An air terminal is intended to intercept the downward-moving stepped leader by launching an upward-going attachment spark. Once this attachment is made, the bulk of the lightning current follows this ionized path. Hence an air terminal is intended to divert the lightning away from crew and electronics. In the absence of a separate air terminal on a boat mast, the highest point, usually the VHF antenna, is the most likely attachment point. Devices that are intended to prevent a lightning strike altogether are also typically mounted on the highest point. However, the effectiveness of these devices has been questioned in several studies, as described in the section on Lightning Dissipators below.
Sharp or blunt?
Almost every lightning air terminal you are likely to see has a sharp point. The theory behind this was that a sharp point causes the largest electric field and hence is more likely to launch the attachment spark. However, recent research indicates that blunt rods are more effective in this task. Following the suggestion of Prof. C. Moore, the next edition of the NFPA code will reflect this change. The full text of his paper is published in Geophysical Research Letters . The most effective air terminal is one with a radius of curvature between 3/16 and 1/2 inch. However, this does not mean that sharp terminals are necessarily avoided: in the absence of a nearby blunt terminal, Prof. Moore found that sharp terminals do intercept the lightning. Also, the top of an aluminum mast is an effective air terminal, but if you want to stand a chance of protecting masthead transducers such as a VHF antenna or anemometer vane, a separate air terminal is a good idea. And in light of the recent research, a simple 3/8" diameter road that is rounded at one end and attached securely (via a flat face) to the mast at the other is all that is needed. Make sure that any transducers are well below the top of the air terminal, and at least within an imaginary cone with 90 degree apex angle.
The idea that a device might be able to prevent a lightning strike is very appealing to the average sailor. Devices that attempt to eliminate or reduce the incidence of lightning strikes generally have a bristly appearance caused by multiple conducting points. Under the influence of an electric field under a thunderstorm, it is undisputed that these multiple points release charge into the air in a similar manner to the phenomenon of St. Elmo's fire. How effective these charge flows are at eliminating or reducing lightning has been the subject of investigations by NASA, FAA, the Departments of Army and Air Force, NFPA, and FDOT. None of these agencies consequently supported their use. Scientific papers by scientists in reputable journals have also been negative. In 1994 Donald Zipse, IEEE Fellow, compared the conventional Franklin air terminal with, amongst other systems, multipoint discharge systems and concluded "The claims of being able to dissipate any and all lightning strokes have been shown to be untrue." In a subsequent study that considered only lightning elimination devices employing the point discharge phenomenon, (that is, lightning dissipators) Abdul Mousa, also an IEEE Fellow, documents many instances when lightning struck towers at Kennedy Space Center and Eglin AirForce Base, and one case of a strike to an FAA control tower in Tampa. In his paper Dr. Mousa concludes that "Natural downward lightning flashes cannot be prevented." His comments on the subject are much more candid in a subsequent posting to the Lightning Safety listserve. His phraseology "natural downward lightning" is carefully chosen, but does cover the case of a sailboat mast.
References
Mousa, A.M., The Applicability of Lightning Elimination Devices to Substations and Power Lines, IEEE Transactions on Power Delivery, 13, 1120-1127, 1998.
Source: Ewen M Thompson. Lightning and Boats, University of Florida Sea Grant.
BLUNT RODS ARE MORE EFFECTIVE.
780- 18 - (3-6. 1): Accept in Principle (Log # 4) SUBMITTEIR: Charles B. Moore, Langmuir Laboratory, New Mexico Tech
RECOMMENDATION: Revise as follows:
3-6.1 Air Terminal Height and Form. The tip of each air terminal shall be not less than 10 in. (254 mm) above the object or area It Is to protect. (SeeFigure3-6.1.) The tip of each air terminal shall be rounded so as to minimize the emission of corona discharges prior to the approach of lightning. The diameter or the rounded tII2 shall be equal to or greater than 3/8 in. (9.5 mm) and I in.(2 5.4 m m).
SUBSTANTIATION: In the years since Franklin Invented the sharp lightning rod, many physicists have shown that, under strong electric fields, the air around sharp rods becomes Ionized, creating space charges that act to weaken the fields. It has also been discovered that the strengths of the strong fields around the tips or sharp rods decrease so rapidly with distance that they become weaker than the fields over blunter rods at distances greater than about 1/4 in. (6 mm). On the other hand, it has been found that the local electric field must become very strong In order that an upward-going streamer propagate from an exposed rod to Intercept approaching lightning. From laboratory and field experiments, we have established that the critical field strengths for lightning Interception develop sooner around blunt lightning rods than around sharp ones around which the field strengths are limited by their charge emissions. In effect. sharp rods tend to protect themselves, by their charge emissions, against participating In
lightning strikes.
In a recent competition between adjacent sharp and blunt rods to determine which type would be preferentially struck by lightning,
teii rods with blunt tips were struck whereas none of the sharp rods were hit although they emitted copious bursts of corona
discharges.
COMMITTEE ACTION: Accept in Principle.
Revise the proposal to read as follows and locate In Appendix A- 3-6.1:
Recent experiments suggest the optimal air terminal tip radius of curvature for Interception of lightning strikes is 3/16 in. (4.8 mm) minimum to 1/2 in. (12.7 MM) maximum. (Reference the journal of Applied Meteorology. Contact Chales Moore, Langmuir Laboratory, New Mexico Tech.)
COMMITTEE STATEMENT: Reworded mandatory text to be suitable as appendix material. Material provided Is better suited as appendix Information accordance with the NFPA Manual of Style.
NUMBER OF COMMITTEE MEMBERS ELIGIBLE TO VOTE: 29 VOTE ON COMMITTEE ACTION:
AFFIRMATIVE: 27
NOT RETURNED: 2 Fowler. Reehl
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