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ADOPTED MANUFACTURER’S INSTALLATION STANDARD
FOR
INSTALLATION OFEARLY STREAMER EMISSION
LIGHTNING ROD SYSTEMS
PROPOSED NFPA 781 - 1994
SUPPLEMENTARY
Report of the Committee on Lightning Protection Systems Using Early Streamer Emission Air Terminals
Andrew J. O’Connor, ChairScience Applications Int’l. Corp., FL
Gerard M. Berger, CNRS-ESE, Gif-Sur-Yvette, FranceBurt J. Bittner, Harris Corp., FLAlexander L. Chaberski, Lightning Research, NYHenry Diaz, MCI Telecommunications, TXThomas P. Dowling, Inst. of Makers of Explosives, DC Rep. Inst. of Makers of ExplosivesN. Floret, Helita SA, Paris, FranceJ.R. Gumley, International Protection Consultants, HobartTAS, AustraliaKenneth P. Heary, Heary Bros. Lightning Protection Co.,NY Rep. Lightning Preventor of America, Inc.Bruce A. Kaiser, Lightning Master Corp., FLEdward A. Lobnitz, Tilden Lobnitz & Cooper Inc., FLDavid E. McAfee, Westinghouse Savannah River Co., SCRobert W. Rapp, National Lightning Protection Corp., CORobert Allan Richardson, Reynolds, Smith & Hills, Inc.,FLMichel Roubinet, Franklin France, Ozoir la Ferriere,FranceTimothy E. Russell, Underwriters Laboratories Inc., ILDavid E. Wilson, Wilson Electrical Sales, OH
ALTERNATES
Marie Bakker, Franklin France, Ozoir la Ferriere, France (Alt. M. Roubinet)Max Goldman, CNRS-Ecole Superieure d’Electricite, Gif-Sur-Yvette, France (Alt. G. Berger)Dale A. Hallerberg, Underwriters Laboratories Inc., IL (Alt. T.E. Russell)Frederick M. Heary, Lightning Preventor of America, Inc.,NY (Alt. K.P. Heary)Arnaud Lefort, Indelec, Douai, France (Alt. N. Floret)
MANUFACTURERS OF EARLY STREAMEREMISSION AIR TERMINALS
Helita SA, Paris, FranceInternational Protection Consultants, Hobart TAS,AustraliaHeary Bros Lightning Protection Co., Inc., NYNational Lightning Protection Corp., COFranklin France, Ozoir la Ferriere, FranceLightning Preventor of America, Inc., NYBritish Lightning Preventor Ltd., Nottingham, EnglandDMC 2, Chennevisres, FranceEFI, Geneva, Switzerland
NONVOTING
Salomon Blitstein, Proteccion Electricia Sa, Buenos Aires,ArgentinaC.G. Invernizzi, E F Int’l SA, Geneva, SwitzerlandEduardo Mariani, Proteccion Electrica SA, Buenos AiresArgentina
Staff Liaison: John M. Caloggero
This list represents the membership at the time theCommittee was balloted the text of this edition. Since thattime, changes in the membership may have occurred.
Committee Scope: This Committee shall have primaryresponsibilities for documents on the protection fromlightning of buildings and other structures and recreationand sports areas, other situations involving danger fromlightning to people or property, utilizing early streameremission air terminals. The protection of electricgenerating, power transmission and distribution systemsis not within the scope of this Committee.
The Supplementary Report of the Committee on LightningProtection Systems Using Early Streamer Emission AirTerminals is presented for adoption.
This Supplementary Report was prepared by theTechnical Committee on Lightning Protection SystemsUsing Early Streamer Emission Air Terminals andproposes for adoption a Supplementary Report whichdocuments its action on the public comments received onthe proposed NFPA 781-1994, Lightning ProtectionSystems Using Early Streamer Emission Air Terminalspublished in the Technical Committee Reports for the1993 Fall Meeting.
The results of the balloting will be found in thesupplementary report.
ADOPTED MANUFACTURER’S INSTALLATION STANDARD
PROPOSED NFPA 781--F93 TCD
The following draft of proposed NFPA 781-1994 incorporates the Committee Actions on the Public Comments which make up theTechnical Committee Documentation and which appear on the preceding pages. The draft is presented only as an aid to thereviewer.
Manufacturer’s Installation Standard proposed NFPA 781Standard for Lightning Protection Systems Using Early Streamer Emission Air Terminals
1994 Edition
NOTICE: An asterisk (*) following the number or letter designating a paragraph indicates explanatory material on that paragraphin Appendix A. Information on referenced publications can be found in Chapter 6 and Appendix H.
Chapter 1 Introduction
1-1 Scope. This standard covers protection from lightning for buildings, structures, recreation and sportsareas, and other situations subject to lightning that pose a danger to people or property by utilizing EarlyStreamer Emission (ESE) air terminals. The protection of electric generating, transmission and distributionsystems and explosives manufacturing buildings and magazines is not within the scope of this standard.
1-2 Purpose. The purpose of this standard is the practical safeguard of persons and property from exposureto lightning through the use of early streamer emission air terminals. This document covers a lightningprotection system that utilizes the influence of early streamer emission air terminals on the formation andpropagation of upward-connecting leaders. An ESE air terminal serves as a preferred strike point and acts tooffer an increased zone of protection from lightning.
1-3 Listed, Labeled and Approved Components. Where fittings, devices or other components required by thisstandard are available as listed or labeled, such components shall be used. Otherwise, such components shallbe approved by the authority having jurisdiction.
Chapter 2 Terms and Definitions
2-1 General Terminology. General terms commonly used in describing lightning protection methods anddevices are defined as follows:
Early Streamer Emission Air Terminals. Air terminals that, by design, trigger the early initiation of an upward-connecting leader as compared to a control air terminal under identical conditions. The ESE air terminal shallexhibit a positive time advantage, and be qualified for listing as described in Chapter 5.
Lightning Protection System. A lightning protection system is a complete system of air terminal(s), conductors,ground terminals, bonding conductors, surge suppression devices and other connections or fittings required tocomplete the system.
2-2 Definitions.
Air Terminal. An air terminal is that component of a lightning protection system that is intended to interceptlightning strokes.
Approved. Acceptable to the “authority having jurisdiction.”
NOTE: The National Fire Protection Association does not approve, inspect or certify any installations, procedures, equipment or materials nor does it approve or evaluate testing laboratories. In determining the acceptability of installations or procedures, equipment or materials, the authority having jurisdiction may
Proposed NFPA 781 -- F93 TCD Chapter 2
base acceptance on compliance with NFPA or other appropriate standards. In the absence of such standards, said authority may require evidence of proper installation, procedure or use. The authority having jurisdiction may also refer to the listings or labeling practices of an organization concerned with product evaluations which is in a position to determine compliance with appropriate standards for the current production of listed items.
Authority Having Jurisdiction. The “authority having jurisdiction” is the organization, office or individualresponsible for “approving” equipment, an installation or a procedure.
NOTE: The phrase “authority having jurisdiction” is used in NFPA documents in a broad manner since jurisdictions and “approval” agencies vary as do their responsibilities. Where public safety is primary, the “authority having jurisdiction” may be a federal, state, local or other regional department or individual such as a fire chief, fire marshal, chief of a fire prevention bureau, labor department, health department, building official, electrical inspector, or others having statutory authority. For insurance purposes, an insurance inspection department, rating bureau, or other insurance company representative may be the “authority having jurisdiction.” In many circumstances the property owner or his designated agent assumes the role of the “authority having jurisdiction”; at government installations, the commanding officer or departmental official may be the “authority having jurisdiction.”
Bonding. An electrical connection between an electrically conductive object and a component of a lightningprotection system that is intended to reduce significantly the potential differences created by lightning currents.
Cable. A conductor formed of a number of wires stranded together.
Chimney. A smoke or vent stack having a flue with a cross sectional area less than 500 sq. in. (0.3 sq. m) anda total height less than 75 ft (23 m).
Conductor, Bonding. A conductor intended to be used for potential equalization between metal bodies and thelightning protection system. Bonding conductors are not designed to carry the main lightning current.
Conductor, Main. A conductor intended to be used to carry lightning currents between air terminals and groundterminals.
Control Air Terminal. The control air terminal shall be 13 mm to 20 mm (0.5 in. to 0.8 in.) diameter solidcopper or copper alloy. The conical tip shall have an included angle no greater than 90 degrees.
Copper-clad Steel. Steel with a coating of copper bonded to it.
Fastener. An attachment to secure the conductor to the structure.
First Return Stroke. After connection of the two opposite leaders (downward, upward), initial phase of highcurrent flow.
Grounded. Connected to earth or to some conducting body that is connected to earth.
