It is important to understand that adding a lightning-protection system to your vessel may actually increase the chances of being struck. You are inviting the lightning to come aboard and use your vessel as a path to ground. With this invitation comes the responsibility of installing the correct system—remember that the reason for installing the system in the first place is to minimize damage to the boat and injuries to the crew.
The basic lightning-protection system is based on a theory developed by Michael Faraday, called the "Faraday's Cage." The principle is to provide a grounded structure where all the parts are bonded together and have the same electrical potential. This is accomplished by connecting the mast and any large or dense metal objects with large copper conductors. Copper is preferred over aluminum because it's more conductive per square inch than aluminum and it is also more corrosion resistant; consequently it takes less copper wire than aluminum wire to accomplish the same conductivity.
Equipment requiring bonding to the ground system includes the engine, refrigeration and AC compressors, rails, chainplates, keel bolts, metal tanks, steering pedestals, galley ranges, seacocks, and propeller and rudder shafts; also, shrouds, stays, and all tracks, including the sail track on a non-conductive mast or boom.
Sailboats are at a great advantage when it comes to lightning-protection because of their masts. A spar less than 50 feet in length off the water offers what is called a "cone of protection" or "umbrella" when properly fitted with a lightning protection system. The entire area and any object falling within a 45-degree angle in a straight line off the top of a grounded mast will fall within this cone of protection.
For vessels whose spar is greater than 50 feet off the water, the protection zone 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. Spars over 100 feet off the water do not garner any larger protection zone.
Practical Application The main components of a lightning-protection system 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 order to understand the installation of the lightning protection system, you need to know these terms:
| ||Air terminals are pointed copper or aluminum rods 12 to 24 inches in length located at the top of the spar. They act to attract, direct, and dissipate an electrical charge.|
| ||Ion dissipaters are stainless steel wire brushes, shaped either as a spiral or a feather-duster. These should not be used to replace a well-grounded and bonded lightning-protection system, but can be used in conjunction with it. They may, though, replace the traditional air terminal, although stainless steel is much less conductive than copper or aluminum. The theory behind these devices is that they neutralize the buildup of static charges on the ground by dissipating ions into the atmosphere prior to a lightning strike.|
| ||Main down conductors carry the current from the top of the mast to "ground" and need to be an insulated, flexible, compact-stranded, concentric lay-stranded, or a minimum 20-gauge solid copper ribbon. The American Boat and Yacht Council (ABYC) recommends a minimum size of No.4 AWG copper wire as the main down conductor. This conductor will be attached directly to the grounding plate or grounding strip.|
| ||Secondary conductors which are also referred to as parallel conductors, provide a separate conductive path from shroud and stay chainplates to the ground plate, strip, or equalization bus. They are a minimum size No.6 AWG copper wire by ABYC standards. These conductors should not be laid in close proximity to the vessel's electrical system wiring or electronics ground system.|
| ||Bonding conductors are used to equalize the current potential between large, dense metal pieces of equipment such as keels, tanks, and engine block, and to bond them into the system. They eliminate side flashes (electrical current jumping from one conductor to another seeking an easier path to ground) when such equipment falls within six feet of the main and secondary conductors. They are a minimum size No.8 AWG copper wire.|
| ||Ground plates are mounted on the exterior of the hull, preferably as close to the mast as possible, and should be made of copper, Monel®, or bronze material that is corrosion resistant. This plate transfers the electrical current from the down conductors to the water and must have an area of at least one square foot. Props, rudders, and struts that meet these requirements can be used, but remember that the longer the run of a conductor from the mast, the greater the electrical resistance. The engine and mast should be bonded directly to this plate. A ground strip can be used as an alternative to a ground plate.|
| ||Ground strips of copper or aluminum are installed on the exterior of the hull running longitudinally from directly under the spar to the end of the transom. The total length should never be less than four feet. The copper or aluminum strip should be at least 3/16-inch thick and 3/4-inch wide according to ABYC standards. Secure the strip with galvanically compatible thru-bolts at each end and spaced two inches apart down the length of the strip.|
| ||Equalization buses are made from 20-gauge copper strap that is laid longitudinally in the bilge in close proximity to the ground plate or ground strip and connected to the plate/strip at both ends with bonding conductors. All secondary and bonding conductors not connected to the ground plate or strip are attached to this bus so as to provide a low-resistance path to ground.|
| ||Lightning surge arrestors and air gaps prevent damage from a lightning strike to electronics and electrical circuits by rapidly reducing the voltage surge to a lower level. The air gap is a form of arrestor in which a small air space exists between two metal plates.|
| ||Connectors should be of non-corrosive metals of similar composition to the conductors themselves in order to reduce galvanic action and to maintain an equally low-resistance throughout the system. If for any reason dissimilar metals are used, stainless-steel connectors should be employed.|
Installation Notes A metal mast is a direct and low-resistance path to ground. A down conductor is not required, but an air terminal should be affixed at the top of the spar. Bond the foot of a metal mast to the grounding plate or grounding strip with No. 4 gauge copper wire at deck level if it is deck stepped, and at keel level if keel stepped. If a radio antenna extends above the air terminal, it should be either relocated or removed from the top of the spar.
If the spar is wooden or of a composite material such as fiberglass or carbon fiber, an air terminal should be attached at the top and a heavy gauge conductor should be run down the full length of the spar as straight and directly as possible to a ground plate or ground strip on the hull exterior.
The bonding of shrouds and stays should not have less than the collective conductivity of the main down conductor of No. 4 AWG wire. These secondary conductors should bond directly to the chain plates and from there directly to the ground plate or strip. If a strip is used, the backstay and engine's negative terminal should be connected to the aft end of the strip. This connection to the engine will help alleviate stray currents imposed by the thru-bolts where they may lay in bilge water.
Seacocksand thru-hull fittings should not be grounded to the main down conductor, but to the ground plate, strip, or equalization bus by way of bonding conductors. There is still considerable debate as to whether seacocks and thru-hull fittings should be part of the lightning-protection system since they are sometimes used as part of the vessel's electronics and electrical grounding system.
If possible, surround electronics in a metal casing and bond the case with a minimum No.8 AWG copper wire. Add lightning surge arrestors to all wiring leaving and entering electronic equipment and never ground your electronics to the lightning-protection system.
With the installation of a good lightning- protection system should come the peace of mind that if your vessel is struck, the damage to it and the crew should be minimal. There is no guarantee that there will be no damage, but minimized and controlled damage. Lightning continues to be one of nature's great unknowns, and new theories about the mechanics of lightning are still under study. After all, it was only recently that the "ground to cloud" theory of lightning was put forward, revolutionizing the way we think about the prevention of being zapped.