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Thread: Arc-Flash: Proper I for determining Ct of Upstream OCPD

  1. #1
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    Arc-Flash: Proper I for determining Ct of Upstream OCPD

    Which value is correct current in determining upstream OCPD clearing time ?
    Simple 2 point system
    Panel A has an OCPD that serves panel B.

    bf = boltedfault af = arcing fault
    Panel A : Ibf= 44,940.01 amps
    Feeder to Panel B>>> 100’ - 1 set - #3's - Single conductors - Steel conduit

    Panel B : Ibf= 10,221.30 amps, calc >>>> Iaf = 6,681.32 amps

    REFLECT Iaf = 6,681.32 amps back to Panel A , calc>>>> Isc = 13,497.47 amps

    Utilize Isc = 13,497.47 amps to derive CLEARING TIMEat OCPD at Panel A.
    Agree or disagree ??


  2. #2
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    Keep in mind the highest fault current may not be the largest Ei.

  3. #3
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    Per IEEE 1584, you also have to look at the trip time at 85% of arcing current to determine worst case for incident energy.

  4. #4
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    Your Ibf to Iaf about Panel B looks good to me assuming IEEE 1584 method for arc in box at 0.48kV MCC/Panel w/ conductor gap of .25mm.

    I don't understand what you are doing with the reflect calculation.

    If Panel B has a fault with 6.7kA of arcing current that is the same current passing through the upstream OCPD at Panel A. Why use 13.5kA and how was that derived?

  5. #5
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    Quote Originally Posted by smoothops10 View Post
    Your Ibf to Iaf about Panel B looks good to me assuming IEEE 1584 method for arc in box at 0.48kV MCC/Panel w/ conductor gap of .25mm.

    I don't understand what you are doing with the reflect calculation.

    If Panel B has a fault with 6.7kA of arcing current that is the same current passing through the upstream OCPD at Panel A. Why use 13.5kA and how was that derived?
    since the line is shorted a reflected wave of equal I magnitude is reflected, only slightly attenuated/dampened by the feeder Z
    basically 2 x ifault



  6. #6
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    wbdvt - Thank you, was not aware of that requirement.

    Ingenieur - not clear on your comment....please expand or provide an example.

  7. #7
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    Quote Originally Posted by Ingenieur View Post
    since the line is shorted a reflected wave of equal I magnitude is reflected, only slightly attenuated/dampened by the feeder Z
    basically 2 x ifault
    Not familiar with this. If I were to run this in SKM with 1584 method it would use the arcing fault current (af) - not 2 x af to calculate the trip time of upstream ocpd. I don't think it matters in OP's case as it is probably in the instantaneous region with both values for this scenario but obviosuly in a full study it would come up with quite different results since many ocpds would have quicker clearing times with the higher 2xaf being used.

  8. #8
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    Quote Originally Posted by MyCleveland View Post
    wbdvt - Thank you, was not aware of that requirement.

    Ingenieur - not clear on your comment....please expand or provide an example.

    this applies to transmission lines
    theory for any conductor is similar
    it's the only thing I can think of that relates
    page 4 'short circuited line'
    http://www.snscourseware.org/snsct/f...ember_2011.pdf

    this is factored in HV T & D fault transient fault calcs, not so much LV 480 stuff



  9. #9
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    smoothops10 - you stated....
    "If I were to run this in SKM with 1584 method it would use the arcing fault current (af) - not 2 x af to calculate the trip time of upstream ocpd."
    Do you know this because SKM STATES the I value used in picking off Ct from TCC ? or are you assuming this is the case ?

  10. #10
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    Quote Originally Posted by Ingenieur View Post
    this applies to transmission lines
    theory for any conductor is similar
    it's the only thing I can think of that relates
    page 4 'short circuited line'
    http://www.snscourseware.org/snsct/f...ember_2011.pdf

    this is factored in HV T & D fault transient fault calcs, not so much LV 480 stuff

    If you do not have a transmission line of sufficient length (1 nanosecond per foot, roughly) the energy available from any possible short circuit impedance change will not be significant at the time scale of cycles of 60Hz.

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