Lightning Protection

SailNet · #1 (**permalink**) 09-27-2000

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 for your vessel to minimize damage to the boat and injuries to the crew.

HOW IT WORKS

Lightning is the result of the difference, or potential, between positive and negative electrical charges, and these differences can be measured in hundreds of thousands of volts.
The positive charges are usually higher in a cloud formation while the negative charges are lower, resulting in electrical current movements within a cloud, between clouds, or from cloud to ground.
There is also evidence that the potential can be from ground to cloud, meaning that the positive charges are accumulating on objects on the ground below the cloud.
The negative charges seek their positive counterparts, but because air is a poor conductor of electricity this buildup of potential searches for a conductive path between the positive and negative charges until this energy is released in a sudden flow as lightning.
The basic precept to remember about lightning is that it searches for a conductive path to ground. Only when the insulation of the air is overcome by huge amounts of energy is the electrical potential, or difference between positive and negative charges, released between the cloud and the ground as lightning. In the case of a sailboat on the water, water is the "ground" and the lightning will use the vessel as its path to ground.

THE SYSTEM

Equipment requiring bonding to the ground system includes the engine, refrigeration and AC compressors, rails, chain plates, keel bolts, metal tanks, steering pedestals, galley ranges, seacocks, and prop and rudder shafts. Also incorporated are the shrouds, stays, and all tracks, including the sail track on a non-conductive mast or boom.
The main components of a lightning protection system will be an air terminal, heavy gauge main conductors, secondary conductors, bonding conductors, arrestors, fasteners, a ground plate or ground strip, and equalization bus. It’s not recommended using the VHF antenna for an air terminal because most antennas don’t meet the requirements for conductivity.

TERMINOLOGY

Air terminals, a pointed copper or aluminum rod 12 to 24 inches in length located at the top of the spar. This acts to attract, direct, and dissipate an electrical charge.
Ion dissipaters, Stainless steel or 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 the system. They may, however, 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 #4 AWG copper wire as the main down conductor. This conductor will be attached directly to the grounding plate or grounding strip.
Secondary conductors, 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 #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 bond them into the system. These 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 #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 placed at each end and spaced two inches apart down the length of the strip.
Equalization buses are a 20-gauge copper strap 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 strips 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. If for any reason dissimilar metals are used, stainless steel connectors should be employed.

TIPS

Aluminum spars don’t require down conductors, but an air terminal should be affixed at the top.
Bond the foot of the metal mast to the grounding plate or grounding strip with #4 gauge copper wire at deck level if it is deck stepped, and at keel level if keel stepped.
Wooden or composite material spars, such as fiberglass or carbon fiber, require a #4 gauge copper conductor run down the spar as straight and directly as possible to a ground plate or ground strip on the hull exterior, and an air terminal attached at the top.
If a radio antenna extends above the air terminal, it should be either relocated or removed from the top of the spar.
Down conductors should bond directly to the chainplates and from there directly to the ground plate or strip.
If a ground 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.
Seacocks and thru-hull fittings should not be grounded to the main down conductor, but to the ground plate, strip, or equalization bus by way of secondary conductors. If the seacocks or thru-hull fittings are used as part of the vessel’s electronics and electrical grounding system, do not bond them to the LP system.
Add lightning surge arrestors to all wiring leaving and entering electronic equipment and never ground your electronics to the lightning protection system.

INSTALLATION

Lightning protection systems require an above-average electrical aptitude and electrical toolbox.
The ABYC recommends the installation of a well-designed lightning protection system to minimize damage and injury. The installation should be done by an ABYC-certified electrician.

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