Lightning Protection: The Truth About Dissipators

Posted by Darrell Nicholson at 05:16PM - Comments: (12)

August 19, 2013

There is really nothing you can do to dissuade Thor if he takes a liking to your masthead, but you can take steps to minimize the damage.

About this time of year, when lightning strikes become frequent occurrences, we receive a good deal of mail asking about static dissipators such as the Lightning Master. These are the downside-up, wire-brush-like devices you see sprouting from antennas and rooftops in cities and towns, and more frequently, on sailboat masts. When these devices first appeared on the market, we did a fair amount of research to find out whether they realistically could be expected to spare a sailboat's mast from a lightning strike. The following Special Report first appeared in the July 15, 1995 issue of Practical Sailor. Sailors also will be interested in reading about our discussion of conventional lightning protection systems in “Getting a Charge Out of Lightning.”

All sailors—except those who sail exclusively in the most northern but still liquid reaches of the Arctic Ocean, or most southern parts of the Antarctic Ocean—are well aware of lightning and its inherent risks. Lightning awareness generally takes one of two forms: (1) aware, concerned, resigned, do nothing or (2) aware, concerned, do something, and hope what was done will be more beneficial than harmful. In many ways, our ability to deal intelligently with lightning is little advanced from Benjamin Franklin’s approach. Most boats are built in compliance with the safety grounding and lightning protection recommendations of the American Boat and Yacht Council (ABYC). The highest mast will be well grounded to the sea through a copper wire of suitable size, which connects to a metal plate mounted on the hull’s exterior surface. There may be a lightning protection air terminal mounted at the masthead. The terminal may take the form of a vertical spike with a sharp point or some more exotic shape and construction.

For years, a number of companies have started to aggressively market on-purpose lightning protection devices for use on boats. Although the devices appear to be little different from the forms that have been used on both aircraft and stationary constructions, some of the marketing claims have been rather innovative. Are these claims reasonable in light of what is known about lightning? Is the cost of protecting a vessel with one of these devices a good investment? Can you really placate Thor, the god of lightning?

How Lightning Occurs

First, let's examine what we know about lightning. Lightning is a final result of the natural creation of an electrical charge imbalance in the Earth’s atmosphere. Simply put, the imbalance can occur due to the movement of the air, which like the movement of a person across a carpet, can cause electrical charges to be moved from one place to another. Imbalance in electrical charge causes a potential gradient to develop. This gradient can be measured and is usually expressed in volts per meter. The normal electric (E) field averages about 150 volts per meter. The field can exceed 1,000 volts per meter on a dry day. At this intensity, the potential difference from the head to the toe of a person 6 foot, 3 inches tall can reach 1,800 volts!

Since this is a static charge, it won’t electrocute anyone, but unfortunately, it also can’t be used to power the electrical consumers on a boat. The ability of the atmosphere to withstand or prevent a flow of electrical current when a voltage gradient exists can also be measured.

Objects more than 150 feet above the surrounding terrain are more likely to be hit than those which are shorter (like most sailboat masts).

If, or when, the voltage gradient created by the charge imbalance exceeds the ability of the atmosphere to prevent a current flow, something will happen. In some cases, the charge will be dissipated harmlessly as a flow of ions. This flow may cause a visible affect under some conditions. Seen at night. St. Elmo’s Fire, an ethereal blue flamelike discharge, may be seen around any sharp points on the boat's rig. In an aircraft, the blue glow may trail from wing tips and static discharge wicks (those round, pencil-like tubes seen protruding from the trailing edges of wings and control surfaces). An adventuresome pilot may be able to draw electrical arcs from the windscreen to his outstretched fingers. This type of electrical discharge won’t hurt you because the small electrical current moves through the surface of the skin, not through the internal organs of the body.

On some occasions, the build-up of charge gradient occurs very rapidly, so rapidly that little if any effective dissipation of the charge can occur before the stress applied to the air by the charge overcomes the ability of the air to resist. When this happens, the charge imbalance is relieved very quickly, by what we call lightning. Lightning is always occurring somewhere on the earth. The planet is always losing electrons. Although the current is very small, less than 3 millionths of an ampere per square kilometer, it amounts to an average global current flow of about 2,000 amperes. Nature balances this current flow by creating about 150 lightning strikes per second.

