Nfpa 70e

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zog

zog

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Charlotte, NC
charmon said:
If you are in a live 480v MCC and you are rewiring a bucket what catagory would this be????
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That would be the "I am not qualified to be doing this work" catagory. :)

The NFPA 70E does not allow for this type of work to be done live with out justification, then with an EEWP.

Arc Flash HRC's are not determined only by the voltage, however if you know the avilable fault current and clearing time of the protective device you can determine if you can use the task tables.

I suggest you start by reading these http://forums.mikeholt.com/showthread.php?t=99114

Then you can read this, similar work to what you are doing.

http://www.hss.energy.gov/csa/analysis/oesummary/oesummary2009/OES_2009-04.pdf
 
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charmon

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I meant that the mcc is on but the bucket is off and locked out. We all know that there is still energized parts in the bucket at the top of the breaker.
 
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jim dungar

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Wisconsin
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PE (Retired) - Power Systems
charmon said:
I meant that the mcc is on but the bucket is off and locked out. We all know that there is still energized parts in the bucket at the top of the breaker.
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The incident energy inside of the bucket is the same as that of the busbars behind it, unless there is total isolation. I doubt that an existing MCC would have sufficient barriers to provide total isolation. There are stories of screws and even screwdrivers sliding through the space around the bucket and causing a fault.
 
yanici

yanici

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Atlantis
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Old Retired Master/Journeyman Electrician
jim dungar said:
The incident energy inside of the bucket is the same as that of the busbars behind it, unless there is total isolation. I doubt that an existing MCC would have sufficient barriers to provide total isolation. There are stories of screws and even screwdrivers sliding through the space around the bucket and causing a fault.
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Yep. And, sometimes you can drop a screw or something into the bucket that your working in and it finds that little hole so that it can migrate to a live bucket below. Ask me how I know? Ah, the good ole' days, huh!
 
zog

zog

Senior Member
Location
Charlotte, NC
charmon said:
The bucket is deenegized and locked out. But we know there is still energized parts in the bucket.
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So why not remove the bucket and work on it on a bench in the shop? Of course you still need to suit up to revome the bucket or you can extract the bucket remotely and not need any arc flash protection.
 
T

thosey

Member
As Zog has said first this type of work is not provided for by OSHA since they reference the NFPA 70E as an industrial standard. Second you can not just look at something and determine the HRC (Hazard Risk Category) that it fits in unless you know how to do the calculations and have the prerequisite data available such as the protective device. The arc curves for the protective devices and all data leading up to this point from the Utility on. I would suggest that you hire a competent engineer for calculating these for you as well as suppling a PE stamp for the state you dwell within.
You could also purchase Easy Power or SKM to help you in calculating this data but then you take full accountability for the category that you place upon the equipment.
I have seen transformers that the primary is 480 VAC 3 phase to a secondary 240 VAC 3 phase become an HRC 3 on the secondary side where the primary was an HRC 0. This is common on transformers that are over 125kVA.
I have also seen HRC 2 circuits receiving dangerous ratings after protective devices.
The analogy to look at this is the following.
You are in a room with two water spigots. Both spigots are about three feet off the ground and in the center of each wall. One on the left wall and the other on the right. Both look the same correct? Well you would think that putting a glass of water under one to get a drink would be an easy task correct? But what if on the left side the water pressure was a solid 8000 PSI. And on the right side it was normal city water pressure that is from 30 to 50 PSI. Which one would you put your glass under? If they are not labeled with correct PSI it is a craps shoot and the left side would probably hurt you right. You would be pissed at the person that put that spigot in without warning correct? Well why is it that people think they can take the short cut and look at a piece of electrical equipment and some tables in 70E and get a good picture of what could happen.
Lets break this down. If you were harmed from the spigot on the left you would probably seek litigation from the plumber. Are you going to seek litigation from the person that installed the MCC also?
 
zog

zog

Senior Member
Location
Charlotte, NC
StephenSDH said:
I personally can do on the spot HRC analysis, and your MCC is a class 4.
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You are amazing, please show your work. How did you come up with 4?

What if this is in a paper mill where the MCC's are commonly fed by switchgear with no main breakers and feeder breakers without INST trips, giving the MCC's long clearing times with the already high fault currents?
 
StephenSDH

StephenSDH

Senior Member
Location
Allentown, PA
zog said:
You are amazing, please show your work. How did you come up with 4?

What if this is in a paper mill where the MCC's are commonly fed by switchgear with no main breakers and feeder breakers without INST trips, giving the MCC's long clearing times with the already high fault currents?
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I knew this comment would bend you out of shape. :)
 
T

TxEngr

Senior Member
Location
North Florida
Removing a bucket from an energized MCC can be a high risk operation requiring significant safety evaluation. Working on a deengergized bucket is a better solution. Older MCC's have the connection to the bus somewhat exposed but recently I have been purchasing MCCs that have a plastic barrier that isolates the high volage side of the breaker from the rest of the bucket. You can also purchase retrofit kits to do the same thing. In this case, if the breaker has been opened and verified, the high voltage is no longer exposed nor are you interacting with it since it is protected from accidental contact. If you make provisions to prevent parts from falling into the bucket below, then you should be able to work on the low voltage side of the deenergized bucket wearing minimal PPE.

Comments?
 
zog

zog

Senior Member
Location
Charlotte, NC
TxEngr said:
Removing a bucket from an energized MCC can be a high risk operation requiring significant safety evaluation. Working on a deengergized bucket is a better solution. Older MCC's have the connection to the bus somewhat exposed but recently I have been purchasing MCCs that have a plastic barrier that isolates the high volage side of the breaker from the rest of the bucket. You can also purchase retrofit kits to do the same thing. In this case, if the breaker has been opened and verified, the high voltage is no longer exposed nor are you interacting with it since it is protected from accidental contact. If you make provisions to prevent parts from falling into the bucket below, then you should be able to work on the low voltage side of the deenergized bucket wearing minimal PPE.

Comments?
Click to expand...


Thats why remote extraction is becoming so popular.
 
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