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 Post subject: in the box out of the box criteria
PostPosted: Wed Jan 08, 2014 9:45 am 
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I work in the Generation sector, we had a study performed by a contractor. On our UAT (unit aux transformer) switch gear (13.8Kv/6.9Kv) the study calculated the incident energy somewhere in the nieghborhood of 100 cal because the transformer is fed directly off our generator. They would not consider the Main breaker for the line up as a point of protection because it was "in the box".
This presents a problem with operations (racking in/out) because we don't have remote capabilities and mitigation is also long term. If the Mains were considered "out of the box" my numbers would drop down to the 35 cal range, operations would not suffer. The cubicles are side by side but seperated by steel panels, is this truely "in the box"?


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PostPosted: Wed Jan 08, 2014 12:36 pm 
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Could you post a one line of this situation? It would probably help but I am thinking that the UAT transformer primary is connected directly to the generator terminals and the terminals also feed direct to the GSU??
The breaker you are discussing is the main incoming breaker from the UAT low side terminals?
If this is so, then the next upstream protection is nothing sort of the generator protection shutting it down on something other than overcurrent.

What is the arcing time used? Is the 2 second limit applicable here?

Yes it is a box configuration.

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PostPosted: Thu Jan 09, 2014 7:51 am 
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wdb,
Thanks for the reply. Your sumation is correct, including the 2 second limit. I agree with your determination, problem is the company has decided that they are "out of the box". Apparently they have contacted other utilities that have made the same determination, mainly I believe because the "dangerous" incident energy levels restrict everday tasks such as racking breakers.
Our problem stems from the fact that we were not prepared (remote racking/mitigation) to deal with the numbers we received from the study.
Even if I believed they were "out of the box" there are still other issues I consider to be relevant. Our breaker PM's on the Mains range from 3-13 year intervals, control power is daisy chained from cubicle to cubicle terminated directly above the breaker and nobody has thought about the fault propegating to the line side of the Main breaker.

I would like to hear from other Generation folks: Are you doing the same thing? Are you suiting guys up above the 40cal level?


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PostPosted: Thu Jan 09, 2014 1:34 pm 
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I am interested in why they think they are out of the box. This may relate to using NESC for inside generating stations where I have found that IEEE 1584 is more appropriate as generating stations have a lot in common with industrial facilities. NESC is good for the overhead lines and open air substations where the predominate arc flash is a SLG event due to conductor spacing.
I spent 30 years in utilities, primarily generating stations and substations, and when I did the arc flash analysis I used both the above approaches.
It would be interesting to see how they justify that they were "out of the box" when an unfortunate incident occurs and it results with an individual "in a box" (i.e. a fatality)

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PostPosted: Thu Jan 09, 2014 3:35 pm 
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generation safety adv wrote:
They would not consider the Main breaker for the line up as a point of protection because it was "in the box".
If the Mains were considered "out of the box" my numbers would drop down to the 35 cal range, operations would not suffer. The cubicles are side by side but seperated by steel panels, is this truely "in the box"?

"In the box" and "out of the box" can mean 2 things.
Do you mean "arc in a box configuration", with a partial opening which will focus energy at the worker (which I believe it is since it's in a sheet metal enclosure)?
Do you mean "mains is out of the box" from the racked in/out breaker, meaning the mains is in a separate compartment, so an arc flash occuring in a compartment will not propagate to a neighbouring compartment? That second question depends a lot on the equipment design.


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PostPosted: Fri Jan 10, 2014 7:57 am 
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Vincent your last statement is correct. I'm referring to the meathology (in the box/ out of the box) used to determine the point of protection. Our original study did not count on the main breaker of the switch gear to open, quenching the blast. All points of protection were determined to be breakers upstream of the main breaker. For example, the breaker that feeds the primary side of the transformer which in turn feeds the switch gear where the work will be performed.
Since there are no isolation points between the Generator and the UAT, the study came out at over 100cal. This obstructed day to day operations so our engineers decided that the main for the switch gear where work will be performed is, in thier opinion "out of the box".
Doing this allowed them to adjust relaying on the main, getting the incident energy value below 40cal, suit up and perform those daily tasks.
Sorry about the confusion earlier. It took me three years to convince them that our flash conditions were in fact "flash in a box", which in turn lead to them performing the study. Originally they said follow the tables in NESC, which left employees under suited for the tasks.


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PostPosted: Sun Jan 12, 2014 7:16 am 
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There have been some cases where arcs propagate such as magnetically propelled arcs moving down along an MCC section being most common, and plenty of cases where panelboard arcs propagate to the line side of a main breaker. I haven't turned up anything beyond anecdotal concerns of propagation from section to section within an MCC or switchgear. I'd like to know the circumstances if it does.


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PostPosted: Mon Jan 13, 2014 9:06 am 
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The question revolves around the incoming line Main CB in the switchgear being able to extinguish an arc or would the arc propagate to the line side of the Main CB. For switchgear that is constructed to the ANSI C37 standards, it is my opinion that an arc flash/blast in a downstream cubicle would not result in the arc propogating to the incoming line-side of the Main CB. Thus all of the cubicles downstream of the incoming line main CB would be able to use the main CB interrupting time. However, the Main CB cubicle would have to rely on upstream protective devices for arc interruption so that one cubicle would have that "Dangerous" level.

I don't think the same statement could be made about a MCC. An arc blast in a section next to the main device will definitely affect the adjacent sections. Would an arc propagate to the line side the main device? Maybe. Until its demonstrated that it can't then I'll be using the next upstream device from the MCC.

I get some blowback from equipment rep's on the arc propagation issue in MCC's. I'd like some opinions on using insulated bus in MCC's to eliminate the arc propagation issue. If the horizontal bus is insulated, then in theory, an arc shouldn't be able to propagate between sections.


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PostPosted: Wed Jan 15, 2014 11:44 am 
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Jeff,
I get your point. Unfortunately I haven't found any documentation on the construction of our switchgear. They range in age from 35 to 50 years old. Were they constructed with Arc Flash concerns, doubtful. But how do you view the issue of the control power? The way ours are daisy chained from cubicle to cubicle, located directly above the breaker, if a flash were to occur there is a good chance the leads could seperate, killing the source to trip the main. [ATTACH=full]343[/ATTACH]


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