Standard Of Practice For The

the presence of moisture shall not be used.Current carrying system components mustbe highly conductive. Prevailing site soilconditions will impact in-ground systemcomponents. The system life andmaintenance/replacement cycle is dependenton material choice and the localenvironment. System materials must becoordinated with the structural materials inuse – including flashings, copings, ventilatorhousings, various roofing systems – tomaintain the moisture envelope for theintended life of the building.components for required transitions fromaluminum to copper. These may includelisted products for the purpose, or in somecases stainless steel components. Aluminumcan never come in contact with the earth orsoil. Aluminum should never contactalkaline based paint surfaces or beembedded directly in concrete.If any product is subject to unusualmechanical damage or displacement, it maybe protected with a molding or covering, butcare must be exercised to allow striketerminations and other roof mountedcomponents to serve their function inaccepting attachments. Lightning protectioncomponents below the strike terminals maybe concealed within the building below theroof level during construction or whenaccessible. The speed of lightning currentand splitting the flow among multiple pathswill not permit components to heat to anyinstantaneous ignition temperaturehazardous to typical building materials.Incorporating the system into theconstruction allows interconnection ofstructural metal framing and internalgrounded systems, and provides protectionagainst displacement and maintenance issueswhich are beneficial in extending the life ofa system.Copper, copper alloys (including brassand bronze), and aluminum are the basicsystem component materials. They servethe best combination of function for currentcarrying and weathering. Since aluminummaterials have slightly lower currentcarrying capability and mechanical strengththan similar sized copper products, listedand labeled materials for lightningprotection include larger physical size parts.For example to be considered equivalent, aminimum size air terminal would be ½”diameter in aluminum, versus 3/8” diameterin copper.Materials suitable for use in lightningprotection systems are listed, labeled, andtested according to UL Standard 96.Consideration for conductor design includesmaximizing surface area to carry lightningand flexibility of the configuration to makebends and turns required in installationpractices. Air terminal bases efficientlyaccomplish the transfer of a strike fromtermination device to cable conductor andsecurely mount to various building surfacesunder severe weather conditions. Splicingfittings must maintain contact withconductor lengths adequate to accomplishWater running off copper will oxidizealuminum and galvanized surfaces, socoordination of system design must includegalvanic considerations for potentialmounting problems. Qualified bimetallicfittings are used to coordinate system5

current transfer and weather the exposedenvironment. Grounding electrodes mustprovide the proper earth contact to dispersethe charge and satisfy requirements for lifecycle suitability in various soilcompositions. Bonding devices are sized toprovide adequate interconnection of systemsto create potential equalization throughoutthe structure. Surge Protection Devices arequalified at higher current levels to meet theneeds associated with lightning attachments.relatively balanced ion distribution. Whenwe raise a building into the air, a tree oreven a person to a lesser extent, we changethat electrical balance. The electrical fieldaccumulates to change points in thegeometry of ground mounted objects. Itemslike ridges and particularly ridge ends, edgesof flat roofed buildings and even more thecorners become points of accumulation forions that increase susceptibility to lightningattachments. A proper system of striketermination devices accounts for theserealities by using air terminals in aconfigured pattern designed to use thebuilding’s points of natural ionicaccumulation to pull lightning into theprotection system. The taller the structureand the more severe the planar changes (likea vertical wall to a horizontal flat roof) thegreater the opportunity for attachment atthese critical junctions. Designing a systemof air terminals projecting only 10 inchesabove these structural points of emphasisand along ridges and edges has been proven,in more than a century of practice, toprovide interception of some 95% ofrecorded lightning flashes, including themost violent. Some lower potentiallightning strokes could theoretically attachon flat planes away from strike terminationsdesigned to the Standards, but theconsequences are within acceptable limitsfor ordinary construction. Considering thelower energy level required for a bypass, theother structural grounding componentsincluded in a complete lightning protectionsystem, and the random probability forconnection with a system componentanyway, this method of building protectionis considered most efficient.Strike TerminationStrike termination devices serve thesystem function of accepting the directlightning attachments. They represent theumbrella against penetration of lightning tonon-conductive building materials to guardagainst fire or explosion. Any metallic body3/16” thick or more projecting above astructure will accept a lightning strikewithout burning through. Therefore, insome cases construction elements may beincorporated as strike terminations. Tallmasts or overhead ground wires similar topower transmission line protection mayserve as strike terminations. In most cases,however, small specific purpose airterminals constitute the majority of striketermination systems. These unobtrusivecomponents are preferred for ease ofmounting and aesthetic reasons, and can becoordinated into a most effectiveconfiguration for all typical buildingconstructions.Protecting the highest most prominentbuilding elements with strike terminationdevices, based on a building’s geometry,also provides some level of protection forlower extensions of the structure, or items inThe atmosphere surrounding us iselectrically charged, but free air maintains a6

