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.

 

[JBD]

IEEE 1584-2002 says you can ignore the 208V secondaries of transformers smaller than 125kVA.

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.

 

[rbulsara]

JBD:

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.

 

[JBD]

rbulsara,

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.

 

[dpc]

JBD,

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

 

[rbulsara]

JBD:

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.

 

[rbulsara]

To add to my previous post:

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.

 

[JBD]

rbulsara,
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.

 

[oldfieldguy]

JBD--

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

 

[JBD]

old field guy,

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.

 

[tjg124]

My company estimated arc flash hazards for equipment based upon voltages. We call 50 to 250 volt hot work HRC 1 requiring ATPV 5 clothing. We call >250 to 1000 volt hot work HRC 2 and require ATPV 8 clothing. We did this to just get some program of protection going for our people quickly, and we intend to go back and crunch the individual arc flash energy numbers in later months. At least we have an arc flash protection plan going. It's better than a lot of company's that have nothing because they are amazed by the complexity and cost of the whole program.

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.

 

[dpc]

OSHA says that if you follow NFPA 70E you will be in compliance. I don't think they say that NFPA 70E is mandatory. It's conceivable that a company could have a safety program that did not fully comply with 70E that OSHA would deem acceptable.

 

[bjenks]

I have a question about using the Point to Point Short Circuit Current calculations for three situations and need to know how to address them when calculating the "f" factor

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?

 

[rbulsara]

JBD:

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.

 

[JBD]

You can not simply follow only IEEE 1584 when you are creating an Electrical Safe Work Practice.

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.

 

[rbulsara]

JBD:

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.

 

[JBD]

I have no emotional involvement. But, I do wonder why you seem to put more faith in the IEEE 1584-2002 calculations for <250 systems than IEEE does. What would you tell a customer when the IEEE calculation for their 300kVA transformer is >8 cal/cm^2 but that NFPA 70E considers it to be a hazard category -1?

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.

 

[viper1bw]

After all of my reading, one question that I haven't found a definative answer on is the required Short Circuit Current required to create a arc Flash at 480v. If anyone is able to reference me to it, that would be greatly appreciated.

 

[rbulsara]

viper1bw:

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.

 

[dpc]

Are you asking about the magnitude of available SC current required to initiate an arc-flash?

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.

 

[viper1bw]

-dpc

Yes, I was searching for the magnitude of the current required to initiate an arc-flash at 480v