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Transmission Line Assumptions

Discussion in 'NESC - ANSI C2 - National Electrical Safety Code' started by jscross, Jan 27, 2009.

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  1. jscross Junior Level

    I have been asked to calculate values for our 115 kV and 69 kV transmission lines. If anyone else is /has done this:

    (1) What are you using for arc gap values?
    (2) What are you using for typical working distances?

    I'm looking for a sanity check on some numbers floating around in-house before I begin.

    Thanks.
  2. twh344 Junior Level

    For our transmission lines I have used:

    1) Arc gap - the vertical length of the insulator string (8 bells x 146mm = 1168mm or 46")
    2) Working distance - MAID - minimum air insulation distance. That is the minimum approach distance minus the ergonomic safety factor. If an arc was generated there is a good chance that the ergonomic safety factor was breached.

    However I haven't done the actual calculations yet due to the lack of a high voltage open air arc model. What arc model are you using to calculate incident energies?
  3. jghrist Sparks Level

    NESC Table 410-2 uses an arc gap (in) = Ø-grd voltage (kV) divided by 10, on the basis of the dielectric strength of air being 10 kV per inch.

    Working distance in Table 410-2 is calculated by using the minimum approach distance from Table 441-2 and subtracting two times the arc gap length. This would be 3'-11" for 69 kV and 4'-6" for 115 kV unless a maximum overvoltage other than 3.0 has been determined by engineering analysis.
  4. stevenal Sparks Level

    Big difference. Table 2 suggests that a worker brings a grounded metal object within the air break down distance, holds it there for the duration, and the arc does not lengthen on its own. But using the insulator string strike distance can yield some pretty large incident energy values.
  5. jghrist Sparks Level

    I just noticed that I added two times the arc gap length instead of subtracting. I'll have to look at this again in the morning, but off-hand it appears that this may yield smaller working distances for 115 kV than for 69 kV!
  6. jghrist Sparks Level

    I was right, the calculation of working distance according to the footnotes to NESC Table 410-2 result in a working distance for 69 kV of 2'-7" but a working distance for 115 kV of 2'-3". Doesn't seem to make sense having a shorter working distance at 115 kV than at 69 kV.
  7. Michael Furtak Junior Level

    table 441-2 needed

    Could anybody post a copy of table 441-2 from nesc c2 in pdf or doc formats?

    Thanks
  8. jscross Junior Level

    Transmission Arc Flash Model

    I think the only model to consider is the ArcPro model, but I have several reservations about that one. I'm just trying to do some due diligence before I have to tell my boss "I can't do that".
  9. JamesT Junior Level

    jghrist Error?

    jghrist: I believe that you have used the Phase to ground voltage from table 441-1 (69kV=3' 3"). This distance should be 3' 11" from the phase to phase column for distance to employee. Subtracting 8" would give you 3' 3", not 2' 7".
    Please confirm.... and repost. I think that you are ok for the 115kV line however. I do get 2' 3".
  10. jghrist Sparks Level

    Why use the value for Ø-Ø work in Table 441-1? If you used the Ø-Ø Table 441-3 for 115 kV, you would subtract 13" from 4"-7" and get 3'-6".

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