Arc Flash Calculations
Arc Flash Calculations
(OP)
Has anyone done any of these. I am doing one for the first time using SKM Systems software. I am confused by the intent of the printable signs. I would think you would have one category for each piece of equipment such as a switchgear lineup or motor control center. The software however, calculates by bus, giving different values for different feeders. This would mean I would have to put different signs on each switchgear compartment, because each compartment would have a different requirement. This seems like overkill. Has anyone completed an installation in the field? I would be curious as to how you set up your signs.






RE: Arc Flash Calculations
Mike
RE: Arc Flash Calculations
RE: Arc Flash Calculations
RE: Arc Flash Calculations
RE: Arc Flash Calculations
RE: Arc Flash Calculations
I end up at Category 2 for all the breakers except the one connected to the standby generator which goes up to Category 4.
I will probably have a sign for Category 2 with a separate sign on the breaker cubicle feeding the generator identifying the special requirement if generator is being tested.
RE: Arc Flash Calculations
I would try to keep it as simple as possible.
RE: Arc Flash Calculations
Mike
RE: Arc Flash Calculations
The problem with a task-based approach is that the task often evolves as the work is done and there will always be a tendency to define the task so as to be able to wear the lowest level of PPE.
Another major problem with the task-based approach is that it really doesn't make any sense in a lot of cases. If the door of an MCC bucket is open, it won't matter what task was being done if an arcing fault occurs - the same energy will be released. The arc doesn't care what you were doing, or trying to do. NFPA-70E allows lower levels of PPE for certain tasks because they figure the probability of causing an arc is reduced, not because the impact of any arc created will be less. That's a little like saying you only need to wear your seat belt in inclement weather.
I think we're all trying to make sense out of the NFPA 70E requirements and come up with a workable, logical approach. I expect these requirements will be evolving quite a bit in the next few years.
RE: Arc Flash Calculations
To get around tasks evolving, at our site, we always have a Maintenance Operating Procedure. This has kept our eye on the ball, so to speak. But, I do hear where you're coming from.
Mike
RE: Arc Flash Calculations
RE: Arc Flash Calculations
But seriously, I think what has to be done is you evaluate say everything in an electrical room. If everything in there is Category 0, 1, or 2, maybe you decide to make everything Category 2. Then the maintenance tech will know that to work on anything in that room requires certain clothing, and you put a sign on the door going into the room. Maybe you remain specific to each piece of switchgear and MCC if the categories are higher.
This is not to say that this approach will always be valid. There are always special cases. I just think you need to have a global view and just separate out things that don't fit the envelope.
I don't think it should get to the point where the maintenance guy has to remember what kind of underwear he put on this morning depending on what piece of equipment he is going to work on.
RE: Arc Flash Calculations
Electricians should be required to wear cotton underwear all the time and then they won't have to worry about it. They should also be forbidden to wear watches, rings, necklaces, metal-frame glasses, etc.
The problem with a single PPE requirement for an entire room is when the PPE level is 3 or 4. At these levels, an arc hood is required and these are not comfortable to work in for long periods of time.
At PPE level #2, only a face shield is required and this is much cooler and much less restrictive than the hood.
I'd do whatever you can to get arc-flash energy below 8 cal/cm2.
RE: Arc Flash Calculations
RE: Arc Flash Calculations
You would still have to do some calculations on the bolted fault current to determine if the current limiting fuse will limit the current. For example: Paragraph D.8.6 in Annex D of NFPA 70E, has the equations for CL fuses. For a Class L Fuse, 1601A - 2,000 A, if the bolted fault is less than 22.6kA, the arcing current needs to be calculated and then the time current curves used to determine the incident energy using the appropriate incident energy equations.
It's not as simple as just installing some CL fuses and walking away with what might be a false sense of security and safety.
RE: Arc Flash Calculations
RE: Arc Flash Calculations
Relying on fuses for arc-flash reduction can also create significant coordination problems as well.
RE: Arc Flash Calculations
RE: Arc Flash Calculations
What about arc resistance and it's unpredictable nature: a high resistance fault can cause less current than what would trip the instantaneous pickup. Instead of clearing at a predicted 6-7 cycles, this could increase the 'cook time' (albeit at lower current levels)to 60 cycles or more. We have modelled real life situations where this could occur.
With modern protective relays it is easy to analyze that most ground faults do not reach maximum calculated available fault current. This could have a deletrious effect on true arc flash exposure by extending clearing times.
At this time we are interpreting our assignment as following the recommendations of IEEE which is a substantial safety improvement over no warnings or PPE, but with many disclaimers (as also exist in NFPA 70E) that following these guidelines does not assure absolute safety. Seems like a cop out, but I know of no better.
RE: Arc Flash Calculations
http://www.csemag.com/article/CA602446.html