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4322Re: [campernicholson] Lightning Protection

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  • Graham Norbury
    Jul 4, 2008
      Jim,

      There are a lot of myths and misinformation surrounding lightning
      protection. Fortunately the laws of physics still apply, so there are
      some basic precautions that a vessel owner can take to maximize their
      safety and that of the vessel. Unfortunately there isn't much that can
      be done to protect onboard electronics as these are often damaged by
      high voltages induced on internal wiring connected to sensitive
      integrated circuits.

      1. Provide a suitably sized, low resistance path to ground leading
      directly down from the air terminal.
      In the case of a sailboat, the air terminal is either the top of the
      aluminum mast, or pointed rod mounted ABOVE the highest fixture (e.g.
      vhf antenna) on the mast. ABYC standards dictate the minimum surface
      area of an underwater grounding plate for both salt or fresh water, and
      also the use of minimum 4 AWG conductor size to connect the air terminal
      (base of aluminum mast) to the grounding shoe.
      NOTE: stainless steel is a poor conductor compared to aluminum or
      copper, so relying on your standing rigging as a primary grounding
      conductor is likely to lead to turnbuckle or swage damage.

      2. Provide suitably sized secondary grounding paths, to minimize side
      flashes and crew hazard.
      All exposed metal such as shrouds, lifelines, stanchions, binnacle,
      pushpit & pullpit should be grounded with minimum 6 AWG sized
      conductors, leading to the primary grounding point. In addition, metal
      hardware such as water or fuel tanks located near the primary conductor
      (mast base) should also be bonded to prevent side flashes.

      3. A lightning strike delivery huge energy in a very short duration.
      With properly sized hardware and low resistance connections, there is
      really no reason for anything to heat up enough for damage to occur.
      Most vessel suffering hull damage either had an inadequate ground system
      (if at all), or defective/corroded underwater fittings that became
      impromptu grounds.

      While I don't have the standards book in front of me here at home, if
      anyone wants more information, feel free to contact me directly.

      Graham
      ABYC Certified Marine Electrician
      Oxford Boatyard, Oxford, MD

      JIM TEIPEN wrote:
      >
      > Albert,
      >
      > Articles that I've read on the topic over the years don't seem to
      > have any real consensus on the topic. Ground plates on the bottom of
      > the hull seem to be common, but other sources I've read say this
      > isn't a good idea since lightening can leave a hole at the point of
      > exit to the water. These articles say that its better to run a heavy
      > conductor either from a shroud or from the mast itself directly to the
      > water. In the situation like you described where you were caught out
      > in a storm, you might consider carrying a something like a heavy duty
      > car battery jumper cable to use as a temporary ground for your mast or
      > shroud to the water.
      >
      > Jim
      > Alegria
      > CN 35 - 68
      >
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