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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 PhDEwen Thomson is a recognized expert on marine lightning and was instrumental inwriting the new NFPA watercraft standard He is also the founder of Marine a company that specializes in lightning protection of boats This article isbeing 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 ofa paper1 published in 1991 by the Institute of Electrical and Electronic Engineers IEEETrue to its name this peerreviewed journal publication pointed out several key problemareas then existing in standards published by all major authorities concerning of boats Some such as upgrading the size of a main lightning conductorfrom 8AWG to 4AWG required minor editorial changes while others were that had no clear solution A major issue was the conclusion that a 1sq ftground plate is shown to be hopelessly inadequate to prevent sideflashes in freshwater In 1991 there was no practical solution for this Stretching the grounding areainto a long strip improves its theoretical performance but is difficult to concern which also gives a hint to the solution becomes evident when wecompare lightning protection techniques used successfully in buildings with thosetypically applied with much less success to watercraft In buildings the are placed on the outside and terminate in multiple ground rods also on theoutside On the other hand the requirement that only one ground plate is called for in aboat usually results in a single down conductor running through the middle of the boatWith 2020 predictability internal side flashes frequently form between conductors in thelightning protection system and other conducting fittings These internal side flashes canbe prevented by bonding the fittings to the lightning protection system as mandated inthe standards but bonding also increases the risk of external side flashes from thefittings to the waterThe obvious organization to address these problems is ABYC whose marine standardsform the basis for NMMA certification In recognition of the above problems inherent inits Lightning Protection Standard E4 in its latest rewrite ABYC downgraded E4 to aTechnical Report TE42 However even when the standard E4 existed it was notrequired for NMMA certification During its latest revision cycle the National FireProtection 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 isreviewed on a fouryearly cycle by a committee of lightning protection professionals andcontains not only code language but also several informational annexes explaining the 1underlying principles The result in Chapter 8 of NFPA78020083is a new that is a major departure from the old Instead of a single lightning rod airterminal at the top of a centrally located mast many air terminals may be placedaround the perimeter Instead of a single down conductor following the straightest pathto the water an interconnected grid of down conductors are placed externally to fittingscrew and electronics Instead of a single onesquarefoot immersed ground platemultiple grounding terminals terminate the down conductors also there are several instances where observed damage to boats can be related toshortcomings in the existing standard Let s start with these Then we can show whatchanges needed to be made to the marine standard so that a boat protection systemlooks more like that on a building Finally we will discuss a system that has beendesigned in accordance with the new with previous standardsWhile there were a number of problems with the status quo before NFPA7802008 themajor stumbling block was the mandate for a one square foot ground plate or strip Thiswas frequently interpreted to mean that that was all that was required and the best wayto connect this was by the shortest path possible to a single air terminal on top of amast In my 1991 IEEE paper1 I calculated what typical voltage the lightning would reach if it were connected to a single immersed grounding conductor witha contact area of one square foot In fresh water this voltage was found to be so largethat sideflashes would be inevitable A side flash is an uncontrolled spark that carriescurrent to the water and can do extensive damage to hulls and equipment was done to help explain observations of extensive sideflash damage insailboats even when the mast was grounded to the keel or a ground plate Cases suchas that below from Boat US claim 950447 have necessitated a new term to be added tothe glossary of lightning protection a supplemental grounding electrodethat current into the water in addition to that conducted by a main or ground plate In this case the anchor chain formed sideflashes throughthe hull causing extensive hull damage 2In another case shown below a water tank and an aluminum organizer acted assupplemental electrodes The owner of this sailboat reported not only thousands ofholes in the lead ballast indicating that lightning current had indeed flowed out of theintended grounding conductor but also noted two large holes at about the waterline andoutboard of an aluminum organizer aft and a water tank forward The side flashes thatcaused these holes originated respectively on the backstay and forestay and clearlytook much longer and more tortuous paths than the shortest distance to the lightning does not always