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NFPA Arc Flash & FPB

NFPA Arc Flash & FPB

NFPA Arc Flash & FPB

(OP)
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?

RE: NFPA Arc Flash & FPB

It's based on testing done with the covers removed.  But unless it is arc-resistant switchgear, there's not much practical difference since there is no assurance that the door is going to stay on.  If it is a situation where you are concerned about arc-flash boundary, you probably need to assume the door is open or cover removed.  

RE: NFPA Arc Flash & FPB

(OP)
Does NFPA say anthing about this? (Covers off vs. on).

The calculations that I have done are with the covers off. However, with a properly designed system (CB and panelboards with suitably sized interrupting capacities for the short circuit currents avaialble) there should be no danger wit the covers on (ie: the equipment is designed to interrupt a particular fault current, meaning as a minimum that it shouldn't blow apart while interrupting that fault current, hence specifying a suitable interrupting capacity.)

RE: NFPA Arc Flash & FPB

whycliff-
The calculations assume that the cover is off.  They also do not take into account the effects of relays, breakers, or other foreign matter in the way that may direct or deflect the arc.  The 1584 AFB may not even be a worst case; there are just too many variables.

As far as I understand (not that I understand much) the IAC rating is just related to the ability of the equipment to interrupt the fault, i.e. it can withstand the magnetic forces, etc, associated with the fault.  An arc flash calculation assumes that something else may have gone wrong, and an explosive event has occurred.  The AIC rating does not address the ability to contain the arc flash / blast.  For that there is the arc resistant rating, although I think that's also a little misleading.

So, to get back to your original post, you cannot assume that the flash will stop with the doors closed.  The door may be blown open, but the resulting fireball and pressure increase in the enclosed panel will find an exit.  Whether it is out a loose screw hole right at the workers face, or a conduit opening in the top is anyone's guess.

RE: NFPA Arc Flash & FPB

(OP)
What about Fault Clearing Time (FCT). I'm doing a study w/ 120/208VAC panelboards and Thermal Magnetic Circuit Breakers. The fault currents are really low (some only 1.2kA) and hence the breakers are tripping on their thermal elements (long time trip) are up to 50 seconds, hence the incident energy gets really high (+level 4 arc flash hazard rating).  This sets the flash protection boundary (FPB) at distances like 150 FT!!!! This would indicate that you can't even enter the room with the equipment (let alone walk in there with a T-shirt and laptop and set up the toehr MCCB LSIG breakers).  This seems over the hill to me. Is there a reasonable limit that can be put on FCT? ie: how likely is it tha the arc would sustain itself for that long? In this particular instance the 120/208VAC panelboard is fed by a 45kVA transformer!!  Any thoughts or suggestions on this would be greatly appreciated.

RE: NFPA Arc Flash & FPB

I don't have time to look it up, but I believe you are allowed to set a 2 second limit on the assumption that someone would move out of the way.

RE: NFPA Arc Flash & FPB

Quote:

there should be no danger wit the covers on

If the fault is inside the panel in question, the door or cover can certainly be blown off.  Equipment testing is done with bolted faults, not arcing faults.  With a bolted fault, there is no arc, so the door would stay on.  With an arcing fault, the door or cover offers some protection but there is no assurance that it will stay on.  There are plenty of cases where equipment doors have been blown off (taking the protective relays with them).

RE: NFPA Arc Flash & FPB

IEEE Standard 1584-2002 "IEEE Guide for Performing Arc-Flash Calculations" on  page 6 says "Equipment below 240V need not be considered unless it involves at least one 125 kVA or larger low-impedance transformer in its immediate supply."  

RE: NFPA Arc Flash & FPB

davidbeach is right, you can limit the FCT to 2 seconds according to the IEEE Standard 1584-2002.

RE: NFPA Arc Flash & FPB

(OP)
what are other people's opinions on limiting the fct to 2 seconds (or some other higher arbitrary number) depending on the situation? Is this a common practice when performing an arc flash hazard study?

Additionally, what are people's comments/opinions on this excerpt form IEEE 1584:

Equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger lowimpedance
transformer in its immediate power supply.