Ground Terminal. The portion of a lightning protection system such as a ground rod, ground plate or groundconductor that is installed for the purpose of providing electrical contact with the earth.
Initiation Distance. Distance separating the air terminal tip and the downward leader head at the time of theupward connecting leader initiation.
Initiation Time. The time of inception of the continuous upward propagating leader issued from the air terminal.
Ionization. The multiplication of electric charge in high electric fields, due mostly to collisions of electrons withair molecules.
Proposed NFPA 781--F93 TCD Chapter 2 (a)
Labeled. Equipment or materials to which has been attached a label, symbol or other identifying mark of anorganization acceptable to the “authority having jurisdiction” and concerned with product evaluation thatmaintains periodic inspection of production of labeled equipment or materials and by whose labeling themanufacturer indicates compliance with appropriate standards or performance in a specified manner.
Leader. An electrically conductive propagating channel carrying a high intensity current electric charge andheaded by filamentary discharge streamers.
Downward Leader. A precursor to the main lightning discharge that propagates by steps towards ground from a cloud charge center.
Upward Leader. An upward propagating discharge commencing from a ground point in response to the strong electric field created by an approaching down leader.
Lightning Strike. The entire lightning event, which can consist of one or more lightning strokes.
Lightning Stroke. Lightning strokes are classified according to the direction of development of the first leaderand the polarity of the cloud charge center that discharges. Ninety percent of all lightning strokes are negativedownward strokes from a negative cloud charge center.
Listed. Equipment or materials included in a list published by an organization acceptable to the “authorityhaving jurisdiction” and concerned with product evaluation, that maintains periodic inspection of production oflisted equipment or materials and whose listing states either that the equipment or material meets appropriatestandards or has been tested and found suitable for use in a specified manner.
NOTE: The means for identifying listed equipment may vary for each organization concerned with product evaluation, some of which do not recognize equipment as listed unless it is also labeled. The “authority having jurisdiction” should utilize the system employed by the listing organization to identify a listed product.
Loop Conductor. A conductor encircling a structure that is used to interconnect ground terminals, mainconductors or other grounded bodies.
Metal-Clad Structure. A structure with sides or roof or both covered with metal.
Metal Frame Structure. A structure with electrically continuous structural members of sufficient size to providean electrical path equivalent to that of the lightning conductors covered in this standard.
Point of Attachment. See “Strike Point.”
Shall. Indicates a mandatory requirement.
Should. Indicates a recommendation or that which is advised but not required.
Sideflash. An electrical spark, caused by differences of potential, occurring between conductive metal bodiesor between such metal bodies and a component of the lightning protection system.
Space Charge. Positive or negative charge, or both, distributed in volume due to corona or streamerdischarges.
Spark Gap. Any short air space between two conductors electrically insulated from, or remotely electricallyconnected to each other.
Streamer. A low intensity current that is a filamentary electrical discharge propagating out of corona andpreceding a propagating leader.
Strike Point. The point from which the upward connecting leader originates (point of attachment).
Proposed NFPA 781 --F93 TCD Chapter 2 (b)
Striking Distance. The distance over which final breakdown of the initial stroke to ground or to a groundedobject occurs.
Subsequent Stroke. Repeated discharges that originate at 30 millisecond to 200 millisecond intervals fromalternate cloud charge centers, which utilize the initial lightning path.
Surge Suppresser. A protective device for limiting surge voltages and preventing continued flow of followcurrent while sustaining the ability to repeat these functions.
Test Joint. A disconnecting device used to permit isolated resistance measurements.
Time Advantage. The gain in length of the upward connecting leader of the early streamer emission airterminal. A calculated numerical assignment used to determine a zone of protection.
Zone of Protection. The zone of protection is the space adjacent to a lightning protection system that issubstantially immune to direct lightning flashes.
Metric Units of Measurement. Metric units of measurement in this standard are in accordance with themodernized metric system known as the International System of Units (SI). If a value for a measurement asgiven in this standard is followed by an equivalent value in other units, the first stated value is to be regardedas the requirement. A given equivalent value may be approximate.
Proposed NFPA 781-F93 TCD
Chapter 3 System Design
3-1 General Design Requirements. A lightning protection system using a ESE air terminal(s) shall bedesigned with provisions for inspection and maintenance. Its design should be specified whenever possiblebefore the building is constructed.
Every installation shall be reviewed by a prior study to determine the level of protection needed. (Appendix D provides guidance.)
3-2 Zone of Protection. The positioning of ESE air terminals is determined by the radius of protection, Rp, as afunction of the height, h, of the early streamer emission air terminal above the area to be protected.
The zone of protection is determined by the volume of the cone of the height, h, which has its apex at the top of the ESE air terminal and the radius of which is Rp. (See Figure 3-2.1.)
3-2.1 Radius of Protection. The positioning of the ESE air terminal is determined by the radius of protection,Rp, as a function of the height, h, of the ESE air terminal above the area to be protected.
All structures and buildings that are to be protected shall be located in the zone under a cone of height, h, having its apex at the top of the ESE air terminal and a radius of protection, Rp.
The radius of protection is determined as follows:
Figure 3-2.1 Positioning of the ESE air terminal.
h: Height of the early streamer emission air terminal above the area to be protected.
Rp: Radius of protection of the ESE as function of h obtained using Figures 3-2.2(a), (b) and (c).
3-2.2 Determination of the Radius of Protection, Rp.
The position of the ESE air terminal depends on the radius of protection it provides. The radius of protectioncan be determined by the use of Figures 3-2.2(a), (b) and (c). The figure used depends upon the level ofprotection determined for the structure to be protected as follows:
Level I, structure with low risk: See Figure 3-2.2(a). Level II, standard level of protection: See Figure 3-2.2(b). Level III, structure with high risk: See Figure 3-2.2(c).
Proposed NFPA 781-F93 TCD Chapter 3(a)
Rp is the intersection of the horizontal scale (x axis) by the height at which the ESE air terminal is to beinstalled (y axis), and the curve of Delta L corresponding to the ESE air terminal.
The value of h shall be determined using the equation that follows. The radius of protection, Rp, shall bepermitted to be obtainedby calculation, where h is greater than 16.4 ft (5 m).
Proposed NFPA 781-F93 TCD Chapter 3(b)
Proposed NFPA 781--F93 TCD Chapter 3 (c)
Proposed NFPA 781--F93 TCD Chapter 3 (d)
Proposed NFPA 781--F93 TCD
Chapter 4 Installation of a Lightning Protection System Using ESE Air Terminals
4-1 An ESE lightning protection systems installation consists of the following interconnected parts (SeeFigure 4-1):
(a) One or more ESE air terminal(s) (b) Bonding conductors (c) Down conductor(s) (d) Test joint (e) Grounding system (f) Equipotential bondings (g) Surge voltage suppressors, if applicable.
4-1.1 Materials. Table 4-4.3A and Table 4-4.3B provide minimum sizes and weights for use in early streameremission system installations.
4-2 Air Terminal. ESE air terminals shall be sufficiently rigid to withstand mechanical damage. They shall beof copper, copper alloy, aluminum or stainless steel. The main conductive parts of an ESE air terminal shallhave minimum dimensions in accordance with Table 4-4.3A and Table 4-4.3B.
4-2.1 Installation. Air terminals and their supports shall be mounted in such a way as to withstand thelightning currents, electromotive force, corrosion and effects of weather, heat, humidity, snow and wind.
NOTE: Wind design criteria should be consistent with local building code requirements.
Proposed NFPA 781--F93 TCD Chapter 4 (a)
Proposed NFPA 781--F93 TCD Chapter 4 (b)
Proposed NFPA 781--F93 TCD Chapter 4 (c)
Proposed NFPA 781--F93 TCD Chapter 4 (d)
4-2.2 Where an air terminal or mast is supported by conductive guys, the guys shall be connected from theirpoints of anchorage to the closest down conductor(s) by a conductor equivalent in size to that of the downconductor(s).
4-2.3 The tip of any ESE air terminal shall not be less than 6.6 ft (2 m) above protected open areas andprotected structures including antennas, cooling towers, tanks, roofs and masts.
4-2.4 Open Area. For sports grounds, golf courses, public parks, camping sites, swimming pools and racetracks, the ESE air terminals shall be permitted to be mounted on top of pylons, flagpoles, or lamp standardsor on dedicated supports selected to protect the area best.
4-3 Storage Areas for Flammable and Combustible Liquids or Flammable Gases. Structures containing thesematerials shall be grounded, but grounding alone does not constitute sufficient protection against atmosphericdischarges.
4-4 General. This section is concerned with equalizing potential differences that occur when lightning currentsflow through separately grounded metal bodies and conductors. Proper bonding, grounding and application ofsurge suppression minimizes the possibility of flashover and arcing and their destructive results.