take the straightest path to the water but ratherhas an affinity for the waterline Note the major role of the two intermediate conductorsthe organizer and the water tank in guiding the side flash on its way to the waterline Itdoes not take much imagination to appreciate the probable consequences if a crewmember had been lying in the V berth between the forestay chain plate and the watertankIn other cases an aluminum Ibeam mast support a plumbing fixture gel coat blistersand moisture in the hull all acted as supplemental grounding electrodes In fact inanother welldocumented case the keel ballast appeared to have carried no current atall but four large holes at the waterline implicated the lightning down to the keel bolts as the grounding electrodes which could hardly beinterpreted here as being a side flash does occur through a fiberglass hull carbon atoms are split out of theresin thereby weakening the laminate and this residual carbon now forms through otherwise insulating fiberglass So if the boat were to be struck again it ishighly likely that the carbon traces would provide attractive current pathways but theirhigh resistances would likely result in overheating In other words the risk for serioushull damage is increased if the carbon is not removed during repairs Thus removal ofall carbon tracks should be a high priority during the repair of any fiberglass hulldamaged by lightning Unfortunately finding and repairing these traces is but if there is a side flash exit from the hull you can be sure there arecarbon traces presentSo theoretically one square foot is not nearly enough However the illustration aboveshows that even the area of the lead ballast was not enough in this case Apparently the 3problem is not so much the size of the grounding area but how it is distributed Ratherthan attempting to dissipate the lightning current through just one ground plate we needmultiple exit points The preferred locations for these as indicated from observed exitholes are around the outside of the hull rather than directly below the mastThe single ground plate is not the only major problem The short straight connectionfrom mast base to the ground plate is another This places the lightning charge right inthe middle of the boat increasing the risk of internal side flashes to on the boat In this respect electrical wiring water tanks whether metal orplastic and crew members are all possible conductorsNFPA standard Protect a boat like a buildingSo what to do now Well remember that the standard for buildings has been aroundfor a long time has undergone many iterations under guidance from a committee oflightning experts and works very well The difference is that the building standardplaces multiple lightning rods conductors and grounding rods on the outside of thebuilding Maybe we can do something similar for boatsThis one idea is the basis for the new NFPA watercraft standard3 When the NFPAcommittee on lightning protection saw the types of damage sustained by boats andrecognized the fundamental causes they agreed that there were serious issues thatneeded to be addressed So we examined the existing standard with the intention ofchanging the fundamental concepts to be more in line with those applied to buildingsThe result is a major departure The final text is a comprehensive treatment of a wholelightning protection system that includes details such as how to use existing fittings aspart of the system and introduce spark gaps to minimize galvanic corrosion Three main points stand out in stark contrast to the status quo1 Multiple air terminalsFirst when determining where air terminals should be placed any method that isallowable for buildings can now be used for boats So instead of having to use just thecone of protection method to establish the protective zone the rolling sphere methodcan be used This allows air terminals to be placed around the perimeter and results inmuch shorter lightning rods being required For example in a powerboat with a Ttopthe inverted cone when hung off the Ttop gives a zone of protection that usually doesnot cover the whole foredeck If instead we use the rolling sphere method we can addan air terminal on the bow pulpit such as a metal flag staff so that the foredeck is nowincluded 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 isto string a catenary wire between the Ttop and the air terminal as an provides far superior protection to a vertical rod2 External lightning consistent with the building standard lightning conductors note the plural areplaced preferentially 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 ofthis shield everything is at about the same voltage as the lightning protection systemeven if there is no bonding connection In the new NFPA standard a novel feature is a 4loop conductor that completely encircles the boat This serves as a for the conductor network allowing air terminals and grounding terminals tobe interconnected as well as establishing this protective shield around the interior of theboatThe loop conductor serves the same function as the equalization bus in the old standardand replaces it While equalizing potentials through bonding is a good idea can also initiate side flashes And the old mandate in Section 8613 in the2004 version of NFPA780 that The equalization bus shall be connected