Benjamin Franklin's 1752 lightning protection invention consisted of a rod in the air, one in the ground, and a connecting conductor. Today, this conventional wisdom still offers fire protection in cases of direct flashes to ordinary structures. However, more complex facilities (where electrical systems/electronics, explosives, or volatile substances are present), need better protection (Wiesinger, ICLP, 2000).
Electric power industry experts report 30% of all outages to be lightning-related (EPRI, 1999). Insurance companies categorize some 5% of all paid claims as stemming from lightning effects (III, 1999). The U.S. Department of Energy has recorded 346 known lightning events to its facilities from 1990 to 2000 (ORPS, 2000). In total, lightning causes about $4 to $5 billion annual losses in the United States (NLSI, 1999).
Considering these numbers, it's essential for every organization to analyze facilities and operations to identify lightning vulnerability. Designs and operational means to deflect potential accidents also should be developed. This article suggests the industry should adopt a revised lightning safety design guideline, which you can apply to contemporary environments.
Lightning Characteristics Physics of lightning. Lightning's characteristics include current levels approaching 400 kA with the 50% average at about 25 kA, temperatures to 15,000C, and voltages in the hundreds of millions. Globally, some 2,000 ongoing thunderstorms generate about 50 to 100 lightning strikes to earth each second.
As presently understood, the phenomenology of lightning flashes to earth follows an approximate behavior. First, 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, lightning rods, power poles, people, etc.) emit varying degrees of induced electric activity. In this intensified local field, some leaders will likely connect with some upward "streamers" when they reach the breakdown voltage. At this point, the "switch" closes, the current flows, and lightning flashes to the ground. A series of return strokes follows.
Lightning effects. Thermal stress of materials around an attachment point is determined by heat conduction from arc root, heat radiation from arc channel, and joule heating. The radial acoustic shockwave produced by the current of a lightning stroke can cause severe mechanical damage. Magnetic pressures - up to 6,000 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 superscript .
Voltage sparking is a result of dielectric breakdown. Thermal sparking 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 (e.g., concrete and rocks). Voltage transfers from a lightning conductor into electrical circuits can occur due to capacitive coupling, inductive coupling, and/or resistance (e.g., insulation breakdown) coupling. Transfer impedance, due to loss of skin effect attenuation or shielding, also can radiate interference and noise into power and signal lines. Transfer inductance (mutual coupling) may induce voltages into a loop, which can cause current flows in other coupled circuits.
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. Fortunately, new important information about lightning may offer a solution to protect 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 to 3 miles, but now 6 to 8 miles is considered a safe distance superscript .
Second, some 40% of cloud-to-ground lightning is forked, with two or more attachment points to the earth. This means there is more lightning to earth than previously measured superscript . Third, radial horizontal arcing in excess of 20 m from the base of the lightning flash extends the hazardous environment superscript .
Lightning Protection Designs You can mitigate lightning effects by using a detailed systems approach.
Air terminals. Since Franklin's day, people have installed lightning rods on ordinary structures as sacrificial attachment points - intending to conduct direct flashes to earth. But in 1876, J.C. Maxwell proposed that Franklin rods on buildings attracted a greater number of flashes than their absence. Therefore, you should not install such rods on an explosive storage structure.
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. Air terminal designs, which claim the elimination of lightning or its directed capture, deserve skepticism superscript .
Reputable scientists have investigated and dismissed the merits of radioactive air terminals superscript . Overhead shield wires (also called catenary or Faraday systems) located above the structure and supported by masts are a suggested alternative in many circumstances. These systems are known as indirect air terminal designs. This design presumes to collect lightning above the sensitive structure, thus avoiding or reducing flashover of unwanted currents and voltages to the facility and equipment. Investigation into applicability of dielectric shielding may provide additional protection where upward leader suppression may manipulate breakdown voltages superscript .
If you choose downconductor pathways, you should install them outside of the structure. A rigid strap is preferable to flexible cable due to inductance advantages. Do not paint conductors, because this increases impedance. Gradual bends always should be employed to avoid flashover. If it's practical, you also can use building structural steel in place of downconductors as a beneficial subsystem emulating the Faraday Cage concept.
