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The science and technology of yacht lightning protection
History
When Benjamin Franklin invented the lightning rod in 1750,
he noted that it could also be used to protect ships. It was not long
before the first ships were to benefit from his ideas. In the late
18th century the sailing warships of the British navy were fitted with
lengths of anchor chain to prevent their wooden masts from splintering when
struck by lightning. Franklin himself was unsure of the actual
mechanism, thinking initially that a pointed rod would discharge the
thunderstorm "for if there be a rod sharpened ... the electrical fire
would be drawn out of a cloud" but five years later covering all bases
by adding "pointed rods would either prevent a stroke or would
conduct it so that the building should suffer no damage".
For whatever reason, this technology worked. The discharge physics of
the lightning strike to ground would not be well understood until
research done in South Africa
in the 1930's and later.
In the intervening centuries scientific opinion has come
down squarely on the side of Franklin's
last opinion - that a lightning rod protects a building by offering a
suitable path for the current to flow. Still, modern day refinements
for marine protection somewhat mirror the historical record. Although
a code developed by ABYC definitely improves lightning safety, research
continues into the underlying science. In a paper
published by IEEE (the Institute
of Electrical and Electronic
Engineers) in 1991, Ewen Thomson of the University
of Florida tested this code by
applying the traditional science used in lightning protection systems for
ground installations. The traditional science, reflected in terms
such as "ground resistance" and "step potentials"
models voltage gradients as a consequence of current flow in the ground (or
water). Thomson concluded that key changes were needed. While some changes
were trivial to implement, such as upgrading down conductors from #8 gauge
copper to #4 gauge copper, others were highly impractical. In
particular, Thomson noted that hull damage to sailboats struck in fresh
water was so extensive, even when the boat was well grounded, that multiple
grounding surfaces were needed over an extensive underwater area, much more
than the one square foot ground plate quoted in the code. This requirement
is very difficult to fulfill in practice.
Of even more concern were some types of boat lightning
damage that were impossible to explain with the traditional scientific
model. In the light of these inconsistencies, Dr. Thomson concluded, in a
yet unpublished study, that key assumptions in the traditional model were
invalid Removing these assumptions and reinterpreting the fundamental
science has resulted in a new model that enabled innovative technology to
be developed to overcome the above practical limitations. This
technology is now covered by a patent issued by the University
of Florida and licensed
solely to Marine Lightning Protection Inc.
Problems
The discharge mechanism for lightning strikes to ground
is very well understood and explained in detail in many of Professor Martin
Uman's books. A detailed explanation
of this strike mechanism for a
lightning strike to a boat written with the layman in mind is given in the
Florida Sea Grant publication SGEB17.
From a yacht's point of view, the impending lightning strike begins
when a column of charge has been lowered to within a few tens of meters of
ground in a process called the stepped leader. At this point currents
begin to flow, both up towards the stepped leader, and down into the
water. Current flows either from electronic conduction inside
conductors or in the form of propagating charged streamers following
ionization of air or water. Eventually one of the upward streamers,
termed the attachment streamer, connects with the stepped leader to form a
physical attachment to the boat. At this stage the thundercloud is
effectively shorted to ground by a continuous ionized channel, and the peak
current flows with an amplitude of several tens of kA with a rise time of
about 100ns during the return stroke phase. This peak current
decays in a few tens of microseconds but may be followed by a lower level
continuing current (~few hundred amperes) for perhaps several hundred
milliseconds. While at a much lower level, the continuing current is
the process responsible for the largest heating effects.
The challenges of protecting against damage from this
high voltage discharge in the case of a boat lightning strike are
formidable :-
·
Sensitive electronics transducers such as
VHF antennas, radar dishes, and anemometers are usually mounted at height,
but many upward-going sparks precede the eventual lightning attachment and
this does not necessarily form at the highest point.
·
Crew members, electronics wiring, and carbon
fiber reinforcements are all good conductors and any conductor anywhere in
the boat is a possible origin for spark formation.
