zog
Senior Member
- Location
- Charlotte, NC
Then how do you calculate a value with the covers on?
John M
You dont, you assume the covers are off, have vents, or will fail for an analysis.
Then how do you calculate a value with the covers on?
John M
Then I calculate the 'covers on' incident energy buildup on the grounded switchgear metal. ZERO, which translates to a level ZERO PPE requirement...
And then I default to the table for the more restrictive PPE requirement when someone works at the switchgear with the covers on, and I specify that level of PPE. There's a point where you apply problem-solving engineering skills while working within the regulations, but use common sense in the process.
John M
I guess this means that if you short circuit grounded metal, you get zero current and therefore zero incident energy. This doesn't take into account what happens if there is an arc behind the covers. The arc can move, vents allow energy through, covers can blow off, etc. There's no way to calculate the energy. NFPA 70E 130.7(C)(9) FPN No. 1 says that the collective experience of the task group is that closed doors don't provide enough protection to eliminate the need for PPE.Then I calculate the 'covers on' incident energy buildup on the grounded switchgear metal. ZERO, which translates to a level ZERO PPE requirement...
So it's not an NEC code requirement but a label with PPE level on it, is required by OSHA. As an engineer I run into alot of facility guys who don't want anything to do this if they can avoid it. I'm often adding a single new switchboard to an existing system for which no Arc Flash study has been done and on which there are no labels. They want to keep it that way.
That I should recommend a label beyond the NEC requirement of a generic label is clear. But am I required to get arc flash warning with specific PPE level on the new switchboard that I specify? Must I insist on this for my client.
Thanks,
Mike
Not dissenting this point at all. I say use the "covers-on" value from the table.I guess this means that if you short circuit grounded metal, you get zero current and therefore zero incident energy. This doesn't take into account what happens if there is an arc behind the covers. The arc can move, vents allow energy through, covers can blow off, etc. There's no way to calculate the energy. NFPA 70E 130.7(C)(9) FPN No. 1 says that the collective experience of the task group is that closed doors don't provide enough protection to eliminate the need for PPE.
Stay tuned because I will get to the bottom of this asap. I think my approach is a safe one: open bus-use calcs; covers on - use tables (because I can't calc the incident energy acroos the sheet steel covers!).
But I'm only looking for the answers, because I don't have all of them. I certainly stand to be corrected if and when appropriate.
John M
I personally don't like the NFPA tables. They're based on gambling. As in this case, arc flash incidents can occur because of things you're not even aware of when your not even performing work on energized equipment! We don't determine our fall protection by the strength of the ladder, or how many tools we're carrying in our tool belt, or how wide the girder we're walking out on is... don't play probability games! You wear a harness if there is ANY possibility of falling.
I want to be protected for the maximum possible arc flash at the given location. And because that is a function of fault current and device operating time, you can't tell by looking what that's going to be. You've got to calculate it! Now flash containing gear may be a different matter, but that's a newer innovation, and primarily only used in MV equipment, and I typically see more high category locations on LV equipment.
My personal opinion is that the table should never take the place of a full study, but should only be used in the interim period while the study is being performed. Not only will the study give you worst case results, it will also give you the tools to devise a way to lower the results. You should be able to get any 480V MCC with a main breaker down to a HRC 2 or below. Most can be HRC 1 or 0.
At the bottom line, it becomes a personal matter. And that is part of the purpose of NFPA-70E anyway--to remind the guy working on the equipment that he has a responsibility too! I think the standard gives some leeway regarding covers and the nature of the work, but the calculations don't. They present the worst case scenario. It's up to you if you want to play odds.
Personally, I would check meters all day in jeans and a long sleeve shirt. But I would never tell someone else that it is safe to do so. I have assumed the risk and accepted the odds. If that starter goes out while I'm standing here, I'm toast!
Anyway, there's my opinion on a subject where the standard has obviously left a lot of room for opinions!
Not dissenting this point at all. I say use the "covers-on" value from the table.
Stay tuned because I will get to the bottom of this asap. I think my approach is a safe one: open bus-use calcs; covers on - use tables (because I can't calc the incident energy across the sheet steel covers!).
But I'm only looking for the answers, because I don't have all of them. I certainly stand to be corrected if and when appropriate.
John M
Don't forget 110.10 Circuit Impedance and Other Characteristics. The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit. . .
That makes me wonder at your statement, ". . . then closing the doors on it won't shield you from that energy, and in some cases could exacerbate the problem . . ." Really, then is the equipment selected and installed correctly?:smile:
110.16 says, equipment must be able to withstand a thru fault until the protective device operates. But what if the fault occurs internal to the equipment (i.e. on the bus of a panel or switchboard)? The equipmnet is the fault, therefore how can it be able to withstand the arc, unless it is arc-resistant gear? While the arcing equipment is waiting for the protective to operate, the proper PPE must be worn by any one standing in the 'arc flash area'.Don't forget 110.10 Circuit Impedance and Other Characteristics. The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit. . .
That makes me wonder at your statement, ". . . then closing the doors on it won't shield you from that energy, and in some cases could exacerbate the problem . . ." Really, then is the equipment selected and installed correctly?:smile:
110.16 says, equipment must be able to withstand a thru fault until the protective device operates. But what if the fault occurs internal to the equipment (i.e. on the bus of a panel or switchboard)? The equipmnet is the fault, therefore how can it be able to withstand the arc, unless it is arc-resistant gear? While the arcing equipment is waiting for the protective to operate, the proper PPE must be worn by any one standing in the 'arc flash area'.
But, I feel that interacting with a dedicated/isolated 120V metering compartment is not the same as interacting with other parts of the equipment. For example, in a two high section of MV switchgear where the bottom cubicle is a breaker and the top one is dedicated to metering, the 120V metering compartment could have a HRC=0 (it is fed by a transformer <125KVA) while the breaker cubicle could have a different one. This would not be the case however in equipment, like a switchboard, where the metering is barriered but not isolated.I was about to post the exact same thing, great minds....