NOTE: For basic theory and design information regarding grounding, bonding and shielding for equipment and facilities, see Department of Defense (D.O.D.) Military Handbook 419A, Volume 1 of 2, Grounding, Bonding and Shielding for Electronic Equipment and Facilities.
4-4.1 Surge Suppression. Devices suitable for protection of the structure shall be installed on electric andtelephone service entrances and on radio and television antenna lead-ins.
NOTE: Electrical systems and utilization equipment within the structure might require further surge suppression. Such protection is not part of this standard. NFPA 70, National Electrical Code, and ANSI/IEEE C-62.1, Surge Arrestors for Alternating Current Systems, provide additional information.
4-4.2 General. In determining the necessity of bonding a metal body to a lightning protection system, thefollowing factors shall be considered:
(a) Bonding is required only if there is likely to be a sideflash between the lightning protection system and another grounded metal body.
(b) The influence of a nongrounded metal body, such as a metal window frame in a nonconductive medium, is limited to its effectiveness as a short circuit conductor if a sideflash occurs and therefore, does not necessarily require bonding to the lightning protection system
(c) Bonding distance requirements depend on a technical evaluation of the number of down conductors and their location, the interconnection of other grounded systems, the proximity of grounded metal bodies to the down conductors, and the flashover medium (e.g., air or solid materials).
(d) Metal bodies located in a steel frame structure shall be permitted to be inherently bonded through construction and further bonding shall not be required.
(e) Metal antenna masts or supports located on a protected structure shall be bonded to the lightning protection system using main size conductors and listed fittings unless they are within a zone of protection.
Proposed NFPA 781--F93 TCD Chapter 4 (e)
4-4.3 Materials. Conductors used for the bonding of grounded metal bodies, or isolated metal bodies,requiring connection to the lightning protection system shall be sized in accordance with the bonding conductorrequirements in Table 4-4.3A and Table 4-4.3B
Table 4-4.3A Class I Material Requirements Under 60 ft. in Height
Type of Conductor Copper Aluminum Standard Metric Standard Metric
Air Terminal, Solid Min. Diameter 3/8 in. 9.5 mm 1/2 in. 12.7 mmAir Terminal, Tubular Min. Diameter 5/8 in. 15.9 mm 5/8 in. 15.9 mm Min. Wall Thickness 0.033 in. 0.8 mm 0.064 in. 1.6 mm Min. Size ea. Strand 17 AWG 14 AWGMain Conductor, Cable Wgt. per Length 187 lb/1000 ft 278 g/m 95 lb/1000 ft 141 g/m Cross Sect. Area 57,400 CM 29 mm 98,600 CM 50 mmMain Conductor, Thickness 0.051 in. 1.30 mm 0.064 in. 1.63 mmSolid Strip Width 1 in. 25.4 mm 1 in. 25.4 mmBonding, Conductor, Min. Size ea. Strand 17 AWG 14 AWGCable (solid or stranded) Cross Sect. Area 26,240 CM 41,100 CMBonding, Conductor, Thickness 0.051 in. 1.30 mm 0.064 in. 1.63 mmSolid Strip Width 1/2 in. 12.7 mm 1/2 in. 12.7 mm
Table 4-4.3B Class II Material Requirements Over 60 ft. in Height
Type of Conductor Copper Aluminum Standard Metric Standard Metric
Air Terminal, Solid Min. Diameter 1/2 in. 12.7 mm 5/8 in. 15.9 mm Min. Size ea. Strand 15 AWG 13 AWGMain Conductor Cable Wt. per Length 375 lb/1000 ft 558 g/m 190 lb/1000 ft 283 g/m Cross Sect. Area 115,000 CM 58 mm2 192,000 CM 97 mm2
Solid Strip Thickness 0.078 in. 2 mm Width 1/2 in. 12.7 mmBonding Conductor, Min. Size ea. Strand 17 AWG 14 AWGCable (solid or stranded) Cross Sect. Area 26,240 CM 41,100 CMBonding Conductor, Thickness 0.051 in 1.30 mm 0.064 in. 1.63 mmSolid Strip Width 1/2 in. 12.7 mm 1/2 in. 12.7 mm
4-4.4 Materials. Horizontal loop conductors used for the interconnection of lightning protection systemdownlead conductors, ground terminals or other grounded media shall be sized no smaller than that requiredfor the main lightning conductor. (See Table 4-4.3A and Table 4-4.3B).
4-4.5 Ground Level Potential Equalization. All grounded media in and on a structure shall be connected to thelightning protection system within 12 ft (3.6 m) of the base of the structure in accordance with 4-7.6. Forstructures exceeding 60 ft (18 m) in height, the interconnection of the lightning protection system groundterminals and other grounded media shall be in the form of a ground loop conductor.
NOTE: For structures 60 ft (18 m) or less in height a loop conductor should be provided for the inter- connection of all ground terminals and other grounded media. Regardless of the building height, ground loop conductors should be installed underground in contact with earth. Ground level potential equalization allows use of a ground ring electrode as a ground loop conductor. A ground ring electrode conforming to 4-7.4 may be permitted to be utilized for the ground loop conductor.
Proposed NFPA 781--F93 TCD Chapter 4 (f)
4-5 Metal Bodies. Certain metal bodies located outside or inside a structure contribute to lightning hazards,because they are grounded or assist in providing a path to ground for lightning currents. Such metal bodiesshall be bonded to the lightning protection system in accordance with this section and the following twosections. (See Appendix B for a technical discussion of lightning protection potential equalization bonding.)
4-5.1 Long, Vertical Metal Bodies
(a) Steel Frame Structure. Grounded and ungrounded metal bodies exceeding 60 ft (18 m) in vertical length shall be bonded to structural steel members as near as practical to their extremities, unless inherently bonded through construction at these locations.
(b) Reinforced Concrete Structures where the Reinforcement is Interconnected and Grounded in Accordance with 4-7.10. Grounded and ungrounded metal bodies exceeding 60 ft (18 m) in vertical length shall be bonded to the lightning protection system as near as practical to their extremities, unless inherently bonded through construction at these locations.
(c) Other Structures. Bonding of grounded or ungrounded long, vertical metal bodies shall be as specified in 4-5.2
4-5.2 Grounded Metal Bodies. This paragraph covers the bonding of grounded metal bodies not covered in4-5.1. Where grounded metal bodies have been connected to the lightning protection system at only oneextremity, the formula that follows shall be used to determine if additional bonding is required. Branches ofgrounded metal bodies connected to the lightning protection system at their extremities shall require bonding tothe lightning protection system in accordance with the formula that follows if they change vertical directionmore than 12 ft (3.6 m).
Exception: Where such bonding has been accomplished either inherently through construction or by physical contact betweenelectrically conductive materials, no additional bonding connection shall be required.
(a) Structures Over 40 ft (12 m) in Height. Grounded metal bodies shall be bonded to the lightning protection system where located within a distance, D, as determined by the following formula:
D = h/6n . Km
Where “h” is the vertical distance between the bond being considered and the nearest lightning protection system bond.
The value of “n” is related to the number of down conductors that are spaced at least 25 ft (7.6 m) apart and located within a zone of 100 ft (30 m) from the bond in question and is calculated as follows:
(1) Where bonding is required within 60 ft (18 m) from the top of any structure:
n = 1, where there is only one down conductor in this zone
n = 1.5, where there are only two down conductors in this zone
n = a minimum of 2.25, where there are three or more down conductors in this zone.
Km = 1.0 if flashover is through air or 0.50 if flashover is through dense material such as concrete, brick, wood, etc.
Proposed NFPA 781--F93 TCD Chapter 4(g)
(2) Where bonding is required below a level 60 ft (18 m) from the top of a structure, n is the total number of down conductors in the lightning protection system.
(b) Structures 40 ft (12 m) and Less in Height. Grounded metal bodies shall be bonded to the lightning protection system where located within a distance, D, as determined by the formula:
D = h/6n . Km
Where “h” is either the height of the building or the vertical distance from the nearest bonding connection from the grounded metal body to the lightning protection system and the point on the down conductor where the bonding connection is being considered.
The value of “n” is related to the number of down conductors that are spaced at least 25 ft (7.6 m) apart and located within a zone of 100 ft (30 m) from the bond in question and is calculated as follows:
n = 1, where there is only one down conductor in this zone
n = 1.5, where there are only two down conductors in this zone
n = a minimum of 2.25, where there are three or more down conductors in this zone
Km = 1.0 if the flashover is through air or 0.50 if flashover is through dense material such as concrete, brick, wood, etc.
