~ National Lightning Safety Institute ~
Lightning Safety in the Mining Industry
By Richard Kithil, President & CEO, NLSI
Investigations at various mine locations worldwide disclose many power quality and safety issues traceable to ignoring lightning safety. Some examples are:
With such examples, it is difficult to support a position that lightning incidents are rare. This paper suggests that lightning is responsible for significant economic and personal costs in the industry. How to defend against the lightning hazard at mining operations? We recommend adoption of a lightning hazard mitigation and safety planning process that can be applied to most facilities.
2. Lightning Behavior and Characteristics
2.1. Physics of Lightning
Lightning’s characteristics include current levels approaching 250 kA with the 50% average being about 25kA, temperatures to 15,000 C, and voltages in the hundreds of millions. There are some 10 cloud-to-cloud lightnings for each cloud-to-ground lightning flash. Globally, some 2,000 on-going thunderstorms generate about 50-100 lightning strikes to earth per second. Lightning is the agency that 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, and power poles — emit varying degrees of induced electric activity. They may respond at breakdown voltage by forming upward streamers. In this intensified local field, one or more leaders likely will connect with one or more streamers. The "switch" is then closed and the current flows. Lightning flashes to ground are the result. A series of return strokes follow.
2.2 Lightning Effects
Thermal stress of materials around the attachment point is determined by:
The radial acoustic shock wave (thunder) can cause 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. 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 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.
2.3 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, 1998, National Severe Storm Center). 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, 1998, International Conference on Atmospheric Electricity). Third, radial horizontal arcing in excess of 20 meters from the base of the lightning flash extends the hazardous environment (Sandia Laboratories, 1997). Lightning is a capricious, random, stochastic and unpredictable event. At the macro-level, much about lightning is understood. At the micro-level, much has yet to be learned.
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 AFS path impedance, Z (Z = R + XL, resistance plus inductive reactance). The resistance term will be low, assuming effectively bonded metallic conductors. 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 1,000 ohms. Therefore, achieving the lowest possible impedance serves to minimize the transient voltage developed across the AFS path through which the current is flowing [e(t) = I (t)R + L di/dt)].
3. Lightning Protection Designs
Mitigation of lightning consequences can be achieved by the use of a detailed systems approach, described below in general terms.
3.1 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. This integral air terminal design does not provide protection for electronics, explosives, or people inside modern structures. Inductive and capacitive coupling (transfer impedance) from lightning-energized conductors can result in significant voltages and currents on interior power, signal, and other 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. These designs have been in use by the electric power industry for over 100 years. Investigation into applicability of dielectric shielding may provide additional protection where upward leader suppression may influence breakdown voltages (Sandia Laboratories, 1997). Faraday-like interior shielding, either via rebar or inner-wall screening, is under investigation for critical applications (U.S. Army Tacom-Ardec).
Unconventional air terminal designs that claim the elimination or redirecting of lightning (DAS/CTS - charge dissipators) or lightning preferential capture (early streamer emitters - ESE) deserve a very skeptical reception. Their uselessness has been well-described in many publications, including: NASA/Navy Tall Tower Study, 1975; R.H. Golde, 1977, “Lightning”; FAA Airport Study, 1989; T. Horvath, 1991, “Computation of Lightning Protection”; D. MacKerras et al, 1997, IEE Proc-Sci Meas. Technol, V. 144, No. 1; National Lightning Safety Institute, 1997, “Royal Thai Air Force Study”; A. Mousa, 1998, “IEEE Trans. Power Delivery, V. 13, No. 4; International Conference on Lightning Protection - Technical Committee personal correspondence 2000; Uman & Rakov, 2002, “Critical Review of Nonconventional Approaches to Lightning Protection,” AMS Dec. Radioactive air terminals have been banned in Europe following investigations by reputable scientists (R.H. Golde, op. cit. and C.B. Moore, 2000, personal correspondence).
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.
