70E Arc Flash Analysis

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Electrobe

Member
I am new to this NFPA 70E stuff and still have many questions here is another:

If an Arc flash analysis is done on an 480V MCC bus and the following info is determined (MCC would be labled with this also):

Flash Hazard Boundry: 45"
Cal/cm2 @ 18": 5.4
HRC: 2
LAB: 42"
RAB: 12
PAB: 1"

Does this mean any time you enter the 45" when there is an MCC bucket door open, you need to use HRC 2 PPE, include all the tasks listed in the 130.7(C) Table?

Thanks
 

zog

Senior Member
Location
Charlotte, NC
Electrobe said:
If an Arc flash analysis is done on an 480V MCC bus and the following info is determined (MCC would be labled with this also):

Flash Hazard Boundry: 45"
Cal/cm2 @ 18": 5.4
HRC: 2
LAB: 42"
RAB: 12
PAB: 1"

Does this mean any time you enter the 45" when there is an MCC bucket door open, you need to use HRC 2 PPE, include all the tasks listed in the 130.7(C) Table?

Thanks

Yes, you either use the table method or you do an analysis, you dont mix them. The arc flash analysis is actual incident energy (Ei) measured in calories/cm squared on a recieving surface. This is a hazard based method.

the tables make several assumptions about working distance, fault currents, clearing times (You need to read the notes). The tables are task based.

You could do a study and have different working distances and PPE requirements or your labels, but that would cost more and be confusing to your crew (But I have done it for some plants).
 

zog

Senior Member
Location
Charlotte, NC
Electrobe said:
I am new to this NFPA 70E stuff and still have many questions here is another:

Be sure to read the Safety FAQ's in this section of the forum, lots of common questions answered there.
 

Electrobe

Member
Since we did the analysis for my example and we don't use the table, what PPE is required to operate the switch when the door is closed?
 

zog

Senior Member
Location
Charlotte, NC
Electrobe said:
Since we did the analysis for my example and we don't use the table, what PPE is required to operate the switch when the door is closed?

Same as the door open, unless you have arc rated switchgear. In most of the arc flash aftermaths I have been called out to the doors fail, plus many doors have vents on the front, missing hardware, or not all screw (Latches, bolts, etc) are tight. You should assume the door is not there for PPE selection.
 

Electrobe

Member
zog said:
Same as the door open, unless you have arc rated switchgear. In most of the arc flash aftermaths I have been called out to the doors fail, plus many doors have vents on the front, missing hardware, or not all screw (Latches, bolts, etc) are tight. You should assume the door is not there for PPE selection.

The 130.7(C)(9) table for "600 V Class MCCs" appears to be a HRC 3 (At 18") if you did an analysis using the info in the notes. The table allows you to use reduced PPE for some tasks. If I do an analysis and determine a MCC to be HRC 2 @ 18" it logically seems like you should be able to do some tasks at a reduced PPE???

If the table is used, what HRC would you use on the labels? If you used the highest, it would be HRC 4 for all 480V MCC??
 

zog

Senior Member
Location
Charlotte, NC
For a guy who is just diving into the 70E you sure are asking good questions.

1st off, I dont see any HRC 3 for 600V MCC's, everything else, but not HRC 3. Are you looking at the 2009 70E?

Your logic makes sense, but it dosent fly for the 70E, there were a lot of proposals based around the concept you are discussing, all shot down.

In the tables, the highest HRC is determined for an assumed working distance, and the limitations of the notes. From there, the other tasks are all a "Gut instinct" of the commitee, based on probability of an event and proximity of the worker, the commitee uses the terms "Task based" or "Risk based"

When you do an analysis of the actual system, you calculate actual worst case Ei at a distance you choose. The commitee uses the term "Hazard based", risk or probability is not factored in. Lets say you did 2 studies for a given point, one at 18" (For T/s, removing covers, etc) and another at 36" (For visual inspections, IR scanning, etc) then you could do other tasks at 36" working distances at a reduced PPE level, but otherwise you would be just guessing.

I am part of a Hazard vs. Risk subcommitee that is looking closer at these issues and will be reporting to the 70E commitee with our findings.

Here is an example. At a major automotive plant, they required a 2nd person in the substation for CPR and safety observing for all switching tasks. The arc flash boundary was over 20' so the other person needed to be outside the room or in HRC 4 PPE (As calculated at 24" on thier study and labels). So we calculated the distance that the Ei was <4 cal/cm2 so that the safety person could observe the worker wearing only HRC 1 clothing and safety glasses (This was before the facesheild was added to HRC 1 in 2009), that ended up being around 8-10'. 2 labels were put on the equipment.

Does that help clarify?
 

Electrobe

Member
1st off, I dont see any HRC 3 for 600V MCC's, everything else, but not HRC 3. Are you looking at the 2009 70E?

Yes looking at 2009 code which requires HRC 4 for insertion/removal of MCC buckets.

Does that help clarify?

Yes this helps.... Thanks for the examples
 
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