE: New to Short Circuit / Arc Flash Study
You have not answered my question about how you would reconcile the differences in my examples. I am not advocating using tables instead of engineering calculations. Again for example, that the IEEE standard says a 112.5kVA 208V 3%Z transformer is not important and to ignore it, but the NFPA standard says it has an available fault current above 10kA so calculations are required.
dpc
OSHA cites NFPA 70E as an accepted standard when they apply their "General Duty Clause". You can get more information at the OSHA website.
RE: New to Short Circuit / Arc Flash Study
When in doubt, err on the conservative side. In this case, it's NFPA 70E. If there's an accident your lawyers will thank you...
rbulsara is absolutely right. This about "safety" and it has nothing to do with getting work done...
old field guy
RE: New to Short Circuit / Arc Flash Study
Personally I do not think NFPA 70E task tables are always conservative. I also do not think that calculations are always conservative either (IEEE 1584 admits that it's formulas are not "as accurate" <250V). My point is that some times an educated judgement must be made, and circuits at or below 240V is one of those points.
And of course this is about safety. NFPA 70E is about creating an entire Electrical Safe Work Practices program and not simply about arc flash.
RE: New to Short Circuit / Arc Flash Study
The whole NFPA 70E thing is a good idea to an extent, but it was'nt spoon fed to industry. OSHA just backed up the truck and dumped on industry as they always do, and many of the shell shocked industrials are doing nothing in a state of shock or a state of ignorance or both. It's also what drives businesses (like mine!!) to shift production to Mexico, China, Korea, etc. Some of you guys may do this for a living and that's good for you. For me, it may be the difference between my company building a product here in the states or just shutting down another line and building it in Mexico. You guys who do these calcs for a living remember to turn out the lights when the last manufacturer moves out of the states from the cost of these initiatives. But wait, I forgot, the attorneys that prosecute us industrials that have arc flash accidents will still be burning the midnight oil trying to find another industry to victimize.
My understanding from attending the NETA conference last year in Memphis and listening to Ken Mastrullo with OSHA Boston, is that NFPA 70E is not law, yet. It is the document that OSHA uses to evaluate accidents, so it might as well be law, but they can't come in and cite you for non compliance if you have not had an accident yet. We are still trying to comply here at my company. West Virgina is the most litigious state in the nation, so we'd be crazy not to try to comply.
RE: New to Short Circuit / Arc Flash Study
RE: New to Short Circuit / Arc Flash Study
1) For Line-to-Neutral Faults you assume that the neutral conductor and the line conductor are the same size. Most of the time that is not true, so I am wonder how the "f" factor would change for a smaller size neutral or less conductors on the neutral.
2) Does anything change when you have a 3 phase delta high leg? especially for the high leg?
3) It doesn't look like the PtoP method calculates the Asymmetrically fault current in my design is this going to come back and haunt me on large jobs? Or is the fact that it is based on an infinite bus cover me?
RE: New to Short Circuit / Arc Flash Study
The short answer is simple, if your situation requires the calcs then, do the calcs and see where you end up. Pick the more stringent of the two requirements, if in fact you are following both IEEE and NFPA.
The long and "split the hair" answer could be as follows:
As of today, NFPA 70E has not been adopted as Code (by legilation) in most states, so it is not mandatory to follow NFPA 70E. If you want to use the engineering judgement, I would follow IEEE 1584 vs NFPA 70E any day. Even from liability point of view, I would be comfortable defending that I performed due deligence in evaluating the hazard by following IEEE 1584.
On the other hand, if NFPA 70E is in fact adopted as Code, I would verify that I do meet NFPA 70E, even if I follow IEEE.
"dpc" is right in saying that OSHA would accept compliance with NFPA 70E as adequate measure for safety, but it is not mandatory. I am sure if you talk to OSHA they will readily accept compliance with IEEE as well.
On the other note:
Most 112.5kVA tranfromers will have significant source impedance ahead of them so they will seldom exceed 10kA scc.
I would also not split hair for 10,000A and 10,387A.
RE: New to Short Circuit / Arc Flash Study
IEEE 1584 is simply a tool for determining the incident energy from an arc fault, it makes no recommendations as to any action to be taken. You must go to NFPA 70E to find the correct hazard category and resultant PPE for that arc flash incident energy as well as for the PPE for the voltage.
And as far as your split hair answer, most 150kVA 208V transformers will also have significant source impedance in front of them so why not ignore them as well? Today I evaluated a 300kVA 208V 6%Z transformer with enough source impedance that the secondary fault current is only 9.218kA.
RE: New to Short Circuit / Arc Flash Study
That’s why you perform the calculations.
I think you are letting your emotions getting better of you.
And you keep missing the logic behind recommendations and validity of available equations. You cannot arbitrarily interpolate or extrapolate the equations of IEEE (and NFPA) beyond the limits stated therein as they are not following a law of physics/math, but are curve fit equations based on test results of certain setups.
The advice of ignoring smaller units is primarily based on historical evidence that such installations do not pose significant personnel injury (burn injury) risk and less so on test or theoretical evidence. There is likelihood of getting distorted results, if you perform calculations for smaller units (or for conditions other than they are permitted for), which may require unnecessary PPE.
Your example in fact backs up what IEEE says, if you perform calculation for real life 125kVA, 208V transformers, you will rarely get more than 10kA. Less so on a 300kVA unit.
After all techniques available today are just a beginning and they will be refined as time goes on.
I rest my case now.
RE: New to Short Circuit / Arc Flash Study
I know many things will be worked out in the future, but for now we need to acknowledge that there are gray areas. I have been playing devil's advocate, I am not second geussin gth e answer I gave my customeer. I am interested in what others would advise and the engineering rational for making that choice, not just "because".
I guess I am just fed up with people saying the only way to answer arc flash questions is their way. Only by debating the issues and discrepencies will we be able to develop consensus opinions that will eventually reduce the grey areas.
RE: New to Short Circuit / Arc Flash Study
RE: New to Short Circuit / Arc Flash Study
The answer is Yes. Knowledge of available short circuit current is the starting point of an arc flash calculation.
You must read standards mentioned above that will save you from asking such questions. There is no way around it.
RE: New to Short Circuit / Arc Flash Study
At 480V, it requires very little. If you review the test data used in IEEE 1584, you should find some examples.
Remember that the arcing fault current is always less than the bolted fault current due to the resistance of the arc. At 480 V, it can be much less than the bolted fault current.
RE: New to Short Circuit / Arc Flash Study
Yes, I was searching for the magnitude of the current required to initiate an arc-flash at 480v
RE: New to Short Circuit / Arc Flash Study
To start an arcing event not much current is required, but once an arc is established it could (and it does in most cases) rapidly develop in a fireball and a blast. This is because the inital arc ionizes the air which furhter lowers the fault impdedance and increases the fault current and the cycle continues rapidly growing in a dangerous event.
This is one of the reasons, single phase systems are not evaluated on a 3 phase system, because it is assumed that a single phase arcing fault will quickly develop in a 3 phase fault at any rate.
The knowledge of a short circuit study is necessary to estimate most likely damage (release of incident energy) by an arcing fault at a certain point.
RE: New to Short Circuit / Arc Flash Study