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 Post subject: Littelfuse Article
PostPosted: Sun Dec 15, 2013 12:26 pm 
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I am attaching several items:
1. An article by Littelfuse on Misconceptions About Arc Flash Hazard Assessments
2. My EasyPower model of the Littelfuse example in the above article.

My reason for posting is to solicit feedback on my EasyPower model and maybe some other people can model it in other software such as SKM, ETAP, EDSA, etc. It would interesting to see those results. For my model, I used an infinite source and adjusted the transformer impedance to match what Littelfuse had for the short circuit current. I also assumed steel conduit and THHN insulation.

As you can see, I did have some agreement on the Main bus (Littelfuse: 135.8 cal/cm2, Me: 107.2 cal/cm2), either way Extreme Danger. Things fell apart further on down the model. For example: Littelfuse at Disconnect Motor 4 has a SC 3.1kA, 9.5 cal/cm2, Me at Disconnect Motor 4 has SC 3.4 kA and 0.2 cal/cm2. So, I am confused on how Littelfuse arrives at 9.5 cal/cm2. I forced the arcing current to be 3.1kA and still was around 0.2 cal/cm2. I have included a TCC showing the breaker, and FLSR and LLSR curves.

Comments/discussion would be appreciated.

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PostPosted: Mon Dec 23, 2013 9:46 am 
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Littlefuse has assumed that part of the fault current comes from the motor and does not flow through the protective device. They also take into account the a reduced current (85%). This puts the arcing current into the thermal response portion of the breaker trip curve.


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PostPosted: Mon Dec 23, 2013 10:31 am 
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Yes, but only 195 A comes from the motor and 85% current is ~2.1kA, which still puts the trip in transition region but still less than 0.03 sec to trip

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PostPosted: Fri Jan 03, 2014 7:03 am 
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Well, I am disappointed that no one decided to model the system in the article using other software.

I had a discussion with someone from Littelfuse who was in the department that produced the article. He said the article is dated, was done with SKM software and that the engineer(s) who were involved are no longer available. This because Littelfuse has gotten out of the business of performing studies and now are just back to the roots of their core business.

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PostPosted: Fri Jan 03, 2014 8:10 am 
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This looks suspicious. Even at 2.1 kA (85%) it looks like the current is just slightly to the right of the right side of the instantaneous band. i.e. fast tripping. As soon as the current drops to the left of the right side of the band it would be a long trip but I'm not seeing this. Although it close, it seems more like someone might have taken a few liberties as a "commercial interest". (but we all know that never happens :cool: )


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PostPosted: Mon Jan 06, 2014 10:07 am 
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Just few observations:

The LF article does not show complete single line diagram. In particular, it doesn't show utility short circuit MVA used for the analysis. The EasyPower example assumes 100000MVA 150X/R available on the transformer primary which is quite unrealistic as far as I am concerned. The LF article indicates available S.C. 29.5kA in Main Switchgear which makes me believe they used much lesser then the 100000MVA source for the analysis. Reducing the source MVA in EasyPower model would probably bring Incident Energy on the Main Bus predicted by EasyPower model more in line with the LF article number (135.8 cal/cm2).

Also, the LF article does not indicate whether equipment X/R ratios were considered in the analysis. I am not sure if the EasyPower model takes into account equipment X/R. At least the summary table from EasyPower example does not show fault X/R data for each bus name. Neglecting equipment X/R would have further impact on the amount of predicted SC currents and the predicted SC current margin of error throughout the system.

Next, neither the LF article nor the EasyPower model indicates what S.C. current value was used to determine arc duration: predicted S.C. current or part of the S.C current through upstream protection device?

wbd, with all due respect I am not convinced you are comparing apples to apples. The sure thing is that indeed very small change in fault current through a circuit breaker may have profound effect on the breaker operating time and subsequently incident energy and AFB predicted by IEEE 1584 model.

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PostPosted: Mon Jan 06, 2014 3:57 pm 
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Well, I varied the impedance of the transformer to match the Littelfuse fault current. So it should not matter for this example what the utility feed is capable of as tje short circuit current is limited by the transformer impedance.