Lightning occurs both within the atmosphere, cloud-to-cloud lightning, and from the atmosphere to the earth, sky to ground lightning or the reverse, ground to sky discharge. Regardless of the direction of the lightning stroke, a great deal of energy is released as the electrical charge balance of the atmosphere-earth is restored. An average lightning strike consists of three strokes, with a peak current flow of 18,000 amperes for the first impulse and about half that amount of current flowing in the second and third strokes. Typically, each stroke is complete in about 20 millionths of a second. Once the lightning strike occurs, the air becomes a conductive plasma, with a temperature reaching 60,000 degrees. The heating makes the plasma luminous; in fact, it is brighter than the surface of the sun.

Measurements made of the current flow in the lightning strike show that 50 percent will have a first strike flow of at least 18,000 amperes (18 kiloamps, or kA), 10 percent will exceed 65 kA, and 1 percent will have a current flow over 140 kA. The largest current recorded was almost 400 kA.

Current flows of this magnitude are serious stuff and cannot be dealt with lightly.

The Risk to Structures

People who have boats and those who have towers or tall buildings share a common concern about lightning. Due to the altitude distribution of the air movement in the atmosphere that gives rise to the charge imbalance, things that are tall and stick up into the atmosphere are likely to be attractive targets as nature tries to rid itself of the charge imbalance. Since there are more tall towers than seriously tall boat masts, and since lightning-strike records are kept for these towers, we can use this data to ascertain the affect of tower height on attractiveness for lighting strikes.

The Westinghouse Co. obtained data for isolated, grounded towers or masts on level terrain, in a region that experiences 30 thunderstorm days per year. The number of strikes per tower or mast did not reach two until the height of the tower exceeded 500 feet. With a tower 1,000 feet high, the strike frequency was about nine. Towers more than 1,200 feet high were struck more than 20 times. Although the data may not be accurate for very small towers or masts, it appears that the chance of a typical 60-foot sailboat mast being hit will be quite close to, but clearly not zero. We know that there is always a chance of being hit by lightning; after all, people walking on beaches have been hit.

The ground wire, usually the topmost wire in an electrical power transmission line, is frequently hit. Trees are hit very often, sometimes exploding due to the instantaneous vaporization of moisture within the wood. Concern about lightning strikes on golf courses is sufficient to cause the Professional Golf Association to take special measures to ascertain the level of a threat of lightning and to stop play when the local electrical field strength and other indicators show a probability of lightning.

The earth can supply energy 4,000 times faster than the rate at which a static discharge brush can dissipate the energy.

Charge Dissipation

Some people believe that by constantly discharging the charge build-up on an object, the magnitude of the charge imbalance can be controlled and kept to a level where a lightning strike will not occur. Continuous dissipation of static charge potentials is used in every electronics laboratory that works with sensitive integrated circuits and transistors. The workers wear wristbands of conductive material that are connected to the room’s electrical ground. Charges bleed off before they reach levels that might destroy the electronics.

Unfortunately, what works in a laboratory, with very modest static charge quantities, does not work in nature. Let’s look at the facts that govern the charge dissipation approach to undoing what Thor wants to do—blast us with a lightning bolt.

We can begin with some interesting evidence in nature. Trees have many thousands of reasonably sharp points. These points should operate somewhat like man-made charge dissipation devices. The evidence shows that trees, even small trees, are constantly being hit by lightning. Although trees are not terribly good conductors of electricity, they do in fact conduct to some extent, as witnessed by the lightning strikes they suffer. Suppose we substitute a carefully designed set of sharp points for the branches and twigs of the tree. We will make the sharp points of a material that conducts electricity very well, perhaps metal, or graphite (used in aircraft static wick systems). The idea is to take the electrostatically induced potential in the ground system and convey it to the sharp points where it can create ions in the air.

Sharp points create the greatest possible voltage gradient, enhancing the creation of ion flow. As the ions are created, they are supposed to be carried away by the wind, eliminating or greatly reducing the total potential difference, thereby reducing or eliminating the chance of our object being hit by lightning.

