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 Section 5.1.7:

CONTEMPORARY LIGHTNING SAFETY FOR ENVIRONMENTS CONTAINING SENSITIVE ELECTRONICS, EXPLOSIVES AND VOLATILE SUBSTANCES.
by Richard Kithil, President & CEO
National Lightning Safety Institute

 

Prepared for: Department of Energy Subcommittee on Consequence Assessment and Protective Actions (SCAPA) Las Vegas NV Oct. 2000; Lockheed Martin Aeronautic Co. Weekly Activity Report – F-16 Support and Services WUC-1100, Nov. 2000;
Power Quality Assurance Magazine, January 2001;
US Army Explosives Safety Bulletin, Defense Ammunition Center, Feb. 2001;
US Dept. Energy TA 16 Buildings Assessment, Los Alamos NL, NM Oct. 2001;
US Dep. Energy Explosives Safety Committee, 440.1 Lightning Code Review, Albuquerque NM Dec. 2001

1. ABSTRACT

Franklin’s 1752 lightning protection invention consisted of a rod in the air, one in the ground and a connecting conductor. This "conventional wisdom" today is helpful for fire protection in cases of direct flashes to ordinary structures. For more complex facilities, where electrical systems/electronics or explosives or volatile substances are present, the 248 year old design is questionable. This paper suggests adoption of a modern lightning safety planning process which can be applied to contemporary environments.

2. BACKGROUND

The USA electric power industry reports 30% of all outages to be lightning-related (Electric Power Research Institute, 1999). Insurance companies here categorize some 5% of all paid claims as resulting from lightning (Insurance Information Institute, 1999).The US Department of Energy has recorded 346 known lightning events to its facilities during the 1990-2000 period (US Dept. Energy - Occurance Reporting & Processing System, 2000). In total, lightning is responsible for about $4-5 billion annual losses in the USA (National Lightning Safety Institute, 1999). It is a prudent organizational policy to analyze facilities and operations so as to identify lightning vulnerability. Designs and operational means to deflect potential accidents should be developed. For the lightning hazard, safety should be a prevailing directive.

3. LIGHTNING CHARACTERISTICS

3.1. Physics of Lightning. Lightning’s characteristics include current levels approaching 400 kA with the 50% average being about 25kA, temperatures to 15,000 C, and voltages in the hundreds of millions. There are some ten cloud-to-cloud lightnings for each cloud-to-ground lightning flash. Globally, some 2000 on-going thunderstorms generate about 50-100 lightning strikes to earth each second. Lightning is the agency which maintains the earth’s electrical balance. The phenomenology of lightning flashes to earth, as presently understood, follows an approximate behavior: the downward Leader (gas plasma channel) from a thundercloud pulses toward earth. Ground-based air terminators such as fences, trees, blades of grass, corners of buildings, people, lightning rods, power poles etc., etc. emit varying degrees of induced electric activity. They may respond at breakdown voltage by forming upward Streamers. In this intensified local field some Leader(s) likely will connect with some Streamer(s). Then, the "switch" is closed and the current flows. Lightning flashes to ground are the result. A series of return strokes follow.

3.2 Lightning effects . Thermal stress of materials around the attachment point is determined by: a) heat conduction from arc root; b) heat radiation from arc channel; and, c) Joule heating. The radial acoustic shock wave can cause mechanical damage. Magnetic pressures – up to 6000 atmospheres for a 200 kA flash - are proportional to the square of the current and inversely proportional to the square of the diameter of struck objects (G. A. Odam, GAO Consultancy,1996). Voltage sparking is a result of dielectric breakdown. Thermal sparking is caused is caused when melted materials are thrown out from hot spots. Exploding high current arcs, due to the rapid heating of air in enclosed spaces, have been observed to fracture massive objects (i.e. concrete and rocks). Voltage transfers from an intended lightning conductor into electrical circuits can occur due to capacitive coupling, inductive coupling, and/or resistance (i.e. insulation breakdown) coupling. Transfer impedance, due to loss of skin effect attenuation or shielding, can radiate interference and noise into power and signal lines. Transfer inductance (mutual coupling) can induce voltages into a loop which can cause current flows in other coupled circuits.