I have a lot of 120/208VAC panel boards feed by 45 and 75kVA transformers (does this mean that I am to IGNORE) arc flash with respect to these panelboards?  Is this also common practice?

What are other people doing? commments, Suggestions?

RE: NFPA Arc Flash & FPB

We typically use 2 seconds as a maximum fault duration, since it is suggested in IEEE 1584.  

We follow IEEE 1584 guidelines you reference for arc-flash calculations. It says that no calculations are required for these smaller systems below 240 V.  But IEEE 1584 is just a guide for calculation of arc-flash energy levels.  

NFPA-70E does NOT say that these systems can be ignored.  We typically label all panelboards for arc-flash hazard, using the guidelines given in the NFPA-70E tables.  

Maybe someday these two standards will be better aligned, but for now, this seems to be working for us.  

RE: NFPA Arc Flash & FPB

(OP)
so if you have a 208vac panelboard fed by a 30kVA transformer you still include it?

RE: NFPA Arc Flash & FPB

We label the panelboard as HRC #1 based on the NFPA 70E Table 130.7(C)(9)(a).  We use a Flash Boundary of 4 feet.  

We don't try to calculate it using IEEE 1584.

RE: NFPA Arc Flash & FPB

One caveat on the 2 second FCT, you need to run the study both with the 2sec FCT and without. If you look at the TCC curves, your arcing current might hit a fuse curve at 2.12 seconds so the level would actually be higher than jsut limiting it to 2 seconds. We only use the 2sec FCT once we understand why a location is so unreasonalby high.

RE: NFPA Arc Flash & FPB

Eleceng01

That's really the point of using 2 second maximum - to limit the maximum arc-flash energy calculated.  If it wasn't higher above 2 seconds, there no real reason to limit the calculation to 2 seconds.

The idea is that longer duration arcing faults are unlikely and also any worker will not be sticking around if they can get out of the way by any means possible.  Is it realistic ?- that can be argued, but IEEE 1584 says it's OK (In the Annex) so that's what is normally done.

(Assuming I'm interpreting what you said correctly)

RE: NFPA Arc Flash & FPB

(OP)
ok. i see. so people do limit the operating times. what about the 125kVA rule (ignoring equipment 240V and less that's feed by 125kVA transformers). what are other people doing with that?

RE: NFPA Arc Flash & FPB

As I said, we don't really "ignore" the equipment, we just don't do the IEEE 1584 calculations.  We use the NFPA table.  

RE: NFPA Arc Flash & FPB

(OP)
the nfpa short form table (the one that makes all the assumptions about fault levels and clearing times?)

RE: NFPA Arc Flash & FPB

NFPA 70E Table 130.7(C)(9)(a)

We use this only for the 208 V panels or 240 V single-phase panels that are outside the scope of IEEE 1584.  

RE: NFPA Arc Flash & FPB

(OP)
Does IEEE say that 208VAC panels fed by transformers smaller than 125kVA are outside its scope or to just that you don't necessarily NEED to consider such situations.

So you use the first section of the NFPA table for equipment 50 volts (not necessarily AC)up to(but not including) 208VAC, and from 208VAC to 240VAC where the system is not directly feed by at least one (1) 125kVA transformer?

I'm am doing a study in a CO for a phone company (does this imply that the 52VDC phone exchange equipment should also have a flash hazard analysis done?)  The source pulls over 1000A.   

Additionally, as per our earlier discussion, if there is not difference between 'covers on' and 'covers off' for panels why does NFPA distinguish between this in the first section of its short form table?

RE: NFPA Arc Flash & FPB

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.

RE: NFPA Arc Flash & FPB

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.

RE: NFPA Arc Flash & FPB

(OP)
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.

RE: NFPA Arc Flash & FPB

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.

RE: NFPA Arc Flash & FPB

"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?

RE: NFPA Arc Flash & FPB

(OP)
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).

RE: NFPA Arc Flash & FPB

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)

RE: NFPA Arc Flash & FPB

"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.

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