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 Post subject: The 2 Second Cutoff "Option"
PostPosted: Tue Dec 02, 2008 6:30 pm 
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Location: Charlotte, NC
The 2 Second Cutoff "Option"

I have been pondering (as we are prone to do alot here in the south) over this for awhile and had an extended conversation about it today with one of our engineers. As we are all aware, there is the mention of possibly considering a 2 second cut off option for IE (IEEE 1584 - Annex B) when the worker is not in a position to be trapped.

On further thought, one might argue that if they are in a position to move from reflex, or be blown by the arc blast, then 2 seconds seems like a long time.

If I am free to move, cover up, or be blown by the concussion of the blast do you really think it will take 2 seconds?

Appears to me that if you are going to use the full 2 seconds to move, you will most likely be there for a lot longer.

I have taken a dive to the bottom of a bucket before and I do not believe it took me 2 seconds!

Might seem like splitting hairs, but the difference between say 1 and 2 seconds can be great when considering the IE.

Given that no data has been presented to date to indicate that a problem exists in the utility field, I for one am having some major issues with the calculated values with such little research having been done in this area.

Given also that IEEE 1584 doesn't do a good jod of modelling the phase spacings in a padmounted transformer since we are beyond the upper limit of the phase spacings (153mm).

Doesn't it seem reasonable to expect that at the higher levels of IE, most or all of the damage has been done with the initial blast....which will only take milliseconds?

My Thoughts....How Bout Some Others,
Alan


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PostPosted: Wed Dec 03, 2008 3:38 pm 
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Yes as most of us know by now, there are quite a few holes in all the data. Back in 2002 when 1584 was developed, it was cutting edge and answered a lot of questions that were hanging out there. Now we have had time to figure out what is missing. The list keeps growing but for how this was developed and a shoe string budget, it was as good as it gets.

As I recall from some discussions in the 1584 committee meetings, the 2 second cut off was really trying to take a stab at a cut off in case a low fault current gave you a clearing time that was way out there. Yes 2 seconds seems like an eternity (and I'm sure it is if you are engulfed in a sustained arc flash) but it was a first attempt at justifying some kind of cut off. I understand it is related to the "2 second rule" from reaction time in driving. Yes, it is probably too long for many cases but at the time 1584 was developed, there was nothing else to go on so 2 seconds was used. I have had a few small flashes in my time and yes, when your life is also flashing in front of your eyes you move quickly - much faster than 2 seconds.

There is some preliminary data that supports a shorter cut off in some cases. I just saw some test data recently for meter bases with no upstream protection (common configuration) and the conductor vaporized so fast the arc self extinguished and it was not as bad as people thought. - Of course this is just preliminary info so I would not use it yet. I'll try to dig it up and post it here.

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PostPosted: Wed Dec 03, 2008 5:26 pm 
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Thanks, Jim....do you think that we might get to the point where the initial blast energy becomes more of an issue than the duration of the arc when a person is unrestrained with a high ampere incident?

MVA as more relating to initial power of the blast and debris as opposed to using time to calculate the energy when no one is there?

It does seem to me that time does not matter much if you are initially blasted 10 or 20 feet from the source.

I made the analogy today with one of our guys that if you told me I had to sit on a charge, but I did have the choice of either 1/4 or a whole stick of dynamite. Needless to say the size of the charge was more important to me than the time of the ride.

Alan


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PostPosted: Wed Dec 03, 2008 6:14 pm 
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I tend to agree that the 2 second cutoff can lend itself to questions but for now, it is the only acceptable way there is of bringing some of the results of calculations back to the realm of common sense and experience.

I have seen results where if the 2 sec rule is not used, it is ED or #4 with a AFB of a 1/4 mile. Not realistic.

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PostPosted: Wed Dec 03, 2008 7:41 pm 
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I agree it ain't much but it is all we have for now. Maybe we need to get someone's attention to address some of these issues that we are left to struggle with.

I think that this forum is the best sounding board that we have and I expect that many of the committee members will listen right here.

If Jim could only have imagined what a resource was created when he set this up!

All of us need to get involved and let the committee members hear what we think the real issues are.

After all 2012 is not that far away.

Alan


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PostPosted: Mon Dec 08, 2008 5:21 am 
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2 second rule

From the perspective of a UK engineer I would suggest that the most important issue is the assessment of the working space. We have very little guidance as to what that space is and thats the reason why I advocate the use of IEEE 1584 as an aid to understand the distance at which you would be out of harms way ie FPB. I've been involved with a couple of cases of flash burns recently where I believe that the severity of the injury was increased due to the lack of working space and by the way one was a bucket in front of a sustained LV arc. The guy had to jump to safety in the end. I think that it is very sensible to use a cut off where it can be justified and I know that there are other forum members who have used different cut off values as part of a reasoned judgment. I'm not sure however, if there are many circumstances where I could see the working space in a bucket being sufficient to allow for a worker to move safely outside the FPB.


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PostPosted: Wed Dec 10, 2008 11:33 am 
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The Bucket

Mike,

Thanks, would love to know more of the particulars of the incidents....ie, transformer(s) size, connection, voltage, available fault duty, severity of the injuries, etc. Were safety procedures in place and properly followed? Were they wearing any sort of PPE?

I do believe that in many cases unless a lineman is working with the source directly overhead, the bucket itself can end up as a barrier to the flash energy. While they might not be able to exit an arbitrarily established AFB, the bucket can, and has in many cases, acted as an excellent barrier.

