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 Post subject: Category 0 PPE and Touch Screen Switchgear Controls
PostPosted: Thu Jan 30, 2014 1:50 pm 
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Hello,

We have recently installed 480V generator switchgear at my facility that is primarily controlled via a 15" (resistive input) touch screen display. Our Electrical Safety procedures in place dictate that the operation of these breakers (with the panel doors closed) falls under Cat. 0 with an arc flash boundary of 48". The touch screen is immediately adjacent to 480V breakers which puts the Operators within the 48" boundary.

My question is: Should the Operator be wearing leather gloves when operating the switchgear via the touch screen? This task is somewhat cumbersome in the gloves and could lead to a misoperation fairly easily. Another option we are considering is to mount a touch screen stylus near the panel so that it could be easier to operate while wearing gloves. Has anyone else dealt with an issue similar to this? Thanks in advance!


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PostPosted: Mon Feb 03, 2014 5:36 pm 
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Very hard to get touch screens to even work with gloves on. Category 0 with a 48" boundary sounds ridiculous at 480 Volts. The working distance for 600 volt gear is usually 18 or 24 inches. You can't have a boundary of 48 inches and a 1.2 cal hazard at the working distance. Working backwards from a boundary of 48 inches it would be hard to get that value. It sounds like you are using the old rules from the 2004 edition of 70E. That rule has been removed. The current definition of an arc flash in 70E 2011 edition states that an arc flash hazard exists provided that an employee is interacting with equipment in a way that could cause an arc flash. The "could" is meant in a limited sense...the 70E committee has stated several times in their reports that they do not consider just walking by as dangerous for instance. The case at hand is not much different. If there is no interaction of this type then there is no significant hazard and as a consequence, just as with shock hazards with the doors closed, the arc flash hazard boundary also does not exist.

Might want to consider first whether an arc flash hazard even exists. If so then moving on the tables give as an example reading a meter while operating a meter stick which is probably closest to what you are describing. This is H/RC 0. You may also want to consider that the H/RC 0 category with gloves assumes you have the hands in the typical position where interaction causes an arc flash such as standing with the door open and screwdriver inserted where it does not belong, hand on screwdriver. I'd argue for no gloves.


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PostPosted: Wed Feb 05, 2014 7:22 am 
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Paul,

Thanks for the detailed response. I definitely agree with your statements in regards to considering if this is a credible arc flash initiating scenario. The problem I see is that the PLC/Touch screen interface in this case is basically a breaker control switch since it initiates the breaker operations. In a few cases, the generator main breaker is directly below the touch screen, which puts you within the upper limit of Cat. 0 boundary (~32"). These are considerably large 480V generators (1.5 MVA of capacity each), so the bolted fault values are significant.

The operator must select which bus they would like to transfer to/from the generator at the PLC then finally click an "IMMEDIATE TRANSFER" button. My thoughts (to comply with our current procedure which requires the 48" boundary) are to only have the operator wear the gloves when they push the final transfer button. This has been tested and doesn't seem to be too cumbersome.

I also appreciate the insight into the differences between the recommendations in the 2004/2011 edition. It is possible that our safety procedure should be updated to reflect the removal of that rule. Thanks again.


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PostPosted: Thu Feb 06, 2014 11:58 am 
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Since you are using a PLC, program in a delay. Re-label the button "5s transfer", or such; giving the operator time enough to move out of zone.


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PostPosted: Fri Feb 07, 2014 2:30 pm 
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stevenal: What if the operator trips? That's the snag I ran into. A simpler solution that is usually available since the touch screen has some sort of network port is to simply MOVE the touch screen across the room.


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PostPosted: Mon Feb 10, 2014 10:15 am 
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A delay can also be used for openning. Moving the screen would also work. Are you suggesting a permanent move or a tempory one to be restored to its normal position after the control is performed? The later leaves room operator non-compliamce.


