MCC Arc Flash Labels

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klett@abingdon-va.gov said:
An engineering firm was contracted to perform the analysis. The labels were made specifically for the equipment.

To rephrase the question, shouldn't each bucket have its own label? For example, a section in one MCC has a flash hazard of 306 cal/cm^2 (the section with the main is 37017). So as of right now, I would have to disconnect the entire MCC just to replace a fuse in a bucket that is still in place?
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I took this to mean the section with the main was determined to have an AFC of 37,017A (37kA), which came out to 306 cal/cm2. Makes more sense that way... (actually, looking at it again I guess I should say it makes less nonsense...)

But to your point, does it mean that you must kill the entire MCC bus to change a fuse in a bucket?

No, but you will have to suit up to the appropriate PPE and get a Hot Work Permit.

This is why MCC mfrs are coming out with the ability to disconnect a bucket from the bus, not just open the breaker. What those options do is to significantly limit the exposure to live parts and minimize the time to remove and replace buckets, because the entire bucket, including the stabs, are no longer in contact with anything live and it can be verified from the outside prior to opening the door. Many places are precluding the need for the HWP if it can be documented that there are no live parts inside.

But even then, you cannot open the door if the bus is live without suiting up to the appropriate PPE for the HRC, because you will still be in the Restricted Approach Boundary area (typically 1 ft. for 480-600VAC) of that live bus, even if it is behind an insulator and shutters. Those are electrically safe, but will not even slow down an arc blast if one happens behind them or even in another section sharing the air space. So that usually means PPE 4, even if only for a minute.
 
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mayanees said:
This sounds like the firm that did the Study did not use the industry-standard 2-second timeout.
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That was one of my many thoughts on the reason for the high incident energy.

I took this to mean the section with the main was determined to have an AFC of 37,017A (37kA), which came out to 306 cal/cm2. Makes more sense that way... (actually, looking at it again I guess I should say it makes less nonsense...)
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Yes that is a possibility but I would venture to say it is still unlikely. A 1500kVA transformer with 480V secondary and a 5% impedance is capable of ~36kA short circuit with an infinite bus. If that is the case, the study engineer used an infinite source for the study instead getting the actual fault current from the utility. This is another area of error I have seen in many studies. I was at one site where I saw a label with 1,071 cal/cm^2 on 34.5kV. I questioned that and the owner provided the study to me for review. The firm (a large manufacturer/engineering firm) made an assumption of the utility fault current of 40kA and did not consider the utility protective device 400 ft away. The actual utility fault current at that location was <5kA.

You've paid for the Study. They owe you an explanation.
And as also mentioned in earlier posts, the risk factor hasn't been evaluated, which is now allowed by the 2015 70E.
But you've got inaccurate IE levels, so you should start there.
That's the problem with folks doing these studies. Anyone can buy the software and press the buttons, but there's a lot more to it than that.
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True but one needs to be experienced to know what questions to ask. I agree that the OP needs to go back to the engineering firm to ask questions but it may be prudent to have an experienced engineer in arc flash studies review the study and develop a list of questions for the OP to ask the study firm.
 
OP - I think prior to going forward and performing any work, IMHO you need to be comfortable with the results of the study. Have you had discussions with the study engineer on the assumptions, inputs, and results of the study? If so are you comfortable with it?

If so, then you need to abide by the results and develop work procedures which will keep people safe considering the limits of PPE.
 
I agree that there are some aspects of this arc flash study that should be questioned. There are established best practices in place for arc flash analysis - the first sign that a firm may not be aware of best practices is if they are very flexible about the boundary conditions and assumptions used to perform the study. IEEE 1584.1 and a number of IEEE papers have described the best practices and suggested ways of dealing with unknowns. Mayanees correctly noted that only one incident energy level should be used for the entire MCC, regardless of whether it has a main breaker, main fuses, or main lugs. The standards governing MCC construction do not provide sufficient separation between the line side terminals of the main OCPD and the rest of the structure for us to conclude that the line side of the main cannot become involved in the arcing fault. Likewise, the standards do not define sufficient isolation between the load side of the OCPD in the bucket and the line side, or the bucket and the vertical bus. There has been some limited testing performed where arcing faults were created in a bucket to determine whether they would propagate across the OCPD and whether they would propagate from the bucket to the vertical bus. In both cases the arcs would propagate under realistic conditions.
 
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