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 Post subject: NESC Table 410-1 CT Meters 4 cal?
PostPosted: Thu Mar 06, 2014 9:45 am 
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I have a question regarding the NESC Low Voltage Arc Flash hazard table (Table 410-1). I work for a utility that has only 480 V and lower low voltage installations across their system. We have varying sized CT metering cabinets rated from 400 A up to over 2000 A. In conducting arc flash calculations using the IEEE 1584 formulas, IÔÇÖm finding that energies can exceed 100 cal/cm^2 is several locations. Energies are also close to 6 cal/cm^2 for other more numerous locations. We do not have network protectors or three phase panel boards over 100 A above 250 V on our system, nor do we have metal-clad switchgear or motor control centers except in very few locations. I know that the industry testing has shown that CT meters do not exceed 4 cal/cm^2, but I am somewhat hesitant about using this value as a cutoff where some locations calculate with energies near 100 cal/cm^2. Do you have a good understanding about the CT meter testing methods? We have a variety of CT meters which contain thick busbar ranging from around a foot high to 2-3 ft at the 2000 A units. I would appreciate any insight you might have as to whether the NESC tested results on CT meters can be safely applied to all CT meter encloser sizes. I've reviewed the EPRI study results that confirm the NESC table for CT meters, but the CT encloser they used looked smaller the types we typically use on our system. Thanks for your time and feedback.


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PostPosted: Thu Mar 06, 2014 9:48 am 
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400 to 600 A CT Meter


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PostPosted: Thu Mar 06, 2014 1:38 pm 
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There have been some discussions on these forum before on the utility values vs industry values on arc flash hazards but I know one centered around 208V metering.
What method/software did you use to arrive at 100 cal/cm^2 values? Curious as this may have an effect.

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PostPosted: Thu Mar 06, 2014 1:44 pm 
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I used the IEEE 1584 equations to calculate these values. It was on a transformer's secondary only protected by a primary fuse (blind spot situation). The transformer size was 1000 kVA underground padmount.


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PostPosted: Thu Mar 06, 2014 1:55 pm 
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You are looking at two totally different situations.

If you are looking at the secondary side of the transformer itself, it is very typical especially with 480 V systems to find arc flash values that are enormous on the secondary side of a transformer. Your only options to address this are either to try to lower the primary side protection as much as possible (usually bumps up against the transformer inrush), or to put in bushing CT's on the transformer and use these to trigger a breaker on the primary side. The "blind spot" effectively becomes an infinitesimally small area between the transformer housing and the CT housing.

The secondary side of the primary transformer would be for instance what you would use if you are using split core CT's and attempting to install or remove them live. This would apply for instance if you buy flexible CT's for a meter and wrap them around the bus bars manually. It is certainly possible to cause an arc flash if you are doing this with bare hands but if you are doing it with voltage rated gloves and the CT has no exposed parts, very likely there is very little likelihood of an arc flash. That's a different issue from projecting what would happen if someone were to brush up against two bus bars simultaneously with bare skin.

If you are looking at the CT leads, then this becomes actually another transformer. The arc flash value is from the CT leads themselves, which is the secondary side of the CT. The CT easily saturates which is the reason that arc flash values are very low. Thus the NESC values are indeed correct. This would be the case if the CT is already installed and you are just working with the CT leads and perhaps one of the leads breaks, leading to a significant arc due to the very high voltage available on CT's but not much of an arc flash.


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PostPosted: Thu Mar 06, 2014 2:22 pm 
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What do you believe the NESC table 410-1 "CT meters" referes to? The first time I read this I thought it only refered to meter cans with control wiring from CT's. After reading some recent EPRI testing of CT cabinets, they performed tests on the actual CT cabinet busbar and recorded that energies would be limited to 4 cal/cm^2, because fo the arc's inability to sustain. So, this leads me to believe the NESC is refering to CT cabinets when it refers to "CT meters". This was hard for me to believe, that an enclosure with such significant busbar (with assumed great AF energy capacity because of large transformers and primary protection) would be limited to just 4 cals. That is the reason for this post. I'm wanting to gather some confidence that these CT cabinets are actually limited to 4 cal. We will have folks opening these cabinets and using a tuning fork ammeter or a cable ammeter and also voltage probes, to perform meter verifications/testing.