·
Voltages of just a few tens of volts are
large enough to destroy solid state electronics whereas available voltages
are large enough to launch discharges of any length between conductors and
to the water.
·
Destructive sideflashes spark from
conductors at high potential but fresh water is so poorly conducting that a
low impedance ground is impossible. Huge potentials are inevitable
when the peak return stroke current flows.
As far as a boat is concerned, the major processes
are:
·
attachment at an air terminal
·
charge accumulation and current flow on the boat
·
charge dissipation into the water
·
side flash formation from and between on board
fittings
Attachment
Since there is no scientifically proven method to repel it, the fundamental problem in yacht
lightning protection is how to deal with lightning when it strikes.
Where the lightning channel attaches to a boat is determined by the geometry
of topside conductors on the boat and the location of the downward-going
stepped leader relative to the boat. For example, if the stepped
leader is heading for the water behind the boat, aft conductors are more
likely to be struck. In general, the taller the conductor, the higher
the probability that its upward streamer will be the one that connects with
the stepped leader, thereby completing the ground channel for the
lightning. The conductor in the lightning protection system intended to
make this connection is termed the air terminal, or, more commonly, the
lightning rod. In this respect, research reported by Dr. Charles
Moore and associates in New Mexico only two years ago finally resolved that
blunt lightning rods are actually more effective
than the traditional sharp pointed rods. The tendency of a tall conductor
to attract the lightning strike, by preferentially launching the final
connecting streamer, has resulted in the idea of a "cone of
protection". This somewhat flawed idea holds that a vertical
conductor forms an effective cone of protection, the apex of the 90 degree
cone being at the top of the conductor, and protects the circular area of
the cone's base. The idea is flawed in that a vertical conductor does not
eliminate the electric field on the ground within this
"protected" circular area. Any conductors inside the area,
people included, may give rise to upward streamers if this electric field
reaches breakdown strength. A better
arrangement is to have conductors arranged around the area to be protected,
or, better yet, forming an umbrella overhead, where the outer edges of the
umbrella are connected to down conductors leading towards the water.
Hence the major concern regarding the lightning
attachment is to ensure that the lightning attaches to, and causes current
to flow in, only an air terminal, or other termination conductors, rather
than more vulnerable conductors such as crew members, electronics, etc.
Charge accumulation & current flow
After attachment, conductors, and
usually all conductors on the boat, conduct current, even those not
connected directly to the lightning protection system. This includes
wiring and internal circuitry in and between electronic items. In addition,
charge accumulates on conductors and can initiate sparks to form new
conducting channels as the lightning charge strives to lower its potential
energy. Since the absolute voltage of the lightning source is high
enough to ionize air over a distance of several miles, sufficient energy is
available to form a spark of any length on the boat if needed to bridge an
air or fiberglass gap. Simple circuit concepts such as the path of
least resistance are of limited use in this electromagnetic maelstrom but
the non-linear nature of this high voltage, high current situation means
that the electromagnetics are too complicated for complete analysis.
Add to this the necessity for some sensitive transducers to be placed near
the mast head and others immersed in the water, and it is no wonder why insurance
underwriters use the term an "Act of God" when CDI ignition
systems are destroyed, knot meters are blown out of the hull, and masthead
antennas are vaporized, rendering the boat immobile, sinking, and without
means of communication. This is the complicated physical background
governing the design of the conductor system to conduct the lightning
charge from the air terminals to the grounding conductors.
So, the role of the down
conductors, those connecting the air terminals to the grounding conductors,
is to conduct the lightning current towards ground, preclude all
sideflashes, protect transducers, and minimize electromagnetic coupling
into electronics.
