Lightning Protection: The Truth About Dissipators
August 19, 2013
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.
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.
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).
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.
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.