(c) Isolated (Nongrounded) Metallic Bodies. An isolated metallic body, such as a metal window frame in a nonconducting medium, that is located close to a lightning conductor and to a grounded metal body influences bonding requirements only if the total of the isolated distances between the lightning conductor and the isolated metal body, and between the isolated metal body and the grounded metal body, is equal to or less than the calculated bonding distance.
A bonding connection is required where the total of the shortest distance between the lightning conductor andthe isolated metal body and the shortest distance between the isolated metal body and the grounded metalbody is equal to or less than the bonding distance as calculated in accordance with 4-5.2 Bondings shall bemade between the lightning protection system and the grounded metal body and shall not be required to runthrough or be connected to the isolated metal body.
NOTE: In addition to the bonding of metal bodies, surge suppression should be provided to protect power, communications and data lines from dangerous overvoltages and sparks caused by lightning strikes.
4-5.3 Use of Aluminum. Aluminum systems shall be installed in accordance with other applicable sectionsand with the following requirements.
(a) Aluminum lightning protection equipment shall not be installed on copper roofing materials or other copper surfaces or where exposed to the runoff from copper surfaces.
(b) Aluminum materials shall not be used where they come into direct contact with earth. Fittings used for the connection of aluminum down conductors to copper or copper clad grounding equipment shall be of the bimetallic type. Bimetallic connectors shall be installed not less than 18 in. (457 mm) above earth level.
Proposed NFPA 781--F93 TCD Chapter 4(h)
(c) Connectors and fittings shall be suitable for use with the conductor and the surfaces on which they are installed. Bimetallic connectors and fittings shall be used for splicing or bonding dissimilar metals.
(d) An aluminum conductor shall not be attached to a surface coated with alkaline-base paint, embedded in concrete or masonry, or installed in a location subject to excessive moisture.
4-5.4 Corrosive Protection. Precautions shall be taken to provide the necessary protection against anytendency towards deterioration of any lightning protection components due to environmental conditions.
4-5.5 Mechanical Damage or Displacement. Any part of a lightning protection system that is subject tomechanical damage or displacement shall be protected with a protective molding or covering. If metal pipe ortubing is used around the conductor, the conductor shall be connected electrically to the pipe or tubing at bothends.
4-5.6 Substitution of Metals. Metal parts of a structure such as eave troughs, down spouts, ladders, chutes, orother metal parts shall not be substituted for the main lightning conductor. Likewise, metal roofing or sidinghaving a thickness of less than 3/16 in. (4.8 mm) shall not be substituted for main lightning conductors.
4-5.7 “U” or “V” Pockets. Conductors shall maintain a horizontal or downward coursing, free from “U” or “V”(down and up) pockets or shall not rise at a rate exceeding 3 ft (76.2 mm) to connections with groundterminals.
4-5.8 Conductor Bends. No bend of a conductor shall form an included angle of less than 90 degrees, norshall it have a radius of bend less than 8 in. (203 mm).
4-5.9 Conductor Supports. Conductors shall be permitted to be coursed through the air without supports for adistance of 3 ft (0.9 m) or less. Conductors that are coursed through air for longer distances shall be providedwith a positive means of support that prevents damage or displacement of the conductor.
4-5.10 Down Conductors Entering Corrosive Soil. Down conductors entering corrosive soil shall be protectedagainst corrosion by a protective covering beginning at a point 3 ft (0.9 m) above grade level and extending forits entire length below grade.
4-6 Down Conductors
4-6.1 A single down conductor shall be permitted to be used where the length of run does not exceed 65.5 ft(20 m).
4-6.2 If the horizontal length is greater than the vertical length, two down conductors shall be required andseparated as far apart as is possible.
(a) For lengths greater than 65.5 ft (20 m), two conductors shall be required.
(b) For masts, a single down conductor only shall be required.
(c) If electrically conductive, the mast shall be permitted to be used as the down conductor.
(d) If electrically continuous, structural components shall be permitted to be used as the down conductor.
(e) Test joints shall be provided for each down conductor. They shall be accessible and located as near as practicable to the ground termination. [See Figure 4-6.2(c)].
4-6.3 Masonry Anchors. Masonry anchors used to secure lightning protection materials shall have a minimumoutside diameter of 1/4 in. (6.4 mm) and shall be set with care. Holes made to receive the body of the anchor
Proposed NFPA 781--F93 TCD Chapter 4(i)
shall be of the correct size, made with the proper tools and preferably made in brick, stone or other masonryunit rather than in mortar joints. When the anchors are installed, the fit shall be tight against moisture, thusreducing the possibility of damage due to freezing.
4-6.4 Connector Fittings. Connector fittings shall be used at all “end-to-end,” “tee,” or “Y” splices of lightningconductors. They shall be attached so as to withstand a pull test of 200 lb (890 N). Fittings used for requiredconnections shall be of the bolted, welded or high compression type.
4-6.5 Conductor Fasteners. Conductors shall be fastened securely to the structure upon which they are placedat intervals not exceeding 3 ft (1 m). The fasteners, attached by nails, screws, bolts or adhesives, asnecessary, shall not be subject to breakage and shall be of the same material as the conductor or of a materialas resistant to corrosion as that of the conductor. No combination of materials shall be used that forms anelectrolytic couple of such nature that, in the presence of moisture, corrosion is accelerated.
4-7 Grounding. A study shall be conducted to engineer a grounding system to a maximum of 10 ohmsresistance in accordance with IEEE fall of potential ANSI/IEEE 141, Recommended Practice for Electric PowerDistribution for Industrial Power Plants, method. The material for the grounding system shall be selected to besuitable for the soil conditions and other side factors such as cathodic protection.
4-7.1 Ground Electrodes. Ground electrodes shall be of copper, copper-clad steel, electrolytic and stainlesssteel, or both, or galvanized steel and shall be utilized so as to achieve the 10 ohm maximum resistance toground.
4-7.2 Ground Rod Terminations. The down conductor shall be attached to the ground rod by welding(including exothermic welding), brazing or clamping. Clamps shall be suitable for direct soil burial.
(a) Each down conductor shall terminate at a ground terminal. The design, size, depth and number of ground terminals used shall comply with 4-7.1.
(b) Ground terminals located under slabs or in crawl spaces shall be installed as near as practicable to the outside perimeter of the structure.
(c) Electrical system grounding electrodes shall not be used in lieu of lightning ground rods. This provision shall not prohibit the required bonding together of grounding electrodes of different systems.
NOTE 1: For further information, see NFPA 70, National Electrical Code which contains detailed information on the grounding of electrical systems.
NOTE 2: Research demonstrates that stainless steel is very susceptible to corrosion in many soil conditions. It is critical that a proper soil analysis be performed before this type of rod is used.
NOTE 3: The ground rod material should be selected only after thorough investigation of soil type, corrosive effects on various metals and other factors to ensure longevity of the rod system.
NOTE 4: Ground rod or ground ring electrodes should be installed below the frost line, where practicable.
4-7.3 Concrete-encased Electrodes. Concrete-encased electrodes shall be used only in new construction.The electrodes shall be located near the bottom of a concrete foundation or footing that is in direct contact withthe earth and shall be encased by not less than 2 in. (50.8 mm) of concrete. The encased electrode shallconsist of the following:
(a) Not less than 20 ft (6.1 m) of No. 2 AWG bare copper conductor.
Proposed NFPA 781--F93 TCD Chapter 4(j)
(b) An electrode consisting of at least 20 ft (6.1 m) of one or more steel reinforcing bars or rods not less than 1/2 in. (12.7 mm) in diameter that have been effectively bonded together by either welding or overlapping 20 diameters and that are securely wire-tied.
4-7.4 Ground Ring Electrode. A ground ring electrode encircling a structure shall be in direct contact withearth at a depth of not less than 2-1/2 ft (762 mm) or encased in a concrete footing in accordance with 4-7.3. Itshall consist of not less than a continuous length of 20 ft (6.1 m) of No. 2 AWG bare copper conductor.
4-7.5 Concrete-encased Electrode Terminations. The down conductors(s) shall be permanently attached tothe concrete-encased electrode system by welding ( including exothermic welding), brazing or clamps.
4-7.6 Common Grounding. All grounding media in or on a structure shall be interconnected to provide acommon ground potential. This shall include lightning protection, electric service, and telephone and antennasystem grounds, as well as underground metallic piping systems. Such piping systems include water service,well casings located within 25 ft (7.6 m) of the structure, gas piping, underground conduits, undergroundliquefied petroleum gas piping systems, etc. Interconnection to a gas line shall be made on the customer’sside of the meter. Main size lightning conductors shall be used for interconnecting these grounding systems tothe lightning protection system.
