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JeffBlichmann

Post subject: "Rule of Thumb" Studies Posted: Wed Jun 21, 2017 8:14 am 

Joined: Tue Apr 17, 2012 9:17 am Posts: 10

Our facility has more 208/120V and 240/120V lighting panels than I could possibly count. I've been asked to run some sensitivity studies to determine rules of thumb for what arc flash could be at a lighting panel.
For instance, we'd like to come out with a blanket statement that as long as the transformer is less than XkVA and less than X feet from the panel and fed from X fuse, the incident energy is less than 1.2 cal. I know IEEE has the exception stating less than 125kVA, etc., but it sounds like that may be changing and we're just trying to stay ahead of the game.
My question is this... what should I model as the source for these sensitivity studies? I don't think I want to use an "infinite" source as that could skew the results too much one way or another. My plan would be to use one source for different kVA lighting transformers (480V to 208/120V) along with different feeder lengths and see how it affects the arc flash on the panel.
Has any one else ran these sensitivity type studies to develop rules of thumb? Any big "watch outs" I should be aware of?


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Jim Phillips (brainfiller)

Post subject: Re: "Rule of Thumb" Studies Posted: Wed Jun 21, 2017 12:01 pm 

Plasma Level 

Joined: Mon Sep 17, 2007 5:00 pm Posts: 1501 Location: Scottsdale, Arizona

Up near the service when specific short circuit data is unknown, I have advocated an iterative approach. Begin with the transformer secondary current with an infinite source and see what PPE levels you have. Then reduce the current by 5 or 10 percent increments and rerun the study. The incident energy should go down but at some value of current it will go up – due to the current dropping below a protective device’s instantaneous. For small transformers, this approach may be difficult because the big issue is the clearing time typically defined by the primary protective device. The clearing time can be quite large leading to an incident energy calculation that is also quite large.
The 125 kVA “Exception” was for this reason. It assumed an arc flash would not sustain so a low fault current combined with a minimal duration of a few cycles at most could be ignored as part of the study. Using a “typical” statement works for short circuit studies with small transformers, It would be a bit more difficult for arc flash studies.


