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Michael Faraday developed a basic theory, now called the "Faraday's Cage”, on which a lightning protection system is based. The principle is to provide a grounded structure where all the parts are bonded together and have the same electrical potential, which is the ability to move an electrical charge from one point to another. For the basics of lightning and boats, see the MariSafe article Lightning Safety. To establish a Faraday cage on a boat, any large or dense metal objects are connected with large copper conductors, including the mast of a sailboat.
Equipment requiring bonding to the ground system includes the engine, refrigeration and AC compressors, rails and stanchions, metal tanks, steering pedestals, galley ranges, seacocks, and propeller and rudder shafts. If you own a sailboat, you must also bond the chainplates, keel bolts, shrouds, stays and all tracks, including the sail track on a nonconductive mast or boom.
Sailboats are at a great advantage over power vessels when it comes to lightning protection systems because of their masts. A spar less than 50 feet above the water offers what is called a "cone of protection" or "umbrella.” The entire area and any object falling within a 45° angle in a straight line off the top of a grounded mast will fall within this cone of protection. In order to protect an entire powerboat, it is necessary to erect an air terminal high enough to protect the ends of the boat.
For sailboats with a spar greater than 50 feet off the water, the “cone of protection” is based on a different set of values. The lightning strike zone is defined as a concave arc with a radius of 100 feet drawn from the top of the mast to a tangent point on the water. Thus a mast precisely 100 feet off the water's surface would have an arced protection zone that would extend 100 feet in front of the mast's base and masts over 100 feet off the water will not garner any larger protection zone.
Powerboats can use what is referred to by the American Boat and Yacht Council (ABYC) as a “temporary lightning protective mast.” This can be a temporary structure that’s raised during lightning storms. Outriggers may also be used but must have the same conductive equivalent to No. 4 AWG copper wire. The same guidelines used for sailboat masts less than 50 feet above the water apply to the temporary mast for a powerboat.
A lightning protection system’s main components are an air terminal, heavy-gauge main conductors, secondary conductors, bonding conductors, arrestors, fasteners, a ground plate or ground strip and equalization bus. It is not recommended that the VHF antenna be used for an air terminal because most antennas don't meet the requirements for conductivity. In fact, to be effective, the air terminal must be higher than any other object on the boat, including the antennas.
Understanding these terms will help you understand the installation of your lightning protection system:
Air terminals are pointed copper or aluminum rods 12 to 24 inches in length located at the top of a sailboat’s spar or on a powerboat’s temporary mast. Their function in the system is to attract, direct and dissipate an electrical charge.
Ion dissipaters are the stainless steel wire brushes you see on the top of some sailboat masts and are shaped either as a spiral or a feather-duster. Ion dissipaters should not be used to replace a well-grounded and bonded lightning protection system, but can be used in conjunction with it. They may replace a traditional air terminal, although many are buil |
