It is currently Fri Aug 19, 2022 7:02 pm

Author Message

 Post subject: Arc Gap Assumptions in 1584Posted: Wed Jun 11, 2008 11:40 am

Joined: Wed Jun 11, 2008 11:15 am
Posts: 3
I am an engineer for a small Midwestern consulting firm that provides services to municipal, cooperative, and investor-owned utilities. We have been receiving numerous requests for arc flash studies from our clients and are determining how we want to approach these studies. I have been given the task of analyzing this approach.

I have numerous questions, but have one specific to IEEE 1584. I was wondering if anyone knew how IEEE chose the arc gaps shown in Table 4 of the standard?

Top

 Post subject: Posted: Wed Jun 11, 2008 3:51 pm
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1664
Location: Scottsdale, Arizona
The gaps are based on NEMA phase conductor spacing requirements. If the equipment in question has different spacing than the gaps in IEEE 1584, then those gaps should also be examined. In general the arc voltage is somewhat proportional to the arc gap length but the arc resistance that results is not linear although it does increase with length. This can have a big impact on the arcing current and incident energy.

_________________
Jim Phillips, P.E.
Brainfiller.com

Top

 Post subject: Posted: Wed Aug 27, 2008 9:12 am

Joined: Mon Nov 05, 2007 10:10 am
Posts: 48
Location: Mid-West
Gear Considerations

Another item to watch is the rating of the Gear if you are using the standard â€œgapsâ€ for example we had a system 4.8Kv, that utilized 15Kv gear there is a difference of approx. 40mm in the gap from 4.8Kv to 15Kv gear so make sure even if your system voltage is 4.8Kv or whatever it may be the gap is defined by the voltage rating of the equipment.

Top

 Post subject: Posted: Wed Sep 24, 2008 2:21 pm

Joined: Mon Sep 22, 2008 1:43 pm
Posts: 3
Typical Bus Gaps in 1584 less than C37.32 minimum Clearances

What should be used as a gap in Arc-Flash calculations. The way both IEEE and NESC have applied gaps, they appear to not meet the minimum spacing requirments as defined in C37.32-2002, Table 5. In otherwords, you cannot build to the gaps that have been utilized in the NESC tables or using "typical gaps" as identified in IEEE 1584. What am I missing?

How are these standards determinining the arc gap? If you cannot build to these gaps, then how can the standards imply or state that they are typical? Since I am constructing to larger gaps, when does the gap exceed abilities of Arc-Pro or 1584 to give reliable results?

Top

 Post subject: Posted: Wed Sep 24, 2008 2:23 pm

Joined: Mon Sep 22, 2008 1:43 pm
Posts: 3
Typical Bus Gaps in 1584 less than C37.32 minimum Clearances

What should be used as a gap in Arc-Flash calculations? The way both IEEE and NESC have applied gaps, they appear to not meet the minimum spacing requirements as defined in C37.32-2002, Table 5. In other words, you cannot build to the gaps that have been utilized in the NESC tables or using "typical gaps" as identified in IEEE 1584. What am I missing?

How are these standards determining the arc gap? If you cannot build to these gaps, then how can the standards imply or state that they are typical? Since I am constructing to larger gaps, when does the gap exceed abilities of Arc-Pro or 1584 to give reliable results?

Top

 Post subject: Posted: Wed Sep 24, 2008 5:20 pm
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
If you are intending to do an arc flash analysis on equipment that is owned, operated, and under the control of an electric utility you should be looking primarily at the 2007 NESC Subpart 4 for your arc flash mitigation guidelines.

We are using ArcPro for our high voltage calcs since it appears to be what was used to create the NESC tables.

Alan

Top

 Post subject: Posted: Thu Sep 25, 2008 7:25 am
 Arc Level

Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
How does NESC arrive at the gaps used in their tables? For 1-15 kV, the NESC uses a 2" gap. The tables are for phase-to-ground faults. The minimum clearance to support arms for 0-8.7 kV is 3" (Table 235-6).

Top

 Post subject: Posted: Thu Sep 25, 2008 7:46 am
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Arc Gaps

jg

Would be nice to know.....they use a simple 2,4 and 6 inches through the 3 basic distribution voltage classes. Could have come from the ArcPro folks? Maybe Jim has some insight. I do think the arc gap is referencing the gap to ground in order to calculate the arc voltage/impedance once the insulation medium has been bridged to create a phase to ground arc. Talking with some on the comittee it is clear that some of the other values, and maybe the arc gap as well, was pulled from.......well let's just say that they are not based on hard and fast calcs or data.

Alan

Top

 Post subject: Posted: Thu Sep 25, 2008 8:26 am

Joined: Mon Sep 22, 2008 1:43 pm
Posts: 3
What is the validity of the NESC subpart 4 tables if no utility can design to the clearances/gaps in these tables? Also, for switchgear the only reasonable available method is IEEE 1584.

When moving into the gray area of 25kV and 35kV switchgear, what should be used as a gap to be consistent with how standards are defacto defining the gap. At best I see that it has been defined as being the phase-to-ground voltage of the circuit divided by 10, with this being based on the strength of air being 10kV per inch. Does this mean I exclude the remaining portion of the gap that is present for lightning and switching surges? Do the standards have the gap wrong in the tables, or is there a new definition for gap?

