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 Post subject: 208V AFH Assistance
PostPosted: Fri Jan 01, 2010 9:34 am 
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Hello everyone,

I am currently struggling with the results of a AFH study. In this study there many Cat #3 areas that are primarily at 208V but fed from transformers >125 kVA. The Arc Faults range from 1.5 kA to 3.3 kA. A 2 sec cutoff was used.

The protective devices are MCCB and have fixed instantaneous trips due to smaller size of the breaker.

My concerns are that this is relatively low fault currents but due to TCCs of breakers result in the 2 sec cutoff being reached, therefore high incident energy. Intuitively, it seems like Cat #3 PPE is overkill.

I'm sure other people have run into this situation and would appreciate their input on how they handled this. This is a hospital so it is not easy, if not impossible to de-energize circuits to work on.

Thank you all

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PostPosted: Sat Jan 02, 2010 2:47 pm 
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Hmm, you may find something in the forum search function. This has been a complex issue for most of us. You want protection but overprotection has its own hazards.

What are your options?
1. Could the IE be reduced?
2. Notify the hospital Management of the hazard and the sign off required.
3. Wear the PPE.
4. Develop procedures for task.
5. Train workers.

The hazard may appear limited, however the calculations of the IE is all you have to go by. Best of luck.


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PostPosted: Sat Jan 02, 2010 6:37 pm 
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Yep, you figured out 2 things lots of places are figuring out. The suprisingly high Ei's on LV systems and why those types of CB's were so cheap when they were purchased.

A careful evaluation of your system by someone who specializes in mitigation solutions might be worth the investment. Replacing a few MCCB's at key points with adjustable trips may help, but hard to tell from the limited info you have provided so far.

What tasks need to be done energized?

Remote switching may be a good solution too.


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PostPosted: Mon Jan 04, 2010 10:41 am 
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We have the same issues. Given some of the recent independent testing we’ve discussed clamping the results at 1 second in place of 2. Most of the independent tests have shown a 208V arc will not last anywhere close to 2 seconds.

We’ve talked with others who are not evaluating down to the 208V level; they do not believe the (IEEE 1584) results at this level.
This is an area that must be addressed ASAP; they're too many people asking the same questions over and over again.


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PostPosted: Tue Jan 05, 2010 6:38 pm 
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208v Post

I just searched the phrase 208v and seen a lot of old discussions. You may get some background into this subject that may help.

It's an in-depth subject that until NFPA or IEEE gets the testing results completed it's all subjective.
I do agree with the previous message on the 1 second rule, at least with < 125KVA. I am unaware of >125KVa faults and the extended hazards, so I don’t want to give the impression there is no hazard. As of now I have seen very little evidence of a sustained fault at 208v. Saying that, there is a lot of independent testing going on now and hopefully more data will be published soon.

Safety is the main concern and I stand by the options above to consider. There is no easy answer as too many variables exist.


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PostPosted: Tue Jan 05, 2010 7:10 pm 
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Additional info from OP

Hello,

Here is an example:

A 208V panel is fed from a Square D Q2-H 200A breaker. The conductor is 1c 3/0 and is 80 feet long The panel fed is MLO. The bolted fault is 4.61 kA, arc fault is 2.5 kA resulting in an IE of 15 cal/cm2 for a HRC #3.

However, if I was to use Table 130.7(C)(9) in NFPA 70E-2009, the tasks for panelboards 240V and below the highest risk category is 1. This situation meets Note 1 of the table which is max 25 kA available and max 0.03 sec clearing time. In this case fault is 4.61 kA and clearing time is 0.017 sec.

Now I have seen on this forum that people have said you can not mix and match AFH analysis with using the tables.

Any advice?

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www.workplacesafetysolutions.com


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PostPosted: Wed Jan 06, 2010 6:52 am 
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wbd wrote:
A 208V panel is fed from a Square D Q2-H 200A breaker. The conductor is 1c 3/0 and is 80 feet long The panel fed is MLO. The bolted fault is 4.61 kA, arc fault is 2.5 kA resulting in an IE of 15 cal/cm2 for a HRC #3.


How do you get 15 cal/cm^2 for that fault?

Using IEEE 1584, from Ia=2.5 kA, V=0.208 kV and t=0.017 s, assuming G=25 mm, D=455 mm, X=1.641, k1=-.555 and k2=-.113, I get En=0.62 J/cm^2 and E=0.54 J/cm^2, so E=0.13 cal/cm^2.


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PostPosted: Wed Jan 06, 2010 8:42 am 
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Vincent B.

Thank you for your response but the trip time is 0.017 secs for the bolted fault current of ~4 kA not the arcing fault of ~2 kA. The arcing fault is in the region of the long time trip band (don't have the file with me but can provide that time tonight), therefore the trip time is in seconds. However, I used a cutoff time of 2 seconds per IEEE 1584.

The analysis is being performed using EasyPower Ver 8.

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PostPosted: Wed Jan 06, 2010 9:19 am 
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wbd wrote:
Thank you for your response but the trip time is 0.017 secs for the bolted fault current of ~4 kA not the arcing fault of ~2 kA. The arcing fault is in the region of the long time trip band (don't have the file with me but can provide that time tonight), therefore the trip time is in seconds. However, I used a cutoff time of 2 seconds per IEEE 1584.


Ok. I didn't understand how you got such a high IE with a fairly low Ia and 1 cycle.


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PostPosted: Sat Jan 09, 2010 6:57 pm 
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An option is change the breakers to low peak fuses. This will drop the IE considerably.


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PostPosted: Mon Jan 11, 2010 9:03 am 
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Chet Davis wrote:
Attached is a link to a recent paper we wrote on calculating arc flash energies for systems rated <250 volts and served by transformers rated less than 125 kVA. This paper is based on the PG&E testing as well as the Ferraz Shawmut tests which is the best available data we have at this point in time. We feel this is just a stop gap until the IEEE takes the PG&E and Ferraz data as well as their own comprehensive series of tests to formulate new equations. There is more to this problem than just current and time, that is why the IEEE equations do not work well in this voltage range and you end up with energies much greater than at 480V. I believe we will need to go to an energy equation or a more detailed model (which will have its own data problems).

http://www.easypower.com/arc_flash/arc_flash_resource_center.php

Chet E. Davis, PE
chet_davis@easypower.com


This was copied from another place on this forum and could help you.

You could do an advanced search for under user name Chet Davis for the discussion.

As someone said above about reduced max duration time of <.25KV and spoke of in this document found on this page by Chet Davis PE a very knowledgable and respected power professional and President of ESA the developer of Easypower software.

You may find that .5 seconds <.25kv may serve as a good cut off for now.


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PostPosted: Mon Jan 11, 2010 2:19 pm 
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wbd
Check out this OSHA letter of breaker operation.


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PostPosted: Mon Jan 11, 2010 2:20 pm 
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:eek: try this[url="http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=25973"]http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=25973[/url]


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