Charge dissipation into
the water
Fiberglass is such a good insulator that it is used to
make insulators for high voltage installations. Nevertheless, the
lightning voltage is more than enough to cause electrical breakdown through
a fiberglass hull if no alternative path is provided, and frequently even
if one is. Each penetration leaves a charred hole and much more
extensive internal damage. Grounding conductors (electrodes) are
intended to form a bridge into the water to eliminate this hull
damage. However, a single ground plate is inadequate to prevent
sideflashes, necessitating multiple interconnected conductors. These
cause a whole new set of problems:
·
accelerated galvanic corrosion or loss of
sacrificial zinc's
·
electrolytic erosion in marinas with ground
currents leakage
·
many mounting bolts and hull penetrations,
each one raising the risk of water seepage
·
additional drag since plates should have
exposed edges
Through-hull transducers, fittings, and all immersed
metal, including outboard drives, also inadvertently act as lightning
grounds. A typical scenario for an ungrounded smaller powerboat, such
as a 20' fisherman, is for lightning to attach to the VHF antenna
(vaporizing it), spark through the electronics panel (destroying all
electronics), travel into the battery ground or control cables into
the outboard solid state ignition (rendering it inoperable), and then spark
into the water through the drive unit. Any transducer such as a knotmeter
is also likely to be blown out, possibly leaving a hole where it was
mounted. This scenario assumes that no crew member is unlucky enough
to be bridging a gap along the way.
Carbon deposits after lightning strikes trace out
the paths followed by sparks forming from immersed conductors, both those
grounded and those that are isolated. A detailed discussion of
this effect is given in a letter recently
published in Professional Boatbuilder. Briefly, charge accumulates on
all conductors on the boat, even when current is flowing into the
water. The charge density is largest close to the water and on sharp
corners and edges of conductors, which is thus where sparks are most likely
to start. So sharp corners are highly desirable on the outside of
grounding plates and are recommended in most standards. As well as
initiating current flow, spark formation reduces the grounding resistance,
thereby lowering the voltage of the whole protection system.
In summary, the major problem with charge dissipation
into the water is how to provide the appropriate number and distribution of
grounding conductors, to eliminate sideflashes, while minimizing the
corrosive effects of multiple immersed conductors that are bonded together.
Sideflash formation
While sharp corners are beneficial on grounding
conductors, this is definitely not the case for conducting fittings that are
not supposed to act as grounding electrodes. Any spark from these is
a sideflash, that can injure crew, blast a hole through the hull, and
destroy electronics. Uncontrolled sideflashes are to be avoided. On the
other hand, the location or shape of fittings frequently cannot be changed.
For example, a sideflash from a chainplate to the water is a very common
occurrence. The figure below shows what happened in one case. Even though
current definitely flowed out of the grounded cast-iron keel, as evident from
"thousands" of holes in the keel, sideflashes still developed
from both forestay and backstay. Both ended up holing the hull near the
waterline having been diverted by onboard conductors - an aluminum
organizer aft and a water tank forward. Obviously what was needed in
this case were at least two more grounding conductors, one fore and one
aft. In other cases multiple holes have been blown out, usually
associated with onboard conducting fittings., and frequently at the
waterline.
By analyzing case studies such as these we have gained
an understanding of the causes of sideflashes, their association with
standard fittings, and how they form and propagate. We identify two
distinct types of sideflash:
- Internal
sideflashes form between on-board conductors.
- External
sideflashes terminate in the water.
For example, in the above case an internal sideflash
formed between the forestay and the water tank, and an external sideflash
formed between the water tank and the water.
Solutions
The only viable solution is an integrated approach. Key elements in this are:
- air
terminal placement to ensure attachment;
- down
conductor layout and size to ensure conduction to the grounding
electrode without overheating and minimal emi;
- bonding
conductors between fittings to minimize both internal and external
sideflashes ;
- grounding
terminal placement to reduce external sideflashes and allow for
lateral current flow into water.
Unfortunately, the general consensus for designing a
marine lightning protection system fails on all of the above counts. This is represented in the figure below
that is roughly consistent with present standards published by ABYC, NFPA
and ISO.
The main features of this are:
·
an air terminal that is high enough to give a
zone of protection for the whole boat based on a 90 degree cone;
·
a down conductor orientated vertically;
·
a one square foot immersed ground plate or
strip directly below the air terminal;
·
metallic fittings bonded to the down
conductor or ground plate/strip.