the “shadow” of the higher fully protectedareas. A zone of protection exists from anyvertical strike termination device and morethan that from a vertical fully protectedbuilding level. Zone of protection isdescribed in the lightning Standards using a150 feet (45 meters) radius sphere model toidentify items under the protection of highersystem elements or building extensions todistances that require further protection byadditional strike terminals. This is likerolling a 300 feet (92 meters) diameter ballfrom grade up against and then over abuilding to the opposite grade level in everyconceivable direction. If the ball touchesinsulated building material, then anadditional strike terminal is added. Areassupported by strike terminals, a striketerminal and grade, and vertical walls arethen under the protection of properlydesigned system elements. This geometricmodel for protecting total structures is basedon the last step in the lightning attachmentprocess, and again covers well over 90% ofconceivable strikes. On more criticalstructures, like those containing explosivesor flammable liquids and vapors, the modelis reduced to a 100 feet (30 meters) radiussphere that covers in excess of 98% ofrecorded lightning strikes.conductivity and suitability to exposure toweather. Qualified prominent metallicbuilding elements may also serve thefunction. In special circumstances wherelightning cannot be allowed to penetrate, theuse of tall masts and overhead ground wiresused in a reduced zone model can provideadditional protection. Protecting things likelighting standards or trees can provide somearea protection based on the zone model.Strike termination design configuration isthe first key element to providing a completelightning protection system.ConductorsThe conductor system component ofcomplete lightning protection includes mainsized cables, the structural steel of abuilding, and bonding or interconnectionwires to internal grounded building systems.The main conductors perform the currentcarrying function from the strike terminationdevices to the grounding system. Maincables are highly conductive copper oraluminum that perform well in an externalenvironment. Lightning seeks a path towardground, so even with very conductivematerials, the routing of cables needs to bemaintained in a horizontal or downwardcoursing. This is similar in concept to thegravity flow of water on sloped flat areas toroof drains or in gutter to downspoutsystems. Cables need to be routed usinglong smooth bends of no less than 90degrees. Lightning will place significantmechanical force on cables, and sharp bendsor corners can be damaged or lightning canarc over in the worst cases. This mechanicalforce can be compared to sendingpressurized water through a fire hose – theconductor will try to straighten itselfcreating a damage concern for splicefittings, fasteners, or the conductor itself.The strike termination system defends thestructure against lightning attachment byproviding preferred attachment points.Copper or aluminum air terminals arepreferred in most cases based on their7