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PostPosted: Thu Mar 06, 2014 2:49 pm 
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A couple of items:
1. What was the EPRI voltage? Most of the test reports I read were 208V CT metering
2. What bolted fault current did you use? Was it the fault current that would be available with an infinite bus or was it using the actual fault current available from the utility?
3. Did you use a 2 second cutoff time, if appropriate?

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PostPosted: Thu Mar 06, 2014 3:46 pm 
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The EPRI Study results are available at : http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000000001022002
1. The EPRI voltage was 480 V
2. Bolted fault current = 38,012.8 A. This is located at a specific location without infinite bus. The transformer impedance was 2%. The 85% value of arcing current was clearing at 2.6 seconds so I used the 2 second cutoff.
3. The 2 second cutoff was used and it is appropriate for the location.


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PostPosted: Sat Mar 08, 2014 4:42 am 
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That is a common test voltage in the 1584 data set that you get if you buy the standard. The test conditions you are using are a very low impedance transformer. The only difference is that the size of the test enclosure is supposed to approximate an MCC bucket for the "enclosed" tests but even the open air results are way above 4 cal. EPRI results definitely do not sound like they simulated a full 480 phase to phase fault.


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PostPosted: Sat Mar 08, 2014 5:42 am 
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After looking carefully at the EPRI report, the spacing in the cabinet is around 3.5 inches, and the cabinet is 36 inches tall with the CT's spaced at least 5.5 inches above the bottom. EPRI makes the argument that spacing from the bottom is important. Recent IEEE/NFPA tests also hint at this. It is clear that EPRI is looking at equipment specific cases. As long as your equipment and currents are similar, I see no reason EPRI test data could not be used as a better alternative. The only thing though that makes me a bit nervous is if you look at the analysis of arc currents vs. probability early in the report and similar data from IEEE 1584 report itself. Most of the time, arc flashes are fairly low level similar to EPRI results. Once in a while though arcing currents are much higher. You can't do anything statistically with only a handful of data points as EPRI/NESC has done. This means there is a small chance that NESC is recommending something questionable. 1584 itself only gives a statistical 95% guarantee. But if it makes you feel any better I would not be afraid to use the NESC tables if I knew my equipment met the criteria.


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PostPosted: Mon Mar 10, 2014 6:02 am 
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I believe that the term "CT meters" means meters that are connected to CT secondaries instead of directly connected to the line. It does not refer to the CTs themselves. Note that the first row of Table 410-1 is "Self-contained meters" has a IE of 20 cal/cm┬▓.


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PostPosted: Mon Mar 10, 2014 7:56 am 
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Quote:
I believe that the term "CT meters" means meters that are connected to CT secondaries instead of directly connected to the line. It does not refer to the CTs themselves. Note that the first row of Table 410-1 is "Self-contained meters" has a IE of 20 cal/cm┬▓.


I thought the same thing until I read the EPRI study. That is not the case at all. They are actualy looking at a very large CT cabinet with 480 V CT's and all that is in that cabinet is bus bars are CT's. They ran two different types of tests. In one test they used the typical "fuse wire" to initiate an arc flash. In the second type of test they clamped a set of vice grips on the outer phase and laid it so that the handle overlaps the middle phase.

In either case they ran pretty high on the available fault current (38 kA) and close distances (15" instead of 18") and the result is that it never got above 4 cal/cm^2 which is what is on the table.

There are two reasons for this discrepancy compared with IEEE 1584. First, the panel size is quite large so it is somewhere between the open air and enclosure-based tests. Second and more importantly, the arc length is quite long which leads to a very weak and unstable arc. Most of the IEEE 1584 tests were done in the region where arcs are very stable and repeatable (small enclosures, short arc gaps) such as 25 mm (1") spacing between bus bars, in a relatively small enclosure. This is more representative of panelboards and some MCC's than it is for a lot of utility gear.

This is a major reason that we can't honestly limit arc flash modeling to only consider IEEE 1584.


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PostPosted: Mon Mar 10, 2014 10:14 am 
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I would submit that we cannot use the referenced EPRI study to gage the intent of the NESC, since the study in question actually references NESC 2012, and therefore was written later. The NESC footnote 5 simply speaks of industry testing without providing a full reference, although B31 is a likely suspect. Anyone have the paper? As the NESC is written, I'll go with jghrist; the "CT meters and control wiring" is something very different from the CT enclosure. The 12 guage potential leads have dropped the IE considerably.


