All - I've tried searching but haven't come up with much. We have a continual debate going on regarding remote operating of devices. In summary we have a lot of locations where we don't have the proper calculations and exceed the tables regarding AFC at the building so we have gone the route of ropes, pulley's, etc. to allow our personnel to be outside the room when the breaker, disconnect, fused switch is operated. Is there anything out there that would help define how much of a barrier is needed to "shield" someone from the blast of an event that was at a level of even 100 cal? From my vantage blast is the only concern at this location so if I'm missing something please let me know. If I could even get something that said an 8" block wall was enough I would be at a starting point. Thanks for any input you can provide.

So since you are remotely operating equipment such as switches and breakers that are designed to be operated, I assume you did a risk analysis that shows that operating the equipment as designed is a hazard. Typically a risk analysis would take into account items listed in NFPA 70E-2015 Table 130.7(C)(15)(A)(a) to determine if Arc Flash PPE is required. This would be items such as is the equipment properly maintained, is it installed properly, are all the covers secured and is there any signs of impending failure?

So if operating remotely something in your risk analysis says that AR PPE is needed as typically operating a breaker or switch does not require PPE. Now racking the breaker or opening the door to the disconnect switch does require PPE.

If you know you have exceeded the AFC, then some sort of study must have been done to know that as typically the maximum available fault current is not available (infinite bus). The fault current from the utility may be far less and result in low fault currents inside your facility. Of course the next issue is the clearing time.

If a study is done, I would highly suspect that except for the switchgear directly connected to the utility transformer, you don't have that high of incident energy especially the further downstream from the main switchgear you get. Also a study may be able to provide alternate settings to reduce the hazard.

Did you consider the fact that "remote operators" themselves have a very high incidence of failures? And I'm not talking about for instance when certain companies such as GE try to sell motorized remote operators that literally fall right off the doors of the equipment they are supposed to work on when activated. What I'm talking about is the fact that many of them have been known to jam drawout mechanisms or stripped the teeth out of the drawout mechanism on breakers in particular to the point where they won't work anymore and the only safe recourse is to open the mains and disassemble the section to try to extract the breaker after it jams.

Second, I can offer you this slight hint. One of the PI's I submitted to 70E was to remove the special wording in the informational note about "over 40 cal/cm^2" posing some sort of extra hazard that doesn't exist below that point since there is a myth floating around that you will be liquified inside an arc flash suit even if it's a 100 cal/cm^2 rating if you are exposed to an arc flash over 40 cal/cm^2 or some such silliness. Believe it or not there are actually many consultants that actually spread this kind of false statements around. Based on the response from the committee it appears that this informational note is going to be removed in the 2018 edition.

OP used the word "blast". Incident energy is expressed in terms of energy per unit of area or cal/cm^2. It is not a pressure. It is based almost entirely on testing which measures the amount of radiant thermal energy emitted by an arc, NOT pressure. As such, I can think of three examples of what would be an adequate barrier other than outright distance:
1. The various "arc flash blankts" on the market. The manufacturers have plenty of design data for using these. It's a great option for places where exposures are high and there is no chance of getting away such as in manholes.
2. Anything that would qualify as a "firewall" such as with a 1 hour burn rating. See NFPA 101 for a lot of detail on what constitutes acceptable fire walls. Even drywall (because it contains so much moisture) would pass. Drywall works as long as it retains its moisture and given the "2 second" cutoff for arc flash calculations, there is no chance that the moisture in the drywall will do anything more than flash into steam which is just as effective as water absorbed into the gypsum.
3. One or two layers of aluminum siding. Aluminum has an emissivity of less than 10%. That is, it not only reflects about 90%+ of the radiant heat that hits it but it also doesn't transmit thermal radiation very well either. Cement, lime, glass, and iron/steel industries routinely make home made heat shields by using two layers of ordinary aluminum siding or roofing with 1" unistrut spacers which can knock down 800-1000 F temperature radiant heat down to around 120 F or less, and this is under continuous exposure conditions.

Be careful about being "too close" with these barriers though. There are some formulas floating around covering "burn through" damage which could play a factor, and all 3 of them need to be secured fairly substantially, especially the arc flash blankets.

Thanks for the input. I think I may need to reword this. Let's suppose you know you have a switch you need to operate and you have been told the IE is 100cal but you have no arc flash PPE. Assume you know you can operate the switch with a rope and pulleys safely. What distance do you need to be away and or what barrier do you need between you and the switch to be able to do this in your birthday suit "in a code compliant manner."