4-7.7 Common Ground Bonding. If electric, telephone, or other systems are bonded to a metallic water pipe,only one connection from the lightning protection system to the water pipe system shall be required, providedthat the water pipe is electrically continuous between all systems. If the water pipe system is not electricallycontinuous due to the use of plastic pipe sections or for other reasons, the nonconductive sections shall bebridged with main size conductors, or the connection shall be made at a point where electrical continuity isensured.
Proposed NFPA 781--F93 TCD Chapter 4(k)
Proposed NFPA 781--F93 TCD Chapter 4
4-7.8 Structural Steel Systems
4-7.8.1 General. The steel framework of a structure shall be permitted to be utilized as the main conductor ofa lightning protection system if it is electrically continuous or is made electrically continuous.
(a) Steel Structure Terminals. Ground terminals shall be connected to approximately every other steel column around the perimeter of the structure at intervals averaging not more than 60 ft (18 m). Connections shall be made near the base of the column, with a surface contact area of not less than 8 sq. in. (5200 mm2), or by welding or brazing the ground terminal conductor directly to the column. Bonding plates shall have bolt tension cable connectors and shall be bolted, welded or brazed securely to the column so as to maintain electrical continuity.
(b) Connections to Framework. Conductors shall be connected to cleaned areas of the structural steel framework by the bonding of plates having a surface contact area not less than 8 sq. in. (5200 mm2) or by welding or brazing. Bonding plates shall have bolt pressure cable connectors and shall be bolted, welded or brazed securely to the structural steel framework so as to maintain electrical continuity.
4-7.8.2 Air Terminals. Air terminals shall be connected to the structural steel framing by direct connection,by use of individual conductors routed through the roof or parapet walls to the steel framework, or by use of anexterior conductor that interconnects all air terminals and that is connected to the steel framework. Wheresuch an exterior conductor is used, it shall be connected to the steel framework of the structure at intervals notexceeding 100 ft (30m).
4-7.9 Metal Bodies. Certain metal bodies located outside or inside a structure contribute to lightning hazardsbecause they are grounded or assist in providing a path to ground for lightning currents. Such metal bodiesshall be bonded to the lightning protection system in accordance with 4-5.2.
4-7.10 Concealment in Steel Reinforced Concrete. Conductors or other components of the lightning protectionsystem concealed in steel reinforced concrete units shall be connected to the reinforcing steel. Concealeddown conductors shall be connected to the vertical reinforcing steel. Roof conductors or other concealedhorizontal conductor runs shall be connected to the reinforcing steel at intervals not exceeding 100 ft (30 m).
4-7.11 Down Conductors and Structural Columns. Down conductors coursed on or in reinforced concretecolumns or on structural steel columns shall be connected to the reinforcing steel or the structural steelmember at its upper and lower extremities. In the case of long, vertical members, an additional connectionshall be made at intervals not exceeding 200 ft (60 m). Such connections shall be made using listed clamps orlisted bonding plates or by welding or brazing. The use of PVC conduit or other nonmetallic chase does notnegate the need for these interconnections unless sufficient separation is provided to satisfy the bondingrequirements of Sections 4-4 and 4-5. Where this is not the case, provisions shall be made to ensure therequired interconnection of these parallel vertical paths.
PROPOSED NFPA 781--F93 TCD
CHAPTER 5 ESE AIR TERMINAL QUALIFICATIONS
5-1 General. These tests evaluate the ESE air terminal in comparison to a control air terminal of the sameheight under the same ambient conditions. This evaluation provides a value for the “Delta T” of the ESE airterminal, which then is used in the calculation of the Delta L and radius of protection.
5-2 Required Tests. The following tests shall be performed:
(a) 100 Operation test (b) Field confirmation test.
5-3 100 Operation Test. Two alternate methods are acceptable for the 100 operation test, single rod test, ordual rod test, one of which shall be used. The following conditions shall apply to both methods:
(a) The ambient temperature in the test cell shall not vary more than 9oF (5oC) during the test.
(b) The absolute humidity shall be not less than 10 g/m3 throughout the test and shall not very by more than 10 percent during the test.
(c) The control air terminal shall be 18 mm to 20 mm (0.7 in. to 0.8 in.) diameter solid copper or copper alloy freestanding, 1 meter (39 in.) in length. The conical tip shall have an included angle of not greater than 90 degrees.
(d) Each air terminal shall be mounted vertically under a horizontal upper plate.
(e) The tip of each air terminal shall be located not less than 1 m (39 in.) above the floor of the test cell. The distance between the air terminal tip and the upper plate shall be not less than 2 m (79 in.).
(f) The upper plate shall be solid, mesh or interconnected conductors. Suitable preparation of the upper plate (such as polishing) shall be made so that no down leader initiates from it during the test. It shall overhang the air terminal(s) by a distance not less than the distance between the air terminal tip and the upper plate.
(g) Each air terminal shall be grounded.
(h) With the air terminal removed, the upper plate shall be negatively biased at 10 kV/m to 20 kV/m. Distance for this bias is from the upper plate to the floor of the test cell. The bias voltage shall not vary by more than 5 percent during the test.
Exception: Proper biasing of the upper plate shall be permitted to be determined with the air terminal in place by producing a negative air terminal corona current of 0.1 to 10 microamperes. The bias voltage shall not vary by more than 5 percent during the test.
(i) One hundred switching impulses shall be applied to the upper plate. The rate of rise of the voltage immediately before the breakdown shall be 108 V/m/s to 109 V/m/s. Breakdown shall occur when the electric field is 100 kV/m to 500 kV/m. The rate of impulses shall not exceed one per minute and shall remain as constant as practical.
(j) The tip of the air terminal(s) shall be positioned at the same height ± 0.00787 in. (± 0.2 mm) for each shot.
Proposed NFPA 781--F93 TCD Chapter 5(a)
5-3.1 Single Rod Test. Both the ESE air terminal and the control air terminal shall be tested independently,25 shots at a time, then exchanged, for a total of 100 shots on each air terminal.
The upward leader initiation time shall be determined for each breakdown and averaged for the 100 shots. The Delta T for the ESE air terminal is calculated as follows:
Delta T = [Tic-Tiese]
Delta L = 106 m/s x [Delta T]
106 m/s = the velocity of lightning
Tic = Average initiation time for the control air terminal
Tiese = Average initiation time for the ESE air terminal
The result shall be rounded to the nearest whole number and reduced by 3.
5-3.2 Dual Rod Test. * The dual rod test shall be preceded by a check of the parallelism of the upper plate.For the dual rod test, both the ESE air terminal and the control air terminal shall be mounted in the test cell,separated by a distance not less than either of their heights. The air terminals shall be grounded identicallyand interconnected. Fifty shots shall be made and then the air terminal positions shall be exchanged for theremaining 50 shots. The difference in upward leader initiation time shall be determined for each impulse andaveraged for the 100 shots. The Delta T for the ESE air terminal is calculated as follows:
Delta T = [Tic - Tiese]
Tic = Average initiation time for the control air terminal
Tiese = Average initiation time for the ESE air terminal
Delta L = 106 m/s x Delta T Formula B
Delta T = Average difference in initiation time
The result shall be rounded to the nearest whole number and reduced by 3.
5-4 Field Confirmation Test. This controlled, instrumented, 20-event field test is performed to confirm theDelta L determined during the 100 operation lab test. Due to the length of time necessary for outdoor tests,this test shall not be required for initial qualification of the ESE air terminal but shall begin at that time.
The upward leader initiation time shall be compared for the ESE air terminal and a control air terminal for a minimum of 20 events. The Delta L for the ESE air terminal shall be determined in accordance with 5-3.2. The Delta L determined at the conclusion of the field confirmation test shall supersede the Delta L determined from the 100 operation lab test.
The following conditions shall apply to this test:
(a) The ESE air terminal and control air terminal shall be mounted vertically outdoors. (b) The ESE air terminal and control air terminals shall be the same height with their tips not less than 8 m (26 ft.) above the ground. (c) The control air terminal shall be the same as specified in Section 5-3 (c).
Proposed NFPA 781--F93 TCD Chapter 5 (b)
(d) Each air terminal shall be grounded identically and interconnected.(e) Upward emission currents in excess of 10 amperes shall be indicative of upward leader initiation.When both ESE and control air terminals create upward leaders, Delta T shall be the time advantage measured at the points where each achieves emission levels of 10 amperes. In cases where the control air terminal does not initiate an upward leader, Delta T shall be the difference in time between commencement of the return stroke and the prior achievement of 10 ampere emission from the ESE terminal. Delta L is obtained by inserting Delta T in formula B.
Chapter 6 Referenced Publications
6-1 The following documents or portions thereof are referenced within this standard for informational purposesonly and thus are not considered part of the requirements of this document. The edition indicated for eachreference is the current edition as of the date of the NFPA issuance of this document.
