Large bus bar for MRBF fuses?

Discussion in 'OnBoard Electronics & Controls' started by Steelboat, Nov 7, 2024.

  1. Steelboat
    Joined: Feb 2022
    Posts: 87
    Likes: 14, Points: 8
    Location: Seattle

    Steelboat Junior Member

    I am putting in a new bus bar, and would like to use the compact MRBF fuses. I don't see what I want from blue seas, so thinking custom made from some 5/16 x 1 copper stock I have on hand. Basically drill and tap for 5/16 stainless bolts, and thread in from the backside so they work as studs. Then a few 3/8 bolts for short unfused conductors to switches. Is there any existing product like this?
     

    Attached Files:

  2. DCockey
    Joined: Oct 2009
    Posts: 5,287
    Likes: 658, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Participant

    Where would the MRBF fuses be located?
     
  3. gonzo
    Joined: Aug 2002
    Posts: 17,492
    Likes: 2,027, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

  4. Steelboat
    Joined: Feb 2022
    Posts: 87
    Likes: 14, Points: 8
    Location: Seattle

    Steelboat Junior Member

  5. Steelboat
    Joined: Feb 2022
    Posts: 87
    Likes: 14, Points: 8
    Location: Seattle

    Steelboat Junior Member

    On my bus bar design the MRBF slides over a 5/16" stud. Blue seas has a similar bus bar, just much smaller.
     
  6. DCockey
    Joined: Oct 2009
    Posts: 5,287
    Likes: 658, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Participant

    The devices Gonzo linked use the fuse to provide overcurrent protection. The stud that the fuse is goes over is insulated from the short bar. The ring on the bottom of the fuse makes contact with the short bar. The ring on the top of the fuse makes contact with the terminal. The rings are large enough that they do not make contact with the stud. The current from the short bar goes through the fuse, then to the terminal.


    If the MRBF fuse is dropped over a bus bar stud, and the terminal is also dropped over the stud, the fuse is nothing more than a spacer. The current goes through the stud from the bus bar directly to the terminal, bypassing the fuse. (Prior to 2008 a special stepped, insulated washer and nut could be used so that the MRBF fuse was installed directly over the bus bar stud, and the stepped, insulated washer was installed between the terminal and the metal washer/nut to isolate the terminal from the stud. This method is dependent on the terminal not shifting during installation and contacting the stud. If that happens the fuse is bypassed. https://www.bluesea.com/resources/640 )
     
    fallguy and Steelboat like this.
  7. Steelboat
    Joined: Feb 2022
    Posts: 87
    Likes: 14, Points: 8
    Location: Seattle

    Steelboat Junior Member

    Thanks David. My design was to use nylon insulating washers and bushings to isolate the stud from the upper contact ring. I have used these components before when rebuilding alternators (rectifier bridge studs) . It does seems to put a lot of reliance on the insulators. A fiber-reinforced washer would be mechanically superior, like phenolic. I will check McMaster Carr.
     
  8. Steelboat
    Joined: Feb 2022
    Posts: 87
    Likes: 14, Points: 8
    Location: Seattle

    Steelboat Junior Member

    I can't find a fiber shoulder washer in that size :(. Worried the nylon will "extrude" at high torques. If Blue seas will sell me the washers it will all work I think.
     

    Attached Files:

  9. DCockey
    Joined: Oct 2009
    Posts: 5,287
    Likes: 658, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Participant

    It will work with if the washer-nut that Blue Sea is available. Disadvantages compared to the designs with insulated studs are you cannot stack terminals on the stud, and there is a real risk of the fuse being bypassed if the terminal shifts during installation.
     
  10. Steelboat
    Joined: Feb 2022
    Posts: 87
    Likes: 14, Points: 8
    Location: Seattle

    Steelboat Junior Member

    I have never seen an insulated stud bus, good idea. It would be relatively easy to execute with a shoulder sleeve washer instead of threading stud to bus. Fuse bypass risk would be very low. Is this design commercially available?
     
    Last edited: Nov 10, 2024
  11. DCockey
    Joined: Oct 2009
    Posts: 5,287
    Likes: 658, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Participant

    The video linked above has several configurations with insulated studs.
     
    Steelboat likes this.

  12. Steelboat
    Joined: Feb 2022
    Posts: 87
    Likes: 14, Points: 8
    Location: Seattle

    Steelboat Junior Member

    Thanks I see it now! I found some nylon shoulder washers, and drilled a test piece of copper. Installed with a 5/16 bolt, ss flat washers and a hex nut. Torqued it to 150 inch pounds (well beyond the fuse rating of 106lbs ) and saw very little distortion of the washer. Zero ohms. Ready to fabricate. Thanks for the feedback.
     

    Attached Files:

Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.