NFPA Arc Flash & FPB

[whycliffrussell]

When calculating the the flash protection boundary (FPB) it is based upon the panels of the equipment removed (ie: someone has the covers removed working on it). Is the FPB different if the covers on the the equipment? ie: do they terminate at the surface of the enclosure if the covers are on?

 

[dpc]

IEEE 1584 says that 208V system less than 125 kVA do not need to be calculated.

I'm just telling you what we do. You can follow whatever procedure you want.

There is no accepted calculation method for arc-flash energy on dc arcs.

The short-form table is task-based. With the covers off, the risk of doing something that would initiate an arcing fault is much higher than with covers on.

 

[Electic]

We have found that if we do NOT limit the FCT to 2 seconds, in some cases the approach distance will calculate to about a quarter mile. To maintain credibility we have used the 2 second limit.

 

[whycliffrussell]

Electic:

I agree. We have had the same problem in the past with some large industrial facilities.

I'm currently working on a commercial facility with a small service size (1500kVA at the main service entrance board), and I’m running into problems with level 4 arc flash at load panels way downstream which are feed by 225kVA transformers (even with the 2 second limit imposed). On paper the results are very self explanatory – Low fault currents (in the range of 1-2kA) leading to CBs tripping on their overload as opposed to short circuit elements. This is resulting in a long arcing times and hence high incident energies available. Many commercial facilities have load panels scattered throughout the building (in hallways, corridors, and sometimes office/cubicle areas). At level 4 some of the flash protection boundaries work out to 10 and 15 feet. Does this imply that individuals cannot occupy the flash protection boundary w/out the appropriate PPE (This would imply that some hallway/corridors would be inaccessible/off limits to ordinary office workers)? (See earlier part of thread regarding flash protection boundary and covers on vs. covers off electrical panels) Am I the only person that this seems absolutely ridiculous to? Additionally, At 208VAC the limited approach boundary is still 3.5 ft, this would still make some hallways/corridors, etc inaccessible to non-qualified individuals.

How are other people performing arc flash hazard studies dealing with this? We want to make sure that individuals are warned of the potential hazards, but still maintain credibility.

 

[stephenw22]

We don't do any calculations for equipment under 480V at our site. We just use the NFPA 70E recommendations.

As far as time limits go, I agree that it should be looked at on a case-by-case basis. Many of the "dangerous" locations on our site are at the ends of long 480V and 600V runs, generally for yard lighting or remote switchracks. The arcing fault current is low and won't trip the feeder breaker at all. In those cases, the short-term "fix" is to limit the time to 2 seconds. In the long term, we plan to add ground-fault monitoring to feeder breakers as we have the opportunity.

 

[Zogzog]

"We don't do any calculations for equipment under 480V at our site. We just use the NFPA 70E recommendations."

You do realize you have to calculate fault current and know the clearing time of the protective device to use those tables right?

 

[whycliffrussell]

I agree with Zogzog. This is an disagreement that I have been having with some people (they want to use the NFPA tables when they yield lower results) as they believe that they are somehow more accurate or more realistic than the calculations. The NFPA tables would only appear to work in limited circumstances (for the give FCTs and short circuit current ranges that they note at the bottom of the table).

 

[WDeanN]

whycliff -
"At level 4 some of the flash protection boundaries work out to 10 and 15 feet. Does this imply that individuals cannot occupy the flash protection boundary w/out the appropriate PPE (This would imply that some hallway/corridors would be inaccessible/off limits to ordinary office workers)?"

To maintain credibility, we have adopted the position that you may walk in front of, or stand in front of a panelboard, MCC, or switchgear, regardless of the posted incident energy and FPB, provided you or someone else are (is?) not "working on" the equipment.

The thought process is sort of like the 70E. Unless you are performing one of the tasks outlined, you do not need to dress out. We have addressed the issue by saying that if you (or someone else) are (is) either: changing the state of the device, or there are exposed, energized parts, you must dress to the posted level of PPE.

stephenw22 - I'm not sure that you can use ground fault protection as a valid excuse to reduce the arc flash energy. Anyway, the topic has come up at my workplace as well, so I have posted a new thread to address this. (

http://www.eng-tips.com/viewthread.cfm?qid=200900&page=1)

 

[Zogzog]

"To maintain credibility, we have adopted the position that you may walk in front of, or stand in front of a panelboard, MCC, or switchgear, regardless of the posted incident energy and FPB, provided you or someone else are (is?) not "working on" the equipment."

That is exactly right, I had this discussion with some of the 70E group a few years ago and they use the term "Interaction" with the equipment as to when the FHB applies.