Arc flash spreadsheet 2

[stevenal]

Anyone try using the spreadsheet that came with IEEE 1584. I just tried it for a 69kV 9.2KA bus and the PPE requirement is off the chart. Same data input into flux.exe gives 9 cal/cm^2, while the spreadsheet gives 700. Double checked all my input values, and there's still 25.4 mm in an inch and 60 cycles in a second if I'm not mistaken. The 9 value is also more in line with the tabulated values in the NESC preprint proposals that came from Arcpro. Anyone find the error in this spreadsheet?

 

[mpparent]

It certainly sounds odd....I punched in your numbers on a program I wrote, and while my program is a bit "fixed", ie I use the standard bus distances, etc., I get about 16 cal/cm^2 with an 18" worker distance. I've never used the spreadsheet that came w/ my copy, but it sounds like you've came across an error.

Mike

 

[Laplacian]

What is your clearing time? I've found for longer clearing times (>30 cycles), the IEEE values seem way out of proportion; especially at higher voltages. If you examine the data points used as the basis for IEEE calculations; you will see there are very few points indeed and also that clearing times are fast (< 15 cycles) and voltage is pretty low.

Further testing should be done in my opinion; specifically at higher voltage and longer clearing times.

 

[stevenal]

Trip time is 0.9s, opening time is 0.14s, working distance is 694mm (minimum approach distance less 2X the arc length per NESC proposal), class is 1, grounding type is 1, protective device type is 0, motor contribution is 0, portion of bolted fault current that flows through protective device is left blank.

So I tried using a shorter clearing time. One right off the NESC proposal chart under the 5-cal system chart. IEEE says 212 cal/cm^2.

 

[stevenal]

A little more research shows that the spreadsheet transitions to the Lee method at 15kV. The formula looks correct according to 1584, where the formula is given twice. Putting a 15 kV bus one row below a 14.9 kV bus with all else equal, the 15 kV system delivers 202 cal/cm^2 compared to 10 cal/cm^2 for the 14.9. According to 1584 the Lee method delivers conservative results, but this is ridiculous.

 

[mpparent]

stevenal,

I must have fat-fingered a button, cause when I ran my program on my handy-dandy calculator again, I came up with the same results as you. You're right though, Lee's method is a bit ridiculous for HV calculations.

Mike

This has been an interesting discussion. Definitely more work needs to be done to iron out the issues w/ Arc Flash Analysis.

 

[stevenal]

I took my concern to the chair of the working group. He says that they are aware of the discontinuity, and that the Lee assumption that half the total voltage drop is across the arc doesn't really apply at the higher voltages. He suggested ArcPro, but I had already made out the requisition a little earlier today.

 

[arcflash99]

What is your distance and is this in open air?

 

[stevenal]

694mm in open air.

 

[stevenal]

So I've got Arcpro up and running. As a check, I'm trying to reproduce some of the NESC proposal tabulated values. These are identical to the proposed OSHA tabulated values, except the NESC properly shows fault current in kA rather than OSHA's kV. See

http://mywebpages.comcast.net/dwallis2/subv.pdf

tables 8 and 9. Now instead of being overly high, my values are low. Process: convert system voltage to line to ground, find arc gap using line to ground voltage at 10kV per inch, distance is the NESC/OSHA minimum approach distance less twice the arc gap. At 46.1 kV system voltage, 20kA, 25.5 cycles, Arcpro says 3.7 cal/cm^2. In the table this is "12-cal" (I assumed that meant cal/cm^2). Any ideas?

 

[Electic]

Interesting that the IEEE committee chair would recommend Arcpro above their own formula.

1) For liability reasons we have chosen to use IEEE formulation, but also determined terrible results. In some cases the calculated Arc Flash Boundary around a 480V pad mount was over a quarter mile. To preserve credibility, we placarded such locations :"Do not work energized".