The easiest way to appreciate the fundamental issues
with this approach is to design a lightning protection system for a
building using the same principles.
The building system would look something like this:
Obviously, buildings are not protected this way. Rather, the typical building system has
multiple air terminals around the roof edges, down conductors on the
outside, and ground rods around the perimeter. Also, the air terminal heights and
locations are typically determined using a rolling sphere model rather than
an inverted cone. Placement of air
terminals, down conductors, and ground rods on the outside, guides the
lightning current outside the structure, minimizing step potentials and
electromagnetic interference (emi).
Translating these concepts to a marine system results in the design
illustrated below, where the lightning conductors are shown in blue:
We call this arrangement of conductors on the outside of
the boat the ExoTerminalTM system. The new aspects of this design are:
·
Conducting fittings such as handrails and
T-tops can be used as air terminals, allowing for a larger zone of
protection than the 90 degree cone.
·
A potential equalization loop encircles the
hull, preferably at deck level.
·
Multiple grounding electrodes are installed
at about water level, the preferred location for external sideflashes to
exit.
·
Grounding electrodes are also installed
close to large metallic fittings that are close to the water, such as
keel-stepped masts.
A suitable design for a sailboat is shown below:
These principles have been incorporated into the latest
standard published by the National Fire Protection Association (NFPA780-2008) Our discussion of this new standard is
available on a podcast hosted by Professional Boatbuilder. See our Seminars
page for details.
More details of grounding techniques using sparking
electrodes are available as follows:
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Document
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Contents
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Grounding
guidelines
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General description of grounding system using
SiedarcTM electrodes
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Grounding
concepts
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Physical basis for grounding electrode placement and
type
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Product
info
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Pictures, description and prices of SiedarcTM
electrodes
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General approaches
While the above concepts can be adapted to any type of
boat, each one has its own particular set of problems. General
approaches for different classes of boats are as follows:
·
small open boats
·
trailerable sailboats
·
powerboats with metal
superstructure
·
cruising sailboats
·
passagemakers & blue water power yachts
·
hovercraft
·
instrumented buoys
Small open boats
The vast majority of lightning deaths on boats occur on small
open boats. In other cases boaters have been knocked unconscious but
eventually recovered, as do the majority of strike victims. Hence administering CPR to an unconscious
crew member is the top priority. An
excellent source of information for lightning survivors is Michael Utley's website. Hence in the case of an open boat, a good
lightning protection system may indeed save a life. The principles
are the same as for any boat – air terminals to provide a strike
point, external conductors to form a protective cage around the boat, and
multiple grounding terminals to disperse the current away from the
boat. Overhead conducting fittings
such as T-tops and biminis can be life savers when they are integrated into
a complete system. Otherwise they
are potential sideflash hazards. We
are developing products and techniques that can be used on small boats to
dramatically lower these risks. Our
collapsible air terminals are designed to bolt directly onto a handrail,
bimini top, or the deck, thereby attracting the strike point precious feet
away from the crew. SiedarcTM electrodes, installed above the
waterline, provide a low-drag option for grounding. And our through-hull connecting studs and
knowhow enable existing metallic fittings such as bowrails, T-tops, and
biminis to be interconnected to form a protective conducting shield. However, given the extreme nature of the
hazard and limited space available, practical solutions still involve a
high degree of danger and no one can give any guarantees. Discretion definitely should prevail.
Close to shore, an understanding of the partial
protection that can be afforded by nearby tall objects such as trees is
useful. That is, a nearby tree tends to attract lightning away from
the ground around it. Note the words "tends to" rather than
"will". However, the common sense rules of land also apply
here. In particular, do not get too close to the tree (within about
20 feet of the trunk or directly below overhanging branches) since a
sideflash may form from the tree if struck. The cardinal rule is to
avoid being on an open boat during a thunderstorm, even if the fish are
biting! Get out of the boat and into a metallic vehicle or large building,
but avoid any unprotected small shelter, especially one with a metal roof
and no lightning protection.