As far as X/R, in regards to arc flash hazard analysis, Jim Phillips responded in a thread from December 2010:
Image

As far as arc flash, other than the extremely small change in short circuit current, X/R does not enter into the arc flash equations (yet). There is some thought that the X/R may have some minor (?) effect but nothing has been done as of yet. - See more at: http://arcflashforum.brainfiller.com/threads/1432/#sthash.k8hy3Wbo.dpuf
As far as arc flash, other than the extremely small change in short circuit current, X/R does not enter into the arc flash equations (yet). There is some thought that the X/R may have some minor (?) effect but nothing has been done as of yet. - See more at: http://arcflashforum.brainfiller.com/threads/1432/#sthash.k8hy3Wbo.dpuf
As far as arc flash, other than the extremely small change in short circuit current, X/R does not enter into the arc flash equations (yet). There is some thought that the X/R may have some minor (?) effect but nothing has been done as of yet. - See more at: http://arcflashforum.brainfiller.com/threads/1432/#sthash.k8hy3Wbo.dpuf
According to EasyPower, the column on the spreadsheet for Estimated Arc Fault (kA) is:
Estimated Arc Fault (kA): This is the estimated current flowing through the trip device,
which contributes to the total arc current. This value is used by the program to estimate the
trip time. For calculating the incident energy, the total fault current at the faulted bus is
used.

So, when one lines this current up on the respective TCC, it is still a fast trip time in the instantaneous region, so again there is no way I see to acheive the high cal/cm^2 that was noted in the article.

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PostPosted: Mon Jan 06, 2014 9:23 pm 
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Quote:
Well, I varied the impedance of the transformer to match the Littelfuse fault current. So it should not matter for this example what the utility feed is capable of as tje short circuit current is limited by the transformer impedance.


Please correct me if I am wrong but the Littelfuse article shows 29.5kA short circuit on the Main Switchgear bus while your Easypower study shows 30.71kA for the Main under the 'Bus Bolted Fault' column. The difference could explain the discrepancy between IE value reported in the Littelfuse article (135cal/cm2) and your EasyPower model (107cal/cm2) as it takes more time for the upstream protection device (KLPC 2000A) melt and clear lower fault current.

Quote:
As far as X/R, in regards to arc flash hazard analysis, Jim Phillips responded in a thread from December 2010


I was talking about X/R impact on the amount of predicted short circuit current only and not the X/R in regards to arc flash analysis. What I meant is that if you neglect equipment X/R ratios in your short circuit analysis and operate with equipment impedance only you introduce significant error. What is even worse, the error can't be quantified and therefore the results can be used as scientific guess only. As an example adding jX=1Ohm reactance and 1Ohm resistance in series produces combined 1.73Ohm impedance. If you ignored either the equipment X/R or the equipment reactance, you would end up with combined 2Ohm or 1Ohm impedance respectively. Similar observations apply when adding equipment impedances in parallel. Neither the Littelfuse nor your Easypower study lists X/R or fault power factors that's why I'm not convinced in the presented short circuit studies.

Quote:
According to EasyPower, the column on the spreadsheet for Estimated Arc Fault (kA) is: Estimated Arc Fault (kA): This is the estimated current flowing through the trip device, which contributes to the total arc current. This value is used by the program to estimate the trip time. For calculating the incident energy, the total fault current at the faulted bus is used.


I don't see the 'Estimated Arc Fault (kA)' column in your EasyPower study unless I am missing something. Also, IEEE 1584 provides for IE calculated based on predicted arcing current and predicted arcing current reduced by %15, not the total fault current at the faulted bus. Are you sure this is exactly what EasyPower is doing?

Quote:
So, when one lines this current up on the respective TCC, it is still a fast trip time in the instantaneous region, so again there is no way I see to acheive the high cal/cm^2 that was noted in the article.


I believe you are right about the high cal/cm2. However, even a small deviation in arcing current used in determining arc flash duration could prove you wrong and the Littelfuse findings right. I would still make sure having accurate short circuit current values (both total SC and part of the current through protective device) to begin with before making such unconditional conclusion.

The biggest challenge I see with circuit breakers in arc flash analysis is that even a small inaccuracy in calculated SC current, other system parameters (for example conductor gap), the breaker margin of error, environmental conditions, aging etc. could lead to such a drastic change in IE and AFB. I am personally more a fan of fuses in arc flash analysis as they offer much more gradual change of IE as function of system parameters comparing to breakers.

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