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 Post subject: Arc-flash boundary
PostPosted: Mon May 05, 2014 9:46 am 

Joined: Thu Oct 25, 2012 10:37 am
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Hi there,

I am new to the site and wonder if you can guide me to the right direction or directly response to my silly concern:

1- If I calculate the Ei and Arc-flash boundary of an Network Protector (vented / submersible), does those values apply when the door close as I walk by?

2- If PPE is chosen base on AFHCS, does one need to suite up only opererated with door open/close or event just open door for thermal and IR inspection?

Thanks
Tho Tran


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PostPosted: Mon May 05, 2014 11:22 am 
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Tho Tran wrote:
I am new to the site and wonder if you can guide me to the right direction or directly response to my silly concern:


Not silly. And its an issue that has been wrestled with a lot, and you've thrown an extra monkey wrench into it by saying network protector and it appears you are talking about a utility scenario.

There are two issues to contend with: the severity of the injury and the likelihood that it will occur. For instance although being struck by a meteor is generally fatal, the likelihood is so small that we're not going out and erecting meteor shields for personal protection on this planet.

Right now we have lots of hazard assessment procedures which give an indication of severity and what to do to lower the injury to an acceptable level assuming that the PPE works. One such standard is IEEE 1584 which gives a 95% success rate according to the standard. Others are tables in NFPA 70E and the NESC.

On the likelihood front, as of right now there is not much to go on. OSHA has proposed final rule making for utilities in OSHA 1910.269 and includes a task table, and there is a similar task table in the draft version of NFPA 70E for 2015. I'd recommend looking at either one. The previous guidance in 70E (2012 edition) is not very good and I'd recommend against using it.

In either case, we have to consider the condition of the equipment and the task. While performing certain tasks such as moving jumpers are 100% dependent on the likelihood that the worker performing the task does not make a mistake. Humans are not even close to the reliability level that is expected for potentially fatal hazards so these tasks are simply not acceptable without precautions. Thus we assume that the hazard will occur even if the likelihood is fairly low (say once in 100 years) and move on to wearing whatever PPE the hazard calculations/tables suggest. In analyzing the hazard, it may be the case that for instance, the worker is working equipment "remotely" such as from the end of a hot line tool or using remote controls at which point even if an arc flash occurs, the worker could not become injured. At this point the second issue is the installation and maintenance of the equipment. If the equipment was not installed or maintained properly, then the calculations which rely on properly functioning equipment are probably invalid. At that point, the best you can do is make some worst case assumptions or de-energize using known, propoerly installed and maintained equipment just to get through the maintenance aspect.

There are certain activities which categorically don't pose a hazard because they do not involve interacting with the equipment in any meaningful way. Examples of this type of activity include performing inspections without making physical contact and outside a distance where someone could accidentally move and come into contact with equipment, which would include just walking by and taking photos with IR cameras, checking settings or reading meters, and similar functions.

Finally we have cases where the skill of the worker is almost immaterial to the task such as most switching activites.

All types of drawout equipment (breakers, MCC buckets, and meters in meter sockets) tend to be very unreliable when it comes to inserting or removing equipment into the cell. Categorically wear appropriate PPE when performing these tasks with this type of equipment.

Opening doors can be a problem. There are numerous cases where conductive dust, lliquids, and tools fell into enclosures when the doors were opened. Similarly doors which are held on entirely by bolts other than circles (manholes) can and have fallen into the equipment in the process of being removed. Granted it is certainly possible to control the environment such that these tasks can be rendered safe in many cases. Finally, a quick assessment of the equipment condition can usually indicate impending problems. For example equipment that is covered in water or has water running into it, which is badly rusted and perhaps even has holes showing through, or has obvious discoloration, black soot marks, or paint bubbles clearly indicates a possible hazardous condition inside the door. If any of the situations I just described exists, then pretty much any task that involves interacting with the equipment has an elevated likelihood of an arc flash. Otherwise electrical equipment is designed to be safe and reliable and the risk of injury due to perform a task of this type should not be considered a risk that would require PPE.

As an example of this sort of thing for underground gear, installing or removing right angle disconnects which are designed to be just as reliable as other types of disconects, is probably not an arc flash hazard. There have been reported cases of failures of the load break type in Northern climates but this appears to have been abated over the years (ask your supplier).

So I'm a bit confused by your second question...
Quote:
1- If I calculate the Ei and Arc-flash boundary of an Network Protector (vented / submersible), does those values apply when the door close as I walk by?


No. In fact the 70E Committee has stated over and over again in proposals that just walking by does not pose a significant hazard. In other words, electrical equipment is not inherently unsafe.

Quote:
2- If PPE is chosen base on AFHCS, does one need to suite up only opererated with door open/close or event just open door for thermal and IR inspection?


This is where it gets confusing. Switching operations are listed as acceptable without PPE with the caveats I described earlier under the standards being drafted for OSHA regulations and in the draft NFPA 70E-2015 edition. Knowing that breakers have a fixed number of electrical operations that they are designed to work under, if I come up to a breaker with soot marks all over it or perhaps with liquid pouring out of the cabinet in a non-submersible design at the bottom, it would obviously be utterly stupid to think that the breaker is safe to operate. Otherwise, the equipment should be considered safe to operate.

Right after equipment is faulted, knowing that larger breakers especially when designed by IEC/ANSI standards only have to survive a single fault at maximum short circuit current, again it is prudent to suit up before opening/closing the breaker. A worker was injured due to arc flash at the site I work at today after a molded case breaker was operated several times during troubleshooting, and finally closed in one more time after repairs were made to a pump motor WITHOUT ever inspecting the breaker even one time. It finally failed catastrophically after maintenance "completed" repairs but never inspected the breaker, completely contrary to NEMA AB-4 which is the maintenance standard for every molded case breaker produced for the North American market.

