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 Post subject: calculate Arcflash boundary and incident energy for HV CB
PostPosted: Sun Apr 15, 2018 1:35 am 

Joined: Wed Nov 01, 2017 6:09 am
Posts: 27
Hello!
I am trying to a reasonable calculation on large circuit breaker at 132kV(as shown in the picture but with different phase distances 130cm instead of 175cm.)
This is a circuit breaker located in a grid with a coil in the neutral point. Then I guess single phase arc is out of the question and the focus is three phase?
For this I am using Arcpro with the following input:
Current: 5kA(three phase short circuit)
Duration: 18 cycles(300ms)
Arcgap: 126cm(I actualy whanted to use 130cm but the ARCPRO max is 126cm= 49.6 inch)
Source voltage: 120000V(Its 132kV between the phases, 120kV is max in arcpro)
Distance to arc: This is difficult I think. Any suggestions? (for instance on the drawing: Distances from ground to phase-person hight=3337-1800=1537)
Arc type: Three phase open

If I use 100cm distance to arc(39inch) I get 6.3 cal/cm2 at the distance and a AFB on 2.4m(98 inch).

Anyone have experience if this are relevant numbers to present or do you normally just use tables on this voltage levels?

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 Post subject: Re: calculate Arcflash boundary and incident energy for HV C
PostPosted: Sun Apr 15, 2018 6:21 am 
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Realistically can you get enough heat transfer to initiate an arc at those distances on a second phase? I can easily envision phase-to-ground/neutral arcing initiated by weather or damage and I can easily envision phase to phase arcing which is a lot more rare but still happens (think bird with large wingspan). But other than that should it ever be a 3 phase arc at transmission voltages? I haven't heard of one in outdoor/overhead equipment. It might be possible but I'd have to see evidence of it. Even lightning strikes at transmission voltages are pretty rare. Even at medium voltages in the upper end of things (20-35 kV) based on years of monitoring data on overhead systems in a mining operation that was particularly susceptible to lightning (due to mine dewatering soil resistivity was horrendous), and bird strikes (nesting blue herons in the area), as well as "typical" wild life (bear, snakes) and Category 2 hurricane zone, I can't recall ever seeing all three phases tripped simultaneously with roughly 6 foot line spacing except at equipment locations. Once in a while we'd see flags for 2 phases but not all three. So just a thought.

Second and this is where you'll run into trouble. Arcpro is designed to simulate a vertical single phase arc in open air, generally simulating conditions in distribution systems, not transmission. I know it can go higher because the values given in NESC for 115, 230, etc., come from Arcpro but it seems like some of your conditions must be hitting a boundary condition within the software which is limiting your system, arc gap in particular. In addition though I wouldn't worry about the voltage for one very simple reason: Arcpro doesn't actually use voltage. It's really just a reported value, not used for simulation at all. So you can use 69 kV and get the same results as 120 kV or 132 kV. Arc gap is pretty important...longer arc gaps result in higher incident energy because there is more arc to radiate but as reported by EPRI that started preliminary work in this area, the arc gap that you are looking at simply has no data whatsoever on it. You are at the limit of what science can provide answers to, and that will in some way be part of your reported result. You can view the EPRI studies on arc flash at transmission voltage levels for free. I suggest using it as a reference. I highly doubt any other organization is going to ever get enough funding to sponsor research in this area. OSHA requires you to protect employees against recognized hazards, in other words those for which there are valid scientific studies and information. That's why arc flash itself wasn't really addressed formally until about 20 years ago...no science. Transmission line arc flash really isn't a big issue on anyone's radar because the inherent voltages (and MAD) force workers so far away and the long gaps and distances for insulation reasons plus the fact that as voltage increases arc flash hazards decrease dramatically means that it's pretty unlikely that this would be a significant hazard so hence the reason there is a lack of scientific research and interest in funding it.

