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Old 06-13-2004
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Understanding Lightning, Part Two


Perhaps the best way to avoid having your vessel struck by lightning is not to venture out in conditions that promote the possibility of a thunderstorm.
In my most recent article on Lightning (Lightning Examined), I probably succeeded in scaring the heck out of you. At least it scared me when I re-read it! If the part describing millions of volts wandering around your vessel didn't send a quiver down your spine, perhaps the following paragraph might. If not, it's probably a certainty that your landlubber friends privately exchange comments about you being 'not quite all there.'

There really are no good means of ensuring that your vessel won't be struck by lightning. So what I present in this article by way of a protection system is definitely not the ultimate solution, and I certainly can't offer any guarantees. There is no such thing as lightning-proof. However, we can simply do our best to avoid being a prime target of Thor, and utilize methods believed to be effective in practice and theory. As I mentioned before, common sense is an underutilized weapon against lightning threats. Don't go sailing in a thunderstorm! We cannot always exercise that kind of control so here are some ideas used to keep a boat from suffering excessive damage from an encounter with lightning.

Fool Mother Nature? If we find ourselves at sea in a thunderstorm, is there a way to keep from being a lightning target? I believe a simple type of ‘air terminal' has an effect that reduces the possibilities of the top of a mast from participating in the formation of the ionized path lightning follows. To explain the function of this device, first I will describe a more commonly used air terminal.


Steel brushes like this one set atop a masthead can be useful for helping a spar dissipate its electrical charge and thereby become less of a target for lightning.
To prevent damage to the roof or structure of a building, or equipment on the roof, air terminals are placed along the highest point of a peaked roof or around the edge of a flat roof. These air terminals are usually made of aluminum, sharpened to a point, and connected to the earth by way of a large cable. The sharp point concentrates the ions of the charge building up just prior to the ionization of the air and the actual lightning strike. This concentration of a large charge encourages the lightning to strike at the air terminal, drawing its destructive potential away from the equipment or building itself. The inexpensive air terminal is usually destroyed if connected to a lightning event, and is easily replaced; whereas the air conditioning units, solar collectors, or roof structure itself would not be cheap to repair.

The type of air terminal to use at a masthead consists of hundreds of metallic points at its end. The built-up charge divides into smaller differences of potential on each of the very fine ends of the wires of this stainless steel brush. These smaller charges are not enough to cause the formation of an ionized air path for the lightning to follow, but still concentrated enough to get them to leap off towards the opposing charge in the skies. As the hundreds of wires lose their ions in a random sequence, the mast (and everything connected to it) dissipates its total electric field charge, and the boat ceases to be a target for the lightning event.

Many do not agree with this dissipation theory of operation. To demonstrate how difficult it is to realize a consensus of experts concerning lightning theory, I will present an example involving the National Fire Protection Association–the authority in the US concerning wiring codes. In 1993 the NFPA declined to approve proposed NFPA 781, which would have set Early Streamer Emission standards for a controversial “improved” type of air terminal, and presumably given the ESE industry a shot in the arm. ESE makers sued, claiming NFPA 781 had just as much scientific backing as NFPA 780. In a settlement the NFPA agreed to have ESE technology reevaluated by an outside panel. The panel confirmed that there was no scientific basis for NFPA 781. But it also said there's no scientific basis for NFPA 780 (traditional lightning rods) either.

Now the traditional ‘Franklin' type rods have been used for 250 years! As I said, there are no guarantees, and it is not nice to fool Mother Nature! There's always a chance that lightning might still connect with your mast. So, let''s see if we can make a possible disaster into a dockside story. 

When lightning connects with a sailboat (usually near the top of the mast), it raises the difference of potential between the mast and everything not directly connected to it. Once connected to the lightning, the best thing would be to get the raised potential off the boat as quickly as possible. When it is truckin' down your mast at the speed of, well, lightning, you can imagine it doesn't appreciate making a quick turn. It is looking for a way to ground itself into the earth (water) beneath the boat in a straight line. We can help it disembark in an orderly manner.


The grounding plate on this hull is placed near the mast step in order to present an unobstructed path to ground for a potential lightning strike.
Here's another situation where it's important to follow all the proper bonding and grounding procedures. Place the ground plate as near the base of the mast as possible. If your boat's mast is deck-stepped, run a very straight wire down to the hull and the ground plate. If your mast is wood, run a large copper conductor from the top of the mast to it's base and connect it to the ground plate. If your mast is aluminum, connect the base of the mast to the ground plate with a section of large (say three-eighths or half-inch) copper tubing. Tubing is best because very high changes in voltage and high-frequency currents are carried in the outer extremities of a conductor. (RF currents occur in a lightning event due to huge magnetic fields generated by the rapid rise and fall and polarity changes of the voltage.) Flatten the ends of the tubing and drill a hole for the connection. This conductor from the mast should be run as straight as possible to the ground plate connection. This is the single most important feature for protection of the boat's hull from lightning damage.

Now having a smooth curve is OK to get off to the side of the keel. But just imagine 100 million volts coming down your mast and getting lost at the step–right next to your hull! You want to make it as easy as possible for that difference of potential to get to that ground plate and off your boat. (A note here concerning a lightning strike through a carbon-fiber mast: replace the mast! Carbon cloth is a relatively good conductor of electricity and will attempt to conduct the destructive current through all of its fibers with disastrous results.)

As I mentioned, there are no guarantees, but these ideas have worked to protect vessels (and other structures) in all types of lightning situations. They most probably will help in protecting yours. Next month I will conclude the bonding and grounding system series.

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