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PostPosted: Sat Mar 19, 2011 7:17 am 
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Zog wrote:
However, most MCCB's are only rated for one short circuit interuption at rated current. And I doubt this breaker has a trip unit with indication of the type of trip that occured. So, what happens if the breaker trips? What sort of fault did it see? If it was a short circuit and the breaker is only designed and rated for one of those interuptions, is it being operated under "normal operating conditions"?


I have continuously stated my discussion is about the routine on-off operation not resetting of a circuit breaker rated/listed/tested to be used as a switch.

A main breaker panelboard located on the 208Y/120V secondary of a 150kVA transformer will most likely have an HRC=3, yet the maximum bolted fault current it could see would be 18,043A if the transformer had an impedance of 2.3%. The proper breaker to choose would be one rated for 22kAIC. Given a circuit length of as little as 5' of #10 copper, the maximum bolted fault the 22K breaker could be closed on would be 10,218A which is less than 50% of its rated current.


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PostPosted: Thu Mar 24, 2011 4:38 pm 
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JBD wrote:
Absolutely not according to the NFPA 70E definition you are quoting.


The definition says it is "not likely to pose an arc flash hazard". It does not say that an arc flash will not happen. In my opinion, that means that there is a potential for an arc flash to occur.


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PostPosted: Fri Mar 25, 2011 5:03 am 
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A 120V residential panel is "not likely" either, but here is one that was.


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PostPosted: Fri Mar 25, 2011 5:07 am 
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JBD wrote:
I have continuously stated my discussion is about the routine on-off operation not resetting of a circuit breaker rated/listed/tested to be used as a switch.

.


I don't think I ever said you did not say that. So, as far as labels, do you put different HRC's on your labels for "normal operation" and "reseting"? If the calulated Ei is a HRC 3 like in the OP what method do you use to determine the HRC for "normal operation"?


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PostPosted: Fri Mar 25, 2011 7:56 am 
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Zog wrote:
I don't think I ever said you did not say that. So, as far as labels, do you put different HRC's on your labels for "normal operation" and "reseting"? If the calulated Ei is a HRC 3 like in the OP what method do you use to determine the HRC for "normal operation"?


The label would say Cat 3, the facilities Electric Safe Work Practice procedures would define what is normal operation versus what is 'interacting in a manner that could cause an arc'.

This is similar to the way many facilities deal with labels based on 'backup/emergency' generator power, choosing to cover these conditions in their ESWP.


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PostPosted: Fri Mar 25, 2011 8:08 am 
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holliday wrote:
The definition says it is "not likely to pose an arc flash hazard". It does not say that an arc flash will not happen. In my opinion, that means that there is a potential for an arc flash to occur.


Today the lottery is over $300 million, are you going to buy a ticket because you are likely to win? There is always a chance that a vehicle will run a red light, do you never cross a street because you are likely to get hit?

The code was written using the modifier 'not likely', maybe they should have used 'not probable' or 'not possible'?


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PostPosted: Fri Mar 25, 2011 12:26 pm 
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ZOG wrote:
I don't think I ever said you did not say that. So, as far as labels, do you put different HRC's on your labels for "normal operation" and "reseting"? If the calulated Ei is a HRC 3 like in the OP what method do you use to determine the HRC for "normal operation"?



JBD wrote:
The label would say Cat 3, the facilities Electric Safe Work Practice procedures would define what is normal operation versus what is 'interacting in a manner that could cause an arc'.

This is similar to the way many facilities deal with labels based on 'backup/emergency' generator power, choosing to cover these conditions in their ESWP.



If you are doing the an "Incident Energy Analysis" (IE) then you do not use the "Hazard/Risk Categories" (HRC) per NFPA 70E 2009 Secton 130.3 (B) & 130.3 (C).


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PostPosted: Fri Mar 25, 2011 12:36 pm 
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Kimo wrote:
If you are doing the an "Incident Energy Analysis" (IE) then you do not use the "Hazard/Risk Categories" (HRC) per NFPA 70E 2009 Secton 130.3 (B) & 130.3 (C).


Well the task tables are not to be used if you have done the calulations, but the HRC's are still commonly put on the labels for simplification. You take your calculated Ei and select the HRC from table 130.7(C)(11).

This is another one of those confusing areas of 70E, but this one is getting clarified in the 2012 version.


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PostPosted: Mon Mar 28, 2011 7:18 am 
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I think the simple fact that you experience guys can debate this issue for 5 pages of posts is a clear sign that 70E should be written more clearly than it is. It is tremendously subject to interpretation.

As for what a lawyer will do, if you have financial resources a lawyer will try and sue you - no logical basis required. If you are flat broke, they will leave you alone.


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PostPosted: Mon Mar 28, 2011 9:35 am 
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>>I think the simple fact that you experience guys can debate this issue for 5 pages of posts is a clear sign that 70E should be written more clearly than it is. It is tremendously subject to interpretation.<<

I agree with George. The NFPA70E is a trail mix of confusing conflicting information. While the intent of 70E is sound, the delivery is flawed. The fact that one question can generate 5 pages of posting and still no agreed answer must tell you something? Looks like NFPA 70E is going the route of NEC... give 10 people the same NEC code question and 10 different answers. :eek:

I would vote to remove the PPE vs. task tables entirely from future NFPA editions. IMHO the tables are dangerous. The tables were a stop gap measure in the early editions of 70E and, at the time, were better than nothing. Unfortunitly it tries to paint with a broad brush and cover all situations. If you've done enough studies based on actual system data you quickly realize the tables have serious flaws.