Not saying that some AFB's may not need to be established, and if they are, 70E and IEEE 1584 will be a source for info. We are looking at those as well.

Thanks,
Alan
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PostPosted: Tue Feb 10, 2009 9:30 am 
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So 1584 suggests a longer cutoff may be needed for bucket work. Any suggestions for a time to use?


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PostPosted: Tue Feb 10, 2009 1:00 pm 
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Extended Clearing Times

Based on what we are seeing so far, for the overhead primary 12 and 25 kV systems, you could take the time to 4 or 6 seconds and still not make an appreciable difference on the energy. I have checked numerous systems from 150% of pickup up to 500 +% and seldom find energy levels above 6 cal. We have recommended that the standard FR clothing have a rating of at least 8 cal. We are using Arcpro for those calcs. And if you have an inst. set at about 2x the time pickup, energy will probably be less than 1 cal.

The real problem we see is lack of a good model for 480 volt, open air, pole mounted banks.

That said, once all was done, we ended up with recommendations that essentially mirrored the proposed 2012 NESC.

Hope it helps,
Alan


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PostPosted: Tue Feb 10, 2009 2:10 pm 

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Any further thoughts on this? I just realized the question I posted earlier today is asking pretty much this same question.


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PostPosted: Tue Feb 10, 2009 2:52 pm 
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Can you be more specific?

Thanks,
Alan


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PostPosted: Tue Feb 10, 2009 5:16 pm 
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Where I would like to apply a cutoff time, it does make a difference. Long clearing times occur for a fault on the distribution side side of a fuse protected substation transformer, and no instantaneous is possible. If an unimpeded worker on the ground can get clear in 2s, how long for the guy in the bucket?


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PostPosted: Tue Feb 10, 2009 6:03 pm 
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2 Second Cutoff

stevenal wrote:
Where I would like to apply a cutoff time, it does make a difference. Long clearing times occur for a fault on the distribution side side of a fuse protected substation transformer, and no instantaneous is possible. If an unimpeded worker on the ground can get clear in 2s, how long for the guy in the bucket?


If I understand correctly, you have a sub transformer with high side fuse protection and nothing on the low side except for downstream fusing. In that instance the clearing times could get long. You could 1) Take a good look at your high side fusing to see if you can do any better, 2) Consider putting an overcurent device on the secondary of the transformer. Not knowing the exact application it is difficult to give you more.

I will say that unless it is an extremely large transformer or a very low impedance source, you should not have energies that high. Are we talking about an overhead line? What voltage? Available fault duty at the secondary? What is the IE calculated for the line for the clearing time and what are you using for the calcs?

As far as time for the bucket, I think for most or all apps the 2 seconds is adequate, but if you are aware of specific circumstances/data that would lead you to suggest longer times then it is what it is. I worked as a lineman for 8 years prior to going back to school and feel that based on that experience, 2 seconds is long enough. There are those however, that do not agree and that is a good reason to debate it here! As long as we are going to do high voltage energized work, we will have to accept a certain amount of risk. We should make an honest attempt to protect for the majority of cases, but if we are going to put them in FR that still allows the job to be done, there will always be the risk of the exception. Hard to design for that.

Any info will help,
Alan


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PostPosted: Tue Feb 10, 2009 8:58 pm 
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I've been using 5 seconds for pole or bucket truck work. Based on conversations with linemen, I believe this is a conservative estimate.

I agree with Alan that this is seldom a limiting factor in substations. You generally find that the overcurrent protection for a power transformer will not allow a thru-fault to persist more than 2 seconds. The frequent fault damage curve for a transformer is at 2 seconds for an infinite bus bolted thru-fault.

Where this comes into play is on the secondary of a pole-mounted transformer bank, which as was noted, does not have a good model available for analysis. You get extremely high incident energies at either 2 or 5 seconds using IEEE 1584 equations.


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PostPosted: Wed Feb 11, 2009 5:20 pm 
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jghrist,
Fuses generally cover only the infrequent fault area, since a fault in this zone is very rare. The feeder breakers or main breaker, if present, cover the frequent fault curve. One 10 MVA 115 to 12.5 kV transformer on our system with 9.7% impedance protected by an 80E very slow fuse clears an SLG bolted fault at 9.9 s. Cutting off the exposure to half that time reduces the IE by half as well. Many thanks for your suggestion of 5s. That's the one I was using.

Alan,

It's not the low impedance transformers that cause the problem, it's the high impedance ones. The low current, long clearing times create high IE levels if no cutoff time is used. I haven't run any calcs on the mobiles yet. One runs 12%.

We can do better with relays and circuit switchers. Until the stations are revamped, though, the fuses need to remain.


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PostPosted: Thu Feb 12, 2009 9:52 am 
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Yes there are almost unlimited possibilities depending on the %z of the source and the size/%z of the transformer combined, as well as the type of high and low side protection used. With fault current and time being the variables and since we can usually not control the fault current, time becomes the most controllable variable. Are your 80E's slow or fast type? And if they are slow speed, can you live with a faster curve and would that solve your problem? If it is on the low end, I suspect not.

Another consideration is, if it is on the low end, is it reasonable to expect the arc to persist for an extended period?

Our job in the past (and still is) is to keep the lights on to the extent practical while protecting the public, the equipment, and acheiving adequate protective device coordination. We, you, and many others are already finding out that we now have another task in the equation.....arc flash hazard mitigation....which just might not be possible with existing equipment.

Alan


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