Lightning protection starts here!. There is some confusion as what protection can be installed. This can never be an achieved using a single method, and a number of measures should be used to minimize the risks and best security involves a holistic and overall systems approach.
The main objectives of any lightning-protection systems are as follows, and while not being ever a 100% given the power of lightning adoption of measures at least will minimize the effects:
1. The capture of the lightning strike at a nominated point, i.e. the mast head.
2. The conduction of the strike current to ground safely using a well installed down conductor that reduces side flashes.
3. The dissipation of the strike energy to ground, through a low impedance ground system so that rises in ground potential are minimized
4. The equipotential ground bonding of all relevant systems and components
5. The protection of power supplies from high voltage transients and surges that may damage equipment
6. The protection of conductors both power and signal from induced surges that may damage equipment
The most reliable lightning protection system is one that grounds any strike directly and the principles are as follows:
The Air Terminal
Most marine classification societies, ABYC, NFPA and other advisory bodies generally recommend lightning-protection in the form of a directly grounded mast and spike. The first element is the air terminal, and ideally this should be a copper rod with pointed tip.
To avoid metal interaction, stainless rods are commonly used but should be of a thicker section than the more conductive and lower resistance copper. The spike should be at least six inches higher than any other masthead equipment, including VHF aerials; this means a terminal 12-24" in height.
Many commercial units (Dynarod and Seaground) have an offset in the rod, which although not being the required straight section would be satisfactory. The purpose of the point being sharp is that it facilitates what is called point discharge. Ions dissipate from the ground and effectively cause a reduction in potential between the cloud and the sea.
In many cases the strike may be of lower intensity or not occur at all. Note that a stainless steel VHF whip aerial does not constitute any lightning protection. Lightning protection starts here!
The air terminal should be mounted clear of all other equipment and gives a cone of protection below it, to capture the strike. This protective cone prevents strikes to adjacent areas and metalwork. The cone base is the same as the mast height. This lightning protective cone prevents strikes to adjacent areas and metalwork, which in a yacht can mean stays, rails or other items lower than the mast head. Lightning protection starts here!
The Down Conductors
The purpose of the down conductor is to safely conduct the strike current through a low impedance circuit suitably rated to carry the strike current to the ground point and to eliminate side flash dangers, to minimize induction into other conductors, and to maintain the strike period to the minimum possible.
Much of the damage in a strike can result from heat, as the large current flow into even a low resistance down conductor cable can act as a large heating element. The subject of voltage drop is particularly relevant here. It is essential that cable cross sectional area is sufficient, typically 4 AWG but I have always preferred larger cables. The overall resistance of the cable must not exceed 0.02 ohms maximum as electricity follows the path of least resistance, and this reduces side flash dangers if energy looks for alternative paths. This means that if a ground circuit is 2 ohms overall and a communications ground 1 ohms, the energy will divert through the communications ground.
A low resistance grounding circuit is critical to the performance of the protection system. Any resistance will cause significantly greater heating effects and strike energy will seek shorter and lower resistance ground paths.
High resistance circuits contribute to side strike activities. Ideally welding cable is the best off-the-shelf cable as it has a higher voltage insulation in comparison to battery cables. In shore installations special purpose tri-axial cables are used and the multiple screens reduce the large radiated fields that are generated, however this is expensive and an option only on large super yachts. Lightning protection starts here!
The bonding cable to the ground plate should be as straight as practicable without sharp corners as side discharges occur and this is called corona discharge. It is also useful to enclose the conductor internally with PVC flexible conduit normally used in shore electrical systems to increase the insulation levels, as DC battery cable will break down under high voltage conditions.
It is common to have high levels of static build up on multihull and large monohull GRP decks in hot, dry, low humidity wind conditions that have caused significant static electricity shocks, so they require dissipation.
In some cases the process of arcing to ground as charges build up is a source of radio interference. It is one contention that ungrounded vessels actually promote strikes to the vessel due to this condition. Additionally grounding the mast dissipates this charge, and in the process removes a common cause of radio (RFI) noise that occurs as small arc occurs as the static charge goes to ground.
It is often easier to bond the base of an alloy mast to the mast step, and then bond this to the compression post. The bottom of the compression post is then bonded to the ground plate or keel. Keel stepped masts can be directly bonded to the ground plate or keel with a short and heavy gauge conductor. Lightning protection starts here!
Timber masts ideally should have a conductor fastened externally to the mast. Some use a flat copper strip rather than a thick conductor, also bonding the external sail track. There are calls by some to secondary bond the stays and chain plates to the ground point, however no organization recommends this. Creating what is a higher impedance path down the stays and chain plates can result in crystallization of the stainless steel and possible loss of the rig under any tension. This is to be avoided.
One major spar manufacturer in fact voids all warranty on masts if they are struck by lightning as the heat can alter the metallurgical properties. It is imperative that a single, low resistance grounding system is installed, then such dangerous "alternative" methods are not required.
It is common in very closely moored vessels and crowded marinas to have lightning strikes literally jump from vessel to vessel as it attempts to find ground on ungrounded vessels. Usually the strike exits from stays, chain plates and spreaders. In many cases the lightning strike energy will go to water from the chain plates causing serious damage to hull and fittings. Lightning protection starts here!
Do you want more lightning protection and related boat electrical systems, such as corrosion? This article was extracted from The Marine Electrical & Electronics Bible and this is available through Amazon.com Lightning protection starts here!