Hello,

I am a summer intern in a small utility, and here, all of our arc flash calculations were always done internally, so several studies for our substations, generation plants and big customers were already made some time ago. Now we just started working on our overhead distribution systems and some hard tasks just came out.
We are currently using SynerGEE, since all of our distribution circuits were already modeled with this software and we could periodically update all of them using GIS Data. Then, calculating the incident energy definitely wasn't an issue for us, our difficulty is how can we classify all the circuit branches of all circuits and inform the crews that will perform the jobs at each location, since we are finding some "hard to deal" results for our larger circuits.
In almost all of our circuits our higher incident energies are happening for relatively low fault currents (3kA to 900A) branches, farther from the substations and that are being picked up by the feeder breaker after a long time, more than 2 seconds, and that we don't want to reduce to the cut-off value, because the worker will be in a bucket truck and won't have a "way to escape" in a timely manner. If we start doing the calculations considering that the line crew will work with a hot stick, the current PPE they have would be enough to work almost everywhere, but since they're used to work gloving the wire we are trying to find a way to avoid taking this freedom they have in as many regions as we can. They already perform several jobs with hot sticks, they would be happy about not having to use 40 cal suits to work on a line, but their arms/shoulders wouldn't be very happy about using hot sticks that much.

I was wondering if any of you ran into the same kind of problem or something similar and if you have figured out a good way to solve it. Should we define a cut-off time? We know that for most cases not even the arc would be sustained for the time we have to clear the fault, but we don't have any standard to rely on. And do you have any general rule to classify and keep the crews informed about the incident energies on each branch of your circuits?

OSHA has weighed in and clearly recommends ArcPro for single phase faults and anything over 10 kv. See the report on the 1910.269 update. Since the ArcPro code is proprietary most software defaults to IEEE 1584 under 15 KV which is 3 phase and gives higher values, or Lee method which gives crazy high numbers for anything above about 600 V.

The 2 second "rule" is kind of arbitrary but used almost without question. If you read 1584 it is sort of vague on this point. But more importantly even that is probably invalid for overhead lines. An arc will be magnetically propelled away from the power source so the actual exposure need not necessarily occur at the initial fault location, nor should it last until a trip occurs. This has been studied by EPRI but so far no results we can rely on. If this pans out, overhead line work may be practically exempted from arc flash except at places like the end of a line.

That being said, I ran the numbers for hot stick ranges and it is almost always very low. For low currents you run out of detection for distance relays. You could extend protection with fuses or reclosers though, if the values are legitimate. You may also want to calculate fault to ground values since those are more likely for most overhead line tasks. I have kind of ignored it because our practice is to attach or remove a saddle first while gloved then attach a sub/transformer clip with a hot stick. Changing insulators or lightning arresters is similar. If the worker is fully insulated (in a bucket) and you use line hose or blankets, the second phase is not exposed anyway which makes a safer work space and blankets are cheap and reusable.

So be careful to consider the task and whether there is a hazard in the first place. 1910.269 REQUIRES a task risk assessment now, so just arbitrarily calling all tasks arc flash hazards is no longer accepted practice. Lots of work does not require arc flash PPE.

Ditto to what PaulEngr has said. You need to use ArcPro for your utility system. As noted, due to spacing and other considerations, the most likely fault on an overhead system is a line to ground event. I have done a number of studies for smaller utilities and even for gloving, the resultant incident energy values do not result in onerous PPE requiremtnets.

Read Appendix E to OSHA 1910.269 for guidance. Note that OSHA says to use a 15 inch worker distance for gloving not the 18 inch typically found in software default values. Additionally if you have metalclad swtichgear in the substations for your feeder takeoffs, you would need to treat that as a 3 phase enclosed fault.

Thanks guys,

These were really helpful replies. Now I see that acquiring ArcPro is more than strongly recommended, I saw both of you recommending it here before, but I thought I could go through with the SynerGEE.
I've read the Appendix E to OSHA 1910.269, I saw that relying on the IEEE 1584 would be acceptable for 601V to 15kV, single or three-phase (conservative for single), and comparing the results that I was getting from SynerGEE with the ones shown on table 7 I knew that my results were really conservative. Anyway, I thought it wouldn't imply in enough changes to make it worth the acquisition of a new software.

Paul, so if I got it right, what you're saying is that if we analyze carefully all of our overhead line tasks and integrate some of these safety measures to our operation (blankets and line hoses) we can reduce the risk to a point that it wouldn't be necessary to calculate the incident energy for every location? Or did I get wrong? Would these measures be sufficient to comply with OSHA's requirements? And to keep everybody safe? Considering what you said, for what parts of the system beyond the switchgear do you usually perform the calculations? Sorry for the bunch of questions.

