Saturday, May27, 2017

A New Concept For Lightning Protection

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This article was published in the October 2007 edition of Exchange
A New Concept for Lightning Protection of Boats
Protect a Boat like a Building
Ewen M Thomson PhD
Ewen Thomson is a recognized expert on marine lightning and was instrumental in
writing the new NFPA watercraft standard He is also the founder of Marine a company that specializes in lightning protection of boats This article is
being published in the interest of disseminating new information about marine and is not meant to imply endorsement of any product
A Critical Assessment of the US Code for Lightning Protection of Boatswas the title of
a paper1 published in 1991 by the Institute of Electrical and Electronic Engineers IEEE
True to its name this peerreviewed journal publication pointed out several key problem
areas then existing in standards published by all major authorities concerning of boats Some such as upgrading the size of a main lightning 8AWG to
4AWG required minor editorial changes while others were that had no clear solution A major issue was the conclusion that a 1sq ft
ground plate is shown to be hopelessly inadequate to prevent sideflashes in fresh
water In 1991 there was no practical solution for this Stretching the grounding area
into a long strip improves its theoretical performance but is difficult to concern which also gives a hint to the solution becomes evident when we
compare lightning protection techniques used successfully in buildings with applied with much less success to watercraft In buildings the are placed
on the outside and terminate in multiple ground rods also on the
outside On the other hand the requirement that only one ground plate is called for in a
boat usually results in a single down conductor running through the middle of the boat
With 2020 internal side flashes frequently form between conductors in the
lightning protection system and other conducting fittings These internal side flashes can
be prevented by bonding the fittings to the lightning protection system as mandated in
the standards but bonding also increases the risk of external side flashes from the
fittings to the water
The obvious organization to address these problems is ABYC whose marine the basis for NMMA In recognition of the above problems inherent in
its Lightning Protection Standard E4 in its latest rewrite ABYC downgraded E4 to a
Technical Report TE42 However even when the standard E4 existed it was not
required for NMMA During its latest revision cycle the National Association NFPA has taken on a comprehensive rewrite of their standard for
watercraft based on the simple concept that the system on a boat should resemble that on a building The NFPA standard is
reviewed on a fouryearly cycle by a committee of lightning protection professionals and
contains not only code language but also several informational annexes explaining the
underlying principles The result in Chapter 8 of a new that is a major departure from the old Instead of a single lightning rod at the top of a
centrally located mast many air terminals may be placed
around the perimeter Instead of a single down conductor following the straightest path
to the water an grid of down conductors are placed externally to and electronics Instead of a single immersed ground grounding terminals terminate
the down conductors also there are several instances where observed damage to boats can be related in the existing standard Let s start with these
Then we can show what
changes needed to be made to the marine standard so that a boat protection system
looks more like that on a building Finally we will discuss a system that has been
designed in accordance with the new with previous there were a number of problems with the status quo before the
major stumbling block was the mandate for a one square foot ground plate or strip This
was frequently interpreted to mean that that was all that was required and the best way
to connect this was by the shortest path possible to a single air terminal on top of a
mast In my 1991 IEEE paper1 I calculated what typical voltage the lightning would reach if it were connected to a single immersed grounding conductor
a contact area of one square foot In fresh water this voltage was found to be so large
that sideflashes would be inevitable A side flash is an uncontrolled spark that to the water and can do extensive damage to hulls and equipment was
done to help explain observations of extensive sideflash damage in
sailboats even when the mast was grounded to the keel or a ground plate Cases such
as that below from Boat US claim 950447 have necessitated a new term to be added to
the glossary of lightning protection a supplemental grounding current into the water in addition to that conducted by a main or ground plate In
this case the anchor chain formed sideflashes through
the hull causing extensive hull damage
In another case shown below a water tank and an aluminum organizer acted electrodes The owner of this sailboat reported not only thousands of
holes in the lead ballast indicating that lightning current had indeed flowed out of the
intended grounding conductor but also noted two large holes at about the waterline and
outboard of an aluminum organizer aft and a water tank forward The side flashes that
caused these holes originated respectively on the backstay and forestay and clearly
took much longer and more tortuous paths than the shortest distance to the lightning does not always take the straightest path to the water but