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PaulEngr

Post subject: Re: "Rule of Thumb" Studies Posted: Sat Jun 24, 2017 8:49 am 

Plasma Level 

Joined: Tue Oct 26, 2010 9:08 am Posts: 2174 Location: North Carolina

First, did you consider whether or not you can even have an arc flash? Granted we have the IEEE 1584 "exception" which implies <1.2 cal/cm2 but we also have the IEEE C2 "exception" which essentially sets a floor of 4 cal/cm2 for 250 V or less circuits. Since 240/120 circuits are NOT excepted under the IEEE 1584 exception, this one is a little more relevant. A second caveat is that neither one is any kind of absolute exception...laboratory data exists that indicates that these "rules" may not be iron clad but so far no documented "violations" exist in the real world that I'm aware of.
The neat thing about this approach is that while IEEE 1584 has an upper limit on the size of the transformer, IEEE C2 does not have such a limit. Although there are two table entries, the upshot is that if everyone is dressed in "PPE Level 1" (FR shirts and pants, and industrial "standard" PPE), typical of what they already are required to wear at most oil and gas, coal mining, iron & steel, and other similar "heavy" industries, there is no need to do further analysis. You will get push back from electricians but I can state positively from experience working in more than one of these industries that the PPE requirement is "survivable" even in the humid Southeastern United States where I live. Recommend that if you do take this approach and you do need to switch PPE, do it in the fall/winter. That way the transition (acclimation) is not as drastic for those who might be used to wearing say shorts and Tshirts as standard workwear.
There was an ESW paper on this exact subject I think published last year.
Basically we first have to consider the protective device's characteristics compared to the incident energy. When the device is in a "definite time" region, and this includes socalled "instantaneous" tripping regions, the incident energy decreases as current decreases. That's what we would expect to happen. This also occurs if the current is so low that it's below the device's tripping characteristics or if trip times get so excessive that the "2 second rule" applies.
The tricky part is in the inverse time (curved) regions of operation. In this region it kind of depends on the slope of the curve. Incident energy can go down with decreases in current, stay the same, or actually increase. In practice it almost always increases.
This provides two different directions that you can look at things. The first one is that as long as we also account for a certain amount of loadside transient sources (motors, cables, transformers contributing fault current), we can determine an upper limit on incident energy WITHOUT looking at anything other than the overcurrent protective device. This might be jumping to conclusions but I'll bet you can model the largest motor transient fault current that you could possibly have with a given device and get a pretty good idea of what this maximum value is. Given this type of analysis, you could literally just have a table of devices and maximum incident energy possible.
The second approach is that if you do a very simply calculation including transformer impedance fed from an infinite bus, and again the biggest inductive source possible, you can determine a maximum available short circuit current. This gives a starting point on a given protective device's curve and knowing this, the incident energy MUST be less than the incident energy determined at this point and all lower points. There might be more entries to the "table" but again you could derive tables for all your protective devices that given a very simple napkinmath approach (ANSI based short circuit calculation) can determine maximum possible incident energy.
The ESW paper is really good and the only criticisms I have against it are: (1) It doesn't have a table to save the rest of us some time (2) It doesn't include the effects of transients.
I believe that this idea in some form (I've looked at it closely and it appears to be there but is tough to find explicitly) is also inherent in the "Flash Tables" approach, which is a little more commercialized.
There is also a "boundary method" out there in a couple IEEE publications that is similar but only indirectly so. In this method the idea is to plot incident energy curves onto a timecurrent curve. If you fix the system voltage and a couple other parameters, you can easily plot a constant curve for say 1.2 cal/cm2 or 4 cal/cm2 and then by comparing to circuit breakers and/or fuses, you can select devices more or less "directly" to target specific incident energy results. So it is essentially the inverse of what you are looking to do. I know that the online software package "coordinaide" on S&C's web site specifically allows you to plot a constant incident energy curve on top of a TCC for some common distribution equipment. I think I've seen this capability before in other power system analysis software but I don't remember all the details.


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wbd

Post subject: Re: "Rule of Thumb" Studies Posted: Sun Jun 25, 2017 3:33 am 

Plasma Level 

Joined: Wed May 07, 2008 5:00 pm Posts: 822 Location: Rutland, VT

PaulEngr  can you post the title and other info on the ESW paper that you mention so others can read it? Thanks
_________________ Barry Donovan, P.E. www.workplacesafetysolutions.com


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JeffBlichmann

Post subject: Re: "Rule of Thumb" Studies Posted: Mon Jun 26, 2017 7:18 am 

Joined: Tue Apr 17, 2012 9:17 am Posts: 10

I would be interested in that paper as well.
I'm pretty comfortable approaching this from a highest fault current available standpoint with an infinite source and all that. My concern is the wide variety of situations we have around here. Our lighting panel transformers are fed from fused disconnects in MCCs and the MCC could be anywhere from 50 feet worth of cable from the source to a thousand (literally). Then you take all that variety and add in the variety of lighting panel transformer sizes, secondary conductor lengths, etc. If you look at short circuit available at the MCC, we have everywhere from 50kA to 10kA. I just seem to have a lot of variables and I need to find the best way to narrow these down while still covering all situations. There comes a point where you study so many "whatifs" that you may as well have just studied every lighting panel individually, and I definitely don't want to end up there.


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engrick

Post subject: Re: "Rule of Thumb" Studies Posted: Mon Jun 26, 2017 8:36 am 

Joined: Fri Apr 15, 2011 7:43 am Posts: 177 Location: Colorado


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PaulEngr

Post subject: Re: "Rule of Thumb" Studies Posted: Mon Jun 26, 2017 9:28 pm 

Plasma Level 

Joined: Tue Oct 26, 2010 9:08 am Posts: 2174 Location: North Carolina

That's the paper I mentioned. The idea has been around but that paper is the easiest to follow.
Effectively it just "inverts" the empirical equations. If voltage, enclosure, and gap are fixed, and we fold the TCC in, the only remaining independent variable is current. You can find formulas for ANSI inverse time curves in the SEL manuals among other places.


