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 Post subject: OSHA 1910.269(l)(8)(ii) Note 2
PostPosted: Thu Oct 09, 2014 6:27 am 
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OSHA 1910.269(l)(8)(ii) Note 2 contains this statement: "For example, the employer could estimate the heat energy just outside a substation feeding a radial distribution system and use that estimate for all jobs performed on that radial system.

Therefore, it seems that one could go maybe a couple pole spans outside the substation and determine the incident energy and use that for entire circuit. My concern would be that as one goes farther out on a distribution circuit the fault current lowers and the clearing time increases so that it could be possible that the incident energy is higher away from the substation.

Thoughts/opinions on using an estimate near the substation for the entire circuit?


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 Post subject: Re: OSHA 1910.269(l)(8)(ii) Note 2
PostPosted: Thu Oct 09, 2014 11:10 pm 
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I am glad you brought this up WBD. I agree with your thoughts and had the same concerns. I understand the intent but one could think of a few cases where this could create a possible problem. I guess "Use with care" might be suitable words to add. There are other areas in the new 269 that also raised an eyebrow.


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 Post subject: Re: OSHA 1910.269(l)(8)(ii) Note 2
PostPosted: Tue Oct 14, 2014 10:40 am 
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Been there, done that.

For current, it does go down a lot. I can't use the typical 12.5 kA equipment close to the main substation but go even a few hundred feet away and there's no longer an issue.

Conversely as stated, arc flash does strange things. At long distances, the result is essentially dominated by available fault current which keeps going lower. At close in distances, it is essentially dominated by arcing time. So in practice depending on the conditions at your site, you could see arc flash decresaing with distance, or increasing with distance (up to a point). When most of my portable substations (mining application) were around 6-8 miles out, arc flash was getting up around 12 cal/cm^2 or so. Now that they are close in (2-3 miles), it's gone down to under 8 cal/cm^2. Similarly the ones that are way out (10 miles) are also under 8 cal/cm^2.

Try it with a simple model using say the IEEE 1584 spreadsheet and realistic values at 13.5 kV where doing so would be valid.


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 Post subject: Re: OSHA 1910.269(l)(8)(ii) Note 2
PostPosted: Tue Oct 14, 2014 4:21 pm 
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I've done similar analysis and also found that changes of a few miles have a significant effect at 15 kV and below. Conversely at 35 kV and 69 kV the line impedance has a lesser impact and for distances of a few miles the short circuit currents may be more consistent, so it may be possible to generalize results as suggested.


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 Post subject: Re: OSHA 1910.269(l)(8)(ii) Note 2
PostPosted: Mon Oct 20, 2014 11:41 am 
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If the distribution feeder has reclosing with an instantaneous overcurrent relay for the first trip, then the worst case will be at the substation as long as the instantaneous pickup is below the minimum fault current on the feeder. Fault time will be the same regardless of current. If your maintenance practice is to lockout after the first trip (defeat reclosing) if anyone is working on the line, then you don't have to consider delayed trips. Reclosing cutout switches are normally provided for this purpose.


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 Post subject: Re: OSHA 1910.269(l)(8)(ii) Note 2
PostPosted: Tue Oct 21, 2014 8:03 am 
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jghrist wrote:
If the distribution feeder has reclosing with an instantaneous overcurrent relay for the first trip, then the worst case will be at the substation as long as the instantaneous pickup is below the minimum fault current on the feeder. Fault time will be the same regardless of current. If your maintenance practice is to lockout after the first trip (defeat reclosing) if anyone is working on the line, then you don't have to consider delayed trips. Reclosing cutout switches are normally provided for this purpose.


Not true at all. As line impedance increases at a given fault (work site) location, current decreases. If you are still within the instantaneous overcurrent point then this is still true. But once the current drops down below this point then we are looking at a "reaching" problem for the relay. It may still trip on time overcurrent, negative sequence overcurrents, distance relaying, etc., depending on your particular configuration. But the upshot is that we are looking at increased time to trip. Trip curve shapes are for the most part based on equipment damage curvess and this means that even though current is decreasing, trip time is increasing and when we trade current for time on a given trip curve we end up with higher incident energies. This also applies to fuse curves for instance.


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 Post subject: Re: OSHA 1910.269(l)(8)(ii) Note 2
PostPosted: Tue Oct 21, 2014 8:29 am 
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PaulEngr wrote:

Not true at all...


Pretty harsh. jghrist did qualify his statement with pickup, so I find it perfectly true as stated. In addition it is easy enough in modern relays to use the reclosing cutout switch to bring in an extra sensitive instantaneous element into the picture.