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PostPosted: Mon Feb 10, 2014 12:08 pm 
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I mean physically placing the controls permanently outside of the arc flash boundary. OP's arc flash boundary seemed nonsensical in the first place but even at 48" when you need at least that much to meet NEC clearance requirements in the first place, you'd be talking about relocating a network cable and a power cable to a wall somewhere else in the substation.


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PostPosted: Tue Feb 11, 2014 8:17 am 
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First I want to say that I like the practical solutions that occur on this forum. There is alot of theory that is discussed but this place is also where conversation occurs for real world solutions.

As an operator I would prefer to use a stylus for the entering. I would be able to wear the PPE and have accurate entries onto the screen. It would be great to relocate the screen completely out of the boundary but with some facilities it could be months before a PO is generated, bids are submitted , and the job awarded depending on how your location operates. I can go to the office store, get a stylus, and mount it near the screen with a lanyard before most places generate the PO. The financial department/ managers may buy you lunch if you give them the two options and then say that you are going with the low cost one. :)


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PostPosted: Tue Feb 11, 2014 9:49 am 
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KEC wrote:
First I want to say that I like the practical solutions that occur on this forum. There is alot of theory that is discussed but this place is also where conversation occurs for real world solutions.

As an operator I would prefer to use a stylus for the entering . . . . The financial department/ managers may buy you lunch if you give them the two options and then say that you are going with the low cost one. :)


You have to be careful using a stylus. We have had customers stab through a touchscreen with a stylus before, and I mean repeatedly. They replace a screen and before you know it they have to send the new screen back because it has a hole jabbed through it.

PPE should be the last resort. If a task can be made safer by relocating the touchscreen, I would urge management to take that resolution. It certainly looks cheaper to buy a stylus to use, but if you have an employee get an up-close and personal experience with an arc flash, that PO that looked to be so expensive will be nothing compared to that cost.


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PostPosted: Tue Feb 11, 2014 11:01 am 
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Finally, if you want the more complicated answer, consider working distance another way.

Keep in mind also that the working distance (48" in this case...again, still question that) is based on the location of the anticipated arc. The standard IEEE 1584 distances (18", 24", 36") are based on the distance from the bus bars which are located at the back of the cabinetry and are remarkably very similar to manufacturer to manufacturer, to the face and chest area of a worker located in front of the equipment performing a maintenance task on the equipment. The logic behind using the face and chest area is that these are the areas that are most sensitive to a burn injury, where only about 9-10% of body surface area has to be burned (2nd degree or greater) to be fatal. Loss of an arm or leg is generally not fatal, and IEEE 1584 and NFPA 70E are designed to prevent fatalities only, not to allow one to walk away. So far though it looks as though the standards are conservative enough that this has generally been the result.

Now this would be for instance the working distance for say turning a lug with a screwdriver with the hand outstretched and reaching into the cabinet. If the cabinet doors are closed and the worker is for instance standing in a comfortable position operating a touch screen or manually operating a breaker, the working distance would need to be recalculated for the new location which is significantly farther away than working directly on the equipment in an open cabinet.

As has been stated before, don't try to take credit for any "shielding" that the doors may or may not provide. If the doors blow off, there is no shielding anyway. Even if the doors stay intact, it will warp the door and the hot gases will jet out around the edges. None of these effects are currently taken into account by any model so the best approach is to simply pretend that the door does not exist from an incident energy calculation point of view.


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PostPosted: Wed Feb 12, 2014 7:16 am 
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Paul said "IEEE 1584 and NFPA 70E are designed to prevent fatalities only" Do you have a reference for this?


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PostPosted: Wed Feb 12, 2014 10:09 am 
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Technically I think 70E is designed to prevent anything worse than a second degree burn, but that said 70E is more concerned with protecting the core and head than it is with protecting the less important appendages.


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PostPosted: Wed Feb 12, 2014 10:59 am 
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Quote:
Do you have a reference for this?


To be more precise, I am only partly right. NFPA 70E does not specifically recommend any particular arc flash hazard calculation method. Annex D in the 2012 edition references several of them, and there are more that are available but not referenced. The general consensus to date is that IEEE 1584 is the most accurate and has the backing of actual test data (a feature missing in many others) but has limited application since it is an empirical method.