to theunderwater lightning grounding strip at both endsvirtually guaranteed that the buswould be centrally located and well below the waterline two conditions that increaseside flash risk Instead the new standard in Section 8431 states A main size loopconductor shall be routed to form a continuous conducting loop outboard of crewedareas 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 weaknesses3 Grounding terminals near waterline around perimeterThird the multiple lightning conductors coming down the outside need to be terminatedin multiple grounding terminals preferably close to the waterline Distributing the downconductors and grounding terminals uniformly around the hull promotes current flowaway from the boat This minimizes voltage differences in the water below the boat andhence considerably reduces the risk of sideflashes from conducting fittings even thosethat are close to the waterHowever 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 toconvince someone to bore holes through the hull below the waterline for installing evenone immersed ground plate let alone many Doing this would appear to increase therisk of sinking after a lightning strike rather than decreasing it In particular if there hasbeen 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 immersedIf a sailboat heels or powerboat comes to a plane the ground plate can become airborneSo when the new standard mandates multiple grounding electrodes this could causeserious implementation problems Note that grounding electrodeis NFPA s new termfor a grounding terminal in that it is a conductor through which current is passing at theinterface between the lightning protection system and the grounding medium waterhere Fortunately the damage we showed earlier indicates that lightning does notnecessarily share this preference for immersed grounding conductors In fact thecorners of water tanks plumbing fixtures metallic fittings and anchor chains seem towork just as well and frequently much better The same is true for such as metallic through hulls and propeller shafts that may have contactareas much less than one square foot The waterline is a very popular target forsideflashes and multiple exit points is the norm especially in fresh that onboard fittings frequently act as inadvertent grounding electrodes wehave introduced the idea of a supplemental grounding electrode one that has a contactarea of less than one square foot including zero The standard still requires at least one 5main grounding conductor with an immersed area of at least one square foot but nowsmaller 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 squarefoot Alternatively smaller fittings specifically designed to act as grounding electrodescan be added as we have done for John Henry below Note that ABYC TE4 alsoallows that Rudders external ballast keels or any metallic fitting with at least oneexternal face can be used for supplemental grounding so long as they meet in this bulletin Lightning protection system on John HenryWe have applied all of these new concepts in John Henry a Great Harbour 47passagemaker built by Mirage Manufacturing and displayed in last year s show The annotated photo below shows the main features The red linesshow 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 coveragewe placed air terminals on the bow pulpit on top of the fly bridge arch and on top of thehandrails at the rear of the fly bridge deck We also connected the dinghy davit to thelightning protection system The blue lines show the additional lightning were made of 2AWG tinned copper marine battery cable These were connectedto existing conducting fittings the handrails on both main and fly bridge levels and thebimini to form two conducting loops one around the main deck level and the otheraround the fly bridge deck Down conductors connected to these loop conductors wererun 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 groundingEach 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 boatThis placement allowed the down conductor to this strip also to be run down the inside ofthe hull external to all conducting fittings and equipment As an additional precautionall through bolts for the grounding strip were contained inside a watertight lazarette Thecost of such an installation is around 60007000 John Henry has not been struck bylightning yet so that the effectiveness of this system has not been tested And whileJohn Henry s owner may hope the boat is never hit we would find such a strike wouldadd tremendously to current data 6References1 A Critical Assessment of the US Code for lightning Protection of Boatsby EMThomson IEEE Trans EMC Vol3 pp132138 19912 TE4 Lightning Protection 2006 edition ABYC 613 Third Street Suite 10 AnnapolisMD 21403 20063 NFPA 7802008 Standard for the Installation of Lightning Protection Systems 2008Edition Chapter 8 National Fire Protection Association 1 Batterymarch Park QuincyMA Thomson is founder of Marine Lightning Protection His PhD is in Electrical Engineering and he has nearlythree decades of experience as a lightning researcher and university instructor including20 years at the University of Florida The main author of the revision that led toNFPA7802008 he was also involved in the development of ABYC TE4 7


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