The problem with this approach is that the earth can supply a charge far faster than any set of discharge points can create ions. A bit of math will show that a carefully designed static discharge wick or brush can create a current, in an electrical field of 10,000 volts per meter, of 0.5 ampere. This is equivalent to a 20,000 ohm impedance (R=E/I: R=10,000/0.5 = 20,000). The impedance of a site on hard ground is typically 5 ohms. The ratio of the ability of the earth to supply a static charge is inversely proportional to the impedance of the conductor. In this example, the ratio of impedances is 20,000 : 0.05 = 4,000:1.

The earth can supply energy 4,000 times faster than the rate at which a static discharge brush can dissipate the energy! The impedance of saltwater is a great deal less, on the order of 0.1 ohms, making the theory of protection from use of static wicks even more suspect.

Another concept quoted by advocates of lightning prevention through the use of static discharge devices is that the wind will carry off the ions being released by the wicks or brushes. Not only will the wind-blown ions not prevent a strike, they may present a converse affect when there is no wind. In this case, they may migrate upward, making the air more conductive and possibly creating an attractive point of attachment for a step leader which is lurking above looking for a place to strike. Data indicates that step leaders, the precursor of the main lighting strike, don't pick out a point of attachment until within about 150 feet of an object.

Scientific evidence of the behavior of the step leader indicates that it moves in steps about 150 feet long. This indicates that objects more than 150 feet above the surrounding terrain are more likely to be hit than those which are shorter (most sailboat masts). Until 1980, it was assumed that a grounded mast would provide protection against a direct lightning strike for all objects within a 45-degree cone whose apex was at the masthead. From that date the National Fire Protection Association has advocated that a different assumption be used (NFPA Code#78). This code recommendation assumes that a 96-percent protected volume exists adjacent to a grounded mast, with the boundary of the protected volume described by a curve having a radius of 150 feet (the length of one step in a step leader).

Guarantees

Makers of static discharge devices often quote “evidence” of many installations that once equipped, have never been hit by lightning. Unfortunately, these reports must be considered as anecdotal, not scientific proof of the value of the system. The fact is that the chances of a given mast or tower of the dimensions of a typical sailboat mast being hit by lightning are exceedingly small. The willingness of some makers of these systems (notably Island Technology, maker of No-Strike devices) to offer to pay the deductible amount on an insurance policy, or a fixed amount if there is no insurance coverage, is good financial accounting on their part rather than proof of the scientific value of their device.

For example, if you assume that the chances of an equipped vessel being hit by lightning are 1 in 1,000 (much higher than actual probability) and you charge purchasers as little as $10 more than “normal” for the product, you will have accumulated a $10,000 reserve from which to pay the $1,000 deductible amount on an insurance policy.

This income to cost ratio of 10:1 is somewhere between very good and wonderful. Given the price being charged for some of the devices, which offer to pay up to $1,000 toward the deductible in the event of a lightning strike, the ratio of income to probable cost for payout in the event of a lightning strike is more on the order of 100:1, or greater.

Recommended Practices

What should you do to protect your boat from lightning? The best advice available today is to follow the practices recommended by the ABYC for both lightning protection and grounding. Installation of a good lightning protection system won’t hurt. If you like the idea and appearance of a particular kind of static discharge device, sharp points, brush or whatever, install it.

When in an active thunderstorm area, you may wish to have all personnel stay as far from shrouds and the mast as practical, and refrain from using electrical equipment. Some skippers may wish to disconnect electronic devices from all connections to the boat, power and antennas, although in the event of a direct strike, even this may not protect the increasingly sensitive solid-state devices used in this equipment.

And If You Play Golf...

The real risk from lightning appears to be greater for those who play golf than for sailors. The practice at most golf tournaments held in areas where lightning is common is to employ various weather monitoring systems to provide some advance warning of a coming storm or likelihood of lightning. A company appropriately called Thor Guard offers a lightning prediction system that monitors the electrostatic field in the nearby atmosphere. The system compares the monitored data with a stored data base and predicts the probability of a lightning hazard in an area up to 15 miles in radius from the monitor. This system is really not practical for use on a boat, although it could be used to provide warning for an area in which a small boat race was being sailed. It would appear reasonable that, with the very large amounts of money involved in delaying a major golf tournament due to the chance of lightning, static dissipation devices would be sprouting from the fields and woods if they could be shown to work.