Air boats
An air boat has the same risk factors as small open
boats – a direct strike to the boat occupants being the major danger.
In fact, since the operator is
typically on a raised seat the risk may be even higher than for an open
boat. Fortunately the solution for a
metal-hulled air boat is very straightforward. This involves adding conductors (air
terminals) that can be attached directly to the deck or motor housing to
provide alternative attachment points for the lightning strike. See our airboat
page for more details. For an
airboat with a fiberglass hull, a grounding system similar to that in a small
open boat is also needed.
Trailerable sailboats
In contrast to a small open boat, the metallic mast and
rigging of a sailboat forms an effective in-built protection against direct
attachment to crew members. Also, the typical aluminum mast usually
has sufficient cross section to act as a reasonable down conductor (but, in
a small boat, the stainless steel rigging does not). The mast is, however,
also a hazard since a sideflash from the mast base is possible. In
one case, sideflashes occurred from the mast base through the hull in a
cat-rigged sailboat (in salt water) whose mast base was well bonded to a
keel bolt. Add to this the typical
location of the mast base above the living quarters and the risk to crew
becomes obvious. In order to conduct
the lightning current from the mast to the water without involving the
crew, the best solution is to route all lightning conductors around crewed
areas and provide multiple grounding paths as explained above. The type of ballast also is an important
factor. A metallic swing keel
effectively brings the water potential inside and becomes a hazard if it is
not bonded to the mast. That is, a
sideflash could spark from the mast base through a crew member to the keel
if these potentials (mast and keel) are not equalized by bonding. A water ballast presents a whole new set
of problems. Since the water in the
ballast is a conductor, it presents a similar sideflash hazard to a metallic
keel but bonding is impossible.
Also, if the sideflash does connect to the ballast water, exiting
sideflashes from the ballast to the water outside are likely to blow holes
through the hull. In this case
external lightning conductors become crucial. The availability of HStripTM immersed grounding strips
and SiedarcTM electrodes to terminate
these external conductors provides for a practical and reasonably priced solution
to this particular problem.
Powerboats with metal
superstructure
The principles
for effective lightning protection on smaller boats are the same as those
for larger boats. That is, form an
ExoTerminalTM system.
Grounding terminals and interconnections around the perimeter shield
the region inside, while attachment conductors (air terminals), also around
the perimeter, attract the strike point away from the boat's interior. Superstructure such as a metal-framed
bimini, metal outriggers, and metal cabin framing act in a similar fashion
to the metal rigging of a sailboat.
They can be incorporated into a lightning protection system to act
as air terminals and form part of the conducting cage. Some, but not a lot, of modifications are
in order. Consider the typical
fisherman. While there is usually a
lot of deck-top metal, not all of it is above head height, which is an
obvious problem. For example, a
wraparound bow rail is in the optimum place to be used as part of the
system but knee-high is nowhere near high enough to attract the strike that
precious few feet away from a deck hand.
We are presently developing a solution to this – a lightning
rod that attaches to the bow rail but that can easily be laid flat for
docking, etc. While this does not
guarantee the safety of the deckhand, it considerably skews the odds in his
or her favor. Other techniques such
as overhead catenary wires can skew
the odds even more. In the absence
of connections to a good grounding system, however, this same metal
superstructure becomes instead an electrical hazard since it acts as a
launching pad for uncontrolled sparks.
Also, to minimize the shock risk to the helmsman, the engine and
steering controls should be bonded to the lightning protection system to
ensure that everyone floats at the same potential. The grounding
system on any boat should comprise multiple grounding terminals that
include at least one square foot of immersed area. For the immersed area, outboard and stern
drives can be used, or additional grounding strips
can be added. SiedarcTM
electrodes complete the grounding
network. How many electrodes and
where they are best placed depends on the individual boat. However, some locations are no-brainers,
such as directly below each outrigger on a fisherman. Owing to the proximity of the lightning
attachment point to occupants, even the best possible lightning protection
system still presents an extreme risk on a small boat so that there can be
no guarantees. A lightning
protection system hence should be regarded as a good insurance policy that
could just save your life. Also,
when your fisherman is put to bed on its boat lift, our ZzapStrapTM
is the best way to ensure that hazardous potential differences do not form
between the boat and the lift.
