NESC Table 410-2: Determining Arc Gaps

Hi,

in the NESC-2007, at the bottom of Table 410-2, it states that

Arc gap—calculated by using the phase-to-ground voltage of the circuit and dividing by 10. The dielectric strength of air is taken at 10 kV per inch. See IEEE Std 4-1995.

I have the Standard IEEE Std 4-1995. In chapter 17, I see some documentation about gaps. I'm sure not sure what section is used to determine those arc gaps for Arc Flash calculation.

Has anyone search for default arc gap for transmission voltage? I'm not sure I understand the calculation.

When reading the footnote, I wondering, does it literally means that:

Arc gap = Vlg / 10

example: 69 kV L-L
Arc gap in inch = 69 / sqrt(3) / 10 = 4 inch?

Doesn't look right.

Thanks

I have the same question.

For a 345 kV bus, is the arc gap = 345/1.73/10 = 19.9 in ?

If anyone has an answer for this it would be greatly appreciated.

I believe you are correct. The assumption is that the gap will be the distance that would breakdown at the Ø-grd voltage. 199 kV / 10 kV/in = 19.9 in. I used this assumption with ARCPRO to get the max clearing time value given in Table 410-2.

Don't forget that the table was changed via a TIA. The new tabulated results can be verified with ArcPro, the original ones could not.

The new one is the one I verified.

69kV line 4 inches apart ???

Have anybody seen 69kV overhead lines 4 inches ( 10cm ) apart ??? Or 345kV lines separated by 19.9 inch ( 0.5 meter ) gap? I don't believe 10kV / inch dielectric strength for air value is practical. It might probably apply to dry, still air without contaminants, even electrical field etc. The practical dielectric strength would be a fraction of the intrinsic dielectric strength seen for ideal, defect free, material. I remember 1kV / cm rule of thumb from school. Could anybody please provide with a copy of IEEE std 4 page proving the 10kV per inch value?

The arc gap has nothing to do with the phase spacing of the line. It is the "assumed" gap for the purpose of calculating the IE associated with a line to ground arcing fault. After all if there was no gap, there would be no arc.
________
Water Bong

I have similar question regarding arcing gap in new CSA Z462 standard article D.8 derived from NESC tables 410-1 and 410-2. For visualization purposes, check a plot displaying arc gap as a function of system voltage - all according to the tables 410-1 and 410-2 from NESC or tables D.6 and D.7 from CSA Z462



The jump at 46kV point is most obvious and alarming. The problem with the 10kV/in number and arc gap calculated based on it is that it is used in calculation the distance from the arc to the worker, namely CSA Z462 Table D.7 Note 2 states the distance from the arc to the worker is calculated using the minimum approach distance from Table 441-2 of IEEE C2 and subtracting two times the assumed arc gap.

Any comments?

Since I have not had a need to use an arc gap above 35 kV so far, I have never checked to see what a gap for 46+ kV would be. Sure seems to be a dis-continuity in the arc gap calcs as you have noted. This should probably be submitted to the committee for consideration during the comment period.
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ExoticPetite live

Thanks, Alan. I will escalate this issue to the CSA Z462 powers.

- Michael

Arcad,

The unstated assumption in table 1 (as I've understood it) is that rubber gloving is used. Table 2 says it is for live line tool use. Not sure it explains the discontinuity, but it is an important difference.

ARCPRO result with Table 410-1, 9" gap and 15" clearance, 30 kA, 30 cycles, 26.6 kV = 63.8 cal/cm²

ARCPRO result with Table 410-2, 2.66" gap and 15" clearance, 30 kA, 30 cycles, 26.6 kV = 37.9 cal/cm²

ARCPRO result with Table 410-2, 2.66" gap and 2'-3.68" (2'-9" minus 2 times gap) clearance, 30 kA, 30 cycles, 26.6 kV = 11.2 cal/cm²

Could anybody having an access to ARCPRO please provide the ARCPRO incident energy results for:

1) 9" gap and 15" work distance, 20 kA, 60 cycles ( 1 sec ), 46 kV

2) 2.7" gap and 33" work distance, 20 kA, 60 cycles ( 1 sec ), 46.1 kV

3) 9" gap and 20" work distance, 20 kA, 60 cycles ( 1 sec ), 46.1 kV

Thanks

M. Furtak

78.7, 8.6, 45.1 cal/cm^2

with copper electrodes. Note that the distance in Arcpro is not a working distance to conductors, but distance to the arc.

thanks stevenal. I thought the

energy would equal to 12.4 cal/cm^2 based on Table 441-2. What you say Arcpro produces even smaller number ( 8.6cal ). Do you know how to explain almost 10X reduction in incident energy from 78.7cal/cm^2 to 8.6cal/cm^2 for 46kV 20kA 1 sec duration arc when moving 18" = 33" - 15" further away from the arc? I would expect factor of 4X when working distance was doubled but ten (10X) times reduction in incident energy is somehow hard to believe. Could you please let me know what I am missing here?

Table 410-2 has a range of voltages from 46.1 to 72.5, with the results shown for the highest voltage.

The arc gap makes a large difference also. The 33" working distance with a 9 inch gap would give 16.8 cal/cm², closer to 1/4 of 78.7.