Shielded ground/negative

Discussion in 'OnBoard Electronics & Controls' started by fallguy, Jul 2, 2021.

  1. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    A number of my electronic devices came with a shielded ground and a negative wire. Along with that some of them also include a case ground.

    Do they all go to the same place? Can they be combined in a butt conmector or do they all need their own ring terminals (means more wire). Further, is there any reason case grounds don't share the same bus?

    Does anyone know if the screw posts on a Blue Sea 2300 allow multiple rings? On that same line of thought, a ten gang bus with four case grounds doesn't really have any current flowing from those 4, so is it fair to supply a similar sized wire as the hot side or does the negatice bus always generally need to be setup for more current?

    While we are at it, my console has a metal fitting with a bonding lug. All of this can go to the same bus? Yes or No?

    I say yes, but wanted to verify.
     
  2. missinginaction
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    missinginaction Senior Member

    This might help you fallguy. How to Wire Shield Conductor in Fiberglass Boats - Moderated Discussion Areas https://continuouswave.com/ubb/Forum6/HTML/003737.html

    As for the Bluesea 2300. You can stack a few ring terminals but IMO it's not good practice. Why not just install a larger buss bar? I believe that ABYC allows four terminals to be stacked but I'd have to look that up to be sure.

    I don't see why a bonding lug on a metal console can't be grounded with the other device specific grounds. It's the same idea as a case ground.

    Regards,

    MIA
     
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  3. fallguy
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    fallguy Senior Member

    I don't need more than 40 connections.

    on balance, thanks for the good read on the shielding, in case the link ever dies, the advice is to tie shields to the negative bus
     
  4. sdowney717
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    sdowney717 Senior Member

    Not looking at article, but having the case also grounded separate from the negative wire is a good idea. You dont want shorts to ground running all through a black negative wire which means it also runs through the circuit boards of the device. You want the case to not build up any charges. Maybe can help if you have a lightning strike, some of the energy goes places beyond the main strike. Pictures of lightning strikes show lots of little streamers moving off the main bolt.
     
  5. fallguy
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    fallguy Senior Member

    how can it be grounded separately? The grounds all land same busbar
     
  6. sdowney717
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    sdowney717 Senior Member

    They do but stray currents branch down many paths. If the outer case becomes energized, you want that current to ground through a separate case ground wire as much as possible, and not all of it running through the black grounding wire attached to the motherboard in the device. Running the extra ground wire is not used for normal operation, just when something bad happens, goes wrong. To me it makes logical sense. If the entire case is plastic, then it would not be needed. They want a separate ground wire on metal cases of high power devices like inverters too to help prevent a fire hazard.
    And it has to be a big as the ground wire running to the battery, so double grounds. When I read that I thought it is dumb, the electrical experts say it is required.
     
  7. sdowney717
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    sdowney717 Senior Member

    Grounding and Circuit Protection for Inverters and Battery Chargers - Blue Sea Systems

    It was found that faults in the DC wiring or the DC side of chargers could generate fires because high current could flow back from the batteries, so a fuse was added between inverters or chargers and the battery system. As the capacity of chargers increased, and with the introduction of inverters, these DC fuses became quite large. It was then determined that a fire hazard exists when a DC fault in a charger or inverter can pass DC current into the AC safety ground wire. The AC safety ground was not sized for the high DC currents, so a high capacity DC grounding wire is now required by standards A-20 and A-25.

    Now three critical grounding wires for these systems have been identified. This may seem excessive, but this combination of grounding conductors has been shown to give protection against a wide variety of faults.
     
  8. sdowney717
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    sdowney717 Senior Member

    A grounding wire is NOT a current carrying conductor, (like the black negative wire). A ground wire is only there for when something bad goes wrong, then the normally non current carrying ground wire can, if sized correctly, safely carry the current to the ground buss, it is not BS, it is a requirement today. If it can not safely carry away the fault current, then it may catch fire and burn you and your boat up in flames.
    I bet a lot of people who wired up their own stuff, messed up these extra grounds by leaving them out.

    Think about AC power, you have 2 current carrying conductors, a hot wire, a neutral return wire and a normally non current carrying ground wire, it is very similar safety idea with DC power.
     

  9. sdowney717
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    sdowney717 Senior Member

    I have had my 3000 watt inverter apart. Their is a tiny wire that joins the negative wire to the case on the inside, some kind of internal fault and that wire is toast, be like a fusible link, it is only maybe a 16 gauge wire, and the inverter negative wire is 4/0 cable. Then the case is not grounded at all except for an also very small AC grounding wire. Now what happens if the 4/0 negative cable develops a high resistance connection or becomes disconnected? The 14, 10, gauge whatever AC grounding wires gets to carry hundreds of amps, may turn red hot and now you have a fire in the wiring. This can happen.
     
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