Cruising sailboats
Any liveaboard yacht requires a high standard of
protection to ensure crew safety and hull integrity. Further, any
yacht that ventures into blue water needs to be self sustaining after a
strike. Larger sailboats have the advantage that a large interior
volume means that the lightning protection system can be routed away from
living quarters. However, complex control systems, dispersed
electronics sensors, and carbon fiber or metal reinforcing means many
conductors, each being a potential source for a sideflash. While
immersed conductors such as propellors, bronze seacocks, and metal ballast
have the potential to be incorporated into the grounding system, doing so
raises the risk of corrosion and their grounding effectiveness is
compromised if the surfaces are painted or covered with fiberglass. Also,
dezincification of bronze through-hulls leaves them porous and weak and
unsuitable for lightning grounding. Judicious placement of grounding
electrodes is vital to ensure that the only lightning exit points are
those that are planned.
Passagemakers & blue water power yachts
An ocean-going power yacht has more risk factors than
any other type of boat. Large open deck spaces with an absence of
natural lightning rods raise the risk of a direct attachment to anyone on
deck. Any on-deck spa or pool further increases the odds of
electrocution since a wet human body is more likely to conduct a lethal
current than a dry one. Without adequate air terminals, at
least upward-going streamers, and possibly direct lightning attachment, is
likely for elevated transducers such as antennas, radar, and weather
sensors. The natural path to ground in this case is then via on-board
wiring through the main instrumentation cluster, likely destroying most
other electronic systems as well. This may include electronic control
systems for steering and engines that are much more susceptible to
lightning damage than manual ones. Hence mechanical redundancy is
crucial. In-built conductors such as water in tanks, carbon fiber
reinforcing, metallic fittings, and power plants are all potential sources
for sideflashes, perhaps through crew, passengers or the
hull. As with cruising sailboats, there are many immersed
conductors that may be incorporated into the grounding network but doing so
raises the risk of corrosion and their grounding effectiveness is
compromised if the surfaces are painted or covered with fiberglass.
Our proprietary grounding technology is designed to fix this problem.
Passagemakers do also have some advantages. Extensive weather sensing
capabilities can be supplemented with thunderstorm and lightning sensors to
warn the crew of electrical hazards. Also, extensive metallic
superstructure and handrails on the periphery of the vessel are already in
the ideal location to form part of the external conducting shield that we
call the ExoTerminalTM grounding system. Just add air terminals, six SiedarcTM
electrodes, one square foot of immersed ground strips, surge suppressors
for antennas, and interconnections, and a superior lightning protection
system is the result. Our Passagemaker Packages give two options, including
everything you will need to complete the job with the exception of cable
and installation, and our included installation directions and consulting
advice give all the guidance you will need.
Sample installations are available for a Mirage Great
Harbour Pilothouse and a Nordhavn
55.
Hovercraft
The principles for protecting a hovercraft against
lightning are exactly the same as those applied to boats with one
exception: since the hovercraft is
not in contact with the water, the concept of an immersed ground is
meaningless. This should make no
difference to the layout of lightning conductors, which still need to
envelop the craft. Since the skirt
is nonconducting, we can place the grounding terminals - in this case they
will all be SiedarcTM electrodes - at the lowest points on the
rigid body.
Instrumented buoys
An instrumented buoy is very similar to a yacht in that
it is a floating isolated platform with sensitive electronics, usually
including submerged transducers and raised components such as solar cells
and/or telemetry antennas. Even if the buoy itself is metal, these
submerged and raised components are very sensitive to damage even from
close lightning strikes. The consequence of a disabled buoy might
range from loss of valuable data while the equipment is being replaced in a
scientific sensor, to a shipping hazard in the case of a navigation
buoy.
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