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 Post subject: Choosing the worst result from IEEE or NFPA Ralph's Lee Equations
PostPosted: Mon Jan 27, 2014 3:22 pm 
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I am reviewing a project that is choosing to print in the labels the worst case incident energy from two calculations methods. IEEE or the NFPA Ralfph Lee equations. Whichever result is higher, that is the value going to the label.

This feels very wrong to me and I would never mixed these two calculation methods. The question is:

How can I argue using the NFPA 70E 2012 or IEEE standards that this mixing of methods is inadequate?

Thanks.


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PostPosted: Tue Jan 28, 2014 8:52 am 
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RECS wrote:
I am reviewing a project that is choosing to print in the labels the worst case incident energy from two calculations methods. IEEE or the NFPA Ralfph Lee equations. Whichever result is higher, that is the value going to the label.

This feels very wrong to me and I would never mixed these two calculation methods. The question is:

How can I argue using the NFPA 70E 2012 or IEEE standards that this mixing of methods is inadequate?

Thanks.

There are actually several methods:
IEEE 1584
"NFPA" which is based on a 2007 paper that preceded IEEE 1584
Ralph Lee's Method
Arc Pro or Heat Flux (software for systems > 15 kV)

IEEE 1584 is used almost exclusively for three phase arc flash up to 15 kV. It is based on the latest and greatest. (almost - we are working on the next generation of equations right now)

Above 15 kV a method other than IEEE 1584 should be used which is either Ralph Lee Equations, Arc Pro or Duke Heat Flux (software). The problem with the Lee method is although it was great back in the day (several decades ago), it is very theoretical and gives extremely high results as voltage increases. Most people would use Arc Pro or a similar program in this case.

If I understand what you are saying i.e. they are choosing the larger of IEEE 1584 or Ralph Lee results, I would question the capability of the person performing the study because this is all pretty common knowledge for people that have performed studies before.

Good Luck!

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Jim Phillips, P.E.
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PostPosted: Tue Jan 28, 2014 1:25 pm 
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RECS wrote:
I am reviewing a project that is choosing to print in the labels the worst case incident energy from two calculations methods. IEEE or the NFPA Ralfph Lee equations. Whichever result is higher, that is the value going to the label.

This feels very wrong to me and I would never mixed these two calculation methods. The question is:

How can I argue using the NFPA 70E 2012 or IEEE standards that this mixing of methods is inadequate?

Thanks.


Personally, I would not mix the methods - especially not with the sole intent of going with the higher results. Just because a result is higher does not automatically mean dressing to that higher level is any safer.

Hedging your results can end up with workers operating with higher than necesary PPE, working with reduced dexterity, and actually creating problems and increasing the risk of an arc flash event.

I feel like I am stating the obvious

There is a 'real' cal/cm^2 value associated with any exposure. We try to calculate what this value actually is using formulae. And we come up with a number that we think expresses the level of danger involved. How close to the 'real' number did we get?

The right approach is to validate the equasions we use to make certain our answer is close to the 'real' value. I would certainly prefer that there were two methods where the numbers agree in the end, and we could check our answers. (Unfortunately I do not have any software other thanan excel spreadsheet with the equasions keyed in, so I can't work the equasions then run it through the sowtware)


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PostPosted: Wed Jan 29, 2014 4:13 pm 
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Jim Phillips (brainfiller) wrote:
If I understand what you are saying i.e. they are choosing the larger of IEEE 1584 or Ralph Lee results, I would question the capability of the person performing the study because this is all pretty common knowledge for people that have performed studies before.

Agreed! It is a pretty black and white decision. IEEE 1584 < 15 kV and something else > 15 kV like Arc Pro. Ralph Lee equations are a bit silly since they can be off by orders of magnitude.

If the person doing the study would let "orders of magnitude" override IEEE 1584 as the worst case, then you might want to look for someone else.

Anyone else see a problem with this?


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PostPosted: Thu Jan 30, 2014 10:33 am 
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I would personally only consider using Ralph Lee's method when no other method is available for several reasons:

1. It is not a consensus safety standard, except in so much as it is provided for documentation purposes in IEEE 1584 as a fall back when working outside the parameters of the IEEE 1584 model.
2. It produces results that are at least 3 times higher and frequently even further out compared to actual test data, and the error increases as the voltage and/or current increases.
3. It will definitely result in a "conservative" rating, as in a wildly higher number than the real world. However, increasing the level of PPE does NOT result in a safer work environment. "Overprotecting" in terms of arc flash PPE also results in:
A. Reduced visibility and hence greater risk for causing an arc flash due to increasing mistakes while performing a task.
B. Reduced mobility and hence greater risk for causing an arc flash due to increasing mistakes while performing a task.
C. Increased risk of heat related injuries (heat exhaustion, heat stroke, heat cramps, etc.).

Thus I would strongly recommend against using Ralph Lee's method EVEN for cases where IEEE 1584 is inadequate. Over 15 kV for instance your options would be Arc Pro software, tables in IEEE/ANSI C2-2012 edition (developed from Arc Pro), or Duke Heat Flux software. In addition, Ralph Lee's equation is also NOT applicable except in open air cable scenarios. It is not appropriate for use on switchgear, even in the theoretical condition. For those conditions I would fall back on the most appropriate standard which would again be IEEE/ANSI C2-2012. The tables in that standard are also referenced in Annex D of NFPA 70E.


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