3.3 Behavior of Lightning. Absolute protection from lightning may exist in a thick-walled and fully enclosed Faraday Cage, however this is impractical in most cases. Lightning "prevention" exists only as a vendor-inspired marketing tool. Important new information about lightning may affect sensitive facilities. First, the average distance between successive cloud-to-ground flashes is greater than previously thought. The old recommended safe distance from the previous flash was 1-3 miles. New information suggests that a safe distance should be 6-8 miles (Lopez & Holle, National Severe Storm Center, 1998). Second, some 40% of cloud-to-ground lightnings are forked, with two or more attachment points to the earth. This means there is more lightning to earth than previously measured (Krider, Intl. Conf. Atmospheric Electricity,1998). Third, radial horizontal arcing in excess of 20 m from the base of the lightning flash extends the hazardous environment. Lightning is a capricious, random, stochastic and unpredictable event.

3.4 When lightning strikes an asset, facility or structure (AFS) return-stroke current will divide up among all parallel conductive paths between attachment point and earth. Division of current will be inversely proportional to the path impedance, Z = R + XL. The resistance term will be low assuming effective bonding. The inductance, and related inductive reactance, presented to the total return stroke current will be determined by the combination of all the individual inductive paths in parallel. Essentially lightning is a current source. A given stroke will contain a given amount of charge (coulombs = amp/seconds) that must be neutralized during the discharge process. If the return stroke current is 50kA – that is the magnitude of the current that will flow, whether it flows through one ohm or 1000 ohms. Therefore, achieving the lowest possible impedance serves to minimize the transient voltage developed across the path through which the current is flowing (E = L di/dt ). (Hasbrouck, letter to author 2002.)

4. LIGHTNING PROTECTION DESIGNS

Mitigation of lightning consequences can be achieved by the use of a detailed systems approach, described below in general terms.

4.1 Air Terminals. Since Franklin's day lightning rods have been installed upon ordinary structures as sacrificial attachment points, intending to conduct direct flashes to earth. In 1876 JC Maxwell suggested that Franklin rods on buildings attracted a greater number of flashes than their absence. Such rods should not be located on explosives storage structures. This integral air terminal design does not provide protection for electronics, explosives, or people inside modern structures. Inductive and capacitive coupling from lightning-energized conductors can result in significant voltages and currents on interior power and signal conductors.

Overhead shield wires and mast systems located above or next to the structure are suggested alternatives in many circumstances. These are termed indirect air terminal designs. Such methods presume to collect lightning above or away from the sensitive structure, thus avoiding or reducing flashover attachment of unwanted currents and voltages to the facility and equipments.

Investigation into applicability of dielectric shielding may provide additional protection where upward leader suppression may manipulate breakdown voltages (Schnetzer et al, Sandia Laboratories, 1997). Unconventional air terminal designs which claim the elimination or redirecting of lightning (charge dissipators) or lightning preferential capture (early streamer emitters) deserve a very skeptical reception (NASA/Navy Tall Tower Study; 1975, R.H. Golde "Lightning" 1977; FAA Airport Study 1989; T. Horvath "Computation of Lightning Protection" 1991; D. MacKerras et al, IEE Proc-Sci Meas. Technol, V. 144, No. 1 1997; National Lightning Safety Institute "Royal Thai Air Force Study" 1997; A. Mousa "IEEE Trans. Power Delivery, V. 13, No. 4 1998; International Conference on Lightning Protection - Technical Committee personal correspondence 2000). Merits of radioactive air terminals have been investigated and dismissed by reputable scientists (R.H. Golde op cit and C.B. Moore personal correspondence, 2000).

4.2 Downconductors. Downconductor pathways should be installed outside of the structure. Rigid strap is preferred to flexible cable due to inductance advantages. Conductors should not be painted, since this will increase impedance. Gradual bends always should be employed to avoid flashover problems. Building structural steel also may be used in place of downconductors where practical as a beneficial subsystem emulating the Faraday Cage concept.

