TIA comments due Feb

I just learned of an NFPA TIA that has been posted on NFPAs web site to delete completely the exception to arc flash analysis for equipment fed from a single 240 volt and less transformer rated less than 125 KVA. The reasoning provided does not match my reading of IEEE 1584, which would support this exception if changed to less than 240 volt. Previous forum posts and some arc flash software support the concept of small transformers not requiring arc flash analysis. I also do not agree with TIA assertion that it is in conflict with NFPA 70E table for arc flash PPE selection for 240 volt panelboards, as table covers 240 volt equipment with up to 25 KA short circuit. Is anyone planning on commenting on this TIA?

TIA NFPA 70E Exemption

The issue is that this exemption as written doesn't make it clear that you must have and's between the clauses. Must be 240V and below AND one feed AND 125kVA transformer. The way it is written it seems to exempt most 208V systems which can pretty easily sustain an arc. Might have been better to make the TIA to have this statement agree with IEEE 1584. Daleep has good understanding of the issue and most 120V-240V systems have little arc flash potential but some do so they shouldn't be ignored.

The arc flash is a high speed video of an arc flash in 240V meterbase. They were only 2 cycles in time but they did give about 5 cal/cm2 to the face and close to 15 cal/cm2 to the hands. There wasn't enough energy to the chest in the three of these we did to ignite a shirt at the chest but it might have ignited a long sleeved HRC 0 shirt. The fault current was about 20kA.

Can't ignore 240V in all cases.

I know there is the need to clarify the difference in the two, But Hugh....if I understand correctly, the test results that we have been presented with so far seem to indicate that an arc at 208 volts is difficult to sustain and would seem to support the elimination of the low power 208 volt systems at a minimum?

I just did the calcs today for a 75 kVA, 208v bank for which the infinite source was a whopping 4800 amps.

What's up with this?

Sustaining a LV arc

Basically we have to remember the IEEE 1584 version 1-2 raised about $500,000. Only $60,000 was spent on lab time. There is VERY little data. I have see meter blasts that ignited 100% cotton 11 oz with 208V when they went 3 phase. After the arc plasma crosses 300V (which it can in a 208V system that goes to three phases) the arc can self sustain.

To NOT calculate 208V systems is a mistake I know you haven't made.

Some people are moving back to HRC 0 using the exemption as an excuse. We will have better guidance from the new IEEE 1584 because it will have more data. If the trend holds NFPA and IEEE's money will cover the overhead and most of the rest of it can go to data to build a much better model.

Have you looked at PG&E's arc data on meterbases. They had trouble sustaining too but could get 50 cal/cm2. Fault current is the culprit. Just depends on the transformer feeding the system and equipment configuration. The exemption is too broad.

Hugh,

Please explain:

A three phase fault will cause the local 208 to sag. Where do you see >300 V?

Thanks.

I would have to re-visit the PGE info to be sure, but I thought all of their tests (at least initially) were at 480 volts.

Is my memory also failing me?

I reckon so!

Happy and Safe New Year to All!

PG&E Tests

Sorry, I was trying to find all the data. This is also preliminary work. In our recent work on 240V we found short clearing times but with the 1000 FPS camera we could see the arc energy hit mostly the hands and face of the mannequin. There were no sensors there but an estimate could be made from the faceshield and glove shrinkage. It could be an issue if you are using HRC 0. I just wish we didn't have HRC0. That would save more lives than all the silly arguments about HRC 1 and 2.

This is another example of NFPA getting it wrong. NFPA70E is heavily comingled with IEEE 1584. To write a statement in direct conflict with IEEE, especially after an attempt to mimick IEEE, just makes it confusing. They should have just copied the IEEE verbatum.

Even with the technical update, the statement in NFPA that analysis can be based on various methods, still stands, and the IEEE 1584 is 'THE' most standard method. So, if you are following the analysis method than the IEEE exemption still stands.

Trying to regulate to the point that 'an arc is not sustainable', again in my opinion, is excessive. There needs to be some reasonable probability of serious injury to have mandated regulations. This is the premise of how OSHA came to being. If you only sustain an arc 1 in 50 attempts, then I don't see that as a real threat to the industrial community at large (at least no more than the worker had driving to the work site in his car).

Daleep's Response

I let Daleep Mohla know about the site but he hasn't been able to log on yet so I'm posting his reply on the TIA. Daleep is a member of NFPA 70E and a long time friend and generally reasonable fellow.