The chances of being hit by lightning are very low. There is really nothing you can do to dissuade Thor if he takes a liking to your masthead. You might install an electrostatic field strength meter, or calibrate the hair on the back of your head. When the needle indicates a high enough field strength, or when your hair stands up straight enough, give everyone except the helmsman their favorite drink and invite them to watch the show.

Comments (12)

I have an aluminum briefcase acting as a Faraday cage in which I keep a backup computer, VHF handheld radio, GPS chartplotter, flashlight and batteries. I haven't tested it out yet, and don't plan to, but if I do get hit I'll report back and let you know if it worked.

Posted by: Joseph H | September 3, 2013 3:17 PM    Report this comment

Despite have a brass "bottle brush" lightning protection device, I suffered a direct lightning strike in September 2012. We were aboard, and I felt the boat shudder and immediately knew we had a direct hit. Everything on the masthead EXCEPT the lightning protection device was destroyed.

The lightning protection device was grounded to an external plate with a #10 wire. The wire exploded as if it had been filled with gun powder. The 1/4" bronze bolts attaching the grounding plate blew apart just outside the hull. The depth transducer (a bronze mushroom filled with electronic potting material) has all its potting material blown out. There was no electrical path to the transducer other than miniscule data transmission wires, so how did significant amperage get there?

Apparently, fiberglass hulls with metallic thru-hulls often sink when a lightning strike blows out the thru-hulls. If the current exits through the drive shaft, the zincs are usually blown off.

The VHF coaxial antenna cable also exploded. A 12v DC voltmeter had its display blown 8 feet from the nav station. All electronics were destroyed.

The insurance adjuster assessed my damage as a 5 on a scale of 10. I asked what a 10 would do. He replied "It would melt the top of the mast."

I was surprised at the number of other boaters who thought having electronics turned off would protect them from damage. Any current flow creates a magnetic field, and conversely, a magnetic field will cause a current flow in certain circuits. Lightning bolts create very large magnetic pulses. Integrated circuits exposed to a very large magnetic pulse can generate enough internal current to destroy the circuit. A new electronic device sitting in its box on the shelf at a warehouse could be destroyed by the magnetic pulse from a lightning strike. The only protection would be placing the circuit in a Faraday Cage.

Those who think they can somehow control or even influence the current flow in the event of a direct strike to their masthead are woefully math-impaired. What size wire and grounding plate would be required to provide a path to ground for 18,000 amps? Like one wise old sailor said to me, "The lightning came 3 to 5 miles to find your boat. What you do in the last 70 feet is completely immaterial."

Since my lightning protection device did not help me, I do not believe they prevent a strike. But they may be considered similar to a car alarm. If a thief wants your car, he will get it. A car alarm will not prevent theft. But if the thief has several cars to choose from, your alarm may make your car less attractive. The same may be true with lightning protection devices: they merely make the next boat a more attractive target.

Posted by: Bruce M. | August 26, 2013 1:07 PM    Report this comment

I wouldn't hold up the ABYC recommendations as the Holy Grail of standards as far as lightning protection goes. Having a ground plate on the bottom of the hull is the worst part of that idea, guaranteeing to blow a hole in the bottom of your hull when it explodes upon a lightning strike. #8 wire is like a strand of hair compared to the amperage expected to flow through it.

Posted by: Raymond S | August 26, 2013 10:42 AM    Report this comment

Thanks for the math on the ratio of charging to disipation. It always seemed obvious, based upon the huge difference in conductivity of air and sea water, but I never researched the math.