Barry, I was already using the 15 inches to do the calculations (because I saw you commenting in another post and confirmed reading the OSHA App E) and even like that most of the incident energies calculated were already really low, only for some points we got huge values, for example, for the pieces of line connected upstream a lateral fuse, protected by the distance relay far from the substation feeder or for a couple of circuits that we don't have instantaneous settings on our relays.

Thank you very much, the both of you.

It may be worthwhile to look at upgrading your relays to ones where you can use a Hot Line Tag type feature that would use a faster trip curve while linemen are working on the system. Not sure where you are geographically but working with a faceshield and balaclava can be uncomfortable.

Glad we were able to help and keep the questions coming!

Your point seems to be to some degree that the results you are getting from Synergee's software pushes you towards "ridiculous" requirements for PPE and trying to argue against doing this. Although this would seem to be an argument against doing the "safe" thing, quite the contrary. If nothing else you will have an uphill compliance battle if you start from an overly "conservative" point of view. Although the general attitude with line workers in general is a pretty rough and tumble bunch that actually relish the idea to some degree that they are in risky jobs with a very real danger of being seriously injured or killed. Being a hero and saving the day in spite of obvious, clear, and very present danger is what makes the job exciting and gets guys to show up to work doing a job that takes a heavy toll on your body. So somehow we have to temper that drive in our role as engineers by providing guidance on how to do the job safely in a way where we can get buy-in that it's the right (and safe) way to do things.



Interesting...you keep using the word "conservative" but your first post clearly acknowledged that the results from the Synergee software were in a word, overkill. That is precisely the reason that I recommend against using IEEE 1584 and especially the Lee model for overhead lines. However there is a general problem with ArcPro in particular. IEEE 1584 in its current form is a public document. Quite literally anyone that can write software to perform a detailed power system analysis can simply add in the extra equations from that document (or the Lee paper) to produce an incident energy value, and generally speaking the differences between software A and software B are often small.

However when comparing models against one another, we see differences. The Lee equation is almost the simplest one that is widely used, and it is based on the theoretical maximum power that can be generated in an arc. At low voltages and currents, it actually works quite well. As the voltage/current rises to "utility" levels though it becomes overkill and the problem keeps increasing as voltage or current increases. IEEE 1584 takes tha approach where rather than attempting to do any kind of theoretical model, it is simply a curve fit to a set of test data. It does fairly well for around 300-15 kV based on known test data, although there are issues at the "edges" such as predicting an arcing current higher than the bolted fault current. But all of the test cases are 3 phase and the majority of them are designed around industrial environments with equipment that is relatively "spacious" inside but enclosed nonetheless. ArcPro and Duke heat flux are two other well known models. Both models are based on some kind of numerical analysis based on a theoretical approach to arcs. ArcPro is known to produce results that are close to measured tests with single phase open air arcs. Both models are based on published papers but both models are proprietary so like it or not, we don't really know "whats in there" and it has to be treated like a black box. That's a problem for competitors such as Synergee because there is simply no way for them to duplicate ArcPro's results and since ArcPro doesn't sell a "library" version that could be plugged into someone else's software, the only way to get a result from the ArcPro model is with their software.

This is what I meant to say earlier but hit the submit button too fast.



Two different questions in my mind. One is what do you have to model and what not, and the second is how my comment about tasks plays a role. Taking the former first...

Let's keep in mind my environment. It's a private utility with about 70 miles of overhead lines and a 50 MW cogen providing about 60% of the power. The rest is imported from Duke's transmission system. Before Duke acquired Progress Energy, it was the largest customer on Progress' system. I'm not sure where it stands now after the Duke acquisition. Now on that system we have about 200 substations, and about 50-60 of those move every 3-6 months. Roughly 2-4 miles of power line on average is constructed and about the same amount (on average) is removed annually. It's a mine, so the power system moves with the mine. So it is impractical for a lot of reasons to perform an arc flash study as it is typically done (create single lines in software, run the analysis, and produce labels/procedures based on the results). Instead we have to basically put substations approximately in the model and test them at various distances to arrive at conservative results. A lot more numerical modelling has to be done because the whole power system changes literally daily.

On top of that I have the advantage that I have a few parallel feeds but generally everything is either single or double ended, and I don't generally have anything resembling a "network", but that's a common utility distribution model. If you have a network, then theoretically you'd have to simulate every single combination of switch arrangements to determine worst case (again, conservative results) or else somehow do this in real time. Again, not really practical. Instead you will quickly find by running several scenarios what the major drivers in the model are and reduce the number of cases that have to be tested. Often this means for instance you may have a "normal" and some special cases that rarely (if ever) actually happens. As an example typically you don't close all the ties in a ring or double ended bus arrangement, if nothing else than to prevent circulating currents, but this arrangement does typically produce the highest arc flash condition.