Hugh

Daleep wrote:

"I am impressed with the quality of technical viewpoints raised by submitters on the TIA to remove 70E- 2009 Section 130. 3 Exception 1. It may be helpful to know the background of this TIA.
This was a product of task group appointed by the NFPA 70E Chair with a charter to review and re-evaluate technical basis and substantiation provided in 70E-444 for the acceptance of sec 130.3 Exception 1 in the 2009 70E document. Daleep was appointed the chair of the four member task group. After a long, sometimes very energetic discussion, over emails and conference calls, and review of IEEE 1584- 2002, the task group reported their findings to the 70E Chair. Their findings and conclusion was that technical basis and substantiation for including this exception was not appropriate. The recommendation by a 3- 1 vote was that 130.3 Exception 1 be deleted on an emergency basis.

Technically, this exception is incomplete and flawed. Arc flash hazard (incident energy) depends on two variables: Short circuit current available and operating time of the upstream device to clear the fault. As all of you may know that transformer kVA by itself does not determine the available short circuit current. You also need available bolted fault current on the primary side and transformer impedance to determine fault available. Second you also need to opening time of the overcurrent device on the primary of the transformer. Highest incident energy and arc flash hazard is when the arcing fault takes place between the transformer secondary and the overcurrent device on the secondary of the transformer. The overcurrent device on the primary of the transformer needs to clear the fault. Normally this device is set to maximize the use of the transformer capacity and is set to protect the transformer. In addition you also need the cable length between the transformer and the overcurrent device. Longer secondary conductors will reduce the fault available and increase the opening time of the primary device thereby increasing the incident energy. None of these were mentioned in the exception.
IEEE 1584 was not only misquoted but all partially quoted. I believe it happened inadvertently and not intentionally. It happens when you condense a complete standard in 3 line statement. IEEE 1584 is only applicable to 3 phase systems but this exception applies to single phase and 3 phase systems. IEEE 1584-2002 testing was done with vertical electrodes in free air and in 20 inches cube boxes. Most of the equipment at site, such as panel boards have electrodes embedded in insulating type materials that tend to sustain arc at lower voltages. IEEE 1584-2002 section 9.2 clearly indicates that in smaller enclosures 208 V can cause serious burn injuries.
Users need to read the IEEE 1584 document and see if their site conditions to determine if flash hazard analysis is required. OSHA general duty clause requires Employers to provide a safe work employment to its employees. Only evaluation by competent and qualified personnel can determine the extent of the hazard. For this everyone needs to conduct a hazard risk analysis. Hazard analysis should include equipment conditions and use. One factor is if equipment has been properly maintained as per manufacturer's instruction. Using an exception and not performing the arc flash hazard analysis may be convenient but not necessarily safer.
I hope this background and information is helpful in understanding the extent of work went in the evaluation of technical basis of the 130.3 Exception 1 and technical basis for this TIA."

Daleep Mohla

Anyone know how to comment on this? I saw nothing about the process.

Calculating AF Energies (<250V and <125 kVA)

Attached is a link to a recent paper we wrote on calculating arc flash energies for systems rated <250 volts and served by transformers rated less than 125 kVA. This paper is based on the PG&E testing as well as the Ferraz Shawmut tests which is the best available data we have at this point in time. We feel this is just a stop gap until the IEEE takes the PG&E and Ferraz data as well as their own comprehensive series of tests to formulate new equations. There is more to this problem than just current and time, that is why the IEEE equations do not work well in this voltage range and you end up with energies much greater than at 480V. I believe we will need to go to an energy equation or a more detailed model (which will have its own data problems).

http://www.easypower.com/arc_flash/arc_flash_resource_center.php

Chet E. Davis, PE
chet_davis@easypower.com

Welcome my friend Chet. You could greatly add to this forums discussions. Glad your onboard.

Ditto, glad to see you here Chet.

Jim, thanks for the update.

I am starting to wonder now if this TIA puts us in the position of having to analyze 240V single phase systems. I see no reference to three phase systems in 70E, only <50V.

Are we now put in a position to have to analyze every job trailer with a 240V single phase drop?
What about every lighting panel, every receptacle?
Will it be necessary now to post labels on house panels if someone runs a business out of their home?

This seems like it's going too far. We can debate whether or not someone should wear arc flash PPE in a house panel all day. Does this TIA mean that someone now must wear arc flash PPE? And it must be labeled?

Who's going to perform the analysis? While this is great for those of us who do this for a living, and maybe just got our engineering license and looking to make a little more money on the side , it will also bring out a whole lot of scammers and arc flash witch doctors.

Me, I'm going to hook up with some contractors in my area, and quick!

240V Arc Flash Issues

This is what was seen in the original IEEE 1584 testing. Sometimes it sustains and most of the time it doesn't. Thus why I'm against cotton in 240V but not really for protection to the highest possible. When the equipment configuration allows for 480V to be involved in 3 phase configurations all bets are off unless it is arc resistant and being worked on in such a way that it's protected. High resistance grounding makes a big difference too.