Lightning ground

Discussion in 'Electrical Systems' started by goodwilltoall, Feb 25, 2018.

  1. goodwilltoall
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    goodwilltoall Senior Member

    Pic of bottom keel with 4" × 21" × 1" thick ground plate (ground ribs in to get more surface area) with a 3/4" × 8" silicon bronze bolt.

    Epoxied whole plate to bottom at same time, bolt was put through 7/8" × 3" hole at bottom and epoxied around. Remainder of bolt went through additional 4" of 1 1/2" widened hole above.

    Unstranded 4.0 copper wire and oriented it around the remaining 4" of SB bolt. Melt silver solder and poured into hole. Balance of hole had epoxy pour on top of that.

    Question: Should the copper wire have been left unstranded and made tight against the bolt?
     

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    Last edited: Feb 25, 2018
  2. goodwilltoall
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    goodwilltoall Senior Member

    20180225_091010.jpg Pic inside
     
  3. gonzo
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    gonzo Senior Member

    So a 4/0 non-stranded is basically a 1/2" copper rod. In general, lightning protection uses rather fine stranded wire. The best is "basket weave" type. Most of the current will flow on or close to the surface of the conductor. A solid conductor will actually have a much lower conductivity. The second choice, for economy, is flat strap which has a much larger surface area per volume than a rod.
     
  4. goodwilltoall
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    goodwilltoall Senior Member

    I did mention it is stranded (yes 1/2") 4" was unwound around the bolt but not tight then 1.5" of solder poured in.

    Concerned there will be to much resistence and lightning blow a hole rather than efficeintly conduct through.
     
  5. fallguy
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    fallguy Senior Member

    I contacted Ewen Thomson for assistance with my boat, thanks for this post!
     
    Last edited: Feb 25, 2018
  6. gonzo
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    gonzo Senior Member

    Solder will melt if you get hit by lightning. That is why the usual connection is either crimped or clamped. Grounding plates may reduce the damage lighting produces, but the EMF generated will most likely damage electronic equipment. I have investigated maybe 30-40 lighting hits for insurance claims. The grounding path is often unexpected. I saw one where the blackened trail went from the back stay chainplate, under the fuel tank, along the bottom of the toerail, through the electrical panel and finally down the keel bolts.
     
  7. goodwilltoall
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    goodwilltoall Senior Member

    More concerned about avoiding. a hole or fire. Btw, the epoxy would melt first.

    Gonzo, how many of those boats had a dedicated line 4' above all else go directly vertical to ground plate?
     
  8. gonzo
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    gonzo Senior Member

    Most did, that is what a mast acts like. However, an average lightning bolt is about 5 miles long, so it can jump from the mast to anything in the boat of surroundings. The worst I saw was a Cape Dory with no ground protection, also they have an encapsulated keel. The hull from the waterline down looked like some madman had shot it with a shotgun. It was completely riddled with holes where the lightning connected to the water. About the epoxy, it is a thermoset and won't melt but soften and then burn. I think the jury is still out about whether it is better to attract the lightning into a grounded rod or use a diffuser to try to prevent the strike.
     
  9. CloudDiver
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    CloudDiver Senior Member

    I read in Don Casey's book 'Sailboat Electrics Simplified' that dissipaters potentially generate more static than they diffuse, so they are probably worthless and there is no evidence that they reduce the risk of a strike. He recommends the same as wooden boats do, mast spike to heavy-wire to ground plate. I'm not saying this is right or wrong; I am saying I am pretty confused about the varying opinions, science (or lack there of), and sea-stories about lightening strikes.
    Around my neck of the coast (so Cal) very few boats have dedicated protection because strikes are incredibly rare here. I'd like to understand this more because in the future my sailing will no be restricted to coastal California/Mexico.
     
  10. M&M Ovenden
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    M&M Ovenden Senior Member

    The lightening conductor isn't in place to help avoid a strike, it's duty is to conduct a strike safely. The AYBC guidelines are the accepted best practice, and it's a good idea to follow them. I just finished building a couple wood masts for our boat and laminated in a 2/0 copper tinned / stranded conductor. It exits the mast via solid copper. Connections were crimped and soldered, the conductor ends are epoxied in (with the connections) so gives strain relief. If the resistance is low, the solder joint won't heat and melt.
    Cheers,
    Mark
     
  11. gonzo
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    gonzo Senior Member

    The resistance of a 2/0 is high for the current on a lighting strike. The average is 30,000 A.
     
  12. M&M Ovenden
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    M&M Ovenden Senior Member

    It all happens in microseconds. 2/0 AWG is much bigger than what is required.
     
  13. gonzo
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    gonzo Senior Member

  14. CloudDiver
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    CloudDiver Senior Member

    M&M; I didn't say the conductor was there to avoid strikes, I said a static dissipater, Gonzo said diffuser (same thing) .... does that resolve any confusion?
     

  15. M&M Ovenden
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    M&M Ovenden Senior Member

    Hi CloudDiver,
    I was just responding to the original poster about the solder melting, and giving some background on what we have done and why.
    Cheers,
    Mark
     
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