As to opening and closing doors, which should be treated as a separate task, I stated my concerns earlier. Since there are no doors exposing conductors on vaults, I'm not sure where the question is coming from. This would be the same as asking whether or not it is safe to open the door to my office, knowing that there is a light switch and some electrical fixtures on the other side of the door. If however the door conceals exposed equipment, then my previously stated concerns based on actual documented cases stand.

Keep in mind that doors themselves are not really a viable protection against arc flash. If the door remains intact, there is some reduction but there is no guidance as to how much it will be. If the arc is large enough to cause a significant arc flash, it will send the door flying which in itself becomes a significant hazard (though I did not find any specific references to this in the OSHA database so credibility here is rather weak). So from a pure hazard point of view alone, ignore the doors. The doors only come into play when consideration is being given as to the likelihood of an injury. For instance with exposed, energize bus bars, using a wrench within a certain distance to those conductors poses a significant arc flash hazard. Without exposed, energized conductors such as with doors closed, that hazard goes away.

Finally, as to IR scans themselves, if a worker plans on standing outside an appproach distance where harm could come to them including inadvertent movement, then there is no inherent danger in that portion of the task. If the worker plans on taking a tool and probing around in a bundle of wiring that is suspect for instance then this is a different task entirely and should be considered separately.

Sorry that this is a bit long winded but as of right now, the standards that are out there (NFPA 70E-2012 edition, NESC -2012 edition) don't really lend a lot of help to the question of likelihood. This is changing. Expect it to get a lot better in the next revisions.

For my own part, I'm currently putting together a summary of every case of arc flash injury from 2007 to 2012 in OSHA's accident investigation database, publicly available on their web site, consisting of 114 cases. As per OSHA these are a sampling of data and thus not a complete summary of all accidents in the U.S., and the sample set obviously has bias issues. That being said, what we have are 114 examples of how someone has actually received an arc flash injury rather than simple conjecture of what might be possible and arguments about likelihood. None involved anyone just walking by nor by someone doing noncontact inspections. A few examples exist of opening covers where either the cover fell apart, fell into the enclosure, or external tools or materials fell in during opening. One involved a breaker that was previously improperly repaired and then subsequently exploding.


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PostPosted: Mon May 05, 2014 4:29 pm 

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Thanks Paul.


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PostPosted: Mon May 05, 2014 8:04 pm 

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Paul,

By the way, you mentioned about IEEE 1584 I wonder if I need to get the entire set of standard which are IEEE 1584-2002, 1584a and 1584b?

Thanks


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PostPosted: Tue May 06, 2014 12:53 am 
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Location: the Netherlands
Quote:
No. In fact the 70E Committee has stated over and over again in proposals that just walking by does not pose a significant hazard. In other words, electrical equipment is not inherently unsafe.

This however does not mean that a breaker room is a meeting place. A few years back I got a tour at a power plant and we were only allowed to enter the breaker room because there was no planned switching (you can control this, unplanned you obviously can’t). The only PPE we were wearing was a helmet, steel tipped boots and a high visibility FR resistant coat. This is about 8 years ago I was 16 at the time so I do not know the coats rating :p Funny is that arc flash wasn’t really a term back then (at least not here in the Netherlands) but still the rule I describe above was aimed at the risk off an arc flash only it still was called short circuit.
Anyway it was standard practice that only few people had the key to the room and you are not allowed to linger inside the room and there were blast vents to vent away the hot toxic air in case off an arc flash and to prevent from a door being blown out off its hinges.


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PostPosted: Tue May 06, 2014 5:42 am 
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IEEE 1584 has been updated over time but the references you are seeing (1584-2002, 1584a, 1584b) are where some relatively small wording changes were made. The core standard has not really been updated in the last 12 years. The "a" and "b" materials are essentially error correction/update type documents.

As to making "breaker rooms" off limits, this is standard practice in SOME areas in the States. Some of this is a holdover from years past. At one time, switchgear was more or less an "open" design whether it was indoors or outdoors. Bus bars were wide open and the actual breaker mechanisms and blades were wide open as well. Even today, the standards still require locked fencing or rooms to protect unqualified personnel or the general public from getting anywhere near this type of equipment. Some older gear and higher voltage equipment is typically still of this design. Then over time it was decided that the safer approach is to just enclose everything inside grounded metal housings. There are still arguments about the value of metal clad (each phase is in a separate grounded compartment) vs. metal enclosed gear (limited to fused switches in the past but now you can buy anything in this configuration) and the purported safety benefits of the metal clad design. Later, standards were even created to have vented gear which can prevent harm to someone standing in front of the gear in the event of an arcing fault using various "blast doors" to vent the equipment. Throughout all of this however, once the doors were off, the equipment designs inside were largely unchanged. The only significant difference in terms of safety for a technician actually working on the equipment is that there was a lot more time involved in physically removing all of the various barriers to actually access the equipment for maintenance purposes, and the doors themselves could become hazards if they were merely bolted on. Finally in recent years we have started to see more use of elbow connectors which form a continuous shielded exterior down to the termination, "finger safe" terminal strips, moves towards lower voltage controls, and various other features that are finally making the internal components of the equipment as safe as the external housings.

Thus there is a holdover from years past where switch rooms are considered off limits to most employees.

Granted there are also some advantages to this. It is much less likely that there will be a multiple injury case if there is only one person standing next to the gear if it explodes. Although we still cannot lose site of the value of having a second person available to render emergency aid (CPR, emergency release or rescue) in the event that an accident occurs.


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