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Thread: short circuit calculation & soft loading for Equipment Evaluation

  1. #1
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    short circuit calculation & soft loading for Equipment Evaluation

    I am doing an arc-flash study. Here is the situation I am coming across and am trying to see how to go by it and understand it.
    When analyzing the closed transition scenario (utility parallel to generators), few breakers are failing Equipment Evaluation. However, the client is saying there is soft-loading and the time is limited to 20sec, so not to include this closed transition scenario.

    My understanding is that soft loading is for transfer of loads from one source to another. If for 20sec (during soft loading) the system will be in closed transition, I would think we would have to analyze this closed transition scenario for equipment evaluation when doing arc-flash study. Am I correct in saying soft loading will have nothing to do with the short circuit calculation in an event of a fault at 20second or less.
    Any suggestions pls..............

  2. #2
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    Any closed transition for any amount of time gets used for short circuit. Worst case arc flash will be on one source
    Ron

  3. #3
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    20 sec in parallel sounds wrong
    it is usually <10 cycles
    irrc the std asco time is 100 ms

    for 20 sec both sources must be considered

    might help
    http://m.csemag.com/index.php?id=957...498fd9a9384181

    In reviewing the NEC Handbook, Articles 110.9 and 110.10 are a matched pair of requirements. The Code Commentary for Article 110.10 states that “Literature on how to calculate short-circuit currents at each point in any distribution system generally can be obtained by contacting the manufacturers of overcurrent protective devices or by referring to IEEE 141-1993 (R1999): IEEE Recommended Practice for Electrical Power Distribution for Industrial Plants (Red Book).” Furthermore, the Code Commentary for Article 705.1 states that “Article 705 sets forth basic safety requirements for the installation of generators and other types of power production sources that are interconnected and operate in parallel as distributed generation.” Clearly, the use of closed-transition equipment does not equate to distributed generation.



  4. #4
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    Ingenieur's quoted article is an often referenced one.

    Industry opinion: the NEC leave this as a EOR, AHJ or utility interpretation.

    The closed transition is normally restricted to 100ms. If there is a fault, the maximum fault will happen within the first 2-3 cycles. That is under the 100ms.

    Some say the chance is unlikely that a fault would happen to be during a closed transition maintenance activity. My response to that is “do you mean during the first time one is done, like during an acceptance test?”

    If you have a conversation with all, owner included, and proceed with modeling open transition, document it and distribute to all parties.

  5. #5
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    Quote Originally Posted by Ingenieur View Post
    20 sec in parallel sounds wrong
    it is usually <10 cycles
    irrc the std asco time is 100 ms

    for 20 sec both sources must be considered
    For closed transition (make before break), it could be <100ms or for soft loading it could be longer. Depending on the load, it sometimes is 45 - 60 seconds, depending on what the utility will tolerate.

    I've found 705.16 (2014) to be more compelling regarding the contribution of both sources during closed transition.

    As I mentioned in my first post, if it is my stamp/signature, I add both source contributions together for short circuit calcs (although as I read my first post again, I see I am a bad communicator, but this is what I meant).
    Ron

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    Quote Originally Posted by ron View Post
    For closed transition (make before break), it could be <100ms or for soft loading it could be longer. Depending on the load, it sometimes is 45 - 60 seconds, depending on what the utility will tolerate.

    I've found 705.16 (2014) to be more compelling regarding the contribution of both sources during closed transition.

    As I mentioned in my first post, if it is my stamp/signature, I add both source contributions together for short circuit calcs (although as I read my first post again, I see I am a bad communicator, but this is what I meant).
    I have spec'ed at least 100 of installs
    many for mil sites in AK
    many more for water/wasteware/telecom
    maybe 1/3 closed trasition
    most required 3rd party review
    always use largest single source
    never had one kicked back in 30 years
    it is not meant to operate in parallel
    Last edited by Ingenieur; 09-14-17 at 12:41 PM.



  7. #7
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    Quote Originally Posted by Ingenieur View Post
    I have spec'ed at least 100 of installs
    many for mil sites in AK
    many more for water/wasteware/telecom
    maybe 1/3 closed trasition
    most required 3rd party review
    always use largest single source
    never had one kicked back in 30 years
    it is not meant to operate in parallel
    Understood. My experience in the data center world is different.
    Ron

  8. #8
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    Quote Originally Posted by ron View Post
    Understood. My experience in the data center world is different.
    I did 2 Mellons (Denver and Pittsburgh) and Progressives in Cleveland (among other data centers)
    I currently am involved in an Iron Mountain facility, underground, massive
    6 x 2 MW units (they have 2 of these sources)
    they only consider the largest source
    they have parallel ups and static xfer with over-lap
    the engineering firm is one that specializes in data centers/server farms
    it comes down to engineering judgement
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  9. #9
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    There has been lots of discussion over this topic for years in this forum.

    I think, this link was one where Bob recounted a bunch of ROCs he was involved with. Very interesting to see the code making panel's view into the topic.

    http://forums.mikeholt.com/showthrea...sed+transition
    Ron

  10. #10
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    Quote Originally Posted by ron View Post
    There has been lots of discussion over this topic for years in this forum.

    I think, this link was one where Bob recounted a bunch of ROCs he was involved with. Very interesting to see the code making panel's view into the topic.

    http://forums.mikeholt.com/showthrea...sed+transition
    very interesting
    imho I think the NEC deferred from making policy and left it to engineering judgment



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