4.3 Bonding assures that unrelated conductive objects are at the same electrical potential. Without Bonding, lightning protection will not work. All metallic conductors entering structures (ex. AC power lines, gas and water pipes, data and signal lines, HVAC ducting, conduits and piping, railroad tracks, overhead bridge cranes, roll up doors, personnel metal door frames, hand railings, etc.) should be electrically referenced to the same ground. Connector bonding should be exothermal and not mechanical wherever possible, especially in below-grade locations. Mechanical bonds are subject to corrosion and physical damage. HVAC vents that penetrate one structure from another should not be ignored as they may become troublesome electrical pathways. Frequent inspection and resistance measuring (maximum 1 milliohm) of connectors to assure continuity is recommended.

4.4 Grounding. The grounding system must address low earth impedance as well as low resistance. A spectral study of lightning's typical impulse reveals both a high and a low frequency content. The grounding system appears to the lightning impulse as a transmission line where wave propagation theory applies. A considerable part of lightning’s current responds horizontally when striking the ground: it is estimated that less than 15% of it penetrates the earth. As a result, low resistance values (25 ohms per NEC) are less important that volumetric efficiencies.

Equipotential grounding is achieved when all equipment within the structure(s) are referenced to a master bus bar which in turn is bonded to the external grounding system. Earth loops and consequential differential rise times must be avoided. The grounding system should be designed to reduce AC impedance and DC resistance. The use of counterpoise or "crow's foot" radial techniques can lower impedance as they allow lightning energy to diverge as each buried conductor shares voltage gradients. Ground rings connected around structures are useful. Proper use of concrete footing and foundations (Ufer grounds) increase volume. Where high resistance soils or poor moisture content or absence of salts or freezing temperatures are present, treatment of soils with carbon, Coke Breeze, concrete, natural salts or other low resistance additives may be useful. These should be deployed on a case-by-case basis where lowering grounding impedances are difficult an/or expensive by traditional means.

4.5 Corrosion and cathodic reactance issues should be considered during the site analysis phase. Where incompatible materials are joined, suitable bi-metallic connectors should be adopted. Joining of aluminum down conductors together with copper ground wires is a typical situation.

4.6 Transients and Surges. Ordinary fuses and circuit breakers are not capable of dealing with lightning-induced transients. Surge protection devices (SPD aka transient limiters) may shunt current, block energy from traveling down the wire, filter certain frequencies, clamp voltage levels, or perform a combination of these tasks. Voltage clamping devices capable of handling extremely high amperages of the surge, as well as reducing the extremely fast rising edge (dv/dt and di/dt) of the transient are recommended.

Protecting the AC power main panel; protecting all relevant secondary distribution panels; and protecting all valuable plug-in devices such as process control instrumentation, computers, printers, fire alarms, data recording & SCADA equipment, etc. is suggested. Protecting incoming and outgoing data and signal lines (modem, LAN, etc.) is essential. All electrical devices which serve the primary asset such as well heads, remote security alarms, CCTV cameras, high mast lighting, etc. should be included.

Transient limiters should be installed with short lead lengths to their respective panels. Under fast rise time conditions, cable inductance becomes important and high transient voltages can be developed across long leads. SPDs with replacable internal modules are suggested.

In all instances the use high quality, high speed, self-diagnosing SPD components is suggested. Transient limiting devices may use spark gap, diverters, metal oxide varistors, gas tube arrestors, silicon avalanche diodes, or other technologies. Hybrid devices, using a combination of these techniques, are preferred. SPDs conforming to the European CE mark are tested to a 10 X 350 us waveform, while those tested to IEEE and UL standards only meet a 8 X 20 us waveform. It is suggested that user SPD requirements and specifications conform to the CE mark, as well as ISO 9000-9001 series quality control standards.

Uninterupted Power Supplies (UPSs) provide battery backup in cases of power quality anomalies…brownouts, capacitor bank switching, outages, lightning, etc. UPSs are employed as back-up or temporary power supplies. They should not be used in place of dedicated SPD devices. Correct Category A installation configuration is: AC wall outlet to SPD to UPS to equipment.

4.7 Detection. Lightning detectors, available at differing costs and technologies, are useful to provide early warning. Users should beware of over-confidence in detection equipment. It is not perfect and it does not always acquire all lightning data. Detectors cannot "predict" lightning. An interesting application is their use to disconnect from AC line power and to engage standby power, before the arrival of lightning. A notification system of radios, sirens, loudspeakers or other means should be coupled with the detector. See the NLSI WWW site for a more detailed treatment of detectors.

