|
Attachment
As the negatively-charged stepped leader moves downwards, it induces a positive charge on the ground below. When the tip of the leader is about 30-100 yards above ground level, the induced positive charge becomes so concentrated that a new spark forms at the ground, as shown in Figure 2.his positively charged spark is the crucial process as far as the attachment to a boat is concerned. If it starts at the tip of a boat mast, then lightning strikes the mast. Unfortunately, there is no scientifically accepted technique to prevent this spark from forming. Even if a device were effective in diverting the attachment spark, it would not be a good idea to mount it on the masthead as the attachment spark may start elsewhere on the boat or crew. The likelihood of lightning attaching to the masthead is a safety feature as far as the crew is concerned.
Consequently, lightning protection means minimizing the damage caused by lightning in the event of a strike, rather than preventing a lightning strike. In general terms, a protected boat is one in which there is a continuous conducting path from the water to the mast tip. The current needed to feed the attachment spark is conducted through the protectionsystem from the water. That is, the path that the lightning takes in the boat is forced to be that of the conductors in the protection system. If this conducting path is not continuous, for example, in a boat which is not well grounded, there is little difference as far as the top of the mast is concerned. The attachment spark still begins there as this is where the positive charges have concentrated. The difference is what happens where the conducting path, the mast, ends. Since current cannot flow from the ground to feed the growing attachment spark, a negative charge accumulates at the base of the mast and eventually arcs across in the general direction of the water or a nearby conductor. (For this exercise, crew members are conductors!) The result is an unharnessed electrical discharge between the bottom of the mast and the water.
According to the above argument, the likelihood that lightning will strike a boat does not depend on whether the boat is well grounded or not. There is some support for this in the experiences of marine surveyors. Nine marine surveyors in Florida, each of whom had surveyed more than 200 sailboats in their career, reported that between 2% and 67% (on average 34%) of the boats they surveyed for any reason had a lightning protection system. Of the boats that they surveyed because of a lightning strike, they reported that between 0% and 67% (on average 29%) had a protection system. While the individual estimates varied widely between surveyors, there is no support for the argument presented by some sailors that they should not ground 'their sailboat since it will increase the chances of it being struck by lightning.
Sideflashes
Data obtained from sailors whose boats have been struck by lightning are consistent with the above scenario: boats that do not have a protection system do indeed suffer more damage. The type of water, whether salt or fresh, is also important. Damage is much more extensive for boats struck by lightning in fresh water than for boats struck in salt water because fresh water is a worse conductor. Consequently, it is much more difficult to design an adequate protection system for boats in fresh water than for boats in salt water. Figure 3 summarizes these data for a sample of 71 boats that were struck by lightning.he bars show the percentages of boats in each category that received various magnitudes of hull damage. The four categories were boats with/without protection systems in salt/fresh water. The damage indices indicate the severity of hull damage as shown in Table 1.
In boats with a hull damage of 2 or higher the lightning had formed its own path(s) through the boat hull. If a lightning protection system was present it malfunctioned. As the statistics show, malfunctioning protection systems are very common in fresh water: 40% of protected boats in fresh water experienced this effect. The most likely way that this happened was through the formation of "sideflashes". These are sparks that form between the lightning protection system and ungrounded conductors or the water. Basically, in order to dissipate a lightning current in fresh water a much more extensive underwater grounding system is needed than that usually found in 'protected" boats. This is described in more detail below.
Technical Aspects of the Lightning Protection System
Overview
Although lightning protection needs to be designed on a boat-by-boat basis and ideally installed during manufacture, there are three major considerations in a good protection system: (a) grounding, (b) bonding, and; (e) electronics protection. The grounding system is intended to provide an adequate conducting path from the point of lightning attachment, usually the masthead, to a system of conductors in the water, without producing sideflashes. The bonding system protects the crew and consists of conductors that short out large metal fittings so that large voltages cannot develop between them. Electronics protection limits power supply and transducer voltages through a combination of transient protection devices and careful wiring techniques.
Grounding
The idea of the grounding system is to divert the lightning current through a predetermined path so that it does not make its own explosive path through fiberglass, teak, crew members, etc. Figure 4 shows what can happen when lightning strikes an ungrounded fiberglass boat with an aluminum mast.The lightning charges all of the rigging but no conducting path exists to channel the charge into the water. The result is destructive sparks between the lower parts of the rigging, such as the mast base and chainplates, and the water. Wherever these sparks travel through bad conductors (fiberglass hull, teak bulkheads, through-hulls, porta-potties, etc.) sufficient heat is generated to explode the impeding material into a nicely conducting plasma that is hotter than the surface of the sun.