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PostPosted: Thu Mar 20, 2014 6:10 pm 
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Any paper over 10 years old on epri is free and for some reason this one is, too. You can download and read it yourself for free.


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PostPosted: Fri Mar 21, 2014 3:08 pm 
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I did read the one linked above, which was written after the NESC. Are you suggesting another? If so, which?
The NESC table cell in question points to footnote 5, which provides no specific reference from EPRI or anyone else, only a vague reference to industry testing.


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PostPosted: Fri Mar 21, 2014 4:30 pm 
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I agree that NESC is really vague here. Similarly they don't give a reference for the next table with the incident energy ratings above 1 kV except to vaguely reference a commercial software package. We all know it's ArcPro, but they don't come out and say it. I don't know what the standards committee rules are but there are probably very good reasons that unless something is published in a peer reviewed source, it gets the "vague reference" treatment. IEEE 1584 has similar issues where they vaguely refer to sources of arc flash data for current limiting fuses which came from a manufacturer. This is one of those times where it helps to know someone on the committee that was involved if it is really important to know the source.

However from a regulatory/legal point of view, it's much easier. What we have is a peer-reviewed consensus safety document. These things get updated from time to time and they definitely contain errors and discrepancies at times. However they represent the best information that anyone can obtain publicly.

The second source of data would be test work published in peer reviewed publications. This is also pretty good but remember that often this is outright research and until it has been experimentally verified a few times, it is still subject to issues. Can anyone remember the flap about "cold fusion" about 15 years ago before the researchers finally admitted they outright falsified the whole thing?

Finally, we have research that you do yourself or commission yourself. This certainly is the best information from an engineering point of view but has the huge problem that it lacks peer review and repeated testing by others. So it is actually the weakest of the 3 unless there are good reasons that the other 2 do not apply to a particular situation. One that immediately comes to mind is arc flash blankets.

So the argument here is whether to use guidance from one consensus safety standard or another one. Both are widely recognized. However there are some differences. NESC is industry specific. The table in question is not only industry specific but equipment specific. IEEE 1584 on the other hand is only equipment specific when it comes to low voltage fuses. If possible, one should look at the specific reasons why the standards deviate so widely. For example, the theoretical equation in IEEE 1584 (aka Lee's equation) is just that...theoretical. It does not rely on any test data whatsoever. Comparisons to actual test data show that Lee's equation always produces a higher value than actual data, and that the error is least at lower voltages and currents. The error grows dramatically as voltages increase. Thus we can conclude that IEEE 1584 is always better than Lee when confronted with generic conditions within the bounds of where IEEE 1584 is valid. This is to be expected because IEEE 1584 is an empirical (curve fit) equation.

In the case of IEEE 1584's empirical equation, it is also known that it was not tested very deeply with extremely large arc lengths because the arcs are not very stable. The recommended range is only 152 mm. However the actual testing varies by voltage. The test data at 480 V according to the table for 480 V exponents seems to only extend to 40 mm. EPRI was testing CT cabinets with arc lengths of almost 90 mm (3.5"), and finding that the arcs are extremely unstable. Also, IEEE standard tests are with a very small box relative to the size of a CT enclosure. Test results have not been published by the joint IEEE/NFPA study yet but some conclusions are leaking out. One of the very important ones is that enclosure size matters. The smaller the enclosure, the higher the incident energy. The exact relationship is not clear yet though because only the conclusion is available. Since the size of a typical CT enclosure is larger than the IEEE 1584 standard enclosure size (14x12x8), we should expect lower results.

All of these concerns lead to the overwhelming conclusion that IEEE 1584 is going to produce a higher incident energy result than equipment-specific testing. Regardless of whether the source is IEEE C2 with a very vague reference to some nebulous industry testing, or to EPRI's documented testing, the conclusion should be the same: unless there is additional test data proving otherwise, the recommendations in NESC for equipment-specific conditions should be more reliable than IEEE 1584. In terms of legal implications, NESC is ahead by a neck but it's a photo finish.

I am actually considering other parts of this as well. NESC very clearly defers to IEEE 1584 when it comes to the "sweet spot" for IEEE 1584, which is switchgear, MCC's, and similar equipment. However when equipment-specific recommendations apply such as those in IEEE C2, this is superior to IEEE 1584. It's just a matter of finding out again what the specific limitations of the testing are, which is not as easy to do with vague references to work from.


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