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    Mis-match fuses

    I found mis-match fuses installed a PDP bucket at a customers site. The bucket had (2) FLS-R-90 fuses and (1) FRS-R-80 fuse. I've read through NEC 2017 front to back and cannot find a specific code that says this is a violation. Everything tells me this is not acceptable. Can anyone point me to a code that this violates?

    #2
    Are both the 80A and 90A acceptable to the application or is the 90A oversized for the load?
    Ron

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      #3
      I do not know enough to answer that yet. The system has been running in this state for several years, though. The bucket where these fuses are installed feeds a 400A distribution panel that is only being used for a few small circuits which are for pond pumps. Assuming they are appropriately sized, I fear that with the different trip characteristics there could be a situation where a fault occurs and one or two phases are opened but the remaining are left energized. Personnel then begins to troubleshoot and does not realize part of the circuit is still energized. Also, I fear that if a fault occurs some of the phases will be opened but some will remain hot. This could further damage the downstream equipment.

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        #4
        Originally posted by jmoore284@gmail.com View Post
        Also, I fear that if a fault occurs some of the phases will be opened but some will remain hot. This could further damage the downstream equipment.
        That is always the case with fuses, no matter the sizes. Most faults start as L-G, so you will generally lose only one or two fuses, so that is a downside of fuses vs CB's where all three phases trip together (even for a single L-G fault).
        Ron

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          #5
          The code does not require all three fuses be the same rating, as long as they protect the conductors properly.
          Bob

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