The components of the grounding system are: (i) an air terminal at the top of the mast; (ii) downconductors, and; (iii) grounding conductors that are immersed underwater ("ground strips" or "ground plates"). The air terminal is the point where the lightning is supposed to attach, the down-conductors conduct the current from the air terminal to below the water, and the grounding conductors dissipate the current into the water without forming any sideflashes. Usually the aluminum mast is connected in as part of the down-conductor network.
On a sailboat with a VHF radio, the masthead VHF antenna usually serves as a sacrificial air terminal. In fact, one of the first signs that lightning has struck a boat is typically that shards of antenna material are scattered around the deck. The presence of a VHF antenna or other expensive masthead transducers makes a separate air terminal highly desirable, although this will degrade the performance of the VBF. The top of the air terminal should be sufficiently high that the angle from it to any other masthead object is less than 45 degrees. That is, the air terminal provides a 'cone of protection' that attracts lightning (or, more accurately, launches an attachment spark) preferentially to any other object that is below a conical surface whose apex is on the top of the air terminal and that has a 90-degree apex angle.
An aluminum mast is the preferred down conductor, being a much better conductor than stainless stays. If the mast base is on top of the cabin, a downconductor is needed to connect the mast base to the ground strips. Use at least #4 gauge copper with preferably bimetallic copper/stainless connections to prevent galvanic corrosion. Alternatively, make a strong mechanical connection and additionally braze or solder, to improve the electrical contact and lessen the chance of contact corrosion, then paint with an insulating coating. A keel-stepped mast similarly needs to be connected to the keelbolts with at least #4 gauge copper.
The ground strips in contact with the water should be connected to the down-conductors with care to avoid galvanic corrosion. In salt water a single grounding conductor of a square foot or more in area is usually enough. In this respect, a lead keel connected to the down-conductor via the keel bolts is adequate. If the lead is either painted or encapsulated in fiberglass, minor repairs may be needed after a lightning strike. However, the paint or fiberglass does not seriously compromise the ballast lead as a lightning ground. Note that this system does not work in river mouths where there may be a less dense layer of fresh water riding on top of a salt water "wedge". The situation in fresh water is much more complicated as the voltages involved during a lightning strike are about a thousand times larger than those that occur on a boat in salt water. A good start is to lay a flat or 'D' cross section strip of 3/4" x 1/8 ' stainless or brass along the outside of the stem of the boat. Connect this to the forestay, mast base, and backstay with #4 gauge vertical copper down-conductors. However, this is not usually enough. In addition, extra ground strips are needed just outside the hull close to metal fittings such as gas tanks, metal-cored plumbing pipes, wiring, etc. Connect these to the grounding system using near vertical down-conductors. Under no event should these down-conductors run close to the hull except where they penetrate the hull to connect to the grounding strip: otherwise the conductor may cause a sideflash through the hull. The engine, propeller shaft, and propeller should be regarded as part of the grounding system and tied in appropriately.
The manner in which a correctly grounded.boat reacts to a lightning strike is illustrated in Figure 5.燭he lightning charge that flows onto the rigging does not accumulate to the point where it forms destructive sparks, as was the case for an ungrounded boat. Instead, it is discharged into the water over a wide region. The more evenly the charge can be discharged into the water, the less likely it is that a sideflash will occur through the boat hull.
Bonding
The difference between the grounding system and the bonding system is only one of degree since both are interconnected and both will conduct current during a lightning strike. Whereas the grounding system is designed to handle the full lightning current, the bonding system consists of mainly horizontal connections between metal fittings to short out any voltages that might otherwise develop. Bonding is a measure that is intended to protect the crew and enable them to work the boat without getting shocks. This can occur from nearby lightning as well as from direct strikes. Smaller gauge conductors than the grounding system are adequate in the bonding system, down to #8 gauge copper. As with the grounding down-conductor connections, all bonding connections should be made to minimize galvanic corrosion. Metallic fittings that should be bonded to the grounding system, using horizontal connections as much as possible and avoiding the hull, are toe rails, chain plates, steering wheels, motor controls, bow and stern pulpits, antenna bases, the ground wire for the electronics, etc.
The illustrations in Figure 6 show what happens on board a bonded (bottom) and unbonded (top) boat during a lightning strike. On the unbonded boat large voltages develop between the mast, chainplates, forestay, backstay, wheel, rudder post, toe rails, electronics, wiring, metal reinforcing in plumbing fixtures, engine, etc. These make working the boat extremely hazardous, even if lightning is not striking the boat directly. On the bonded boat these voltages are shorted out by bonding conductors. Note, however, that the large magnetic fields associated with a direct lightning strike make the concept of an electrical "short" a misnomer. Appreciable voltages can develop between the ends of long conductors even if the conductors are connected together at their other end. The helm is a particularly dangerous place owing to its proximity to the engine controls, boom, rudder post and backstay. The helmsman in Figure 6 (bottom) would not be smiling if he had one hand on the tiller and the other on the engine controls, for example. (Note that he is steering with one hand in his pocket to minimize the risk of making a connection between two conductors at different voltages. This is not as safe as throwing over the anchor and going below!) For stations such as the helm that are usually manned, it is crucial that the bonding conductors should be kept as short and straight as possible.
