Lightning protection on John
Henry
John Henry lays
the line
Summary
·
External lightning conductors provide protective shield.
·
System consistent with new standards under
consideration by NFPA
·
New patented SiedarcTM grounding electrodes
above waterline.
·
1 ft2 grounding strip below
lazarette at stern.
·
Zone of protection covers all decks.
·
Transient suppressors on VHF antennas.
The Problem with Living Aboard
Wilma and Wink Thornton had experienced the effects of a
close lightning strike a couple of times too many. When contemplating a life as
liveaboards on their new Mirage Great Harbor 47 John Henry, their
vision did not include cowering in a state of deep anxiety at the first
rumble of thunder. And Wink's
profession in the Insurance and Risk Management Industry stood him in
excellent stead to assess risks and mitigation. Consequently, their requirement for lightning protection
was simple –"The best possible". So when Ken Fickett, CEO of Mirage Manufacturing,
mentioned a new type of lightning protection system that showed promise of
being vastly superior to traditional techniques, Wink's response was fast
and in the affirmative.
However, it does not take a close and personal
relationship with lightning to appreciate the hazards involved in living
aboard. Since escaping from
the storm is completely impractical, the only option is to hunker down and
reflect on the safest strategy to avoid potential injuries. After a moment's reflection it will
become apparent that the time to implement the safest strategy was actually
while the boat was being built or during the last refit. An inbuilt lightning protection
system is the only way to assure piece of mind.
But the idea of running a lightning cable right through
the middle of the living area to a ground plate is not particularly
appealing, especially when you realize that the most hazardous place in the
boat is now between that cable and the water. This is right about where you just placed your
recliner. There has to be a
better way to do it than the traditional method.
Now there is.
The Solution
1)
External lightning conductors
First let's address whether the middle of the boat
really is the best place for a
lightning conductor and ground plate.
The answer lies in protection systems that have been developed and
refined successfully over a couple of centuries – those on buildings. Look at the lightning protection
system on any building and you will see a myriad of lightning rods at the
edge of the roof line, and multiple conductors running down the outside of the building, with each
conductor morphing into a copper-clad grounding rod before disappearing
into the soil.
The key concept here is that the lightning conductors
are on the outside. On the
other hand, in the case of lightning conductor running through the center
of a boat, voltage differences ("step voltages") between this and
the water are conducive to dangerous sideflashes, that may involve
conducting fittings or crew members on the way. So the short answer to our question above is a
resounding "No".
Instead, what we need to do is route the conductors close to the
outside of the boat to minimize the risk of sideflashes between the
lightning conductors and other onboard conductors.
The backbone for the external network of lightning
conductors is a continuous loop.
In the case of John Henry, we actually have two loops, one at
flybridge level and another at main deck level. Connected into the conducting loops are the air
terminals and grounding conductors. The zone of protection provided by the air
terminals covers the whole deck area.
One fundamental problem with transferring the building
design onto a boat is the part about the "copper-clad grounding rod ….
disappearing into the soil."
On a boat, or at least one that plans to move through the water
without long copper appendages flailing about, this is not an option.
2)
Multiple grounding conductors
So the next point is grounding. Grounding is the process whereby
lightning charges move off the boat and into the water. In order to do this, contact with
the water is not needed.
Indeed, lightning exits perfectly well from boats that have no
grounding at all, albeit capriciously. In this case the final few feet is bridged by the
lightning making its own path.
On the way, intervening conductors provide attractive conduits, and
the surface of the water is a favorite destination. Damage to boats where this has
happened lays to rest another piece of folklore – that lightning always
takes the straightest path to ground.
So what would happen if we provided a network of
oversize cables that ended just above the water? This would take care of the problem with sideflashes
inside the boat, but would increase the chance of sideflashes from the
lowest points of the conductors.
This is actually a good thing since it lowers the likelihood of
sideflashes from nearby conducting fittings. In fact, what we would like to do is promote a spark from a device that
is designed to handle it safely.
Fortunately, we already have specifications for this type of device
– those for lightning air terminals (or lightning rods).
The six SiedarcTM electrodes on John Henry
are made from ½" copper, the same specification as for an air
terminal. By distributing them
around the hull just above the waterline, we not only complete the desired
external network of lightning conductors, but also induce the lightning
current to flow away from the hull from multiple exit points.
While the SiedarcTM electrodes eliminate the
need to bore holes through the hull below the waterline, an obvious
advantage, we also wanted to adhere to accepted convention in providing an
immersed grounding conductor as well.
An immersed one square foot grounding plate or strip is usually
recommended in the typical protection guidelines as it provides for a
direct conducting path to the water.
On the other hand, there are incidents of ground plates blowing out
of the hull. The most likely
explanation for this is that water in pockets in the hull evaporates into
high-pressure steam when some of the lightning current flows into these pockets.
So, in addition to carefully sealing all through hull
penetrations, in John Henry we took the additional precaution of
placing the ground strip in a watertight lazarette at the aft of the
boat. In the remote chance
that these hull penetrations start leaking after a lightning strike, the amount
of water taken on board will be shallow and contained.
Wink & Wilma
onboard. No anxiety on this
trawler
Media materials for the John Henry protection system
are available at /Media .
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