Posted by: Unknown | August 23, 2013 12:09 PM    Report this comment

I have been personally struck by lightning and can unequivocally state that it hurts like hell. I have had the hair on my body stand on end from impending strikes and had numerous strikes very close to me where trees and telephone poles have been blown to pieces. When looking at lightning protection for my boat I examined what was available commercially and what was recommended by ABYC and others as the minimum for underwater dissipation in the advent of a strike and found them all to be seriously lacking. I designed and built my own system and it works perfectly... I have yet to receive a strike on my boat and hope it stays that way. Since my system was designed to safely dissipate the largest amount of charge possible, received in a strike, not being hit means that my system is untested, but I am confident that if I do receive a strike I have the best chance possible for survival by my crew and boat. Lightning is a powerful force that I feel no preventative measure can forestall so our best option is to limit damage caused when the boat is struck by directing as much of the force possible to a fully wetted and well-designed ground. Not all dissipation grounds are created equal.

Posted by: Bob H | August 22, 2013 12:18 PM    Report this comment

Having been 8' away from a lightening strike to a 80' tall pine tree and with a picture window between the tree and me, I have a healthy respect for what lightening can do. I was stiff and sore for days after, because of the effect on my muscles reacting to the strike - like a massive electostem that physical therapists use. Not to mention the fried air conditioner and other electronics.

Lightening may be one of those things that "when your number's up, your number's up." Have a healthy respect but don't get ridiculous or do something that makes you more attractive to a strike.

I always share the first drink of the evening with Neptune. What is the proper toast to Thor?

Posted by: Dwain L | August 22, 2013 7:26 AM    Report this comment

Fantastic article! Lightning is indeed very strange. Logic can not be applied to any aspect of it nor it's effects. I've personally had three close calls with it. Once while in an aircraft, sitting on the ground with many larger and taller aircraft around. The second while camping, nearby trees got hit but our ten foot high lighting rod, aka tent pole, did not attract a single bolt. And finally, on our sailboat while anchored in a well protected cove. In this situation, three inches of the top of the mast disappeared along with all of the mast head equipment. The strange thing about this event was that all AC powered equipment that were plugged in, were fried. However most the DC equipment was spared. Except the batteries. The seemed to take the blut of the bolt and gave their lives to save the rest of the DC equipment. I've been told that they acted as big capacitors and took want they could until they exploded. We were lucky as no one got hurt. Our two young sons even slept through the whole thing.

Thanks once again for the very well written article.

Posted by: Douglas B | August 21, 2013 7:17 PM    Report this comment

I worked at KSC in Florida for many years and observed quite a few lightening strikes. In my memory no launch vehicle was hit but indirect damage was sustained through land-line conduction and induction from the magnetic field expanding from the lightning stroke. The effects manifest themselves as isolated electronic damage and drawn out searches for possible damage. A large amount of money has been spent on lightning protection especially due to miniaturization of circuits. The effectiveness of these efforts are assumed based on the lack of lightning induced damage to date.

Posted by: George P | August 21, 2013 4:45 PM    Report this comment

Well said! Could one extend the conclusion that 60 foot sailboats are not tall enough to matter, to equate the chance of lightening striking a boat equal to a person standing on the beach? If so, I would love to see that in writing and share it with my crew.

Posted by: DARRELL N | August 21, 2013 12:50 PM    Report this comment

The final word on lighting is this:

If you inbstall the best and most effective protection you may or may not be hit with lightning. If you do nothing for lightning protection you may or may not be hit.

A Faraday Cage (metal cage) provides much pritetion for anything inside that cage. A steel or aluminum boat is a good esample of a Faraday Cage. However, even that does not assure total protection.

Your conclusions are correct in my opinion, but should include some discussion of Farady pincipals.

Posted by: BUDD K | August 21, 2013 12:08 PM    Report this comment

A previous boat of ours was hit by lightning...TWICE! And it had a dissipator (ahem, better known as 'the attractor'). The first time was a nearby discharge that caused intermittent electrical issues. The next time was a direct hit that fried all electronics and blew a melon sized chunk out of the keel fairing. Luckily, we were not onboard. People who saw it said it caused their hair to stand on end on shore.

Posted by: CruisingKitty | August 21, 2013 12:06 PM    Report this comment

Thanks for a very educational, yet tongue in cheek explanation of a topic that's frequently misunderstood. Great reading!

Posted by: Charles R | August 21, 2013 11:29 AM    Report this comment


Add your comments ...

New to Practical Sailor? Register for Free!

Already Registered? Log in

Forgot your password? Click Here.