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Thread: Low Voltage Cable Protection and Coordination Studies

  1. #1
    Join Date
    Aug 2018
    Location
    Baltimore, MD, USA
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    1

    Low Voltage Cable Protection and Coordination Studies

    I have ageneral question about cable protection and Time Current Curves (TCCs). In the past, I haveensured that my TCCs had breaker settings arranged such that conductor thermaldamage curves were always to the right of and above the applicable breaker orfuse curve to ensure protection. In some cases, even when the NEC wouldclassify the feeder cable as being protected (with the ampacity of the cable beingequal to or greater than the breaker long time setting) there might be someoverlap in the instantaneous region (usually an issue with high faultcurrents), or possibly in some other areas of the curve. I would haveclassified this as unprotected cable. I have recently had somediscussions with Schneider Electric where they indicated that the curve overlapwas irrelevant since the conductors were protected per articles 310 and 240 inthe NEC regardless of what the TCC shows.

    I amtalking specifically about conductors at 208V or 480V (not medium voltage).

    Can youshare if there is a consensus on how we should be dealing with this? Canwe ignore overlap as long as the long time setting of the breaker is set below thefeeder ampacity (assuming the breaker fault current rating is adequate ofcourse)?

    Thanksfor any input you can provide.

  2. #2
    Join Date
    Mar 2013
    Location
    Rutland, VT, USA
    Posts
    349
    Quote Originally Posted by Electrical Ninja View Post
    I have ageneral question about cable protection and Time Current Curves (TCCs). In the past, I haveensured that my TCCs had breaker settings arranged such that conductor thermaldamage curves were always to the right of and above the applicable breaker orfuse curve to ensure protection. In some cases, even when the NEC wouldclassify the feeder cable as being protected (with the ampacity of the cable beingequal to or greater than the breaker long time setting) there might be someoverlap in the instantaneous region (usually an issue with high faultcurrents), or possibly in some other areas of the curve. I would haveclassified this as unprotected cable. I have recently had somediscussions with Schneider Electric where they indicated that the curve overlapwas irrelevant since the conductors were protected per articles 310 and 240 inthe NEC regardless of what the TCC shows.

    I amtalking specifically about conductors at 208V or 480V (not medium voltage).

    Can youshare if there is a consensus on how we should be dealing with this? Canwe ignore overlap as long as the long time setting of the breaker is set below thefeeder ampacity (assuming the breaker fault current rating is adequate ofcourse)?

    Thanksfor any input you can provide.
    I can't recall seeing an instance where there was overlap in the instantaneous area of the TCC. Are you clipping the TCCs at the momentary fault current? If not, there area of overlap may not really exist as the fault currents do not go that high.

  3. #3
    Join Date
    May 2008
    Location
    West Chester, PA
    Posts
    575
    I've noticed that SKM has a big blank area on the cable damage curve - it stops at 10 seconds. The next thing you see is a little tick-mark at the NEC ampacity. This is where it will overlap with the short-time range of a typical thermal magnetic breaker. But what is the shape of the actual damage curve between 10 seconds and the long-time asymptote?
    Alternate current
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  4. #4
    Join Date
    Dec 2010
    Posts
    1,427
    See the graph at

    http://www.skm.com/applicationguides4.html

    for some explanation of emergency overload graph note usually show.

    Also some information at

    http://www.cooperindustries.com/cont...otection_2.pdf

    You could plot out the equation shown here.

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    The more they overthink the plumbing, the easier it is to stop up the drain

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