Innovative lightning protection
Peer-reviewed science forms the basis
Lightning
protection in the marine environment presents special challenges. As
the ground attachment path for a 5-mile long spark carrying tens of kiloamperes, the protection system has the task of
safely diverting this current around crew, sensitive electronics, and hull
components.
However,
even when the current is flowing in the water, voltage differences can
cause sideflashes, both inside the boat and between
the boat and the water. These present a shock hazard to the crew, produce overvoltage in electronics systems, and can blast holes
through the hull. Management of the sideflash
problem is the fundamental issue in the design of an effective marine lightning
protection system. See
our Grounding Concepts page
for a technical explanation of the underlying concepts and suggestions as
to how these can be applied to a protection system.
Our
approach to lightning protection is based solidly on scientific theory and
observation. The foundation was established in a paper
published in 1991 in the peer-reviewed IEEE Transactions of Electromagnetic
Compatibility. As a result of this
paper, subsequent renditions of standards published by ABYC
and NFPA upgraded their recommended sizes for
down conductors from #8AWG to #4AWG and noted that a ground strip is a more
effective grounding conductor than a square plate of the same area. In an attempt to find a solution to
another fundamental problem revealed in this scientific work – that a
one square foot ground plate is "hopelessly inadequate" to prevent
sideflashes in fresh water – we have the
sole license for a patent that can provide alternative techniques for
grounding, and have contained to pursue the science. More recently, we have worked with the NFPA 780 technical committee to establish a new
standard based on these ideas, now published as Chapter 8 in the 2008
edition of NFPA780.
Sideflash prevention is the problem
An
interesting feature of hull damage is the tendency for sideflashes
to form around about the waterline. Apparently either the water
surface or the waterline itself causes charges to accumulate, usually on
internal conducting fittings, and initiate sparks through the hull.
The effect is more pronounced in fresh water than salt.
Photo by Dave Edwards
In
lightning protection circles, the conventional solution to a problem such
as this is to add conductors where the damage is observed. In the above case this means placing
lightning conductors through the hull at the waterline. Since it is impractical to install
multiple ground plates in a hull, we developed the SiedarcTM
electrode to provide the necessary exit terminals. This is effectively an air terminal near
the water. In fact, each one is
designed to the same specification as a lightning air terminal.
 In order
to investigate the effectiveness of this concept, we tested an electrode
with a 10kV generator for both salt and fresh water at Kennick
Inc. in St. Petersburg. Even though 10kV is much lower than what
would be expected during a lightning strike, we obtained results that
clearly indicated the promising potential for the method and further
elucidated the best mode of operation.
Specifically, in the photo below, with the electrode about 1/4
"above the surface of salt water, a spark of about 15" in
diameter was produced. Clearly the sparking is contained very close to the
water surface, perhaps even above it, showing the importance of the surface
for current dissipation.
In fresh water, the spark connected all the way to the sides of the
container, about 12" away. In
contrast, when the electrode tip was immersed just below the water surface,
a small (~1/2") glow was observed but no
sparks. The conclusion is that an
electrode can generate a spark that is orders of magnitude longer than the
spark gap in air when placed above the water surface. Hence the optimum placement is just above
the water surface.
The ExoTerminalTM system is the
answer
Providing
exit terminals around the perimeter of the hull is the key to an effective
system design since, in addition to dispersing  the current more uniformly around the boat, it
also enables the lightning down conductors to be routed externally to all
wiring and conducting fittings. This
is illustrated for a sailboat below.
The lightning conductor from mast base connects to both the chainplate and the loop before passing down to a
daisy-chain SiedarcTM electrode just
above the waterline, and from there via an immersed HStripTM
to a keelbolt (and base of a keel-stepped
mast). SiedarcTM
electrodes at bow
and stern provide more exit terminals from the loop to the water. This geometry is mirrored on the port
side, as indicated by the dashed lines.
That is, there is total of two HStrips and
six SiedarcTM electrodes. Thus a conducting grid covers the
interior of the boat and a total of eight exit terminals are distributed
over the hull near the waterline.
For a keel-stepped mast, make another connection from the mast base
to the keelbolt of HStrips.
Guiding
the current on the outside rather than through the middle of the boat
minimizes shock risk and emi. In addition,
a bonding loop around the boat at about deck level equalizes potentials,
provides additional paths for current flow, and can be used for bonding
conducting fittings. In a major
departure from the status quo, NFPA (the National
Fire Protection Association) has recently revised their watercraft standard
(NFPA 780 Ch.8) to include the concepts of a loop
conductor, external down conductors, and perimeter grounding electrodes. See our Standards page for
details. With this new system the
conductor layout more closely mirrors that found on the typical lightning
protection system on a building. For
a graphic demonstration of the shielding effectiveness of a metallic cage,
check out this photo
sequence from the Boston Museum of Science. We call this system of external lightning
conductors and peripheral exit terminals the ExoTerminalTM
protection system.
In
the case of a powerboat, the external down conductors can be connected
directly to air terminals that are around the perimeter. Placed using the rolling sphere model,
these can be much shorter than a single air terminal that covers the whole
boat using the "cone of protection" concept. The annotated photo below shows our first
system on a passagemaker that was installed
during manufacture. The lightning
conductors (the blue lines) are on the inside of the hull and the only
features that might stand out as different are the two air terminals at the
rear of the bridge deck.
Lightning protection system on Mirage Great Harbor 47 John Henry
We
can provide all of the components needed in a marine lightning protection
system.
|
Component
|
Product
|
Description
|
|
Air terminals
|
RMATTM
|
Rod with rail-mount ratchet base
|
|
SMATTM
|
Rod with surface-mount ratchet base
|
|
Rod
|
Aluminum or tinned copper rod with threaded base
|
|
Connections
|
StanConTM
|
Through-connector for stanchion
|
|
MastConTM
|
Connector for aluminum mast base to either aluminum or copper
conductors
|
|
GapConTM
|
Tinned copper connector with air gap for galvanic isolation
|
|
Lug
|
Heavy duty tinned copper closed-end lugs for cable terminations
|
|
HStripConTM
|
Inboard coupling for combining two HStrips
into a single one square foot area
|
|
Conductors
|
Strip
|
Aluminum or tinned copper strip for main conductors
|
|
Cable
|
Insulated tinned copper cable
|
|
Grounding
|
HStripTM
|
0.5 ft2 tinned solid copper grounding strip
|
|
SiedarcTM
|
Spark-promoting grounding electrodes embedded in non-conducting
through-hulls
|
General information on these items and their integration
into a lightning protection system can be found on the History/Problems/Solutions
page, and specific details on these pages:
|
Item
|
Document
|
Contents
|
|
SiedarcTM
|
Introduction
|
Introduction to the SiedarcTM
electrode line
|
|
Grounding
guidelines
|
General
guidelines for using SiedarcTM
electrodes
|
|
Grounding
concepts
|
Physical
basis for grounding electrode placement and type
|
|
Prices
|
Product
price list
|
|
Grounding
strips
|
Information
|
Problems,
solutions & applications for immersed grounding strips
|
|
Specifications
|
Brief
specifications of HStripTM grounding
strips and accessories
|
|
Prices
|
Product
price list
|
We also offer consulting services
for:
·
analysis and recommendations for specific
systems;
·
expert witness testimony
concerning any aspect of lightning on or near water.
Please call or email if you have any questions.
|