So when you are using Arc Pro do you match to the Arc Gaps in NESC, do you use minimum phase to ground clearances, company's recommended phase-ground clearance, or base the gap on typical insulator height.

Top

 Post subject: Posted: Thu Sep 25, 2008 11:37 am
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Arc Gaps

Jim discussed briefly the origin of the 2, 4, and 6" gaps today in his webinar. Referred to some people on the west coast so I am wondering if it came from PG&E? Anyway it is in the table. Keep in mind my previous post that we are talking about single phase arcs, not phase to phase as in the NFPA stuff. The gap has nothing to do with phase to ground or phase to phase design spacings, but rather what gap to use for ie calcs once a worker has bridged the insulation phase to ground and created an arc since obviously we are not using bolted faults for this.

Still trying to figure out what to do with gear, padmounts, transclosures, self contained meter bases, etc. My thoughts are that we will either be forced to use the tables for <1000 volts with which I strongly disagree, or use some parts of 70E for that portion of the systems and do the calcs.

And yes, for the calc portion, we use the arc gap lengths and working distances from the tables as we have no better info to draw from. As for the above 35 kv calcs, just glad I do not have to do them!

Maybe someone else here has and can offer some info?

Thanks,
Alan

Top

 Post subject: Posted: Thu Sep 25, 2008 1:41 pm

Joined: Mon Sep 22, 2008 1:43 pm
Posts: 3
Thanks for the input on the NESC gaps. I still do not see any clear definition on how IEEE 1584 defines the gap. From what I have read it seems to suggest it is the spacing between bus/conductors. However, the maximum allowable gap at 15kV is only 6" and minimum allowable ph-ph spacing is 1'.

This all comes back to developing an approach for 25kV and 35kV switchgear. In the absence of any reasonable method, using the IEEE 1584 calculations for 25kV and 35kV switchgear would be an option. So what do you use for arc gaps at these voltages?

Top

 Post subject: Posted: Thu Sep 25, 2008 2:12 pm
 Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
I completely understand your frustration as well as mine and others. For a realistic approach, once you assume the arc develops into a three phase event, I don't think it matters that it started to ground or what the gap was or is. Once it becomes a three phase event, then we have to use phase spacings. After all, that will be the minimum arc gap at that point. And yes the spacings do not seem to be realistic and that is a source of confusion for me. There in lies the part of the problem I was referring to earlier with the overhead high voltage being fairly straight forward, but where do we go for all of the other equipment and voltages.

70E does not apply and 1584 needs work, but I am using them as a guide to the extent they fit.

Alan

Top

 Post subject: Posted: Fri Sep 26, 2008 6:24 am
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1664
Location: Scottsdale, Arizona
You are correct about the 2, 4, 6 inch gaps, it was PG&E. I know the person that set this up but I never asked what went behind the choice. I'll see if I can find out. Also as far as secondaries <1000V this is giving may people hearburn. Exception 1 in the NESC says use minimum 4 cal clothing. What if there is not any immediate upstream protection like a case I know of where a person was working at 120/208 on a padmount WITH PPE and was pretty badly burned.

My PGE contact also just recently performed a series of arc flash tests on meter bases. They found even without upstream protection, the conductors inside vaporize so quickly it is not nearly as bad as the calculations might indicate. I'll see if I can post their initial results here.

It seems we are going through the 70E thing all over again. Remember around 2004 / 2005 people were thinking how do we handle all of this. Over a few years it began to shake out. Now we get to do it with the NESC.

_________________
Jim Phillips, P.E.
Brainfiller.com

Top

 Display posts from previous: All posts1 day7 days2 weeks1 month3 months6 months1 year Sort by AuthorPost timeSubject AscendingDescending
 Page 1 of 1 [ 13 posts ]

 All times are UTC - 7 hours

 You cannot post new topics in this forumYou cannot reply to topics in this forumYou cannot edit your posts in this forumYou cannot delete your posts in this forumYou cannot post attachments in this forum

 Jump to:  Select a forum ------------------ Forum Library / Articles The Lounge    Question of the Week - What Do You Think?    Arcflashforum.com Feedback and Announcements    Off Topic Discussions    News in Electrical Safety Arc Flash and Electrical Safety    General Discussion    Electrical Safety Practices    Equipment to Reduce Arc Flash Dangers    Personal Protective Equipment (PPE) Arc Flash Studies    General Discussion    Arc Flash Labels    Software for Arc Flash Studies    System Modeling and Calculations    NEW! Electrode Configuration Library – 2018 IEEE 1584 Codes and Standards    CSA Z462 Workplace Electrical Safety    EAWR Electricity at Work Regulations, HSE - Europe    OSHA CFR Title 29    IEEE 1584 - Arc-Flash Hazard Calculations    NFPA 70 - National Electrical Code - NEC (R)    NESC - ANSI C2 - National Electrical Safety Code    NFPA 70E - Electrical Safety in the Workplace    2015 NFPA 70E Share It Here    Arc Flash Photos    Your Stories    What's Wrong Here? by Joe Tedesco
© 2022 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883