I don't have my copy of IEEE 1584 with me but please bear with me as this should be pretty obvious. The references requested are ASTM 1959 and IEEE 1584.

IEEE 1584 is an empirical method to calculate the amount of incident energy at a given distance from an arcing fault. Other arc flash formulas are similar in that they all aim to predict the same threshold. The standard distance that is used in IEEE 1584 is called the "working distance". It is based on the distance from the bus bars at the back of the cabinet (the source of the worst case possible arcing fualt) to the face/chest area of the worker. Note: arms and hands are not considered at all. The focus is on the face/chest. No protection for the hands and arms is predicted or anticipated. The complete IEEE 1584 standard gives a lot more detail on this but a good summary is given in Annex D of NFPA 70E. So if you don't want to pay several hundred dollars you can use that one.

Second, Alicia Stoll subjected several sailors to a test to determine the exact threshold at which a heat source could cause a second degree burn. She equated this back to a copper calorimeter sensor so that today, we don't have to subject anyone to second degree burns to measure the second degree burn threshold. The threshold is given in cal/cm^2. This is indirect information that is used next. The Stoll curve is widely documented on the internet (google it).

The PPE is generally selected via ASTM 1959 for cloth (rainwear and face shields use different standards). The test method suspends a piece of cloth which would be used in manufacturing PPE and subjects it to an arc. A calculation is run to determine the threshold value where the PPE fails to meet the Stoll curve...the point of onset of a second degree burn. The complete standard is kind of dry but explains this in detail. ASTM 1959 does not test "PPE". It tests material samples.

IEEE 1584 contains a numerical calculation that was done where PPE meeting the ASTM 1959 standard was numerically compared to arcing faults with the incident energy calculated according to the IEEE 1584 empirical method. The percentage of failures (to prevent a second degree burn) were determined. 95% of the time, PPE selected according to ASTM 1959 will protect against a second degree or greater burn. Note that this is simply a numerical simulation...it does not represent real world results. Nothing about IEEE 1584 or ASTM 1959 represents real world results directly.

This informatio has to be translated into fatality statistics. There is a method called the "Baux" scoring method that predicts survivability from a burn that is used for triage purposes. The Baux score is equal to the total body surface area burned plus the victim's age. If this score exceeds 100, there is a high likelihood of a fatality. Arms and hands count for about 18% of total body surface area. Once a burn extends to the face/chest area, an additional 17 points is added to account for burns to the lungs, and since the bulk of the body is in the torso area, the likelihood of a fatality goes up dramatically. The Baux score is frequently given in books on arc flash as a chart relating % burned and age. You can find it all over the internet and in Jim Phillips book. So if only arms and hands are involved, a fatality due to burns is not very likely.

Finally, so far there has not been a reported case of a fatality while wearing the proper PPE in the proper manner as calculated by IEEE 1584. There have been reported injuries by either not wearing sufficient PPE or wearing it improperly. See:
"Update of field analysis of arc flash incidents, PPE protective performance and related worker injuries", by Doan, Hoagland, and Neal.

Does this mean that IEEE 1584 and ASTM 1959 guarantee "no injury"? No. Injuries were reported in the study listed above, just not fatal ones. In addition, IEEE 1584 and ASTM 1959 give the 95% performance measure only right at the very threshold. This means that 1 out of 20 cases might fail to be protected, and the above paper only located 30 cases. Arc flash incidents are even more rare than shock incidents (by a 2:1 margin according to BLS statistics analyzed by ESFI). This rarely actually occurs in practice in the real world. For instance if PPE is selected to protect against incident energy from 4 to 8 cal/cm^2, numerically most cases are not going to be 7.9 cal/cm^2. If one looks at an actual ASTM 1959 report, PPE passes 100% of the time if the incident energy is between 0.5 and 1.0 cal/cm^2 below the threshold (sharpness of the line is material dependent). Thus with such low incident rates it might be decades before the 95% performance threshold can be tested in the real world.


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