New to Short Circuit / Arc Flash Study 2

[viper1bw]

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.

 

[rbulsara]

Work you are assigned requires experienced engineers. This forum is no substitute for lack of knowledge or experience.

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.

 

[viper1bw]

Point taken. The reason for a Co-op is to learn. An arc flash hazard analysis has already been completed at the plants. This company is using it as a test to see what I know and what I am able to complete using resources which are available to me.

 

[rbulsara]

Great attitude! Sometimes it is not clear from the first post where the person asking a question is coming from.

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.

 

[viper1bw]

Sadly I don't get to use software. Since the company already has the study completed, the don't want to spend anymore money than they already have for new software,
other than my wages (I believe they hired a company to complete the original Arc Flash Study)
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.

 

[mpparent]

Well..
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

 

[dpc]

Just to clarify, you can ignore transformers smaller than 125 kVA for purposes of arc-flash calculations at 208V. At 240V, a calculation should be done (per IEEE-1584).

Per NFPA-70E, anything over 50 volts is a possible arc-flash hazard, so even the small 208V panels probably still require labeling.

 

[mpparent]

Yep..thanks for clarifying that dpc.

Mike

 

[dpc]

I've been hoping against hope that IEEE 1584 would be revised to eliminate need for arc-flash calcs at 240V, but from what I hear, it's probably going the other direction - extended down to 208 V. What a pain.

 

[davidbeach]

dpc, just more billable hours. ;-)

 

[JBD]

IEEE 1584 says to ignore 125KVA and smaller 208V transformers, but they offer no reason for choosing this size and voltage limits.

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.

 

[dpc]

IEEE 1584 bases its recommendations regarding 208v on the test data they had available. It's really tough to sustain an arc at 208V, but it can be done in some situations, apparently.

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.

 

[JBD]

Page 1 of IEEE 1584-2002 says "Single-phase ac systems and dc systems are not included in this guide."

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.

 

[rbulsara]

JBD:

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.

 

[JBD]

I am aware of the procedures and limitations of IEEE 1854-2002 and NFPA70E. I firmly believe in the need for protection against injuries caused by arc flash.

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)?

 

[rbulsara]

JBD:

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.

 

[JBD]

rbulsara,

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.

 

[rbulsara]

They are not. One may result in more conservative numbers than other but they are not in conflict. Yes you do have a choice where to use NFPA 70 tables or do detailed calc per IEEE 1584 and where possible you are free to apply one they fits your view better.

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.

 

[viper1bw]

So, from what I've gathered here:
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?

 

[rbulsara]

viper:

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.