4.8 Testing & Maintenance. Modern diagnostic testing is available to "verify" the performance of lightning conducting devices as well as to indicate the general route of lightning through structures. With such techniques, lightning paths can be forecast reliably. Sensors which register lightning current attachments can be fastened to downconductors. Regular physical inspections and testing should be a part of an established preventive maintenance program. Failure to maintain any lightning protection system may render it ineffective.

  1. PERSONNEL SAFETY ISSUES

Lightning safety should be practiced by all people during thunderstorms. Measuring lightning's distance is useful. Using the "Flash/Bang" (F/B) technique, for every five seconds - from the time of seeing the lightning flash to hearing the associated thunder - lightning is one mile away. A F/B of 10 = 2 miles; a F/B of 20 = 4 miles, etc. The distance from Strike A to Strike B to Strike C can be as much as 5-8 miles. The National Lightning Safety Institute recommends the 30/30 Rule: suspend activities at a F/B of 30 (6 miles), or when first hearing thunder. Outdoor activities should not be resumed until 30 minutes has expired from the last observable thunder or lightning. This is a conservative approach: perhaps it is not practical in all circumstances.

If one is suddenly exposed to nearby lightning, adopting the so-called Lightning Safety Position (LSP) is suggested. LSP means staying away from other people, taking off all metal objects, crouching with feet together, head bowed, and placing hands on ears to reduce acoustic shock from nearby thunder. When lightning threatens, standard safety measures should include: avoid water and all metal objects; get off the high ground including rooftops; avoid solitary trees; stay off the telephone. A fully enclosed metal vehicle – van, car or truck – is a safe place because of the (partial) Faraday Cage effect. A large permanent building can be considered a safe place. In all situations, people should avoid becoming a part of the electrical circuit. Benjamin Franklin’s advice was to lie in a silk hammock, supported by two wooden posts, located inside a house.

Every organization should consider adopting and promulgating a Lightning Safety Plan specific to their operations.

6. CODES AND STANDARDS

In the USA there is no single lightning safety code or standard providing comprehensive assistance. The most commonly referenced USA commercial lightning protection installation standard is incomplete, out-dated, and largely pre-empted by commercial interests. US Government lightning protection documents should be consulted. The Federal Aviation Administration FAA-STD-019c is valuable. Other recommended federal codes include military documents MIL HDBK 419A, NAV OPSEA 5, KSC STD E0012, MIL STD 188-124B, MIL STD 1542B, MIL B 5087B, UFC 3-570-01 and AFI 32-1065. The British Code BS 6551 is helpful. The new German lightning protection standard for nuclear power plants KTA 2206 places special emphasis on the coupling of overvoltages at instrument and control cables. The European International Electrotechnical Commission IEC 61024 series for lightning protection is the single best reference document for the lightning protection engineer. Adopted by many countries, IEC 1024 is a science-based document applicable to many design situations. Ignored in most Codes is the very essential electromagnetic compatibility (EMC) subject, especially important for explosives safety and facilities containing electronics, VSDs, PLCs, and monitoring equipment.

7. CONCLUSION

Lightning has its own agenda and may cause damage despite application of best efforts. Any comprehensive approach for protection should be site-specific to attain maximum efficiencies. In order to mitigate the hazard, systematic attention to details of grounding, bonding, shielding, air terminals, surge protection devices, detection & notification, personnel education, maintenance, and employment of risk management principles is recommended.

8. REFERENCES.

  1. International Conference on Lightning Protection (ICLP) Proceedings, Rhodes Greece Sept. 2000.
  2. ICLP Proceedings, Birmingham UK Sept. 1998
  3. ICLP Proceedings, Florence Italy Sept.1996
  4. IEEE Transactions on Electromagnetic Compatibility, Nov. 1998
  5. National Research Council, Transportation Research Board, NCHRP Report 317, June 1989
  6. International Electrotechnical Commission (IEC), International Standard for Lightning Protection. See: http://www.iec.ch
  7. Gardner RL, Lightning Electromagnetics, Hemisphere Publishing, NY NY 1991

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Note: Permission to copy and to re-print this paper is freely given. Please credit NLSI as the original author. The National Lightning Safety Institute is a non-profit, non-product independent organization providing objective information about lightning safety issues.

 

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