Electronics
Electronics-killing overvoltages may be introduced through the DC power wires, antenna input, or any other external connection such as a lead to a transducer. Electronics on a small sailboat that are struck by lightning are particularly difficult to protect since it is impossible to divert the lightning current any appreciable distance away from" the electronics. This difficulty, and the pervasive nature of electronics damage, is illustrated in Figure 7 that shows the percentages of boats with electronics damage of different magnitudes.
In this case there is less of a distinction between boats struck in fresh water versus salt water as there was for hull damage, but the same trend is evident: boats with protection systems in salt water fare best and boats with no protection systems in fresh water fare worst. More notably, 96% of all boats sustained damage to at least some electronics items. Apparently a lightning protection system, as installed on the boats in the survey, does not necessarily save the electronics. Note that for these boats "lightning protection" merely meant that the boat was grounded, not necessarily bonded with transient protection devices, as explained below.
In order to protect electronics, more is needed than merely diverting the current to ground (water) without its blowing a hole in the hull. Due to the low voltages typically used in modern marine electronics, just a few extra volts is enough to cause extensive damage. However, techniques that are used to protect computers, cable TV and radio equipment on land can also be used in shipboard DC and AC equipment. Some devices are readily available from electronics stores. Radio antennas can be protected using lightning arrestor hardware designed for cable TV. Connect the "ground' connection to the lightning grounding network. AC transient protection outlets or plug-in metal oxide varistors (MOV) work also on boats but need to have their ground connections connected to the shore ground wire. Ideally this ground should also be connected to the lightning protection ground but this circuit arrangement can cause ground current problems in marinas. As for protection of DC electronics, which are probably the most important, transient protection devices are available to clamp voltages at the point where each piece of equipment is connected to the DC supply. These are available from companies such as General Electric or from mail order electronics distributors. They can be found under the generic name "Transient Suppressors" and are of various types: metal oxide varistor, silicon avalanche diode, and surge suppressor zener diode. It is important to locate this protection device immediately next to the equipment and each piece of equipment should have its own device. The overvoltages that appear at DC inputs can be reduced by using twisted-pair wiring in wiring harnesses, ideally with a conducting sheath that is connected to the bonding system. The overall philosophy here is to minimize the spacing between positive and negative DC lines. If a main control center exists, surround it with a conducting enclosure that is connected to the bonding system. Through-hull transducers are especially vulnerable. Due to the typically vertical alignment of the cables connecting these to their main electronics, they should be regarded as being part of the lightning grounding system. Since the wires used in these cables are of an insufficient thickness to withstand a lightning strike, a #4 gauge copper wire should be placed parallel to any cable that leads to a through-hull transducer. The top of this copper wire should be reconnected to the lightning grounding system and the bottom to a ground strip close to the underwater transducer on the outside of the hull.
As with all aspects of lightning protection, 100% effectiveness cannot be guaranteed, even if all the above measures are taken for electronics systems. Disconnecting equipment in advance of a storm helps isolate it from voltages induced by lightning, and the larger the lead separation the better. Use disconnects in preference to knife switches, and these in preference to switch panels.
Personal Safety
Consider the worst case scenario for a lightning strike to a sailboat - a small boat in fresh water. If the boat has been provided with a well-built protection system it is still an exceedingly hazardous situation. If lightning protection does not exist, the situation is life threatening. In both cases, the areas to avoid are close to the waterline and close to large metal fitting. In the unprotected boat, an additional -danger zone is beneath the mast or boom. Even in the unprotected boat, it is unwise to get in the water, as electrocution is highly probable if lightning strikes nearby. In fact, there is no safe place on an unprotected small sailboat, and in a protected boat only places of relative safety. There is, however, one place that is more hazardous than a small unprotected sailboat, that is a small unprotected boat without a mast. Every year there are multiple deaths of boaters in open boats caused by lightning strikes, but very few reports of sailors in sailboats killed by lightning.
The above general rules also apply to larger sailboats. These are generally safer, if protected, since it is possible to get away from the waterline and large metal objects, and yet still stay dry inside the cabin. As far as unharnessed electricity is concerned, a dry human body is much less attractive than a wet one.