So ultimately keep this in mind...everything you are doing is theoretical. Nobody can challenge your result with factual data. You are guessing at best. Your 6.8 cal/cm2 result is probably about as close as you are going to get and just reiterates the fact that even considering the amount of power involved (MW/MVA), the currents are still relatively reasonable, fault currents are not very high due to inherent limitations in terms of MW/MVA produced during a fault, and the fact that you can only push so many amperes across a conductor which is what pushes the voltage to 132 kV or more in the first place.

When you specify 3 phase, enclosed, etc., it just uses multipliers to estimate it. It also assumes that the ratio of the distance to the victim and the ar c length is comparatively large so that the arc can be treated relatively speaking as a point source. For your longer distances (standing at the ground) this might be accurate and it is probably still accurate at hot stick distances. As far as a 40 inch working distance goes, it begs the question how? The relatively close proximity of the other two phases seems to suggest that live-line technique would be pure suicide but MAD at that voltage suggests double or triple that short of a distance at a bare minimum. I've used 20-40 foot telescoping hot sticks and it isn't fun. That's the reason many utilities have adopted live-line/bare-hands for transmission line work. I'm just not understanding what the work method would be here.

My suggestion is that you need to go back and look at your actual expected working distances (MAD) first and perform your estimates at those distances. Nobody is going to be doing rubber glove work at 132 kV because that's impossible. At live-line/bare hands distances it's the same as direct contact so the hazard is shock and there is no point in estimating arc flash for 0.0"...all models "blow up" (go to infinity) at that distance. Realistically there is no point in even looking at arc flash within flashover distances. At low voltages these tend to be measured in inches so we go to ergonomics (15 or 18 inches) or equipment design limitations (24 or 36 inches). At the upper end of medium voltage and moving into high voltage the flashover distances and the MAD itself gets so large that this becomes the limiting distance. Even at low voltages at 15-35 kV we get two distances. Rubber glove work is 15" as per OSHA guidance and 18" as per NFPA 70E guidance while hot stick (MAD) distances are typically out around 6 to 8 feet. Above 40 kV rubber glove work becomes impossible so the longer hot stick distance is the only one that matters. You can get closer but then it becomes bare-hands/live-line work and at that point no different from evaluating the shock hazard potential. I think you can realistically at that point assume that the victim is probably dead anyways so no need to figure out whether or not the corpse will be cooked to medium rare or well done.


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 Post subject: Re: calculate Arcflash boundary and incident energy for HV C
PostPosted: Sun Apr 15, 2018 8:24 am 
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Location: Rutland, VT
PaulEngr wrote:
My suggestion is that you need to go back and look at your actual expected working distances (MAD) first and perform your estimates at those distances. Nobody is going to be doing rubber glove work at 132 kV because that's impossible. At live-line/bare hands distances it's the same as direct contact so the hazard is shock and there is no point in estimating arc flash for 0.0"...all models "blow up" (go to infinity) at that distance. Realistically there is no point in even looking at arc flash within flashover distances. At low voltages these tend to be measured in inches so we go to ergonomics (15 or 18 inches) or equipment design limitations (24 or 36 inches). At the upper end of medium voltage and moving into high voltage the flashover distances and the MAD itself gets so large that this becomes the limiting distance. Even at low voltages at 15-35 kV we get two distances. Rubber glove work is 15" as per OSHA guidance and 18" as per NFPA 70E guidance while hot stick (MAD) distances are typically out around 6 to 8 feet. Above 40 kV rubber glove work becomes impossible so the longer hot stick distance is the only one that matters. You can get closer but then it becomes bare-hands/live-line work and at that point no different from evaluating the shock hazard potential. I think you can realistically at that point assume that the victim is probably dead anyways so no need to figure out whether or not the corpse will be cooked to medium rare or well done.


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 Post subject: Re: calculate Arcflash boundary and incident energy for HV C
PostPosted: Wed Apr 18, 2018 6:49 am 

Joined: Wed Nov 01, 2017 6:09 am
Posts: 27
I agree that it is not so common to work on the HV equipment. But still you cold have operations where you come pretty close. Like service panel, inspection of the area around the cb(very common) then the boundary distance is relevant.


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