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 Post subject: 120V Control Panels
PostPosted: Thu Feb 20, 2014 10:45 am 
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Location: Idaho
My original assessment went up to and including Motor Control Centers (MCC's), but did not go down to the control panel level. Even though the lighting transformer that feeds the 120V to these control panels is fed from a Category 0 MCC and the control panel is not labeled because my assessment and one line did not go down to that level I have to use the tables in NFPA 70E. These tables state that I need category 1 for voltage testing. Does anyone know if I can use the main feed (again category 0) rating for relabeling these control panels without doing an assessment down to the >50V level??


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PostPosted: Thu Feb 20, 2014 12:46 pm 
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The transformer changes everything. You should not assume upstream incident energy values are applicable at downstream equipment.


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PostPosted: Fri Feb 21, 2014 10:12 am 
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Some of the worst offenders for arc flash we have in our mill are lighting panels. 70E lets you exclude lighting panels from calculation depending on the size of the transformer (there are other discussion threads on that specific topic) or you can do the assessment. You'll often find the arc flash energy higher on the secondary of the lighting transformer than on the primary.


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PostPosted: Fri Feb 21, 2014 11:39 am 
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Voltrael wrote:
70E lets you exclude lighting panels from calculation depending on the size of the transformer (there are other discussion threads on that specific topic) or you can do the assessment.


70E-2012 requires that the lighting panels be 'analyzed', they cannot just be ignored.
There are only two primary industry accepted methods of arc flash analysis: use the Task Tables or perform incident energy calculations.
In the world of calculations, IEEE1584 is by far and away the 'gold standard' however its authors felt that certain transformers were too small to be of concern and calculations would not be required.


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PostPosted: Fri Feb 21, 2014 12:33 pm 
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More to the point, at low voltages, arcs tend to self-extinguish. IEEE 1584 assumed that all arcs are self-sustaining and stable. So if you just apply the IEEE 1584 calculations to 120/208/240 V lighting panels, they may indeed show up as significant arc flash hazards.

In order to accommodate this known issue, IEEE 1584 currently indicates that circuits fed by a single 125 kVA or smaller transformer up to 208 V can be ignored and simply treated as 1.2 cal/cm^2 or less. This exception was removed in 70E-2012 for two reasons. First, more recent test data indicates that the IEEE 1584 "cutoff" is not valid. Second, this exception is specific to IEEE 1584. If using another arc flash hazard calculation method, this exception might not apply.

As to the physics...

Data from a PG&E study from about 10 years ago indicated that arcs were self-extinguishing within a couple cycles for 208 V and below. One of the initial problems with this work was the IEEE 1584 test itself. The design called for a certain size box and a #14 "fuse" wire. In low current/voltage conditions, it is hard to get the "fuse to light" and form a stable arc, which skews the results. Even with a smaller wire the results are still typically lower than what is predicted by the IEEE 1584 empirical method.

The result is that today one is presented with the fact that there is equipment that may be capable of an arc flash over 1.2 cal/cm^2 but no way to predict this due to lack of information. So one either has to follow the IEEE 1584 exception, knowing it is probably flawed, or ignore it and hope for the best.


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PostPosted: Mon Feb 24, 2014 7:43 am 
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PaulEngr wrote:
More to the point, at low voltages, arcs tend to self-extinguish...


At this time we do not have data that is statistically robust enough to publish in an IEEE document stating "below X Voltage and Y Amps of available fault current the arc will probably not sustain."

From a practical perspective, more work is done every day at small lighting panels than the larger, higher voltage, higher hazard locations where we have a better empirical understanding of the physics. For this reason a few of us on 1584 are quite passionate about getting sufficient data to define and set a good lower boundary for practitioners performing arc flash studies.

Up to this point the IEEE/NFPA Arc Flash Research Collaboration has focused on trying to create sustainable arcs in order to measure incident energy, light, pressure, etc. Once this effort is completed and the incident energy model is developed and reviewed, then additional testing can be designed to determine the lower bound of where arcs tend to self-extinguish vs. sustain.


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PostPosted: Tue Feb 25, 2014 8:16 am 
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I have seen studies where the arcing time is reducd to 0.5 seconds. This appears to be a judgement call but it gives a reasonable incident energy for the equipmnet.

Does nayone else do this or something similar?


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PostPosted: Tue Feb 25, 2014 10:40 am 
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I don't know what basis would be used for unilaterally decreasing arcing times to 0.5 seconds, but it is very easy to calculate. Incident energy is linear with time. So if I go from 2 seconds to 0.5 seconds, then I divide by 4 (2/0.5 = 4). Lighting panel breakers frequently have opening times of only 1-2 cycles (0.16-0.30 seconds) but the inherent issue is that the main is in the same panel and this is frequently the only protection on the transformer secondary. This causes the panel to be rated as if there is a dead short on the unprotected secondary side of the transformer which can only be limited by the primary side protection. This usually means that there is almost no protection which causes exceedingly high arc flash values limited only by transformer and cable impedances.

Thus unless someone purposely adds some kind of protection on the transformer secondary side or the engineering (modelling) side of the equation gets much better at predicting low voltage/current faults, we will continue to have this issue.


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PostPosted: Tue Feb 25, 2014 10:42 am 
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We're planning to add fused disconnects to the secondaries of some of our lighting transformers for this very reason.


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PostPosted: Tue Feb 25, 2014 3:56 pm 
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Voltrael wrote:
We're planning to add fused disconnects to the secondaries of some of our lighting transformers for this very reason.

Just make sure there is enough fault current to get the protective devices into their instantaneous/current limiting range. For fuses, this seems to be more of an issue above 200A. Molded case breaker are fairly easy to implement as they normally open instantaneously at about 10x handle rating.


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PostPosted: Wed Feb 26, 2014 9:15 am 
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PaulEngr wrote:
I don't know what basis would be used for unilaterally decreasing arcing times to 0.5 seconds...


Sounds like a legal quagmire to me. If an incident ever occurred and the lawyers found out that the power system analyst had used a 0.5 sec limit instead of 2 seconds, even once at another site, it calls into question whether the analyst followed the other aspects of 1584, 70E, IEEE 399, etc. for all of their other work. Standards exist for a reason - when facilities and consultants start deviating from them without a documented, peer-reviewed reason it opens them up to a lot of tough questions later.


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