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PaulEngr

Post subject: Re: "Rule of Thumb" Studies Posted: Sat Jul 01, 2017 8:17 am 

Plasma Level 

Joined: Tue Oct 26, 2010 9:08 am Posts: 2174 Location: North Carolina

Without getting to far into copyright violations I'll show you a quick excerpt. The first chart attached shows the idea of what the conference article explains in more detail. The dotted line is the conventional "TCC curve" (timecurrent curve). It's an extremely inverse curve with an "instantaneous" trip setting. But superimposed on that it shows the incident energy calculation for every current as the solid line and the "2 second cutoff" as a dashed line.
So if we focus specifically on the incident energy which is the solid line and starting from the left, we see that incident energy is rising as current rises. This should come as no surprise. It keeps rising until we get to the point where the 2 second horizontal line intersects the extremely inverse time curve.
At two second cutoff point the breaker is now tripping faster than 2 seconds. This is where we need to really pay attention though because this curve is typical of what actually happens more often than not. Although current is rising, the corresponding speed of the breaker is increasing faster than the current increase so the NET result is that incident energy actually decreases even though the fault current is rising. Look closely because this result is totally counterintuitive but is exactly what happens with most breakers and fuses.
Finally once we reach the point where the instantaneous trip point is reached, incident energy takes a dramatic dive down (due to a 50 millisecond trip time) and then slowly rises again as we would expect it to (current increasing but trip time remains constant).
There are several implications here to be away of... 1. Lowered fault current in the inverse time region results in INCREASED incident energy rather than decreased incident energy like we would expect to happen. 2. No matter what the fault current is, incident energy will NEVER exceed about 8 cal/cm2 with this set of breaker settings because for any given fault current, that is the maximum incident energy. Out another way, we can actually predict maximum incident energy WITHOUT ever doing any kind of power system analysis at all, only an analysis of the TCC curve. Granted actual incident energy might indeed be lower. 3. Another implication of this technique is that for instance we can quickly see what would happen if the arcing fault current estimate isn't good. If for instance the arcing fault current was overestimated especially near the intantaneous trip point we might estimate incident energy at well under 1.2 cal/cm2. But if the actual fault current is a little lower, suddenly it's in the range of 68 cal/cm2. This should be obvious to anyone that has looked at arc flash mitigation...often small changes to the instantaneous setting of a breaker that have zero impact on "nuisance trips" dramatically lower the incident energy. 4. Another implication is that inexperienced end users frequently try to push doing arc flash analysis onto the vendors. Vendors push back because that's a site issue since they have data only on their equipment and not on anyone else's equipment. But this type of analysis makes it possible to look at incident energy if there is a main breaker/fuse everywhere except at the main breaker/fuse cabinet itself. 4. Do not assume that the curve is always shaped this way. That's what I typically see but there are some cases where it doesn't work. The "second curve" file shows one of those cases. This is definitely a bit of a contrived example of what can happen but in this case the "2 second cutoff" has been increased to 10 seconds and the a moderately inverse curve. In this case the lower and upper segments of the curve are the same as before but instead of a decreased incident energy we get a parabola where only the transition points are peaks. In this case if the arcing fault current and 85% of the current are predicted on either side of one of the transition points, the value in between could result in an increased incident energy. There is a lot more detail than this in the full article, showing cases where incident energy increase all the way except at instantaneous tripping like one would expect to happen, a couple cases with a "concave down" parabola, and several more. Really neat stuff.
At this point I have to say that honestly almost all plants I've run into use either the standard inverse or extremely inverse curves. I just don't see any other curves used in most cases, so a lot of the really strange curves like parabolas never show up.
The only other concern that I have is that this type of analysis does not consider for instance fault current flowing from inductors to the fault after a trip. So it might be possible to have somewhat higher incident energy values if the transient time exceeds the breaker trip time by a significant amount. If this is not the case then this method can indeed predict maximum incident energy without doing a full power system softwae study.
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SparkyFTC

Post subject: Re: "Rule of Thumb" Studies Posted: Tue Oct 03, 2017 2:57 pm 

Joined: Tue Jul 18, 2017 7:26 am Posts: 1

There has been a slightly updated version of this article published here: http://ieeexplore.ieee.org/document/8052570/A couple of graphs were added, as well as some discussion based on feedback. Also, I believe that it will be published in the January/February 2018 issue of IEEE Transactions on Industry Applications.


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