thickness or greater, or a combination.Reinforcing steel or rebar is not acceptableas a substitute for cable conductor, but eachcable downlead must be bonded to thestructural framing at the top and bottom ofeach vertical run. All strike terminationdevices must have a minimum of two pathsto ground to split the lightning alongmultiple paths, so the smallest building musthave two downleads minimum. Downleadsfor large buildings may be calculated at 100feet average intervals for the perimeterfootprint of the building, although systemcomponents for special building designelements may necessitate additional downconductors to meet multiple pathrequirements. It is important to calculate thefootprint of the protected perimeter to getthe proper distribution for downleads forridged roofs which include striketerminations only along the apex.Copper and aluminum main cableconductors for lightning protection aredesigned to a smooth weave or rope-laystandard using smaller gauge individualwires. This construction allows a maximumsurface area per unit weight of conductor toaccommodate lightning which travelsquickly on the surface. This constructionalso allows for easier bending and formingof the conductor system along, around andover building construction elements.Exposed conductors are fastened atmaximum three feet intervals to maintain thesystem in place against wind and weather.All strike termination devices must beconnected to the conductors with aminimum of two paths to the groundingsystem. Strike termination devices coveringvarious areas of a structure must beinterconnected to form a single system eitherby roof conductors, at down conductors, orby interconnection of grounding systemelements for different roof levels orprojections. Lightning cable conductorsmay be concealed below or withinconstruction – in attics and wall spaces, or inconcrete pours – because the speed oflightning lowers the potential for heating ofthe conductors to the spark ignitiontemperature of building materials to wellbelow damaging levels.Providing multiple paths for lightningcurrent has the great advantage of loweringthe total energy on any given conductor.This impacts not only conductor sizing, butalso keeps the lightning to our specifiedpaths to minimize side-flashing to internalsystems and lessen potential internalinduction problems. The lightningprotection Standards call for a minimumnumber on the perimeter, but more paths canbe very beneficial in providing a cage ofDownleads or down conductors are theelements of the main conductor system thatgenerally bring the lightning from the rooflevel system to the grounding system. Thismay include cable conductor, or qualifiedcontinuous steel framework of 3/16”8

protection for equipment and people inside.The fact that steel frame constructioncreates the largest number of qualifiedvertical paths interconnected horizontally onmultilevel structures makes its use asdownleads preferred to give an improvedshield against lightning side-effect intrusion.Although cable conductors are required fordownleads in poured concrete construction,the required bonding of the rebar helpscreate a similar network of protection inhigh rise construction projects.means are necessary. The most effectivesystems consist of an extensive wirenetwork laid on the surface of the rocksurrounding the building to which the downconductors are connected. The resistancebetween such an arrangement and earth maybe high, but at the same time, the potentialdistribution about the building issubstantially the same as though it wereresting on conducting soil and the resultingprotective effect is also substantially thesame. The lightning protection groundingelectrode system serves to take the lightninginto whatever soil strata exists, and route itaway from the structure.GroundingProperly made ground connections areessential to the effective functioning of alightning protection system, as they serve todistribute lightning into earth ground. Thisdoes not mean that the resistance of theground connection must be low, but ratherthat the distribution of metal in the earth, orupon its surface in extreme cases, shall besuch as to permit the dissipation of alightning discharge without causing damage.A grounding electrode network will bedetermined largely by the experience andjudgment of the person planning theinstallation with due regard to the minimumrequirements of the Standards, which areintended to cover the ordinary cases that arelikely to be encountered, keeping in mindthat, in general, the more extensive theunderground metal available, the moreeffective the grounding systemLow resistance is desirable but notessential, as may be shown by the extremecases on the one hand of a building restingin moist clay soil and on the other hand by abuilding sitting on bare rock. In the firstcase, if soil is of normal resistivity, theresistance of a proper grounding electrodewould be expected to be less than 50 ohms,and two such connections to ground on asmall rectangular building have been foundby experience to be sufficient. Under thesefavorable conditions, providing adequatemeans to dissipate the energy of a flashwithout chance of serious damage is asimple matter. In the second case, it wouldbe impossible to make a good groundconnection in the ordinary sense of the term,because most kinds of rock are insulating orat least of high resistivity; hence, in order toobtain an effective ground, more elaborateThe grounding arrangement depends on thecharacter of the soil, ranging from singleground rods where soil is deep, to the use ofmultiple electrodes, ground plates, radials,or buried wire networks where soil isshallow, dry, or of poor conductivity.9