6-1.1 NFPA Publications. National Fire Protection Association, 1 Batterymarch Park, P.O. Box 9101,Quincy, MA 02269-9101.
NFPA 70, National Electrical Code, 1993 edition.
NFPA 780, Lightning Protection Code, 1992 edition.
6-1.2 ANSI/IEEE Publications. Institute of Electrical and Electronic Engineers, Inc., 345 East 47th St.,New York, N Y.
ANSI/IEEE C-62.1-1984, Surge Arrestors for Alternating Current Systems
ANSI/IEEE 141-1986, Recommended Practice for Electric Power Distribution for Industrial Power Plants
6-1.3 U.S. Government Publication. Superintendent of Documents, U.S. Government Printing Office,Washington, D C 20401.
MIL-HDBK 419A, Volume 1 of 2, Grounding, Bonding and Shielding for Electronic Equipment and Facilities, Naval Publications and Forms Center, 700 Robbius Ave., Philadelphia, PA 19111.
Proposed NFPA 781--F93 TCD Appendix A
Appendix A Explanatory Material
This appendix is not a part of the requirements of this NFPA document, but is included for informational purposes only.
A-5-3.2 The check of the parallelism of the upper plate may be permitted to be performed by mounting twoidentical control air terminals in the test cell and subjecting them to a minimum of 20 impulse shots. When thebreakdown is divided equally between the two air terminals, the upper plate is considered to be parallel to thetips of the air terminals.
Appendix B Guide for Personal Safety during Thunderstorms with Suggestionsfor Appropriate Medical Response in the Event of a Lightning Strike
The purpose of this appendix is to furnish a guide for personal safety during thunderstorms. A review ofcommon medical problems initially encountered with a lightning strike and appropriate and basic resuscitativetherapy are discussed.
B-1 Personal Conduct.
B-1.1 Do not go outdoors or remain outside during thunderstorms unless it is necessary. Seek shelter in thefollowing locations:
(a) Dwellings of other buildings that are protected against lightning (b) Underground shelters such as subways, tunnels and caves (c) Large metal frame buildings (d) Large unprotected buildings (e) Enclosed automobiles, buses and other vehicles with metal tops and bodies (f) Enclosed metal trains and street cars (g) Enclosed metal boats and ships (h) Boats that are protected against lightning (I) City streets that are shielded by nearby buildings.
B-1.2 If possible, avoid the following locations, which offer little or no protection from lightning:
(a) Small, unprotected buildings, barns, sheds, etc. (b) Tents and temporary shelters (c) Automobiles (nonmetal top or open) (d) Trailers (nonmetal or open).
B-1.3 Certain locations are extremely hazardous during thunderstorms and should be avoided if at all possible.Approaching thunderstorms should be anticipated and the following locations and situations should be avoidedwhen storms are in the immediate vicinity:
(a) Hilltops and ridges (b) Areas on tops of buildings (c) Open fields, athletic fields, golf courses (d) Parking lots and tennis courts (e) Swimming pools, lakes and seashores (f) Areas near wire fences, clotheslines, overhead wires and railroad tracks (g) Standing beneath isolated trees (h) Use of or contact with electrical appliances, telephones and plumbing fixtures (i) Wearing metal spiked shoes; if wearing them, remove them.
Proposed NFPA 781--F93 TCD Appendix B
B-1.4 In the locations specified above, it is especially hazardous to be riding in or on any of the followingduring lightning storms:
(a) Open tractors or other farm machinery operated in open fields (b) Golf carts, scooters, bicycles or motorcycles (c) Automobiles (nonmetal top or open).
B-1.5 It might not always be possible to choose a location that offers good protection from lightning. Followthese rules when there is a choice in selecting locations:
(a) Seek depressed areas; avoid hilltops and high places. (b) Seek dense woods; avoid isolated trees. (c) Seek buildings, tents and shelters in low areas; avoid unprotected buildings and shelters in high areas. (d) If hopelessly isolated in an exposed area and hair stands on end, indicating that lightning is about to strike, squat down with feet close together, and extend arms to the side (90 degrees at the shoulder).
Proposed NFPA 781--F93 TCD Appendix C
Appendix C Modeling of the Zone of Protection for Lightning Protection SystemsUsing ESE Air Terminals
C-1 Attachment Process.
C-1.1 Determination of the Lightning Strike Point. The overhead arrival of a thunder cloud produces anincrease in the electric field observed at ground level. This electric field can exceed 5kV/m (5kV/3.3 ft), givingrise to corona discharges [see Figure C-1.1(a)] on ground point irregularities.
The lightning stroke is initiated by the formation within the storm cloud of a downward leader that advances in steps towards the ground.
The downward leader carries electric charges causing the field observed at the ground to increase in an exponential manner. The result is that ground points start to exhibit coronas [see Figure C-1.1(b)] that progressively convert into streamers and into upward propagating leaders [see Figure C-1.1(c)] until the two leaders meet [see Figure C-1.1(d)].
As the upward leader progresses, it influences the path of the downward leader [see Figure C-1.1(c)]. Both downward and upward leaders converge until interception occurs [see Figure C-1.1(d)]. The electrical conducting path between cloud and ground then is completed, and the main discharge or return stroke occurs, followed sometimes by one or several subsequent strokes.
Several upward leaders might be initiated from structural projections. The first upward leader to meet the downward leader designates the strike point.
C-1.2 Leader Velocity. Experimental data from nature show that the mean values of the velocities of theupward leader, Vu, and downward leader, Vd, are comparable during the attachment process.
The ratio of the velocities, Vu/Vd, of this natural phenomenon generally is close to 1 (between .9 and 1.1).
We assume Vu = Vd = V = 106 m/s:
Where:
Vu = Velocity of the upward leader Vd = Velocity of the downward leader V = Velocity of lightning 106 m/s.
Proposed NFPA 781--F93 TCD Appendix C(a)
C-2 Advantage in Protection of the ESE Air Terminal.
C-2.1 Gain in Time of Initiation. An ESE air terminal is designed to reduce significantly the statistical timelag associated with the upward leader attachment process. It displays a time advantage when tested incomparison with a pointed vertical rod under identical conditions.
The upward connecting leader issued from the ESE air terminal is the first one to meet the downward leader. The difference in time of initiation is expressed by Delta T, determined during the ESE qualification tests. (See Chapter 5.)
C-2.2 Gain in Length of the Upward Leader. At the moment of initiation of the upward leader at the tip of apointed vertical rod, the head of the downward leader is positioned at a certain height above the ground. Atthat same moment, in the case of an ESE air terminal, the head of the upward leader, traveling at velocity, Vu,is at the distance Vu. Delta T, Delta T being the proper gain in time of initiation of the ESE air terminal. DeltaL is the gain in length of the upward connecting leader of the ESE air terminal.
The increase in zone of protection is determined from the model of protection outlined in the following paragraphs.
C-3 Model of Protection: The Volume of Capture. The calculation of the volume of capture of incidentdownward leaders is adapted from the rolling sphere concept.
The termination of a flash to a specified point depends upon the dual requirements of approach by the descending leader to within at least the initiation distance together with entry by the downward lead (or of a branch of the downward leader) into an effective collection (capture) volume within which interception of the two leaders can take place.
C-3.1 Radius of Protection of a Single Vertical Rod. For a given value of peak current, a spherical surface, S,centered at point A, can be drawn with a radius equal to the initiation distance, D.
Point B is part of S and another surface, S, locus of points such as AB = BC. Any downward leader entering S between verticals passing through A and B will terminate at point A.
Proposed NFPA 781--F93 TCD Appendix C(b)
Where:
Proposed NFPA 781--F93 TCD Appendix C(c)
C-3.2 Radius of Protection of an ESE air terminal. In the case of an ESE air terminal, point B is created froma similar geometric construction as shown in Figure C-3.2(a).
Where:
D = Initiation distance∆L = Gain in length of the upward leader, defined by ∆L = V x ∆T h = Height of the tip of the ESE air terminal above the area to be protectedRp = Radius of protection of the ESE air terminal
h = Height between the tip of the ESE air terminal and the areas to be protected. D = Initiation distance from 4-2.3 ∆L = The assigned numerical value for the ESE air terminal Rp = Radius of protection
Proposed NFPA 781--F93 TCD Appendix C(d)
Where elevated structures penetrate the electric field below a thunderstorm, they, in effect, raise the ground.This creates a nonlinearity in the electric field strength near the upper surfaces of the structure. Lines of equalpotential are compressed severely in such cases.
There is a rapid return to linear conditions as height increases above the structure. In positioning an ESE airterminal, its mounting height should be such that it substantially penetrates the area of nonlinearity. For thisreason, a minimum mounting height of 2m (0.61M) is specified, with a return to more natural conditions atheights above 5m (1.525M). The protected radii for the various protection levels and Delta L ratings have beenmodified accordingly.
