IEEE 1584 Update

The IEEE 1584 Working Group met on Tuesday this week in Toronto, Canada. Here is a summary of the highlights.

The text to IEEE 1584 is presently under revision. The revision process has the standard broken down into different task groups. One of the biggest discussions at the meeting concerned the 125 kVA transformer exception. As many of you have read in this forum, I am chairman of the task group for this issue and it continues to evolve.

The 2012 edition of NFPA 70E has elected to remove the 125 kVA exception and defer to IEEE 1584 for guidance. At the moment IEEE 1584 has the exception where you do not have to perform calculations on circuits fed by transformers 125 kVA and smaller at 208V. This would technically begin with 112.5 kVA transformers. I have made several previous posts in the forum regarding the details of the 125 kVA exception.

Two questions need answered:
1 - what is reasonable low level cut off where calculations do not need to be performed.
2 - what do you do at the areas where you use the exception and there are no calculations.

There has been some independent testing to determine where the low end of arc current sustainability will be. I was involved with some of the testing. Now it appears we have the attention of the IEEE/NFPA collaborative research group. They indicated this might be included in their testing now. This could greatly help move us forward.

As I have indicated before, I believe it will finally evolve into a low level current instead of kVA. There will also be directions on what to do with circuits where calculations have not been performed.

The IEEE 1584.1 Standard that I co-authored is now getting ready to go through the official voting and comment stage. This means it is finally getting close to the finish line. This new standard is to help the industry play on a level field by defining the basics of what an arc flash study should include.

As many of you realize, the standards making process takes quite a long time. That was one of the reasons for creating the forum to help everyone keep their finger on the pulse of what is going on.

Stay tuned as the story continues to unfold!

>>The 2012 edition of NFPA 70E has elected to remove the 125 kVA exception and defer to IEEE 1584 for guidance. At the moment IEEE 1584 has the exception where you do not have to perform calculations on circuits fed by transformers 125 kVA and smaller at 208V. This would technically begin with 112.5 kVA transformers. I have made several previous posts in the forum regarding the details of the 125 kVA exception. <<

Jim,
I don't know if this is a good idea only because using present IEEE 1584 formulas and limiting to a 2 second (120 cycle) cutoff time, can produce some really high IE and cat 3 rating on 208v MLO panels. I've seen some really high calculated IE's on 112.5& 75kva, 480/208, 2.5%Z xfmrs.

If 125 kva becomes the cutoff point is this ignoring a potential dangerous situation?

Easypower has a pretty good paper based on testing that indicates xfmrs >125 kva are more accurately modeled using a 5 cycle cutoff limit.

Lower cutoff on 480V

Jim:

Has there been any discussion on the kA cutoff for 480V?

One extreme example, we recently modeled a steel mill with several 600AT GE AKR breakers feeding open overhead feeders of parallel 500 MCM. For the last 80 years, they have tapped (yes even hot tapped ) these feeders to everything from welding plugs to fan starters and various 30A, 3-phase disconnects on the wall at working level. By the math, these 30A disconnects are HRC 3 or 4 and we labelled them as such. The customer feels that the low fault currents and wire size could not support an arc of this magnitude. While he may have a point, there doesn't seem to be a lot of data points to quantify where this cutoff would be. I think this also gets into the question of how far down the study goes (overhead dock doors, bathroom vent fans...) We are pretty clear that 3-phase 480 gets modeled but is there an industry consensus?

I am not familiar with that paper. I know Chet wrote one but I thought it was for <125 kVA and using a 0.5 second time limit. I am not in my office so I don't have the paper handy.

Yes, you are right 0.5 sec... not 5 cycles. Typo

I handle this type of situation by treating the tap as a branch circuit and looking at the cable impedance, run distance and Isc. 30A/480v disconnects such as welding recpt typically have #8 or #10 AWG wire and are fused in the disconnect. This is also covered in the NEC feeder tap rule. In my experience the calculated Ia is very low (category 1). Its more a shock hazard than an arc flash hazard. I would have to agree with the customer to show me the numbers and reasoning to justify a cat 4 switching suit to plug in a 30A welding plug.

I am still hopeful that we will get our arms around this soon. Where the thinking is right now is ultimately the cut off will be based on short circuit current not transformer kVA. The numbers seem to be pointed somewhere between 5 kA and 10 kA for a cut off on 208V circuits.

The debates is that in order to sustain down to 5 kA (4kA for 240V) a lot of unusual events have to happen together. The arc flash begins as three phase, there is no main, the arc flash begins near the incoming lines and the bus bars terminate in barriers. It is possible - just not likely.

We have also be talking about a default Ei. value for areas where the calculations have not been performed based on the exception. That was part of the reason for this weeks "Question of the Week"

Politically what has been happening is boundaries are being set up between NFPA 70E and IEEE 1584. NFPA focuses on PPE and safety practices and is trying to get away from anything that focuses on calculations (except what is in the annex). IEEE is being asked to stay away from anything that resembles a PPE discussion. Therefore, the 125 kVA exception is being deleted from 70E and it is left up to IEEE.

When we learn more, I'll be posting it here!

btw, the EasyPower paper gives some great insight into this.

We have not done anything yet for 480V low end cut offs. Once we get the 208 / 240 volt cut off worked out, this could be something to investigate - or at least open up the discussion. My thought is this would be way down on the list however, since the thought at the moment is arc can sustain better at 480V.

does the IEEE 1584 under 125 kva transformer exception apply to all types of transformers? For example, oil filled, utility pole type, inside dry type, etc?

Yes, it applies to all types. The 125 kVA value (actually the next size in the U.S. is 112.5 kVA) is used as a cut off assuming the short circuit current will be lower and have difficulty sustaining an arc flash. The thought is that there will still be a hazard, just not as bad.

We are presently considering a revision to the 125 kVA exception. It will likely be based on a lower limit cut of short circuit current rather than a transformer kVA rating. Stay tuned!