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 Post subject: Single Phase 120Vac Greater Than 125kVA Calc Requirement
PostPosted: Mon Jan 22, 2018 9:56 am 
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Hi,

We have a facility with a 167kVA, 480/120Vac, 1ph Transformer. In the past I usually get away without doing any ARC flash study (Incident Energy Analysis Method) for anything lesser than 240Vac with the use of the IEEE 1584 exception as my justification. Most of the transformers I’ve dealt with that has 120Vac Secondaries would always be lower than 125kVA, in my experience anyways.

I think that the application I have this time does not meet the IEEE 1584 exception and IEEE 1584 also indicates that the during their testing they found it difficult to substation voltages lower than 208Vac.

Is it correct to assume that anything lower than 208Vac will not sustain arc no matter what the magnitude of the short circuit current? Or does the greater than 125kVA rating warrant an incident energy analysis method?

To add to my confusion the application is not 3 phase hence using the IEEE 1584 will yield conservative results. Should I just use the NFPA 70E Table method and be done with it?

Appreciate anyone's comment in advance!


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 Post subject: Re: Single Phase 120Vac Greater Than 125kVA Calc Requirement
PostPosted: Mon Jan 22, 2018 3:07 pm 
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This is a difficult one. All there is to go on with IEEE 1584 is what is in the actual standard. Officially (Vice-Chair of IEEE 1584) that is all I can say as well. Single phase is very difficult to sustain at lower voltages like 120V. The main reason is there is only one phase arcing and the current has many zero crossings where the arc can extinguish.

With a three phase system there are 2 other phases arcing while the 3rd goes that can help sustain an arc subject to other variables. The gap distance has a lot to do with it as does the available short circuit current/arcing current, enclosure etc. There is always a hazard - even below 125 kVA/208V, the question is how large of a hazard is there? The NFPA 70E Table that you mention is a viable alternative.

The real problem with all of this is liability. No one that I know of wants to make a judgement that can not be traced back to a standard. That creates situations like yours.


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 Post subject: Re: Single Phase 120Vac Greater Than 125kVA Calc Requirement
PostPosted: Thu Jan 25, 2018 9:01 am 
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Thanks for the reply Jim! I'm with you I prefer to always spec or calculate per the standards.
Can you give some insights as to what the IEEE "exception" will be on the latest version? Are they lowering the 125kVA?


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 Post subject: Re: Single Phase 120Vac Greater Than 125kVA Calc Requirement
PostPosted: Thu Jan 25, 2018 4:20 pm 
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Elec08 wrote:
Thanks for the reply Jim! I'm with you I prefer to always spec or calculate per the standards.
Can you give some insights as to what the IEEE "exception" will be on the latest version? Are they lowering the 125kVA?


I'm really not at liberty to say much. Not that it's any kind of secret - the meetings are open to the public. It's just that it is not final and so I can't really give any definitive direction.

I can say a few things. It WILL change. We are looking at defining an actual current which provides better guidance. That way the low short circuit from a long conductor fed by a large transformer can be treated the same as a low short circuit from a small transformer.

Right now unless a specific transformer falls under the 125 kVA limit (112.5 kVA) there isn't anything to go on even if the short circuit current is low. The other thing is the current is likely to be on the low end - can't say what that will be because different numbers are getting kicked around.

The next edition did go for balloting last year and was APPROVED with many comments (including a debate about the 125 kVA language) and the comments have been getting addressed for the next revision and next balloting. There will probably be a few more revise/re-ballot (officially known as a re-circulation ballot) before it crosses the finish line.


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 Post subject: Re: Single Phase 120Vac Greater Than 125kVA Calc Requirement
PostPosted: Sun Jan 28, 2018 7:29 am 
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According to IEEE 1584, "It was difficult to sustain an arc at the lower voltages. An arc was sustained only once at 208 V in a 508 mm × 508 mm × 508 mm box. In all other tests with that box and the 305 mm × 368 mm × 191 mm box, the arc blew itself out as soon as the fuse wire vaporized”

That sounds promising, but the real question is "is there a hazard". I have to say yes. OSHA has investigated at least one fatality from a 240/120 VAC circuit. Accident #201282837 has the following narrative:
“At approximately 4:00 p.m. on August 18, 2007, Employee #1 and a coworker were removing the 120/240-volt electrical panel at a building in Doraville, GA. The two men had arrived at work at 3:00 p.m. dressed in tank tops and jean shorts and were waiting for Georgia Power to deenergize the circuit, which was scheduled for 5:00 p.m. Before Georgia Power arrived, however, they began to remove the panel by disconnecting circuit breakers and removing the panel's supports. At approximately 4:00 p.m., the coworker was on a ladder above the panel, and Employee #1 was standing on the ground in front of it when there was an arc flash and fire. The workers were transported to Grady Memorial Hospital for treatment. Employee #1 suffered severe burns over 60 percent of his body and died on October 15, 2007. The coworker's legs were badly burned and he was sent home to recover.”

