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 Post subject: Working Distance Mistakes
PostPosted: Sun Jan 13, 2013 5:07 pm 
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Too Close for Comfort
Understanding Working Distance
by Jim Phillips
Published: March 2012 [url='http://www.ecmag.com']Electrical Contractor Magazine[/url]
[url='http://brainfiller.com/library-articles/working-distance-mistakes-in-an-arc-flash-study.33/'][Download PDF of Article][/url]

The term “working distance” appears 20 times in the 2012 Edition of NFPA 70E, the Standard for Electrical Safety in the Workplace. It appears 12 more times in the annexes. The working distance is an important component of the arc flash hazard analysis and is frequently listed on arc flash warning labels and in the arc flash report.
IEEE 1584—IEEE Guide for Performing Arc Flash Hazard Calculations 2002 defines the working distance as “the dimension between the possible arc point and the head and body of the worker positioned in place to perform the assigned task.”

When incident energy is calculated as part of an analysis, it corresponds to a specific distance from the prospective arc flash source, making the working distance a very important piece of information. If a person is located closer than the working distance, the incident energy increases dramatically.
The table above illustrates how incident energy varies with distance. Calculations were performed for distances ranging from 6 to 36 inches for a 480-volt panel. Depending on which distance is used, the incident energy varies from 2.6 calories per square centimeter (cal/cm2) to 48.5 cal/cm2. If the working distance was not included, a worker would not know that 2.6 cal/cm2 is based on being located a minimum of 36 inches away.

Five common (and dangerous) working distance mistakes Even though working distance seems to be a simple concept, it is not always properly considered when performing energized work. The following are five common mistakes:

1) Working too close—Even though the working distance may be listed on the arc flash label, there is sometimes a tendency to move closer for various reasons. This could spell trouble in the event of an arc flash, since the incident energy would be greater. The electrical safety training and job briefing should always emphasize the importance of maintaining a proper working distance and include a discussion of what could happen if this distance is reduced.

2) Parts of the body closer than the working distance—Even if the correct working distance is maintained, it is possible that other parts of the body, such as hands and arms, may be closer to the arc flash source. NFPA 70E requires additional protection to be used for any parts of the body that are closer than the distance used in determining the incident energy.

3) Incorrect input data—When performing arc flash studies using commercially available software, care must be taken to select an appropriate working distance. IEEE 1584 suggests working distance ranging from 18 to 36 inches, depending on the type of equipment and voltage level. Many times, I have reviewed arc flash studies only to find the distance selected is not what IEEE suggests for the equipment. If a larger than normal working distance is used, it can give a false sense of safety by suggesting the incident energy is much less than it could be.

4) Unexpected energized parts upfront—Working distance appears to be a simple concept until you attempt to determine the location of the possible source of the arc. Is the equipment’s energized bus located toward the rear of the enclosure as is usually the case? Or are there also energized conductors located closer to the front? How would you know before opening the enclosure door? The working distance generally assumes the energized conductors are located near the rear of the equipment, but that may not always be true.

5) Rear accessible equipment—Electrical equipment that can be accessed from the rear might result in a big surprise when it is opened. Instead of the energized bus being located at the back, you are now at the back, and the working distance can be very short. Any live work performed at the rear must take this greatly reduced working distance into consideration. Normally, this situation requires a second set of calculations and labels based on the short working distance.

Too close? Distance is everything when it comes to an arc flash. Workers should never be closer than the working distance associated with the incident energy for which they are protected. If a different working distance becomes necessary, new calculations will be required to determine the increase in incident energy and the requirements for possible increased protection. Getting too close could be catastrophic.


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PostPosted: Mon Jan 14, 2013 8:56 am 

Joined: Tue Mar 02, 2010 1:50 pm
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Very good point about the back of switchgear. I hadn't thought about that, but you are absolutely right, often the bus is sometime really close to the back.


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PostPosted: Mon Jan 14, 2013 2:27 pm 
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As I understand it, the working distance is calculated to the face/center of the chest area because unlike say the extremities, it doesn't take a lot of 2nd/3rd degree burns in this area to raise the risk of mortality significantly, as compared to losing a hand or arm which is survivable. At the end of the day after all, IEEE 1584 is based around making an arc flash incident survivable. And I don't really recall any statements in NFPA 70E talking a lot about increasing protection for the extremities (hands/feet). If it is in there then we have a big problem because I'm sure that if I started plugging in working distances for an electrician elbow-deep ito a piece of gear and an arcing fault happens, then with working distances of say 1 inch (20-30 mm) or less, the incident energy is going to be well beyond any available PPE especially for hands.

Before we go there I'd recommend taking a serious look at what the appropriate boundaries would be for the onset of 1st degree burns or the onset of 3rd degree burns, considering whether age plays a factor (it does), and doing a better job than the 90% confidence interval that IEEE 1584 gives on the combined ASTM rated PPE plus IEEE 1584 calculated incident energy result...like being able to derive the probability of serious injury at other confidence intervals or other means of looking at more than just a single point in the analysis.


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PostPosted: Tue Jan 15, 2013 2:43 pm 
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PaulEngr wrote:
As I understand it, the working distance is calculated to the face/center of the chest area because unlike say the extremities, it doesn't take a lot of 2nd/3rd degree burns in this area to raise the risk of mortality significantly, as compared to losing a hand or arm which is survivable. At the end of the day after all, IEEE 1584 is based around making an arc flash incident survivable . . .


That is how I read it as well. The intent is to prevent death. Losing an arm, while regrettable, is better than getting killed outright. In addition, the degree of injury that is still possible in the limited access should serve to remind everyone just how dangerous arc flash is.

It seems to me people who work around electricity have a tendancy to forget this. The longer one goes without getting injured just serves to lessen the perceived danger - even in the face of near-misses.