As for the blowing up 3 phase MCCB breakers I've only seen 3 in almost 40 years in the trade. One was an old 400A Sq D that was well marinated in oil, a 250A mis-wired phase to phase on the load side and an 18kA AIC Fuji on a 42kA available fault current system. BTW... Fuji discontinued the 18kA line,


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PostPosted: Mon Mar 28, 2011 9:56 am 
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More information

Does this help or just cause more confusion?

[url="http://www.nfpa.org/publicColumn.asp?categoryID=&itemID=40828&src=NFPAJournal&cookie_test=1"]http://www.nfpa.org/publicColumn.asp?categoryID=&itemID=40828&src=NFPAJournal&cookie_test=1[/url]


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PostPosted: Mon Mar 28, 2011 3:24 pm 
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100questions wrote:
Does this help or just cause more confusion?

[url="http://www.nfpa.org/publicColumn.asp?categoryID=&itemID=40828&src=NFPAJournal&cookie_test=1"]http://www.nfpa.org/publicColumn.asp?categoryID=&itemID=40828&src=NFPAJournal&cookie_test=1[/url]


It is simply an opinion that is dealing with doors, with which I agree. It does not address my position of if a properly applied device is likely to cause an arc flash simply because it is turned on-off within its 'handle' rating.

Personally, I do not have much interest in following advice given by someone that feels the results of an engineering analysis should be compared to those of the table inferring the table provides superior results.


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PostPosted: Tue Mar 29, 2011 5:08 am 
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The tables are superior in a way. They provide an indication of risk as well as hazard. Incident energy analysis only calculates the hazard and provides no guidance as to the difference in hazard/risk between performing an infrared inspection or racking out a breaker. The analysis is also done on the basis of conditions being similar to the IEEE testing, that is, with direct exposure to the arc. Working with a door between you and the arc does not result in the same incident energy as being directly exposed to the arc.

Granted, the door will not protect you if there is an explosive arc that blows off the door, but the calculated hazard has nothing to do with the dangers of being hit by a door or injured by a blast. It only calculates the heat when directly exposed to the arc.


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PostPosted: Mon Sep 12, 2011 10:39 am 
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we're still talking about this one too

more discussion in this thread

http://www.arcflashforum.com/showthread.php?t=270&page=3

The story is this:

- Panel has high calculated energy and cannot simply be discounted under the small transformer rule (for us, this would be a 208 V panel fed through some station service transfer scheme protected by an 800 A fuse or something. The transfer shceme breakers have no trip coils)

- Nobody can say that the enclosure is arc-proof. I don't care if there are examples of arcs being contained. Sometimes the arc isn't contained and the enclosures are not designed and tested for it. If if blows, it could be bad for the worker.

- Operating the breaker is definitely "interacting with" and could, possibly cause an arc flash. I hadn't though of it before, but yes, simply operating is different from resetting.

- Everyone seems to agree that this is an excellent case where the probability should be factored in, because it seems absulutely, patently absurd to get someone to wear a flash suit to open a moulded-case breaker to change a heater.

I'm about to implement this: Wear one leather glove. Stand off to the side, as far as possible, and reaching across as little panel as possible, (do not reach across the panel regardless of where the hinge is). Look away and take a breath.

Note that it's now required for our electricians to be wearing basic FR clothing in the stations at all times, but I really don't want to be enforcing any kind of high-energy wear for this situation, so I'm hoping that everyone else thinks similarly and does the same!!

I'm not sure what to do about resetting now.....

Jody
Hydro One (Ontario)


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PostPosted: Sun Sep 25, 2011 8:35 am 
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I'm a bit late to the party, but I wanted to comments about the "well-maintained" description that is thrown around quite a bit. If you read most breaker manufacturer's literature, they will state that the MCCB should be mechanically operated periodically (one says every 6 months) to ensure that the mechanical parts of the breaker are free to operate and do not become corroded, stuck, etc. I would be surprised if ANY facility is operating ALL of their MCCB with this regularity - hence we are all not following the manufacturer's instructions and we cannot necessarily consider our system well-maintained.


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PostPosted: Mon Sep 26, 2011 5:19 pm 
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jdsmith wrote:
I'm a bit late to the party, but I wanted to comments about the "well-maintained" description that is thrown around quite a bit. If you read most breaker manufacturer's literature, they will state that the MCCB should be mechanically operated periodically (one says every 6 months) to ensure that the mechanical parts of the breaker are free to operate and do not become corroded, stuck, etc. I would be surprised if ANY facility is operating ALL of their MCCB with this regularity - hence we are all not following the manufacturer's instructions and we cannot necessarily consider our system well-maintained.


"Well maintained" always concerns me when we are talking about arc flash calculations based on everything working as planned. In the real world things don't always go as planned and maintenance does not always happen as scheduled (or at all for some).


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PostPosted: Wed Oct 26, 2011 11:10 am 
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So if we have a switchboard that automatically parallels a generator and the utility by opening and closing the main breakers automatically I cannot be in the arc flash boundary when the doors are closed and secured? This is sequence is automatic.

If we lose utility the utility main breaker opens, the generator starts and then closes the generator breaker to supply the load. When utility returns the utility breaker closes and soft loads off the generator and then the generator breaker opens. If my study says that the we have a dangerous catagory and the arc flash boundary is 20' then I cannot be in front of the switchboard watching the HMI screen while this happens? I cant even be walking past the equipment. Are you saying that I have to suit up to even go in the room as this automatic sequence could happen at any time? What if I have to stand in front of the switchboard to press a button to perform a generator load test?

I think the artical means with the doors open and live equipment exposed.

The same goes for a large ATS. I have a sticker claiming "no safe PPE exists" But I have a bypass feature on the ATS. I have to open the ATS door and rack out the ATS to test it while the load is being fed from the bypass. How can I do this when I have a dangerous catagory? What is the point of having a maintenance bypass!


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