So obviously there is a POSSIBILITY of a serious injury or fatality even at 120/240 VAC equipment. There's also actual laboratory test data backing up this up to show that it is plausible so this isn't just a case of invalid anecdotal evidence. It also took looking through over 15 years of OSHA incident data on electrical accidents to find this one, single case so that should tell you something about how rare this sort of an event must be. Thus asking for Jim's opinion puts him in a tough spot...he can't show all his cards, but I can!

I've accumulated a lot more research in the "Articles" section on the subject. Suffice to say somewhere in the 200-300 VAC range things change because that's where arcs go from nonsustaining to sustaining. So far quantifying this threshold..turning it into a calculation, has alluded everyone that has tried. The best we can do is to look at actual peer reviewed test data and standards that are based on that kind of data (that may or may not be publicly shared) and upper bound it based on those results. That's not a nice, neat calculation, but it is actually more simple because it really just becomes a binary decision or at worst a table. I have found no data to completely clear either the 120, 208, or 240 V values but that does not mean that nothing exists.

A significant amount of actual test data is summarized in “DC Arc Flash. The Implications of NFPA 70E-2012 on Battery Maintenance”, Cantor, Zakielarz, and Spina, Battcon 2012. Calculations were performed using both the Doan and Ammerman DC formulas and compared to actual measurements. The calculated values were less than half of actual measured values. In terms of the reported test cases:

“In the test report, air gaps of 0.5 inches at 130 volts DC and 1 and 2 inches at 260 volts were used. No other voltages were tested. At 130 volts, the arc was not sustainable for gaps above 0.5 inches. Even at 260 volts DC, arcs could not be sustained (< 100 milliseconds) for gaps of 0.5 inches if the arcing current was less than 5000 amps.”

Test results were 1.2 cal/cm2 at 455 mm at 0.8 seconds for a 130 VDC arc at 20 kA available fault current (actual current was 6200 A). All other results are normalized to 2 seconds so it is not possible to determine the actual incident energy without the Kinetrics report they are based on.

This is for 130 VDC. We have a single data point (under 1.2 cal/cm2 for 20 kA or less, and only if the arc gap is really narrow). Since we have all those zero crossings, clearly the 120 VAC case is going to be far less than what this DC case describes but this gives us an upper bound...120 VAC arc flash CANNOT exceed 1.2 cal/cm2 at 20 kA of available fault current. I don't know your transformer impedance but this gives you an upper number which is obviously going to exceed the AC result even though it's actually a DC result.

IEEE Standard C2-2012 (NESC) provides table 410-2 which gives the following ratings.

“Industry testing on [single phase and three phase panelboards, pedestals, pull boxes, hand holes, open air (includes lines), CT meters and control wiring, self-contained meters and cabinets] by two separate major utilities and a research institute has demonstrated that voltages 50 V to 250 V will not sustain arcs for more than 2 cycles, thereby limiting exposure to less than 4 cal/cm2.”

“Industry testing on 480 V equipment indicates exposures on pad-mounted transformers do not exceed 4 cal/cm2.”

“Industry testing on 208 V network protectors indicates exposures do not exceed 4 cal/cm2.”

Note that most of the results in NESC are based on data from PG&E done in independent tests or done under the umbrella of EPRI which is an industry consortium of utilities that does a lot of really great work. EPRI reports become public after 10 years. Prior to that unless you're a member the prices for the reports are crazy high (thousands) so the data for NESC is somewhat out of reach for some of the test results for another few years, so we sort of have to take it on faith of a peer reviewed standard. That's the same status as IEEE 1584 and NFPA 70E, so it has the same credibility as those standards.

So as I see it there are 3 possibilities:
1. NESC (IEEE C2) upper bounds it at 4 cal/cm2. This is kind of like the IEEE 1584 "exception" but it doesn't include the 125 kVA/208 V cutoff.
2. If the short circuit current is 20 kA or less, it can be upper bounded to 1.2 cal/cm2 based on the Battcon paper.
3. IEEE 1584-2002 empirical equation does not apply but Lee theoretical equation does. So you could use that result. Again without knowing short circuit current and other details, it's not possible to run the calculation.

Obviously these are pointers. You have to use your own engineering judgement to decide on the credibility of any of those three methods, just as it is necessary to do so using IEEE 1584. But the credibility is based on essentially the same basis of evidence so this really isn't going out on a limb.


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