Also, you are not going to be issuing PPE for 2, 2.1, 2.2, 2.3...cal/cm^2. Generally you are going to issue PPE at say 4, 8, 12, 40, and say 100 cal/cm^2. So it really doesn't matter if the result is anywhere between say 4.1 and 8 cal/cm^2. So you can pretty quickly lump things together because you are really only interested in thresholds where the PPE grade changes. It is convenient in an industrial setting to model "everything" but neither convenient nor practical to do so in a utility setting, never mind a mining setting.

So this approach to modelling, though it is more of an engineering approach, considerably reduces the number of actual calculations that need to be performed. If you are using ArcPro in particular, this is very useful because with ArcPro as stand-alone software, you will find yourself going back and forth between your power system analysis software (Synergee) and arc flash analysis software (ArcPro) instead of just pushing a button and getting a spreadsheet of results.

Now as to the latter issue, that of task analysis. When you look at arc flash events in general, there are generally 2 causes. The first is that certain tasks or methods in general are highly susceptible to causing an arc flash. For example working on de-energized equipment which is not located near or over the top of energized equipment is not going to cause an arc flash. Doing a corona or IR survey is similar in that it's just not going to happen. And generally speaking using the low voltage controls is not going to cause an arc flash. But installing or removing a jumper on an overhead line without coverup on the other phases while it is energized only requires a small error for whatever reason and you've got a bonafide arc flash. Same thing happens when working inside an enclosed panel with exposed bus bars or studs where a tool can easily drop and fall into the equipment. These are two cases where unconditionally, either arc flash is a hazard or it's not. A third case though involves equipment. Certain types of tasks on equipment such as racking a bucket in/out of an energized bus have been known to go very badly. And any time you are working on equipment that has faulted for an unknown reason or where a breaker may have just tripped for the very last time but hasn't been inspected yet becomes a situation where the equipment is in an unknown state and simply disturbing the equipment for testing/troubleshooting purposes can and has triggered an arc flash. Otherwise under normal conditions with properly maintained equipment, operating disconnects, switches, and breakers, as an example, rarely if ever results in an arc flash. So this is an example of where arc flash PPE may be required on a conditional basis. You can generally do a survey/engineering study ahead of time to determine what equipment is in good shape and what is not, but you need to have a field judgement call made with a set of fairly reasonable conditions (a visual inspection) to determine whether or not the equipment is in a questionable/unknown state (and thus default to arc flash PPE required), such as if you approach the equipment and there are clear and obvious burn marks on the outside of the housing.

So when you do the task analysis you should be able to determine that a large number of tasks especially the PM/inspection kind can be done where there is no arc flash hazard and thus no arc flash analysis is needed. Then there are tasks or equipment where it is needed every time, and there are also some where it is generally not needed but a field inspection is necessary to determine when it is required. And on that last note, use the Appnedix in 1910.269 for guidance on this. 70E is terrible. The "inspection" is so vague that there is no way that field personnel can possibly make any kind of judgement call because it is simply way too vague.

Finally as suggested, with a delta-wye solidly grounded or multi-grounded system as an example you can automatically reduce the voltage for single phase cases by 1.732, reducing arc flash nearly in half, but it simply goes away with resistance grounding or ungrounded systems IF you have put coverup on the other phases so that the only failure type is line-to-ground. Generally speaking again you have to look at the task to determine what is possible. As an example one of my crews was working on installing dead ends on an existing structure because we were going to demo and remove half of the line to reroute it away from an area that was constantly flooded and subject to a lot of heavy equipment traffic. The way it was being done was to attach temporary lashing to the existing lines to take the tension off the part on the insulator, cut the lines free from the insulators, and then reattach them to the newly installed dead ends. All 3 lines were done at once and line hose was not installed. Due to improper installation of the lashing, when one phase was cut, it jerked loose and hit another phase. Fortunately not much else happened but this is a clear case of a tasks where a line-to-line incident can occur. It was still a bad situation but installing cover up would have prevented the arc flash. Fortunately the line had pulled away far enough that exposure was minimal and worst case would involve the lineman somehow coming in direct contact with the energized conductor and for the bucket truck insulation to somehow become bypassed, but this is obviously a real stretch and you'd have to decide if this is even possible in the first place.