has an affinity for the waterline Note the major role of the two intermediate organizer and the water tank in guiding the side flash on its way to
the waterline It
does not take much imagination to appreciate the probable consequences if a crew
member had been lying in the V berth between the forestay chain plate and the other cases an aluminum Ibeam mast support a plumbing fixture gel coat
and moisture in the hull all acted as supplemental grounding electrodes In fact in
another case the keel ballast appeared to have carried no current at
all but four large holes at the waterline implicated the lightning down to the keel bolts as the grounding electrodes which could hardly here as
being a side flash does occur through a fiberglass hull carbon atoms are split out of the
resin thereby weakening the laminate and this residual carbon now forms through otherwise insulating fiberglass So if the boat were to be struck
again it is
highly likely that the carbon traces would provide attractive current pathways but their
high resistances would likely result in overheating In other words the risk for serious
hull damage is increased if the carbon is not removed during repairs Thus removal of
all carbon tracks should be a high priority during the repair of any fiberglass hull
damaged by lightning finding and repairing these traces is but if there is a side flash exit from the hull you can be sure there are
carbon traces present
So one square foot is not nearly enough However the illustration above
shows that even the area of the lead ballast was not enough in this case Apparently the
problem is not so much the size of the grounding area but how it is distributed Rather
than attempting to dissipate the lightning current through just one ground plate we need
multiple exit points The preferred locations for these as indicated from observed exit
holes are around the outside of the hull rather than directly below the mast
The single ground plate is not the only major problem The short straight mast base to the ground plate is another This places the lightning charge
right in
the middle of the boat increasing the risk of internal side flashes to on the boat In this respect electrical wiring water tanks whether metal or
plastic and crew members are all possible standard Protect a boat like a building
So what to do now Well remember that the standard for buildings has been around
for a long time has undergone many iterations under guidance from a committee of
lightning experts and works very well The difference is that the building multiple lightning rods conductors and grounding rods on the outside of the
building Maybe we can do something similar for boats
This one idea is the basis for the new NFPA watercraft standard3 When the on lightning protection saw the types of damage sustained by boats the
fundamental causes they agreed that there were serious issues that
needed to be addressed So we examined the existing standard with the intention of
changing the fundamental concepts to be more in line with those applied to result is a major departure The final text is a comprehensive treatment of
a protection system that includes details such as how to use existing fittings as
part of the system and introduce spark gaps to minimize galvanic corrosion Three main points stand out in stark contrast to the status quo
1 Multiple air when determining where air terminals should be placed any method that is
allowable for buildings can now be used for boats So instead of having to use just the
cone of protection method to establish the protective zone the rolling sphere method
can be used This allows air terminals to be placed around the perimeter and results in
much shorter lightning rods being required For example in a powerboat with a Ttop
the inverted cone when hung off the Ttop gives a zone of protection that usually does
not cover the whole foredeck If instead we use the rolling sphere method we can add
an air terminal on the bow pulpit such as a metal flag staff so that the foredeck is now
included As long as the forward air terminal is higher than head height the of protection now covers anyone working the foredeck An even better
approach is
to string a catenary wire between the Ttop and the air terminal as an provides far superior protection to a vertical rod
2 External lightning consistent with the building standard lightning conductors note the plural are
placed on the outside of the boat What this does is establish a somewhat similar to a Faraday cage around the interior of the boat Inside of
this shield everything is at about the same voltage as the lightning protection system
even if there is no bonding connection In the new NFPA standard a novel feature is a
loop conductor that completely encircles the boat This serves as a for the conductor network allowing air terminals and grounding terminals to
be as well as establishing this protective shield around the interior of loop conductor serves the same function as the equalization bus in the old
and replaces it While equalizing potentials through bonding is a good idea can also initiate side flashes And the old mandate in Section 8613 in the
2004 version of NFPA780 that The equalization bus shall be connected to lightning grounding strip at both guaranteed that the bus
would be centrally located and well below the waterline two conditions that increase
side flash risk Instead the new standard in Section 8431 states A main size shall be routed to form a continuous conducting loop outboard of crewed