2) I am reluctant to not use IEEE as an industry standard, what is "ArcPro" and how much of an industry standard would it be seen as in court?

 

[dpc]

ArcPro is proprietary software intended for high-voltage systems and is certainly not an industry standard. But is has been around longer than the IEEE-1584 standard and is used by many utilities. It uses a line-to-ground short circuit calculation instead of a three-phase fault.

It's basically a glorified, overpriced MathCAD template, IMO. But it may be that the results given are more reasonable for high-voltage outdoor applications than the IEEE-1584 equations.

I don't think you want to use it for 480V.

 

[stevenal]

Also the NESC C8 committee of IEEE also seems to endorse ArcPro, although they consider other methods valid as well. Still don't see how they generated the tables though. Arcpro was developed by Ontario Hydro. Right now it seems that most any attempt to quantify incident energy is considered to be better than no attempt at all.

The lowest tabulated value from NESC/OSHA proposals is 4kV Certainly a 480V fault is more likely to go three phase. The IEEE method for lower voltages also takes into account the energy focusing that occurs within an enclosure. ArcPro is for open air only.

 

[stevenal]

A breakthrough. The table notes claim they are valid for line to ground faults, but were evidently generated using line to line voltage as input into Arcpro. The Arcpro input voltage is the voltage across the gap, line to ground in this case. I've passed my concern on to the chairman of the C8 committee that the tables are in error. Might be too late since publication is in August.

 

[lou1952]

I began reading this particular discussion today. I have been doing arc flash studies for 3 years now and I still learn something new everytime I work on a new plant.

I looked at the OSHA Table 9. Using ArcPro, I tried a couple of different data points and a couple were right on.

72.5kV, 20kA, 10.6 cyc; I got 2.55 cal/cm2 instead of 5.
145kV, 20kA, 12.2 cyc; I got 5.1 cal verses tables 5
362kV, 20kA, 28.3 cyc; I got 5.04 verses tables 5

So, is the table totally wrong, or did they just miss the boat at the lower voltages. Looking at Table V-2 AC Live line work minimum approach distance in the subject document,there are some problems: ie for 41.6-72.5kV they have 3' 3" instead of 3' 2". Not a big deal, but just one example.

 

[stevenal]

Lou,

I confirmed your results except for the last. Inputting 20kA, 209001V, 28.3 cycles, 20.9" gap and 60.2" distance I get 5.5 cal/cm^2.

The above referenced MAD is 3'3" per NESC 2002. It includes a 1" adder for inadvertent movement not used at the higher voltages. So the MAD is 3'2" for 72.6 to 121kV. Seems odd, but the MAD drops an inch as voltage increases across this transition. I believe OSHA is intending to match the NESC on this.

 

[lou1952]

Steve,
You are right, I copied the wrong number for the 363kV level. By the way, and you probably know this, but ArcPro only uses the voltage to ensure the arc is sustained. Whether you run the study at 362kV or 209kV, you get the same answer.

As far as the table values, if what you say is correct, there is an error in NFPA 70E. Table 130.2(C) has the values for 72.5kV and 121kV reversed from the OSHA tables:

46.1 to 72.5kV = 3' 2"
72.6 to 121.kV = 3' 3"

 

[stevenal]

I see that. According to Annex C, the values were taken from the NESC. NESC had lower MADs across the board in '93, but has been using the values that OSHA is proposing to use since '97. More errors in the annex, where "communication worker" is mentioned it should be "supply workers", and the inadvertent movement adder has been left unexplained for all the voltage between 750 and 72.5 kV.

 

[stevenal]

The new NESC arrived today. Looks like they completely redid Table 410-1, and it matches my Arcpro results pretty well. In Table 410-2 they changed the 5-cal system to 4-cal, and changed the clearing cycles accordingly. The other columns remain the same. Some of the results are reproducible if I us the '02 MAD tables. Still cannot get the values they show for 46.1 to 72.5 kV, though.