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 Post subject: Working and opening and closing buckets on a buss duct
PostPosted: Thu Jan 29, 2015 11:18 am 
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Hello everyone.
I have a question that I am hoping someone may be able to answer or provide an opinion on.
We work in an industrial lab. Most of our equipment is fed from buss duct. We had an analysis done a few years back which labeled these duct as Dangerous, but the fused disconnect buckets are rated any where from 0 to 3 through out the plant.

My question is, if the bucket is rated a CAT 0 do we have to where PPE to open and close them when we are using a 10ft hook? Up to this point we have not, but I have recently been cautioned from one of our electricians that services another area that even though we are using a hook we are still interacting with the bucket that is still attached to a Dangerous buss way.

your feed back would be greatly appreciated.


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 Post subject: Re: Working and opening and closing buckets on a buss duct
PostPosted: Thu Jan 29, 2015 5:02 pm 
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TeddyN wrote:
... we are using a 10ft hook? ...


They are correct.
However, in this case, you should be using 10' as the working distance when calculating the Arc Flash Incident Energy, rather than the default 18".

Effectively incident energy drops based on the inverse of the square of the distance.


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 Post subject: Re: Working and opening and closing buckets on a buss duct
PostPosted: Fri Jan 30, 2015 8:31 am 
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Lots of things wrong here. First, you are using terms like "Category 0, 1, 2" etc. Clearly the evaluation was done using the tables in 70E. Those tables are meant for use when you don't have anything else to go on. There are also footnotes on when the tables apply, especially opening time of the overcurrent protection. If you don't meet the requirements, the tables don't apply. The amount of effort needed to verify the table entries is the SAME as using IEEE 1584 empirical equations so the tables have very limited value. Worse still a research project by Doan using data from incident investigations in Dupont showed that the tables work about 50% of the time while so far the IEEE 1584 calculation works 100% of the time. If you are concerned about arc flash at all, I suggest not using the tables except for the limited cases where you don't have all the data. You can't "rerate" things either with the tables. So if you are using a 10 foot stick now and also using the tables, don't bother. This is a quick-and-dirty method, not meant for a detailed analysis.

Second, more to the point, prior to the 2015 edition of 70E, the definition of an arc flash hazard stated that an arc flash hazard may exist "provided a person is interacting with the equipment in such a manner that could cause an electric arc". This is a true statement but somewhat confusing in terms of what type of interacting is meant. The 2015 edition is a bit more clear on this. All electrical disconnecting equipment except for solid state devices by nature forms an arc. But circuit breakers, snap switches, load break switches, relays, contactors, and safety disconnects by nature have mechanical mechanisms in place to rapidly extinguish the arc harmlessly. As long as the equipment is properly installed and maintained and not showing any clear signs of damage, the likelihood of a serious injury or fatality while using the equipment is about the same as the general background risk that exists for the general public (about 1 in a million injuries/year or less). Even when considering all arc flash injuries combined, Cawley at ESFI has compiled OSHA data to show that the injury rate is around 0.1 per 100,000 workers per year, or 1 in a million. Documented arcing fault rates of disconnects are well below that rate, so wearing PPE for this activity is akin to dressing up in a fire retardant racing suit for your morning commute to avoid the risk of burns from a gas tank fire in a car accident.

Third, the use of "DANGER" stickers is incorrect. ANSI Z535 is the standard for signage that OSHA requires. The signal word "DANGER" is supposed to be used when a serious injury or fatality WILL occur. The word "WARNING" is used when death or serious injury MAY occur. 70E states that an arc flash hazard MAY occur so the terminology is incorrect. Also, the organizations that are doing this are using a totally arbitrary and mythological standard. 70E in an informational note (not part of the requirements) states that when the incident energy is above 40 cal/cm^2 "greater emphasis may be necessary" but does not define what that emphasis should be. A cutoff of 40 cal/cm^2 is silly because arc rated PPE is available up to and exceeding 100 cal/cm^2 so it is not based on physical limitations. At around 12 cal/cm^2 is the limit for single layer PPE so if at least that cutoff had been chosen, it would have made more sense. There is a myth that you would somehow be liquified on the spot or your organs explode above 40 cal/cm^2 due to a related phenomena called arc blast but arc blast is related to arc power and enclosure construction, not incident energy, and aside from a theoretical (but disproven) equation for calculating arc blast, I have not seen any evidence whatsoever of arc blast presures reaching the 20+ PSI necessary to cause a fatality due to concussive force. Thus the only other argument for using the word DANGER or indeed doing really anything different at 40 cal/cm^2 or greater does not exist.