Conclusions
Lightning protection on a sailboat means diverting the lightning current into the water without its causing any hull damage, personal injury, or electronics damage. This involves providing a continuous, mainly vertical, conducting path from above any vulnerable masthead transducers to grounding conductors immersed in the water (the grounding system) and a network of mainly horizontal interconnected conductors attached to large metal fittings, including the grounding system (the bonding system). Transient suppressors are needed on each piece of electronics equipment, and wiring should all be twisted pair for protection of electronics.
WHAT CAN YOU DO?
To protect yourself in a boat, the important thing is to give lightning a ground. Boats made of steel, such as naval vessels, have an automatic ground in their metal hulls; but most small boats, usually constructed of fiberglass or wood, prevent the lightning easy access to the water and pose a grounding problem. Small boats may also lack tall objects which could deflect lightning, and even boats that do have tall spars - like sailboats - can run into problems if their spars are not properly grounded. The following grounding system will minimize the risk of lightning damage:
On a sailboat, make a lightning rod using a piece of aluminum which sticks about 1 foot (30.48 centimeters) above the mast. Sharpen the rod to reduce resistance at the top. A radio antenna can also condluct electricity but may lead lightning to the radio and may well vaporize during a strike.
From the rod, lead a wire down a wooden mast; an aluminum mast can serve as its own conductor as long as a wire is led from its base to direct the charge to the ground. (A wooden mast with a metal sail track can be grounded as though it were a metal mast.) The Coast Guard finds a #8 wire adequate, though a larger gauge, say #4, further reduces heat and resistance.
Grounded spars or antennas offer a 60 degree cone of protection. Note that this cone measures from the highest point off the water, not from a mast's full height. Since boats usually heel during storms, the cone of protection will shrink proportionate to the dip of the mast. A bending or slanting whip antenna will considerably lesson the protected zone, as the illustration shows. Adapted from the National Fire Protection Code.
Attach this wire to a copper ground plate mounted on the hull beneath the waterline or dangled underwater. A square foot of copper flashing, easily obtained at many hardware stores, will make an adequate ground plate. If you decide to attach a permanent ground, use large stainless steel bolts, say 1/2 inch, to prevent their cracking under large electrical discharge. To increase safety, some recommend all stays be grounded to the plate.
On a motorboat which has no high spar, use a metal radio antenna (not a fiberglass one), attaching a wire to a ground plate, as described above. If there is a loading coil on the antenna, use a shunt of 31-strand, 17-gauge, bare lightning grounding mesh to bypass it - this will make the whole height of the antenna an effective ground. Quick-release clamps will allow an easy temporary attachment. (On a fiberglass antenna you can clamp the wire to a metal rod, letting that act as a lightning interceptor.) Use a "lightning arrestor" to protect your radio.
When leading the wire to a ground, avoid sharp bends or turns. Any bend in the wire should have a radius of at least 60 degrees.
One alternative calls for leading the grounding wire to a metal strip which runs down the bow or stern and maintains constant contact with the water. This offers the advantage of carrying the charge, at a gradual angle, safely away from the engine and the helm.
Both the mast and the whip antenna will provide a "cone of protection" with a radius of approximately the same dimension as the rod's height. For most small boats, this will include the entire deck area. There may be, however, induced electrical surges created by the lightning, and for this reason large metal objects - engines, for example - should be avoided during a storm. (Remember that an engine will have its own ground in the propeller and the shaft.) Induced electrical charges can cause arcing and electrical shocks strong enough to knock you unconscious, perhaps producing dangerous heart arrhythmias. A lightning strike can also magnetize a keel or other, metal fittings, rendering your compass useless.
FIRST AID FOR LIGHTNING
After a shipboard lightning strike, check to see that everyone is all right - it is, of course, perfectly safe to handle someone who has been hit by lightning. Check for burns, Which should receive normal first aid treatment. (Don't put ointment on severe burns; cover them with clean cloth or plastic to keep out air.)
If someone has been knocked unconscious, act immediately: Check for breathing and heartbeat.
If you feel a pulse, but no breathing, begin mouth-to-mouth resuscitation (a handkerchief over the mouth will help the squeamish). If there is no heartbeat, begin cardiopulmonary resuscitation, a technique every serious boater should know.
PRECAUTIONS
Grounding your boat and unplugging radios and electrical equipment during a storm are good ideas, but the best precaution against lightning is avoidance. Especially in small craft, keep a weather eye out for the coppery haze and building cumulonimbus clouds that signal thunderstorms, heading ashore well ahead of the turbulence. Remember that lightning can lash out for miles in front of a storm, and it can strike after a storm has seemingly passed. Remember, too, that, storms can bring high winds and waves, making a last minute trip to shore a dangerous dash. The best maneuver of all is to think ahead
|