Bonding assures unrelated conductive objects are at the same electrical potential. You should mechanically bond all metallic conductors entering structures (e.g., AC power lines, gas and water pipes, data and signal lines, HVAC ducting, conduits and piping, railroad tracks, overhead bridge cranes, roll up doors, metal door frames, hand railings, etc.) to the same ground.
Exothermic connections are preferred over mechanical connections wherever possible, especially in below-grade locations. Mechanical bonds are subject to corrosion and physical damage. Do not ignore HVAC vents that penetrate one structure from another, as they may become troublesome electrical pathways. Frequent inspection and resistance measuring (maximum 1W) of connectors to assure continuity is recommended.
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 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. Industry experts estimate less than 15% of it penetrates the earth. As a result, low-resistance values (25W per NEC) are less important than volumetric efficiencies.
You'll achieve equipotential grounding when all equipment within the structure(s) is referenced to a master bus bar, which in turn you bond to the external grounding system. Make sure you avoid earth loops and consequential differential rise times. 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 also useful. Use of concrete footing and foundations (Ufer grounds) increase the volume of the earth electrode. Where high-resistance soils, 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. However, these should be deployed on a case-by-case basis.
Consider corrosion and cathodic reactance issues during the site analysis phase. Where incompatible materials join, adopt suitable bimetallic connectors. Joining aluminum down conductors together with copper ground wires is a typical situation.
Transients and surges. Ordinary fuses and circuit breakers are not capable of dealing with lightning-induced transients. Surge protection devices (SPDs) 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 surge as well as reducing the extremely fast rising edge (dv/dt and di/dt) of the transient are typically recommended.
Protection of the AC power main panel, protecting all relevant secondary distribution panels and valuable plug-ins (such as process control instrumentation, computers, printers, fire alarms, data recording, SCADA equipment, etc.) is suggested. Protecting incoming and outgoing data and signal lines is essential. Electrical devices that serve the primary asset, including well heads, remote security alarms, CCTV cameras, and high mast lighting, should be included.
You should install transient limiters with short lead lengths to their respective panels. Under fast rise time conditions, cable inductance becomes important. Avoid long leads.
In all instances, the use of high-quality, high-speed, self-diagnosing SPD components is recommended. Transient limiting devices may use arc gap diverters, metal oxide varistors, gas tube arresters, 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 ms x 350 ms waveform, while those tested to IEEE and UL standards only meet an 8 ms x 20 ms 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.
Uninterrupted power supplies (UPSs) provide battery backup in cases of power quality anomalies, capacitor bank switching, outages, and lightning. However, do not use them in place of dedicated SPD devices.
Detection. Lightning detectors are used to provide early warning signs, but you should beware of overconfidence in detection equipment. This equipment does not always acquire all lightning data. Detectors cannot "predict" lightning. An interesting application is their ability to disconnect from AC line power and to engage standby power, before the arrival of lightning. A notification system consisting of radios, sirens, loudspeakers, or other means should be coupled with the detector.
Testing and maintenance. Modern diagnostic testing is available to predict the performance of lightning conducting devices as well as indicate the general route that lightning current will take through your structure. Sensors fastened to downconductors can reliably forecast the flow of current and help you see where deficiencies may exist. Regular physical inspection and testing should be part of an established preventive maintenance program. Failure to maintain any lightning protection system may render it ineffective.
Codes and Standards In the United States, there is no single lightning safety code or standard providing comprehensive assistance. Several commonly referenced commercial codes and installation standards are incomplete, outdated, or promulgated by commercial interests. U.S. government standards should be consulted. The Federal Aviation Administration Std. 019c is valuable. Other recommended federal codes include MIL HDBK 419A, NAV OPSEA 5, KSC STD 012B/013D, MIL STD 188-124B, MIL STD 1542B, MIL B 5087B, and AFI 32-1065.
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. Adopted by many countries, the European IEC 1024 venue for lightning protection is the single most essential reference document for the lightning protection engineer in the industry. IEC 1024 is a science-based document applicable to many design situations.
Conclusion Lightning has its own agenda and may cause damage despite your best efforts. Any comprehensive approach for protection should be site-specific to attain maximum efficiency. To mitigate the hazard, systematic attention to details of grounding, bonding, shielding, air terminals, surge protection devices, detection, notification, personnel education, maintenance, and risk management is recommended.
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.