Thanks

So you can't use 70E. Chapter 2 makes it very clear that to follow 70E, you MUST perform "proper maintenance". They leave you hanging by not defining it but nevertheless, it is required. Article 130 backs this up by stating in the fine print notes that effectively, your incident energy study is utterly worthless if proper maintenance isn't being done because the breakers won't trip as expected. We do not currently have a code specifically for sites that don't do proper maintenance. When that one comes out, then that's the one you follow.

Try this little experiment to see what could happen. First, take a spreadsheet copy of your arc flash study. Ignore every incident energy that is fuse protected because those don't change (almost all fuses don't rely on mechanical motion for tripping...yes, I'm aware of SM-5's). Since incident energy is linear with time, recalculate any that are under 2 seconds up to what they would be if the breakers failed or failed to trip in a reasonable amount of time. So for instance a 4 cal/cm^2 incident energy with a 0.1 second opening time becomes 4*(2/.1) = 4*20 = 80 cal/cm^2. The reality is that this approach is somewhat overly conservative because in reality you'd have to delete/short each breaker individually and then recalculate because the chances of multiple breakers failing to trip properly is much less than a single failure, but it gives you some idea of just how really bad a system that isn't properly maintained really is in the event of an arc flash. Typically you will find that half of the previously considered "safe" cases are now well beyond available PPE.



OK by ignoring Chapter 2 and not properly maintaining it, you simply can't "get there from here". 70E makes it pretty clear that simply "suiting up" is not an acceptable alternative to practising proper maintenance. This would be akin to a particular Fortune 500 corporation whose policy is that they "don't do energized work". In other words, they follow Article 120 but flat out ignore Article 130. Interesting...I asked them whether or not they test for absence of voltage. Answer was yes, so I asked them how they test for absence of voltage which is energized work without doing energized work. In your case, you are choosing to follow Article 130 while ignoring chapter 2 and even ignoring the notes in Article 130 which specifically tell you that you can't do an incident energy study without doing proper maintenance.

That being said.

The shape of the enclosure matters. Deeper enclosures create a more "directed" radiated heat as opposed to open air conditions where it radiates equally in all directions. This is reflected by the exponent "x" which is in the equipment table in IEEE 1584 and is also listed in Annex D in 70E. This number varies from around 1.3 to 2.0 where the lower the number, the more "directed" the arc flash.

You then calculate:
new incident energy=((old distance)^x)/((new distance)^x)*old incident energy

But there is a vastly simpler approach, one that you probably already have. The "arc flash boundary" which is part of your study gives the distance at which the incident energy is less than 1.2 cal/cm^2, a point where arc flash PPE is no longer required. So stay that far away and you are in the clear. Note that the distance is calculated based on the bus bars in the back of the enclosure to a point roughly around the throat area halfway on a line between the tip of the nose and the center of the chest.

Quite often especially with transformer secondaries this will put you several feet/meters beyond the building or room. So anything that would be considered a decent fire wall is a "barrier". Remember that this is all about thermal radiation (infrared heat) so once you are out several feet/meters from the hot gases, shrapnel, etc., then anything that gets you out beyond "visible" range counts. There is a fallacy to this though...the hot gases will have a tendency to "roll" around corners of barriers especially if they are close by. Watch youtube videos of tests of arc flash blankets and you will see what I mean. Although the arc flash blankets (which are definitely a substantial barrier) more than provide enough of a barrier when sized properly, the arc flash still tends to "roll" around the corner so some arc resistant PPE is still required to protect against this. Again the manufacturers of these products have lots of design information and can help you out in this regard if the building/room itself is not an acceptable solution.

Paul - Thanks for all of the information. While I understand what the code says I also live in the real world. The current code set up has us in a very very bad way. It basically states you must do X or your are in trouble if something happens. Then there is the real world and it states we can only afford to do Y. In you opinion what percentage of businesses actually do proper maintenance on their electrical gear? I know whatever your answer is doesn't make it a good thing but it does help "ground" our conversation. If I took your approach "not saying it's wrong" I would basically need to call our power plant and tell them to shut our little city within a city down today because we don't do the necessary maintenance. I in fact agree with your statements but when I bring up maintenance they "those above me on the totem pole" just look at me like I'm crazy again. The unknowns and simply engineering out the hazards are what have me operating switches outside the room.

Thanks again for all of your help.