Don't forget too that you don't have to do a study anywhere that you won't be working on equipment while energized. I know that sounds silly in a utility environment but depending on work practices it happens. As an example for industrial sites the vast majority of the wiring is from the distribution system to the loads such as motor leads. But usually the practice is that all of those areas are only worked on while de-energized. So there is no reason to do an arc flash study on the motor brnach circuit wiring, only the starter/disconnect where it will be de-energized. Generally the rule is to work energized for utilities but if you have sections of line with no branches or equipment that are typically de-energized to work on them by using network switches to reroute around it, then it's a candidate for not doing an arc flash study.

So you have to do the study but in my mind you might be able to do something as simple as taking a system map and coloring it with some sort of color code based on PPE required. That makes it ieasy to understand.



OSHA used 15" but I'm not sure why and they don't comment on it. I have some trouble accepting that value. If you look at anthropomorphic data, the typical/ideal working distance away from the chest is 15" horizontally. Despite variation in physiology, this distance seems to be fairly uniform and gets used a lot in ergonomic studies. Thus we are looking for a second degree or more severe burn roughly at the bottom of the rib cage/abdomen area which is generally not life threatening as far as burns go by itself.

The threshold value used for arc flash in IEEE 1584 and many others is looking for roughly a 95% success rate at providing protection equal to or exceeding the Stoll curve. That is what the ATPV value on the PPE is based on. The most vulnerable part of the body to 2nd degree or more serious burns is the face and upper chest area. So IEEE 1584 and others using 18" based on the center of this area (basically center of the neck in a plane that consists of the front of the face and the rib cage). This distance adds on another 3" hence the reason for using 18".

OSHA gave no explanation for the rhyme or reason for using 15" instead of 18". It increases the incident energy value by 45% which is a substantial increase, so I'm not seeing any engineering or other reason for supporting the lower value.

First of all, thank you very much for the replies. I was thinking and reading about this for weeks, I read a lot of posts from this forum a lot of other documents and I wasn't going anywhere till now. It's amazing how we can find knowledgeable people like you guys and totally open to help others here, I really really appreciate this.
Now I just regret for not having my questions posted before, since I'll be working here only for one more month that is exactly the ArcPro lead time, so it is very unlikely that I'll be the one working with this for the rest of the project.
Anyway, having more questions and doubts over the process, I know where to go to find the answers.


Sure, Barry! This is one of the questions that we raised here, but unfortunately it isn't something we can do that fast since a lot of investment and studies come with it. Adding IntelliRupters or reclosers, fusing all possible laterals, lowering instantaneous setting on our relays and upgrading old ones was one of our thoughts, but it would certainly imply in a lot of work, money and time. And it's a money that you wouldn't want to spend unless you're sure that this is the right thing to do (and considering the way we're performing our study, apparently that wasn't the right thing to do right now).
And I'm in Michigan, I guess they will be happier working without having to wear balaclavas and face shields, so many weather changes...


Paul, you couldn't be clearer this time. Thank you, I believe now I have all the information I need to start doing this study differently. Very good examples of situations too.
And just trying to clarify some of the things I said and that caused a bit of confusion, in fact, I wasn't sure about how off I was with the SynerGEE results, I knew that they were higher than they should be and I knew that some of them were too high, but I was trusting the standards too much when they say that I would get conservative values, so I was really thinking that I had only "conservative" results (what I imagined as something a little off the target) and not the "overly conservative" results as you very well said.


That was our initial idea, something we could put in our city map or primary circuits drawings that could be easily identified by the ones required to perform the work in a certain area, but it was almost impossible to create general rules with results distributed in all 4 categories and changing branch to branch.


Yes, it looks like they just decided a new value and stated that this is the distance for a worker gloving the wire, coming out with a distance different from all of the other standards. So since you see a good reason to continue using the 18" and they don't have any background information to prove you should be using 15" instead, I guess that's an engineering judgment call you can make.

I started out originally questioning when the face shield/balaclava stuff should kick in. My understanding is that it considers the "back of the head". Looking at the geometry of a head and moving the 18" limit back, it only seems to increase the cal/cm^2 rating by a very small amount, not going from 4 to 8 cal/cm^2 as it does now. Lots of stuff in this whole "proper distance" area really seems to have little to no science behind it and changing arbitrarily from an 18" to a 15" rule is no different. The other one is that 70E proponents support a 1.2 cal/cm^2 cutoff which arises out of the Stoll curve at 1 second while the NESC crowd centers around 2.0 cal/cm^2 which is the Stoll curve at 2 seconds. And there is one poster on here who regularly proposes using heat flux (cal/cm^2/s) but there seems to be little support for that idea. Again...more questions than answers here.

