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Thread: .01 coordination

  1. #1
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    .01 coordination

    The NEC (700.27) indicates that the Emergency system needs to be coordinated. I know that the requirement is to .01 seconds but what I don't know is where that is defined?

    Can anyone help?

    Thanks,

    Mike
    Mike Shields, PE
    Boston, MA

  2. #2
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    Quote Originally Posted by mshields View Post
    The NEC (700.27) indicates that the Emergency system needs to be coordinated. I know that the requirement is to .01 seconds but what I don't know is where that is defined?

    Can anyone help?

    Thanks,

    Mike
    That has been a hotly debated issue. The code does not specify the coordination be to .01 seconds. There are a number of people that say that the systems have to be coodinated to time zero.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  3. #3
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    I agree with Don. Some jurisdictions have limited it to .1 seconds, NYC being one of them.

    If you need the NYC reference and can't find it, let us know.
    Ron

  4. #4
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    hmm

    So what is the argument for it being .01. Is selective coordination defined somewhere as being that OR is it just a contention that selectively coordinated doesn't mean partially selectively coordinated.

    What, in other words, drove all of the equipment vendors to go out and test their breakers for .01 second coordination and publish the results?
    Mike Shields, PE
    Boston, MA

  5. #5
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    A typical TCC goes to .01 seconds. The definition of selectivity in the NEC seems to imply coverage throughout the entire range.

    NYC realized that Bussman and the other fuse guys are not the be all and end all of reliability as too many times there is no spare fuses at the facility.
    Ron

  6. #6
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    .1 sec comes from tradition and other NFPA publications.
    .01 sec comes from the cut-off point on manufacturer's published time current curves.

    The NEC is silent on the exact amount of coordination required.
    Each AHJ seems to establish their own parameters, (e.g. the State of Wisconsin, you must coordinate 'new equipment' as much as practicable).
    Just because you can, doesn't mean you should.

  7. #7
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    0.1 sec in Ddenver

    Here in Denver, we are not requiring the OCPDs to be selectively coordinated down to 0.01 seconds. You only need to coordinate down to 0.1 seconds.

    To be honest, 0.01 seconds is very difficult to achieve without going to almost all fuses, and it doesn’t absolutely guarantee total coordination. The NEC doesn’t provide us with a specific level of selective coordination. NFPA 99 states that selective coordination of the essential electrical systems shall selectively coordinate for the period of time that a fault’s duration extends beyond 0.1 seconds. I think that healthcare facilities are the most restrictive and critical case for OCPD coordination, to prevent loss of life during life-saving procedures such as operations. Although NFPA 99 is not an adopted code by the city of Denver's Building Department (that’s us), it is still used as a guideline. If the most critical type of healthcare facility is only required to be coordinated down to 0.1 seconds per NFPA 99, then I think that it is reasonable to apply an OCPD coordination down to only 0.1 seconds for commercial buildings. Also, full coordination could compromise safety and system reliability, resulting from large current inrushes that are due to instantaneous reestablishment of connections to heavy loads.

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