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 Post subject: Re: OSHA 1910.269(l)(8)(ii) Note 2
PostPosted: Wed Oct 22, 2014 5:56 am 
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stevenal wrote:
PaulEngr wrote:

Not true at all...


Pretty harsh. jghrist did qualify his statement with pickup, so I find it perfectly true as stated. In addition it is easy enough in modern relays to use the reclosing cutout switch to bring in an extra sensitive instantaneous element into the picture.


Most reclosers I've seen use a fast curve, not instantaneous, as the first cycle. If you don't then it tends to nuisance trip on transients.


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 Post subject: Re: OSHA 1910.269(l)(8)(ii) Note 2
PostPosted: Thu Oct 23, 2014 8:12 am 
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PaulEngr wrote:


Most reclosers I've seen use a fast curve, not instantaneous, as the first cycle. If you don't then it tends to nuisance trip on transients.


Transients or transient faults? Never seen one trip on transients (sub cycle events), but they are supposed to trip and reclose for transient faults. If you are attempting to "fuse save" by beating the downstream fuses to the punch, then a true instantaneous is desirable to trade a feeder blink for a longer downstream outage. If attempting to "trip save", disable the instantaneous except when blocking reclose for hot line work. The fast curves are left over from the days of hydraulic reclosers, and I see little use for them any more. Decide whether you wish to save fuses or trips and proceed from there.


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 Post subject: Re: OSHA 1910.269(l)(8)(ii) Note 2
PostPosted: Thu Oct 23, 2014 9:09 am 
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stevenal wrote:
PaulEngr wrote:


Most reclosers I've seen use a fast curve, not instantaneous, as the first cycle. If you don't then it tends to nuisance trip on transients.


Transients or transient faults? Never seen one trip on transients (sub cycle events), but they are supposed to trip and reclose for transient faults. If you are attempting to "fuse save" by beating the downstream fuses to the punch, then a true instantaneous is desirable to trade a feeder blink for a longer downstream outage. If attempting to "trip save", disable the instantaneous except when blocking reclose for hot line work. The fast curves are left over from the days of hydraulic reclosers, and I see little use for them any more. Decide whether you wish to save fuses or trips and proceed from there.


Actual transients. You get multiple lightning strikes in a row spaced out by a few milliseconds. It gets cleared by the surge arrestors but in the mean time when we set our trip curves down to where they tripped in under 100 ms, we tripped on every major lightning strike (enough that the national lightning detector networks showed the strikes). Once we increased this out to 300 ms (looking at the time of an event), the problem went away.

We are "trip save" sensitive here because we're a private utility (large mine with roughly 70 miles of power lines and around 50-60 MW of cogen). In my particular case especially settings are tricky. I have a 2000 A low resistance ground, with a line system design capacity of around 600-650 A. With a nominal setting of around 40% of the resistor to catch ground fault events, or roughly 800 A, using instantaneous would be suicidal in terms of relay engineering. I have to allow a certain amount of delay anyways just to withstand large current events on the line from say a downstream fault to allow it to clear (trip saving, not fuse saving). It's something of a delicate balance.


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 Post subject: Re: OSHA 1910.269(l)(8)(ii) Note 2
PostPosted: Fri Nov 21, 2014 1:11 pm 

Joined: Wed Jun 24, 2009 4:45 pm
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Location: WA State
Regarding the original questions, there is some discussion on this topic of reduced fault currents within the Federal Register, http://www.gpo.gov/fdsys/pkg/FR-2014-04-11/pdf/2013-29579.pdf on page 20478, starting in the first full paragraph of the middle column.

The discussion starts with,
"OSHA recognizes that fault current
lower than the maximum available fault
current can produce a higher incident
energy."

and ends with,
"Considering the evidence in the record
as a whole, the Agency believes that
using maximum fault current in
estimating incident energy will produce
reasonable estimates of the exposures
faced by employees."


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 Post subject: Re: OSHA 1910.269(l)(8)(ii) Note 2
PostPosted: Mon Apr 13, 2015 9:19 am 
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I have been looking at some studies done recently on a number of utility overhead distribution systems and one underground distribution systems to see how the incident energy changes on the system.

On an overhead distribution system, open air, the incident energy will decrease a one gets farther away from the substation. The relay used on this system is a SEL-351A and the incident energy starts out at 5.40 cal/cm2 (3 phase) at the feeder breaker. This is an enclosed 13.8kV switchgear so SLG value not used. At the riser pole, 140 feet from breaker, the IE for SLG is 2.66 cal/cm2. At the end of the circuit, about 2.75 miles from riser pole, the IE is 0.17 cal/cm2. All IE values are at a working distance of 15 inches as this utility gloves 13.8kV. ArcPro was used to calculate the open air, incident energy.