C-3.3 Examples of typical installations using ESE air terminals.
Proposed NFPA 781--F93 TCD Appendix C(e)
Proposed NFPA 781--F93 TCD
Appendix D Risk Assessment Guide
D-1 Introduction. This lightning risk assessment guide is prepared to assist in the analysis of various criteriato determine the risk of loss due to lightning and the need for protection. As a guide, it is not possible to coverevery special design element that might render a structure more or less susceptible to lightning damage.
In special cases, personal or economic factors can be very important and should be considered in addition tothe assessment made using this guide.
Structures with inherent risks usually warrant the highest possible class of lightning protection system, andrecommendations for protecting such structures might be provided in this appendix.
For all other structures, the level of protection required by this standard is applicable and the only questionremaining is whether protection is necessary.
The need for protection is often self-evident, as in the following examples:
(a) Where large numbers of people congregate. (b) Where essential public services are concerned. (c) Where the area is one in which lightning is prevalent. (d) Where there are very tall or isolated structures. (e) Where there are structures containing explosive or flammable contents.
However, there are many cases in which the need for protection is not always clear or obvious. The variousfactors affecting the risk of being struck and the consequences of a strike are discussed in this guide andshould help address such situations. Some factors cannot be assessed, and these may override all otherconsiderations. For example, the desire to avoid all risk to life or to provide the occupants of a building withcomplete assurance of safety might require the use of protection , even though there is no compelling need.No guidance can be provided in such matters, but an assessment can be made that considers the exposurerisk and the following factors:
(a) Nature of the structure’s construction (b) Value of its contents (c) Life occupancy (d) Environment of the structure location (e) External consequences of possible damage to the structure.
The location of the structure in its environment and its overall height are taken into account in the calculation ofthe exposure risk.
D-2 Collection Area and Estimation of Exposure Risk. The probability of a structure being struck by lightningduring any one year is the product of the “lightning flash density” and the “collection area” of the structure.
The lightning flash density, Ng, is the number of flashes to ground per km2 per year. Values of Ng can varysignificantly. The recent site survey network provides more detailed and current data on ground strike densityfor the U.S. and its territories.
Proposed NFPA 781--F93 TCD Appendix D(a)
Figure D-1 Statistics for continental United States showing mean annual number of days with thunderstorms.The highest frequency is encountered in south central Florida. Since 1894, the criteria for recording ofthunderstorms has been defined as the local calendar day during which thunder was heard. A day duringwhich thunderstorms occur is so recorded, regardless of the number occurring on that day. The occurrences oflightning without thunder is not recorded as a thunderstorm. (Data supplied by the Environmental Science ServiceAdministration, U.S. Department of Commerce.)
Lik 5 10 20 30 40 50 60 70 80 90 100Ng .3 .7 1.7 2.8 4.0 5.3 6.7 8.1 9.6 11.1 12.6
Where:
Lik = The lightning frequency isoceraunic levelNg = Lightning flash density
The collection area of a building is the area of the plan of the structure extending in all directions to account forits height. The edge of the collection area is displaced from the edge of the structure by an amount equal todouble the height of the structure at that point. Therefore, for a simple rectangular building of length, L, width,W, and height, H, the collection area has length (L + 4H) and width (W +4H) with four rounded corners formedby quarter circles of radius, H. All dimensions are expressed in meters.
Therefore, calculation of the collection area is as follows:
Ca = (L x W) + (4L x H) + (4W x H) + 4(πH2)
Where:
(π = 3.14)
NOTE: Where the collection area of a structure fully covers the collection area of another structure, the latter isnot taken into account. Where collection areas of several structures overlap, the corresponding common areasare taken into account only once. Three typical examples follow.
Proposed NFPA 781--F93 TCD Appendix D (b)
The probable number of strikes to the structure per year, Ps, is as follows:
Ps = Ca x Ng x 10-6
Ng is derived directly from survey networks maps.
The result of this equation then is weighted by a factor, C1, taking into account the environment of the structure.(See Table D-2.)
The exposure risk then is expressed as the probability:
P = Ps x C1 per year.
Table D-2 Determination of Environmental Coefficient C1
This table takes into account the relative location of the structure.
Relative Location C1
Structure located in a large area of structures or trees 0.5of the same or greater height (tower, forest)
Structure located in areas of few structures of similar height 1.0
Structure located in isolated site or surrounded by small structures 2.0
Structure is isolated on a hilltop or a knoll 3.0
D-3 Overall Assessment of Risk Pr.
Having established the value of P, the probable number of strikes to the structure per year, the “weightingfactors,” as specified in Tables D-3, D-4, D-5 and D-6 then are applied.
Multiply P by the appropriate factors to determine whether the result, the overall risk factor, exceeds theacceptable risk R0 = 5.10-4. The result is Pr, the overall risk probability.
Thus, Pr = P x C2 x C3 x C4 x C5
Tables D-3 through D-6 provide the weighting factors for the relative degree of importance of risk in each case.
Table D-3
Roof Metal MetalStructuralFramework
ElectricallyContinuous
s
NotContinuous Composition Wood
Nonmetallic(other than wood)
1.0 1.7 1.4 3.0
Wood 2.0 1.7 1.4 3.0Reinforcedconcrete
0.5 1.7 1.4 3.0
Structural steel 0.5 1.7 1.4 2.0
Proposed NFPA 781--F93 TCD Appendix D(c)
Table D-4 Determination of Contents Coefficient C3
Content of the structure C3
Noncombustible materials 0.3 Residential furnishings or equipment 0.5 Cattle and livestock 0.8 Combustible materials 1 High value materials or equipment 2 Materials of exceptional or irreplaceable nature 5
Table D-5 Determination of Occupancy Coefficient C4
Occupancy of the structure C4 Unoccupied 0.3 Normally low occupancy 0.5 Normally occupied 1 Occasionally occupied by many persons 1.5 Occupied by immobile or bedridden persons 2.5 Risk of panic or very difficult access 3
Table 6 Determination of Environmental Consequences Coefficient C5
Consequential effects C5
Building with no service continuity necessities 1 Building with a necessary service continuity 1.5 Building containing hazardous materials 5
D-4 Interpretation of Overall Risk Probability. The risk factor method provided in this appendix is intended toassist in solving the difficult problem of risk assessment. If the determined risk is less than 5 X 10-4, then, inthe absence of other overriding considerations, protection is optional. If the risk is greater, the followingrecommendations for protection should be considered:
If Pr < 5 X 10-4 ----> Protection optional
5 X 10-4 < 5 X 10-3 ----> Level I protection recommended
5 X 10-3 < 5 X 10-2 ----> Level II protection recommended
If Pr > 5 X 10-2 ----> Level III protection recommended
D-5 Levels of Protection. As a function of the intensity, I, of the current of the lightning strikes and of thedistance, R, correlated to I, a lightning stroke is relatively easy to capture for a LPS.
Proposed NFPA 781--F93 TCD Appendix D(d)
Proposed NFPA 781--F93 Appendix D(e)
Table D-7 provides the three levels of protection that ensure severe (Level III) to standard (Level I) protection.
Table D-7
Levels of Protection R (m) I (ka)
Level I 60 16.00 Level II 45 10.00 Level III 20 3.00
D-6 Examples of Calculation.
D-6.1 For a rectangular structure, the collection area is calculated as follows:
S = L x W + 4 LH + 4 WH = 4 π H2
D-6.2 For high structures, the radius of which will include the collection of another part of the structure.(See Figure D-6.2)
D-6.3 For high structures of which only a part of the collection area is common to the other part of thestructure.
In the case of a high stack near a smaller building, a factor of 3/4 is added in the formula in order to avoidtaking into account one quarter of the collection area of the stack that is superimposed on the collection area ofthe building. ( See Figure D-6.2)
S = L x W + h (L + W) + 4 x p x h2 + 3/4 x 4 x p H2 = 8869 m2
Figure D-6.2
Proposed NFPA 781--F93 Appendix E
Appendix E Lightning Parameters
E-1 Lightning parameters are widely variable and generally are treated in a statistical manner. Table E-1provides the frequency distributions of the main characteristics of the lightning ground flash.
E-1.2 Current Wave Shape. It should be understood that it is not possible for two similar currents of firstnegative strokes to exist. However, similarities have been established. A mean wave shape was computed byKroninger in 1974 and is reproduced in Figure E-1.2.
Figure E-1.2 Mean current wave shape of negative stroke (Berger etal., 1975). A. Full record length.B. Expanded front. (Reproduced from R.H. Golde, Lightning, Vol. 1, Academic Press, 1977.)
