Arc Flash 208 V panelboard, 2 cycles clearing time in NFPA70E

[tem1234]

Hi, i have a 208 V panelboard fed by a 75 kva transformer which i want to calculate the Hazard Risk Category.

fault level: 4 kA
fault clering time : approx. 1 sec (the fault will be clear by the fuse at the transformer primary)

With the IEEE 1584 standard, it gave me PPE category 3. The 1584 doesn't work for 208V <125 kVA.

If i take the NFPA 70E table 130.7(c)(9)(a), in note 1 it is written " 25 kA available, 2 cycle clearing time". So it can't apply since the fault duration will be longer.

So no standard can apply in that situation. Am I missing something? Because 2 cycle clearing time in the NFPA70E is a little bit short. I don't know many circuit that will open in 2 cycle! What can i do, just put a warning label?

thanks

 

[WDeanN]

I can't find it now, but I thought there was a note in there that you could also base it on kA-cycles, so that if you had a longer clearing time with a smaller fault current, you could also use the tables. Is this maybe in the ROP for 2009 70E?

Although, even if this were the case, if the standard note is 25kA at 2 cycles, you'd still be out of the 50kA-cycle limit at nearly 240kA-cycles.

Generally though, 208V will not sustain an arc very well. Hence the restriction on the 1584 calculations. If you must label it, I would just put the standard warning label on it, without energy levels.

 

[tem1234]

Thanks WDeanN,

that's what i was thinking to do. So on the warning label you wouldn't indicate any PPE, just a warning?

like you said, 208 V will not sustain arc (IEEE 1584 talk about that), but for 600V panelboard, i can use IEEE 1584 for cover off, but for manipulation with cover on, i would like to use the table in NFPA70E but it can't apply since the fault duration is > 2 cycles. That's a little bit strange cause a lot of circuits will have a clearing time >2 cycles. So the table can't apply in the majority of circuit! if you can find the note about kA-cycle it could help me a lot.

thanks

 

[WDeanN]

Well, I couldn't find the note. I may have been thinking about 130.3(A) FPB, which places the FPB at 4 feet for less than 300kA-cycles. I tried looking through the ROP for 2008 70E, but quickly got lost/distracted/confused by some of the proposals.

I'm not sure what to do about PPE. You could probably go with a 4 ft boundary, and use PPE one level above the 70E tables?? Personally, I think this would be exceedingly conservative for a 208V panel. I would probably go with Cat 2 at the most, but if your calculation says Cat 3, how else do you justify stepping down? This is why I prefer not to even look at <240V stuff. ( ) The calculations don't work out right, so in order to CYA you go too conservative and end up with everyone in frog suits all the time!

Just for SAG, what location is this for? It sounds like a panel directly behind a transformer to have such a long clearing time. Could you recommend new fuses on the transformer primary and be done with it? Also, if you’re doing this as part on an in-house analysis, posting levels on 208V equipment may not be necessary. NEC 110.16 only requires a generic warning label. If you’re doing it as part of a contract job, look at the original proposal and SOW to see if <480V is included.

 

[rbulsara]

Search the forum. This has been discussed here before. There is no need to calculate below 125kVA, 240V per IEEE 1584/NFPA 70E.

If you do, you will end up with erroneous results as you already have.

 

[Zogzog]

"If you must label it, I would just put the standard warning label on it, without energy levels."

WDean, one of the 2009 ROP's (Accepted) will require either the Ei or PPE to be on the labels.

 

[Zogzog]

"There is no need to calculate below 125kVA, 240V per IEEE 1584/NFPA 70E."

1584 says an arc will most likely not be self sustaining below these values, dosent say there is not a hazard. 2009 70E again will address this but until then best bet is to use the tables.

 

[rbulsara]

zogozg:

The current formulae do not apply to 125kVA,240 and below. So at least you cannot use them for that purpose.

 

[tem1234]

thanks for the reply

The panelboard is directly behind a transformer, but it's a little complicated to change the fuse because i don't want the fuse to blow up for a short circuit which the circuit breaker at the secondary should operate. There will be a loss of coordination (and i think it's not really necessary).

IEEE says that <240V and <125kVa can be ignore, but not the NFPA. If it was just my decision, i would probably ignore theses pannels (just put a generic label), but it is in the proposal (which i didn't do).

The result of IEEE 1584 (cat 3) is unreallistic. But in the NFPA table it says cat 0 or 1 which i find pretty good for these tasks. The only thing is that the table isn't valable since the clearing time is too high.

So i have difficulty to recommend a ppe since this will be based on my opinion and not a standard.

I'm a little confused about the 130.7(C)(9)(a) table for the pannelboard up to 600V. I don't see where it can be applied except for current limiting device. No CB (or few) will operate in 2 cycle!!!

 

[Zogzog]

"The current formulae do not apply to 125kVA,240 and below. So at least you cannot use them for that purpose. "

Right, but that is not the same as saying it can be ignored, I just wanted to make sure the OP was clear about that.

 

[burnt2x]

Hi!
Try this!