The 13.8kV underground distribution system is another story. The IE was calculated at the padmount switches at various points in the system. At the breaker load terminals, the 3 phase IE is 5.4 cal/cm2 at 15 inches. At the end of the circuit, ~1.6 miles from the breaker the 3 phase IE is 8.4 cal/cm2, close to a 50% increase. A SEL-351A relay was also used on this circuit. EasyPower was used to calculate the IE and enclosed equipment type was used.

In this very limited review, it appears that the OSHA statement about using the incident energy in the substation for the entire circuit is valid for open air, overhead distribution circuits but is not valid for underground distribution systems with enclosed equipment such as padmount switches.

I would be interested in seeing what other people have found in any studies.

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 Post subject: Re: OSHA 1910.269(l)(8)(ii) Note 2
PostPosted: Tue Apr 14, 2015 8:12 am 
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Nit pick: OSHA (and IEEE 1584) use a working distance of 18". If you look at ergonomic studies, 15" is roughly the normal range from where the hands are working to the belly. But IEEE 1584 and similar studies are concerned specifically with the onset of a second degree burn at the face/chest area where burns are most likely to be fatal or much more serious in any event (not that a second degree burn on the arm is not "serious"). The geometry works out to a distance of 18" at that location. That is why the standard set in IEEE 1584 and other places uses a working distance of 18", not 15". You could argue for burns to the belly but why stop there? You could make the same argument for the hands where incident energy is much higher due to closer proximity.

Second, I'm running 23 kV. Using Lee (yeah, invalid but I haven't gotten an ArcPro license yet) at the main substation I'm at around 50 cal/cm^2. Once I get to the 3rd pole I'm down to around 25 cal/cm^2. As I move about 2 miles away it decreases down to around 1-2 cal/cm^2. But then as I move further away to the farthest point in the system (about 8 miles) it increases up to around 10-15 cal/cm^2.. In another area where I go out to about 12-15 miles away, it decreases back down again to under 4 cal/cm^2. These are Lee results so the real world is probably about 1/3rd of that. I also have a large number of motors providing additional fault contribution so this may matter more for my local case.


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 Post subject: Re: OSHA 1910.269(l)(8)(ii) Note 2
PostPosted: Tue Apr 14, 2015 11:19 am 
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PaulEngr wrote:
Nit pick: OSHA (and IEEE 1584) use a working distance of 18".


Not for OSHA. For rubber glove work OSHA cites 15 inches for single conductor in air. Even though this is a cable underground system, it is at a voltage that this utility gloves, so I chose to use the OSHA value for rubber gloving. It might be a little conservative but that's ok.

The point I was making is that I don't subscribe to the OSHA statement about using the incident energy value in the substation for out on the circuits. It looks like you have seen the same. It will interesting to see how your results now stack up against an ArcPro analysis.

As an academic exercise, if you want to send me a couple locations with the fault current, clearing time, LG voltage, and arc gap, I can run it in ArcPro to compare.

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 Post subject: Re: OSHA 1910.269(l)(8)(ii) Note 2
PostPosted: Wed Apr 15, 2015 8:27 am 
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I'll do one better. The 5 year update is going to wrap up in a couple months. Trying to disassemble the model though into a simplified system is not easy.

Three things we're trying to put to bed though:
1. The last time we did this (2009/2010), we had to decide what do do with >15 kV. Obviously Lee results are invalid. We looked at Duke, ArcPro, and we had a corporate rule to take ArcPro results (with or without multiplier as per the instruction manual? Not clear) and just multiply by 2, with no justification whatsoever. We also considered NESC tables and NFPA 70E tables. Without much to go on we arbitrarily picked the 70E tables knowing that it might not be right but at least we're following a standard and not striking out on our own. Now with 2015 edition, the cutoff is 15 kV. And we have OSHA recommending ArcPro. I suggested NESC but it has a note in the tables that says its for outdoor/overhead gear only and we've got indoor/enclosed gear. So we have to consider this one head on.
2. The question of "normal work". Independent of 70E and revised 269 rules there is a corporate policy which effectively creates a "normal work" rule before the revisions came out and I've been pushing for it for a while. The rouble is we have some gear that's in questionable shape and I need to address it, so the field survey is going to try to delineate what gear is in poor shape. Everything else should be subject to the "normal work" rule.
3. What do do with <250 V.


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