Table E-1: Summary of the Frequency Distribution of the Main Characteristicsof the Lightning Flash to Ground
Lightning Characteristics Percentage of events having value of characteristic greater than value shown Unit below ( see Note 1).
99 90 75 50 25 10 1First stroke peak current 1 max 3 12 20 30 50 80 130 kAFirst stroke (di/dt) max 6 10 15 25 30 40 70 GA/sContinuing current 1 max 30 50 80 100 150 200 400 AOverall duration of flash 50 100 250 400 600 900 1500 msAction integral 10 3.102 103 5.103 3.104 105 5.105 A2.s
Note 1: The values shown in this table have been taken from information compiled by CIGRE and have beenrounded in accordance with the known accuracy of these data. Values at the 1 percent and 99 percent levelsare very uncertain and are provided only to indicate an order of magnitude.
Note 2: The action integral, defined as i 2dt for the whole flash, is equivalent to the energy deposited in a 1ohm resistor by the passage of the entire current for the duration of the flash.
Proposed NFPA 781--F93 TCD Appendix F
Appendix F System Design
F-1 The following factors should be considered in the system design.
(a) Dimensions of the building (b) Form and pitch of the roofs (c) Nature of the roofing (d) Metal parts of the roof and the major external metal (e) Items, such as gas conduits, antennas, ventilators and stairways (f) Rainwater gutters and downpipes (g) Projecting parts of the building and the nature of their materials (metallic and nonconductive) (h) Most vulnerable parts of the building (I) Arrangement of the metal pipes (water, electric, gas) of the building (j) Any nearby obstacles that might have an effect upon the trajectory of the lightning, such as overhead electricity lines, metal railings and trees (k) Nature of any atmosphere liable to be particularly corrosive (petrochemical plants, cement works) (l) Presence of any explosive or inflammable materials (m) Presence of any sensitive equipment, such as computers, electronic systems.
Appendix G Maintenance and Inspection of ESE Lightning Protection Systems
G-1 Maintenance and Inspection of ESE Lightning Protection Systems. The effectiveness of a lightningprotection system is maintained most effectively by a quality control program designed to ensure that thesystem in not degraded by age, mechanical damage, or modifications to the protected structure. Many systemcomponents tend to lose their effectiveness over the years because of corrosion factors, weather relateddamage and stroke damage. The physical as well as electrical characteristics of the lightning protectionsystem should be inspected and maintained in order to guarantee compliance with design requirements. Amaintenance and inspection plan should be developed to provide for periodic evaluations and timely repairs.To the extent practical, all necessary repairs should be made immediately upon detection. This ensures thatthe system can be depended upon to deliver adequate and reliable protection against lightning activity.
G-1.1 Frequency of Inspections. All new lightning protection systems should be inspected followingcompletion of their installation. Similarly, existing systems should receive periodic inspections to detect andcorrect for deterioration, damage or inadvertent modifications having detrimental effects on the system’seffectiveness. A lightning protection system should be inspected whenever any alterations or repairs are madeto a protected structure and following a known lightning discharge to the system. To ensure that the protectionsystem is properly maintained, inspections should be performed based on the following minimum criteria:
(a) A lightning protection system should be inspected visually and electrically once a year. (b) Complete in-depth inspections of all systems should be completed every 3 to 5 years. (c) Critical systems should be inspected visually twice a year and inspected electrically once a year. (d) Critical systems should be inspected in-depth every 1 to 3 years, depending on the occupancy or the environment in which the protected structure is located.
G-1.2 Inspection Cycles. To ensure the systems are tested during all four seasons over a 6 year period, 7month and 14 month inspection cycles are suggested.
Proposed NFPA 781--F93 TCD Appendix G
G-1.2.1 Visual (7 Month) Inspection. The lightning protection system should be inspected visually every 7months for evidence of corrosion, broken wires and changes detrimental to the efficiency of the system, amongother problems. Visual inspections are made to ascertain the maintenance requirements of the system and toensure that:
(a) The system is in good repair. (b) There are no loose connections that might result in high resistance joints (bonds). (c) No part of the system is weakened by corrosion or vibration. (d) All down conductors and ground terminals are intact (nonsevered). (e) All down conductors are routed smoothly and contain acceptable bend radii. (f) All conductors are fastened securely to their mounting surfaces and are protected against accidental mechanical displacement as required. (g) There are no new additions or alterations to the protected structure that need further protection. (h) There is no visual indication of damage to surge suppression (overvoltage) devices. (i) The system complies in all respects with this 1994 edition of NFPA 781.
G-1.2.2 Grounding Systems (14 Month) Test. The lightning protection system should be tested electricallyevery 14 months. The test should be conducted in accordance with the appropriate test equipmentmanufacturer’s instructions by personnel familiar with lightning protection system testing. Only thoseinstruments designed for earth ground system testing are acceptable for use in ground resistance testing.Such instruments should be capable of measuring 10 ohms ± 10 percent. The instrument used to measurebonding resistance should be capable of measuring 1 ohm ± 10 percent.
G-1.3 Complete (In-depth) Testing and Inspection. In-depth testing and inspection should include the visualinspections described in G-1.2.1 in addition to the following:
(a) Perform tests to verify continuity of those parts of the system that were concealed (built-in) during the initial installation and that are now accessible for visual inspection. (b) Conduct ground resistance tests of the ground termination system and its individual electrodes, if adequate disconnecting means have been provided. The results of these tests should be compared with previous, or original, results and/or current accepted values for the soil conditions involved. If it is found that the test values differ substantially from previous values obtained under the same test procedures, additional investigations should be made to determine the reason for the difference. (c) Perform continuity tests to determine if suitable equipotential bonding has been established for any new services or construction that has been added to the interior of the structure since the last inspection.
G-1.4 Inspection Guides and Records. Inspection guides or forms should be filled out and maintained by the owner.
These guides should contain sufficient information to aid in the inspection process so that documenting is done onall areas of importance relating to the methods of installation, the type and condition of system components, the testmethods and the proper recording of the test data obtained.
Existing system documentation such as records of previous inspections and test results, as-built drawings, andhistorical maintenance records should be located for convenient review.
G-1.5 Maintenance and Repairs. The mechanical and electrical integrity of the lightning protection system isdependent upon frequent intervals of scheduled maintenance and immediate corrective action on observeddiscrepancies. Care should be exercised to ensure that replacement parts or components and installationprocedures meet the applicable requirements of this standard.
Proposed NFPA 781--F93 TCD Appendix G(a)
G-1.5.1 Maintenance. Preventative maintenance should be performed as an integral part of the inspectionprocess. The frequency of inspections, in effect, determines how often maintenance should be performed.
G-1.5.2 Repairs. Repairs needed to correct discrepancies noted during casual observation, or resulting from aknown lightning or physical event, should not be delayed until the next scheduled inspection. All repairs shouldbe performed as the need is detected in order to maintain system integrity.
G-1.5.3 Special Considerations. Some ESE air terminals contain low levels of radioactive material. Labelingshould conform to federal requirements. Replacement of certain ESE parts and/or system components canrequire special attention to the legal disposal of radioactive substances, lead, or other materials deemed to beenvironmental hazards. It is recommended that the manufacturer be consulted before disposing of anysystem components.
G-1.6 Records and Test Data. The inspecting personnel should compile and maintain records pertaining to:
(a) Past inspection results as detailed in the completed inspection guides and forms. (b) Detailed maintenance records specifying scheduled replacements, repairs and modifications to the lightning protection system. (c) Updated as-built drawings of the lightning protection system. (d) Resistance measurements of the various parts of the ground termination system. (e) Known deviations from the requirements contained in this standard.
Appendix H Referenced Publications
H-1 The following documents or portions thereof are referenced within this standard for informational purposesonly and thus are not considered part of the requirements of this document. The edition indicated for eachreference is the edition dated as noted below:
H-1.1 NFPA Publications. National Fire Protection Association, 1 Batterymarch Park, P.O. Box 9101, Quincy, MA 02269-9101. NFPA 70, National Electrical Code, 1993 edition. NFPA 780, Lightning Protection Code, 1992 edition.
H-1.2 ANSI/IEEE Publications. Institute of Electrical and Electronic Engineers, Inc. 345 East 47th St., New York, NY ANSI/IEEE C-62.1-1984, Surge Arrestors for Alternating Current Systems ANSI/IEEE 141-1986, Recommended Practice for Electric Power Distribution for Industrial Power Plants
H-1.3 U.S. Government Publication. Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20401. MIL-HDBK 419A, Volume 1 of 2, Grounding, Bonding and Shielding for Electronic Equipment and Facilities, Naval Publications and Forms Center. 700 Robbius Ave., Philadelphia, PA 19111.