Each downlead cable shall terminate to agrounding electrode connection dedicated tothe lightning protection system. Electricalor communication system electrodes mustnot be used in lieu of lightning groundelectrodes. The final product must includethe bonding together of separate groundingelectrodes of different systems.Wherever practical, connections togrounding electrodes should be madeexterior to the foundation wall or far enoughaway to avoid buried footings, pipe caps,etc. Grounding electrodes should beinstalled below the frost line where possible.The materials used for grounding electrodesmust be suitable to any alkaline or acidcomposition of soils for long life.Since splitting the lightning along multiplepaths begins at the strike termination pointand follows through the conductor system toground, different resistance values ofelectrodes can upset this function. An inground loop solves this potential problemand provides an extensive wire network toenhance the grounding system. A groundloop is required for every structureexceeding 60 feet in height. If aninterconnecting loop cannot be installed inthe earth, then it may be placed within theconstruction to fulfill this requirement. Thisground level loop also accommodatesconnection with other building groundedsystems.During a discharge of lightning current on asystem of conductors, the groundingelectrodes are to be thought of as the pointsthrough which the heavy current flowsbetween the strike termination system andthe earth about the structure. Therefore,placement with the view of carrying theflow of current away from the structurein the most advantageous manner isimportant. This will be realized by placinggrounding devices at the outer extremities,such as corners and outside walls of thestructure, and avoiding as far as possible theflow of current under a building. In somecases, particularly when additions to anexisting building or zero property lineconstruction are involved, it may benecessary to place downleads and groundinginside and under a structure.All grounding media in or on a structureshall be interconnected to provide acommon ground potential using main sizelightning conductor. This includes thelightning protection grounding electrodesystem, electric, communication, andantenna system grounds along withmetallic piping systems entering thestructure like water, gas and LPG lines,metal conduits, etc. Interconnection to gaslines shall be made on the customer side ofthe meter to avoid defeating any cathodicprotection of service lines. Where all thesesystems are bonded to a continuous metallicwater line system, only one connection isrequired between the lightning protectiongrounding and the water line. Systeminterconnection may be made at multipleA ground loop encircling a structureinterconnecting all downlead cables at theirbase and/or grounding electrode devices isthe best way to equalize potential for anentire lightning protection system. It isalways possible to have varying resistancevalues for individual grounding electrodeseven on the same structure.10

points near structure entrances for systems,or one hard connection at a ground bar maybe used. Bringing all building groundedsystems to the same potential at grade is thefirst step toward protecting internalcomponents and people from lightning. Itbegins the bonding process against sideflashes from system components to internalbuilding systems.is short enough the arc over or sideflashmay occur through air or building materials,either of which creates the potential for fireor explosion.SideflashVIPotential Equalization (Bonding)ErThe major current carrying components ofthe lightning protection system weredescribed in their earliest form by BenjaminFranklin. Modern techniques for componentmanufacture and designs incorporating thesystem on and in a structure have changedthe system look, but the philosophy behindstrike termination, conduction, andgrounding remains similar – accept thelightning and send it to ground. The mostdramatic changes involved in lightningprotection system design come fromadaptations in how we build and outfit themodern building, or what we might call the“indoor plumbing factor”. The modernbuilding counts metallic piping likeplumbing, sewer, and gas systems, alongwith circuitry for electrical andcommunication systems, which all provideinternal paths for lightning to damagecomponents and bring people closer todanger.VoAn electric field can developbetween the conductor andnearby objects, at differentelectrical potential.Vo Once the field value exceedsa breakdown value, Ebr,sideflash, or arcing, canoccur.Air breakdown:Ebr 1 MV/mSince internal grounded building systemspermeate a structure, this potential exists atroof level, on or in building walls, and evenpotentially below grade. Lightning spreadsout from system grounding electrodes nearthe earth’s surface and can return onmetallic pipes or other grounds back into thebuilding. Alternate paths from interiorgrounded circuitry are not designed to carrylightning current (a fire hazard), andjunctions in metallic pipes are not designedas current carrying dev

The lightning protection Standard # 780 is reviewed on a three-year cycle for updating. NFPA 780 includes lightning protection for typical building construction in Chapter 4 as general requirements for structures. The 780 document covers many specialty constructions from haza