areas wiring and electronics Placing the loop conductor well above the and with grounding terminals below it retains the advantages of bus while
correcting for its Grounding terminals near waterline around the multiple lightning conductors coming down the outside need to be terminated
in multiple grounding terminals preferably close to the waterline Distributing the and grounding terminals uniformly around the hull promotes current
away from the boat This minimizes voltage differences in the water below the boat and
hence considerably reduces the risk of sideflashes from conducting fittings even those
that are close to the this poses several practical problems if the only allowable type of is a one square foot immersed ground plate or strip It is
difficult enough to
convince someone to bore holes through the hull below the waterline for installing even
one immersed ground plate let alone many Doing this would appear to increase the
risk of sinking after a lightning strike rather than decreasing it In particular if there has
been any water leakage through these holes a steamboiler type explosion is So if one is a problem this to the point of what about the old
requirement that the ground plate should always be immersed
If a sailboat heels or powerboat comes to a plane the ground plate can become airborne
So when the new standard mandates multiple grounding electrodes this could cause
serious problems Note that grounding electrodeis NFPA s new term
for a grounding terminal in that it is a conductor through which current is passing at the
interface between the lightning protection system and the grounding medium water
here Fortunately the damage we showed earlier indicates that lightning does share this preference for immersed grounding conductors In fact the
corners of water tanks plumbing fixtures metallic fittings and anchor chains seem to
work just as well and frequently much better The same is true for such as metallic through hulls and propeller shafts that may have contact
areas much less than one square foot The waterline is a very popular target and multiple exit points is the norm especially in fresh that onboard
fittings frequently act as inadvertent grounding electrodes we
have introduced the idea of a supplemental grounding electrode one that has a contact
area of less than one square foot including zero The standard still requires at least one
main grounding conductor with an immersed area of at least one square foot but now
smaller additional grounding terminals are also allowed This makes it feasible to grounding terminals using existing metallic fittings such as
struts and rudder posts even those with contact areas less than one square
foot smaller fittings specifically designed to act as grounding be added as we have done for John Henry below Note that ABYC TE4 also
allows that Rudders external ballast keels or any metallic fitting with at least one
external face can be used for supplemental grounding so long as they meet in this bulletin
Lightning protection system on John Henry
We have applied all of these new concepts in John Henry a Great Harbour built by Mirage Manufacturing and displayed in last year s show The
annotated photo below shows the main features The red lines
show the total zone of protection using the rolling sphere method Any person on the deck is inside this protective zone In order to achieve this
we placed air terminals on the bow pulpit on top of the fly bridge arch and on top of the
handrails at the rear of the fly bridge deck We also connected the dinghy davit to the
lightning protection system The blue lines show the additional lightning were made of 2AWG tinned copper marine battery cable These were connected
to existing conducting fittings the handrails on both main and fly bridge levels and the
bimini to form two conducting loops one around the main deck level and the other
around the fly bridge deck Down conductors connected to these loop conductors were
run vertically down the inside of the hull and terminated at SiedarcTM at six locations symmetrically distributed around the waterline The electrode
is a customized fitting designed specifically for lightning of these was installed just above the black stripe One square foot of area was provided
by a grounding strip placed near the stern of the boat
This placement allowed the down conductor to this strip also to be run down the inside of
the hull external to all conducting fittings and equipment As an additional through bolts for the grounding strip were contained inside a watertight
lazarette The
cost of such an installation is around 60007000 John Henry has not been struck by
lightning yet so that the effectiveness of this system has not been tested And while
John Henry s owner may hope the boat is never hit we would find such a strike would
add tremendously to current data
A Critical Assessment of the US Code for lightning Protection of Boatsby EM
Thomson IEEE Trans EMC Vol3 pp132138 1991
2 TE4 Lightning Protection 2006 edition ABYC 613 Third Street Suite 10 Annapolis
MD 21403 2006
3 NFPA 7802008 Standard for the Installation of Lightning Protection Systems 2008
Edition Chapter 8 National Fire Protection Association 1 Batterymarch Park Quincy
MA Thomson is founder of Marine Lightning Protection His PhD is in Electrical Engineering and he has nearly
three decades of experience as a lightning researcher and university instructor including
20 years at the University of Florida The main author of the revision that led he was also involved in the development of ABYC TE4

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