I had a field service engineer ask me while I was presenting some of the statistics regarding arc flash injury/occurances if I was just trying to scare him. This is in no way meant just to scare workers. Workers need to be aware of the potential for injury as well as being aware that donning the correct PPE is not some kind of punishment or some sort of morbid or sadistic joke.


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PostPosted: Mon Jan 21, 2013 9:08 am 
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I don't think unexpected configuration is a hazard very often. Anybody opening up switchgear, especially switchgear from the rear (which implies a fairly involved project) should be familiar with that switchgear and where the bus is located. I realize that is not an 'academic' solution, but it is based on observations from the field: these jobs are generally well considered and anticipated. They don't send an apprentice or inexperienced electrician in to mess with energized gear at this level. And when there is a piece of unfamiliar gear, it is approached with great caution.

Working too close might be a more common mistake; we calculate the HRC at an assumed distance and post that on labels, but often times the workers hands will be closer than that to the arc source. Arc flash gloves or rubber insulated gloves with leather keepers, that are used on the sites I am familiar with generally have a higher HRC than what we have calculated so that offers a bit of protection.

I find it unfortunate how particular arc flash work has become and wonder if greater safety might have been provided by simpler analysis rather than more complicated analysis.


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PostPosted: Mon Mar 04, 2013 11:28 am 

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Gary B wrote:
I find it unfortunate how particular arc flash work has become and wonder if greater safety might have been provided by simpler analysis rather than more complicated analysis.


Gary - Are you inferring that this type of work should be performed de-energized? If you are, then I agree 100%, working de-energized should be the norm. It seems we are always looking for a way to justify working energized.

I have found using arc flash blankets (in addition to a worker's PPE, not in place of PPE) is a good way to mark out a work zone and provide some extra protection to the workers around switchgear. I recently came across 20 KV uninsulated switchgear bus from a major US manufacturer. I didn't think anyone would still send uninsulated MV switchgear out the door, but appears they do.


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PostPosted: Wed Mar 13, 2013 7:35 am 
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CSC wrote:
Gary - Are you inferring that this type of work should be performed de-energized? If you are, then I agree 100%, working de-energized should be the norm. It seems we are always looking for a way to justify working energized.

I have found using arc flash blankets (in addition to a worker's PPE, not in place of PPE) is a good way to mark out a work zone and provide some extra protection to the workers around switchgear. I recently came across 20 KV uninsulated switchgear bus from a major US manufacturer. I didn't think anyone would still send uninsulated MV switchgear out the door, but appears they do.


Sure they do. Portable substations, particularly those used by mining companies, use uninsulated bus and bolted switchgear. Drawout switchgear is utter crap and doesn't hold up to being moved around on a regular basis. It only successfully works (for a while) when it is mounted on a stable concrete, level platform. Add any amount of external movement or especially, work on an unlevel surface, and it is nothing but trouble. The basic specification for a portable substation is to use metal enclosed gear, except that in addition to the typical LBS and fuse arrangements, you also find both breakers and contactors commonly used in this configuration. Only one manufacturer, Linepower, is actually attempting to go back and design a heavier duty rackable switchgear system specifically intended to address the low headroom requirements in underground Appalachian coal where sometimes working heights are limited to 36" at best. In this environment everything needs to be a "slideout" or "tipout" system designed to work in a space of 24" or less so that you can slide it out and work leaning over the top of it. Although insulated bus and even insulated connectors (IEEE 386/separable "elbow" connectors) would be a large benefit, their adoption is only coming along slowly due to lack of experience with them. Insulated bus is also particularly expensive when you are building something that typically only lasts 10-20 years depending on whether it is an underground or surface application. The equipment must be cleaned on a frequent basis anyways so you don't get a particular advantage from insulated bus unless you go all the way to insulated switchgear (underground/vault style) where cleanliness is no longer critical. I've tried with and without insulated bus and saw no performance advantage (no decrease in downtime) whatsoever.

Now, if your comments are directed towards "open" switchgear such as traditional "overhead" substations, again, it's a matter of application. The front end of the portable substations in my facility use jumpers to tie into the overhead line. Directly below that depending on the size of the substation, it has either a gang operated load break switch with fuse cutouts or just fuse cutouts along with lightning arresters. Due to contamination/failure issues we gave up on the indoor version altogether because the overhead versions hold up year after year because they have a natural cleaning mechanism (rain water). If I went back there again, I'd go for a vault-style unit where the entire unit is sealed. When I do this then in the transition from overhead cable to shielded cable, I have to insert surge arrester protection. Once I do this it will be outdoor/overhead rated and I need to be able to disconnect the load and change the surge arresters so I might as well put a switch on the pole, and then at this point the value of the indoor gear tends to disappear because I'm right back where I started from. For indoor switches at 25 kV, I've tried Powercons, ABB, and Square D. I have yet to have a Federal Pacific switch fail but then I get stuck with the ergonomics issues with an SM-5 fuse package compared to an expulsion fuse. Oil immersed switches built into transformers might be a good option but my experience with oil filled <fill in the blank> has been that this stuff is all trouble eventually. Anything that intentionally develops an arc (e.g. a switch) is even more trouble than the stuff that is supposed to be "passive" (transformers).

Due to the safety/failure prone issues with draw out gear and the few times that having it in that configuration leads to increased availability, I'm beginning to consider whether we should just copy the underground utility companies and build all the switchgear in modular units connected by load break or non-load break "plugs" (separable connectors). Then I'd just unplug a breaker and swap it out as needed rather than messing around with draw out gear. This is exactly the design philosophy that Duke Power (largest utility in the U.S.) has gone to with their 15 kV and 25 kV underground gear and it makes a lot of sense.


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