Does anybody know the logic in the NFPA and IEEE using the transformer rating in kVA to allow 240V or lower panels to be automatically assigned category 0? Since the arc fault energy is a function of current and time, wouldn't it make more sense to use a minimum arc fault current value?

The use of KVA appears to give misleading results. Using ETAP to study a distribution system I get a 208V panel fed by a 75 kVA transformer with 2.51kA of arc fault current rated as category 0 if the clearing time is ignored (transformer less than 125 kVA). If you use the breaker clearing time you get category 3 for the panel.

A similar panel fed by a 150 kVA transformer has an arc fault current of 2.56kA and is rated category 3 since the transformer is larger than 125 kVA and the exclusion does not apply.

I understand the logic of excluding small panels at 240V or less due to the difficulty of maintaining an arc, but it doesn't seem right to rate one panel as category 3 and another as category 0 when the arc fault currents are virtually identical. Any suggestions?

Tests show that low voltage arcs with a high impedance source (small transformer) are not sustained for a significant length of time.

Thanks for the reply.

I understand the logic behind excluding small transformers 240V and lower. I'm questioning why the exclusion is based on the transformer KVA rating and not the arc fault current. In the example I gave, a 125 kVA transformer is rated as category 0 using the exclusion, yet a 150 kVA transformer with a lower arc flash current is rated category 3 because of the breaker clearing time. If you use the breaker clearing time to evaluate the 125 kVA transformer, it is also rated category 3.

I'm guessing that 125 kVA was used as a cut off to make it easier on field personnel to evaluate unlabeled equipment. Using an arc fault current value as a cut off would require a short circuit study which is time consuming.

I believe the IEEE language is under 240 V.

We have similar issues with transformers above the 125kVA cut off. We’ve been studying numerous 3-phase 208V switchgear and distribution panel boards which are supplied by transformers ranging from 150kVA – 300kVA. The transformers supply switchgear and power circuit breakers prior to supplying the individual distribution panels (panels are main lugs only). In most cases the calculated, lower, arcing current is below the instantaneous pickup on the breaker trip unit therefore we hit the 2 sec limit in most cases. Protection adjustments are not possible in most cases without retrofitting the breakers or compromising down stream coordination. Everything we have seen (and those we have talked with) indicates 208V arcs are difficult to maintain for more than a few cycles except for very close bus gaps. Using the 2 sec evaluation time typically indicates the need for AF PPE larger than HRC 2. Our opinion is this is extremely over conservative.

Is everyone conducting calculations at the 208V level for sources above 125kVA? We have talked with others who have not and have no plans to do so at this voltage level. Has anyone been using a different kVA cutoff above 125kVA? If so, what was your justification? How about a shorter evaluation time limit i.e. something less than 2 seconds based on test indications? One idea, we're considering, is a blanket requirement of [u]HRC 2 AF PPE for all 208V panel board work [/u]and forgoing studies at this level.

If you search the moderator has spoke about NFPA 70's reported error in the language regarding exclusion of 240v. And the possibility of NFPA issuing a clarification.

I understand the arc flash test done by a Pacific Gas and Electric could not get the arc to maintain and self extinguished in 2007.

Again the way I understand, testing was never established for the voltages under ~ 460v. Therefore software calculations will not calculate the voltages below ~ 460v to any accuracy based on specific test results. At least by the governing authorities. Again I have heard IEEE is doing more testing to get the proper calculations but I would not think it will happen for a few more years.

I draw the one-line put in the data and when the software gets modified for 240v 3-phase or single phase then I will be ready.

Now, I have heard Hugh Hoagland state at LGE they have established sustained arc's at lower voltages. Hugh could explain that.

Myself, if the system is 35KA or less SC current and meets the criteria I change the bus in my software to either exclude or set the cal's by hand to a higher value.

I believe there is no wrong or right answer. Until NFPA and IEEE testing has completed.
Make a plan and stick with it. If your program says in this situation the devices will be a HRC1 put a CAT1 PPE on the label or a 2.0 cal/cm2.

The plan, worker training and documentation will guide the way through this dilemma for now.

Even if the label does say its a CAT 0 panel that does not stop you from wearing a face shield and hard hat if the worker wishes. The label is only a guide not a free pass to work on hot.

This is my understanding only, not any facts. You will find a lot on this subject by doing a search inside this forum. Our facilitator and others may be best at describing this.

I believe IEEE 1584 tests included down to 208 V tests, up to 15 kV (which is why those are the usage limits quoted on Annex D).



Not even talking about EWP...

Vincent, you are correct, I was in error on the calculations. I plan to research this much more. Sorry for the confusion.
I believe that I was stuck in thinking single phase.

IEEE 1584 presents two formulas for calculating arc fault currents, one for use with 0.208-1 kV systems, and the other for systems between 1 and 15 kV.