As to improved switching, I have looked at the Intellirupter. It's a pretty neat idea but in some ways its a solution looking for a problem. During the initial fault it works exactly like any other interrupting system. For this reason the system has to be rated for full fault currents. The difference is that during reclosing, it uses a timing trick to minimize fault current if it closes onto a fault. This would be useful if the equipment didn't already have to have a withstand rating high enough for the initial fault. Without mentioning names there are at least 2 reclosers on the market that are priced around $25K for a 35 kV rated recloser and another pair of names that are down around $15-18K. The Intellirupter stands alone at nearly $40K. So I have a hard time justifying the extra expense. Fuses also run around $10K installed cost for boric acid cartridge fuses mounted on cutouts. They can be coordinated with reclosers as well to reduce the overall cost of reclosers but the price difference is not nearly what it used to be at one time.

A balaclava is a wonderful thing in Michigan in winter, whether it's FR rated or not. We used to call them "ski hats". In summer, not so much. But try a lighter one from say Salisbury. I was very surprised that it is not as hot to wear as it looks and in winter time, iI'd be much happier with my arctic grade hard hat liner. That still applies in North Carolina where I live now, but the cold season is a lot shorter. but a good stiff sea breeze when you are working at heights in January can still chill you quickly.

As to the word "conservative", we can really break this down into 3 counter-arguments against arc flash and shock PPE.

First is gloving. How do I say this...they are rotten. They kill dexterity. They are freezing cold in the winter no matter how good your liner glove is and they are wet, slimy, sticky, and stinky in the summer and you can't possibly use enough powder. They're uncomfortable, make it virtually impossible to handle small tools (fortunately overhead line equipment is generally bulked up to match the gloves), and leave little between you and certain death relative to other methods such as hot sticks, but for a lot of jobs it's faster and easier to use the gloves over handling everything remotely with a hot stick. But I see almost no complaints about it.

In terms of arc rated PPE, there is proven performance data that shows that single layer FR clothing is not thermally any worse than standard cotton industrial work wear (12 ounce cotton shirt and pants). Due to concerns about this on the contrary, most of it is actually lighter than the non-FR counterparts. Once you get into multilayer PPE though that is when the trouble starts and that is when heat exhaustion/stroke becomes a real and significant hazard. The biggest issue is the long sleeves so get Henley style work shirts and roll the sleeves up when not needed. The FR stuff feels DIFFERENT. It is not worse if your crews are already wearing rental uniform pants and shirts or similar materials.

In terms of the face shields (and balaclavas) yes, it increases heat load and sometimes they fog up. And because they are tinted to prevent burning out your corneas in an arc flash, visibility is reduced. This is again where overprotecting (overly conservative) plays a huge factor. And some of the newer face shields have a better tint (grey) that doesn't make wire colors had to see, and they have generally been able to improve light transmission somewhat but there are limits to what can be done. The other argument is that it sticks out or somehow impedes movement. This one doesn't hold water at all. Linemen love the wide brim hard hats but never complain that it gets in the way, and the face shield doesn't stick out any more than a wide brim hard hat aka "lineman hat". So its a mixed bag here.

In terms of compliance, there are two videos out there for training that do a better job than any other one I've ever seen. First is this one specifically on arc flash. It starts out a bit overly dramatic but once you get past the first minute or two and they have the electricians at the Elkins, NC mill on video, it gets much better.
https://www.youtube.com/watch?v=hfnEuRA7-vo

The second is this one which is referring to refinery fires but makes the same case. The version that is floating around has the speaker standing in front of a Union Pacific podium. He talks about cutting off the sleeves of his FR shirt because it was too hot and all the other mistakes he made where he was severely burned. The guy has a heavy Jersey accent.

Good recommendations about PPE, thank you. I'll make sure I'll search about them and provide all the information I get to the responsible people here. And I know the line crews are already used to some FR clothing (they are using 8 cal/cm^2 as standard) and they already perform some operations with a hot stick. Our study now will tell us when/if they will need more than 8 cal suits, face shields and balaclavas to work.

About the IntelliRupters, I mentioned them, because I know it's being a common practice here to install them instead of reclosers as they're used to do before. I actually never asked what was the main reason to do that, since I didn't know that the cost difference was that high. Just checking on the website you see a couple of videos showing some functionalities that can be useful to a utility, like this one:
https://www.youtube.com/watch?v=2VGs7FdrSIE
But I don't know if that's reason enough to choose them, anyway I'll try to ask someone that was behind this decision and let you know in case there was anything else being considered by them to justify the choice.

And I'll try watch the two videos you recommended too, thanks for the hints.