That's not to say that we should not set a reasonable limit for a site. As a general principle in ANSI Z10 and similar standards, we should be using ALARA/ALARP principles to lower the hazards to "as low as reasonably practical". As PPE always has a risk of failing that means doing things to minimize the need for it. A 10 foot pole might be just such a great solution in your situation. I'm all for setting an arbitrary limit or goal for all electrical work. Right now in my plants we use 40 cal/cm^2. In the very near future I hope to drive this down even lower to where ultimately we may be at around 20 cal/cm^2 as an upper limit for instance.


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 Post subject: Re: Working and opening and closing buckets on a buss duct
PostPosted: Mon Feb 02, 2015 9:01 am 
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Paul, the existence of cat X on labels does not necessarily mean that the hazard analysis was accomplished through the use of 70e tables. I know that 2015 edition has made this position indefensible, but rightly or wrongly, many companies used the cat number as a defacto ppe level; cat 0 <1.2 cal/cm2; cat 1=1.2-4 cal/cm2; etc. I think that this position was perpetuated by sales of PPE which were often marketed by category. I suspect to be in conformity to the new 70e standard we will see a lot more "site specific level of PPE" which will probably look very much like categories.


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 Post subject: Re: Working and opening and closing buckets on a buss duct
PostPosted: Mon Feb 02, 2015 11:07 am 
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Roger wrote:
Paul, the existence of cat X on labels does not necessarily mean that the hazard analysis was accomplished through the use of 70e tables. I know that 2015 edition has made this position indefensible, but rightly or wrongly, many companies used the cat number as a defacto ppe level; cat 0 <1.2 cal/cm2; cat 1=1.2-4 cal/cm2; etc. I think that this position was perpetuated by sales of PPE which were often marketed by category. I suspect to be in conformity to the new 70e standard we will see a lot more "site specific level of PPE" which will probably look very much like categories.


I didn't pick up on it just from this. Also see from the original post:

Quote:
We had an analysis done a few years back which labeled these duct as Dangerous, but the fused disconnect buckets are rated any where from 0 to 3 through out the plant.


So we clip a disconnect onto the buss duct. The incoming lines at lesat within the "bucket" unless there is some other assumption being made that faults can only happen on the load side (and load->line propagation of an arc cannot occur) then the disconnects should have the exact same rating as the buss duct. But it has a very different incident energy rating whether it is expressed as cal/cm^2, or HRC/PPE "Levels".

Agreed about PPE manufacturers wanting simpler and more uniform systems but then they keep "one upping" each other on the cal/cm^2 ratings being higher than the HRC/Levels.

Buss duct is inherently a nightmare to do an incident energy analysis on especially in a lab setting because not only is the location of the equipment changing on a regular basis but also the specific loads and cable lengths are changing, too. So some sort of estimation has to be done. Either the engineering analysis has to look at best/worst case, or else appeal to an analysis method that is less igorous than IEEE 1584.. Either way the resulting labels are "generic" (not load/location specific). The best way from a point of view of least liability for the engineering firm to do the analysis would be use of the tables for nonfixed assets. This would then result in a rating of 0-4 for the loads, but a much different rating (>40 cal/cm^2??) for the buss duct.


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 Post subject: Re: Working and opening and closing buckets on a buss duct
PostPosted: Thu Feb 05, 2015 2:25 pm 
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Just for what it's worth- "buss duct" or "bus duct" is jargon. The official ANSI and NEC term is "busway".


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