For systems between 0.208 and 1 kV:

lg Ia = K + 0.662(lg Ibf) + 0.0966(V) + 0.000526(G) + 0.5588(V)(lg Ibf) - 0.00304(G)(lg Ibf)

For systems between 1 and 15 kV:

lg Ia = 0.00402 + 0.983(lg Ibf)

where Ia = arc fault current in kA; K = -0.153 for open-air arcs and -0.097 for enclosed arcs; Ibf = 3-phase bolted fault current in kA; V = voltage in kV; G = conductor gap in mm

The logic is based on the "typical" short circuit current being low downstream of a transformer rated 125 kVA and lower. More than likely this would be a 112.5 kVA transformer. The problem that has surfaced now that it has been a few years since 1584 came out is, what kind of fault currents are we talking about. What if you have a 150 kVA or 175 kVA transformer that has a higher impedance and gives a fault current similar to those less than 125 kVA with a slightly less impedance. There is a never ending list of "what ifs" being developed.

I think a better solution is to define an actualy lower limit current cut off. There has been talk of this and the testing is based on lower short circuit limits at 208 volts. It is difficult to sustain a lower level current at 208 V. As someone pointed out, there was some testing conducted by Mike Lang (I've used his lab before) showing the lower limit might be lower than we originally expected based on bus orientation, phase barriers and other factors that were not previously conisdered.

No answer yet, but I am pretty confident this issue will be resolved when more testing is completed.

I am hoping we can also put the 208 vs. 240V lower limit debate to rest as well. Right now IEEE says less than 240V. NFPA 70E made a mistake and listed it as 240V AND below.

Thanks Mr. Brainfiller. Your right the 208-240 and below thing has confused a lot of people. Thanks again for putting in you knowledge for us.

Your right about the current and I started to talk about that earlier. I believe I saw some engineering companies use <35K amps or less with a .5 sec clearing time as a cut off point, not sure but I did see that written somewhere. If I remember those devices were to be listed as HRC-0.

Arc fault on single phase circuits

The IEEE exemption may be fine for industrial transformers which have typical impedance's 5 to 7 %. However I came across an utility installation where a transformer feeding old services had well over 70,000 Asic. and we are encountering Utility single phase services in this area of 50,000 Asic and above.

I do not believe that the IEEE testing included little if any at 208V. At best it is a guess if the 480V will translate with any accuracy into data at 208V. To go one step further, testing at PG&E and Ferraz shows a major deviation from the IEEE 480V calculations.

Correct! There is language in IEEE 1584 about low impedance transformers that can result in very high short circuit currents for exactly this reason. The problem however is there are no specific details about what a low impedance is.

Putting this back into perspective, the IEEE 1584 Standard was developed going on 10 years ago (yes time flies). It was developed with a minimal budget and many things were learned at the time, lots of questions were answered but many things were not addressed at the time. Now that many years have passed, people are focusing on what was not answered back then. I posted some of this elsewhere in the forum but it is worth repeating here.

I got together with a few colleagues / friends a couple of weeks ago in the Ferraz lab and we blew up quite a bit of 208 and 240 V equipment with low currents. There has also been some similar research over the past year or 2. PGE has also performed some low current / low voltage testing as well. They are making many valuable contributions to all of this.

What we are all searching for is a "bottom" for the current level where an arc flash can sustain at 208 and 240 volts. It looks like the "bottom" might ultimatly be around 5,000 amps plus or minus 1000 depending on more testing and whether it is 208 or 240.

The recent testing was more of a fishing expedition to see what we could find. The sustainability depends on many factors including where the arc begins (closer to the source / near the main is impressive), orientation of the bus bars, barriers, spacing etc. Also it was three phase. I don't believe a line-ground arc flash at this level could escalate, it seems to be more of an attention getting pop.

It is important to define the bottom end as current rather than kVA so if you have long runs of conductors, you can apply the cut off as well.

The kVA cut off in IEEE paralleled what I have used for short circuit studies in the past. In SC studies it is common to cut off the calculations at a particular transformer size recognizing the fault current will not exceed 10 kA /minimum breaker rating.

We sustained some low voltage arcs that resulted in a serious amount of incident energy. The problem with LV arcs is they are more difficult to initiate but once they get going, it can be pretty bad.

All of this is not an "offical" answer to the problem but hopefully it helps everyone understand where this is all headed.

As always, stay tuned into the forum for further updates!

70,000 AIC on a 125 kVA system?



I realize this was posted some time ago, but I am trying to rationalize high claculated energy levels for small systems and I came across this thread - 125 kVA transformers. I know there is a difference in fault values between a utility transformer with a 1% impedance and a transformer with a 5% impedance, but for a 112 1/2 kVA 3 phase transformer we talking about fault currents between approximately 6 kA at 5% and 30 kA at 1%. Are we still discussing 125 kVA sytems?

Bottom Limit for Small Systems

Jim I see in your post above your thoughts on what the "bottom" might look liike in the future.

"What we are all searching for is a "bottom" for the current level where an arc flash can sustain at 208 and 240 volts. It looks like the "bottom" might ultimatly be around 5,000 amps plus or minus 1000 depending on more testing and whether it is 208 or 240. "

I believe I saw in another thread that you are involved with determining that bottom limit for IEEE 1584. I hope that in addition to the AIC for the bottom limit that any revision will also include an upper time limit less than 2 seconds for small 208 volt systems. It hardly seems to make any sense to calculate incident energy levels based on 2 seconds when the arcs can't be sustained for more than a couple of cycles.