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Thread: 240.87 - setting the instantaneous to less than the available arcing current

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    240.87 - setting the instantaneous to less than the available arcing current

    how does one exploit this option. Is arcing current a function of available fault current and in any event, how does one assure at the design stage that the main breaker on a 1200A, 480V switchboard/DP meets this requirement AND coordinates to 0.01 with the down stream devices?
    Mike Shields, PE
    Boston, MA

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    According to this link https://www.ecmag.com/section/codes-...-fault-current
    The Doughty Neal Paper states, for a 480V system, the electrical-industry-accepted value for a self-sustaining arcing fault is about 38 percent of the bolted fault current of the system.

    Many times to ensure coordination of breakers to 0.01 seconds, you need to use the manufacturers tables, and I don't recall if that is based on the main (upstream) breaker having the instantaneous set at a particular value.

    So at a minimum, specify an adjustable instantaneous that can be set to a multiplier less than 38% of your previously calculated bolted fault current, which is pretty low.
    Ron

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