Bonding assures that unrelated conductive objects are at the same electrical potential. Without proper bonding, lightning protection systems will not work. 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, personnel metal door frames, hand railings) should be electrically referenced to the same ground potential. 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 cross-joint measuring (maximum 10 milliohms) of connectors to assure continuity is recommended.
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 equipments 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 buried linear or 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) increases volume. Where high-resistance soils, poor moisture content, 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 and/or expensive by traditional means.
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 downconductors together with copper ground wires is a typical situation promising future troubles.
3.6 Transients and Surges
Electronic and electrical protection approaches are well-described in IEEE 1100. Ordinary fuses and circuit breakers are not capable of dealing with lightning-induced transients. A surge protection device (SPD, also known as TVSS) 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.
It is suggested to protect the AC power main panel, all relevant secondary distribution panels, and all valuable plug-in devices, such as process control instrumentation, computers, printers, fire alarms, and data recording and SCADA equipment. Protecting incoming and outgoing data and signal lines (such as modems and LANs) is essential. All electrical devices that serve the primary asset, such as wellheads, remote security alarms, CCTV cameras, and high-mast lighting, 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 replaceable internal modules are suggested.
In all instances, the use of 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.
An uninterrupted-power supply (UPS) provides battery backup in cases of power-quality anomalies — brownouts, capacitor bank switching, outages, lightning, etc. UPSes are employed as backup or temporary power supplies. They should not be used in place of dedicated SPD devices. Correct Category A installation configuration is as follows: AC wall outlet to SPD to UPS to equipment.
The marketplace provides good/not good/poor products for lightning detection. They are available at differing costs and technologies and can be useful to provide early warnings. They acquire lightning signals, such as RF, EF, or light from cloud-to-cloud or cloud-to-ground or atmospheric gradients. Users should beware of overconfidence in detection equipment. It is not perfect, and it does not always acquire all lightning all the time. Detectors cannot “predict” lightning. Detectors cannot help with “bolt from the blue” events. We recommend the redundancy of a network system, a professional grade detector, and a high-quality handheld detector. A notification system of radios, sirens, loudspeakers, telephone, cell phone, or other communication means should be employed with the detectors. See An Overview of Lightning Detection Equipment for a more detailed treatment of detectors.
3.8 Testing and 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 pathways can be inferred reliably. Sensors that 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.
4. Personnel Safety Issues
Lightning safety should be practiced by all people during thunderstorms. If thunder is heard, the accompanying lightning is within one’s hearing range, and evacuation to a safe location should be immediate. Measuring lightning's distance with detection equipment (see above) is useful. The National Lightning Safety Institute recommends suspending all outdoor activities when lightning crosses a 10-mile radius. Activities should not be resumed until 20-30 minutes has expired from the last observed thunder or lightning. This is a conservative approach — perhaps it is not practical in all circumstances. Safety is the prevailing directive.
Specifically for outdoor workers, we suggest an action protocol of:
When lightning threatens, standard safety measures should include the following: avoid water and all metal objects; get off the higher elevations, including rooftops; avoid solitary trees; stay off the telephone. A fully enclosed metal vehicle — van, car, or truck — is a lightning-safe refuge because of the quasi-Faraday Cage effect. (Go to YouTube’s “Faraday Cage” for a demonstration.) A large permanent building can be considered a safe place. Shipping containers (MilVans/Conex) can be converted to safe shelters. In all situations, people should avoid becoming a part of the electrical circuit.
Every organization should consider adopting and promulgating a lightning safety plan specific to its operations. An all-encompassing general rule should be: “If you can hear it (thunder), clear it (evacuate); if you can see it (lightning), flee it.”
5. Codes and Standards
In the U.S. there is no single lightning safety code or standard providing comprehensive assistance. NFPA-780 is a general installation guideline for lightning protection systems. U.S. government lightning protection documents should be consulted. The Federal Aviation Administration's FAA-STD-019d and the U.S Air Force's AFI 32-1065 are valuable. The IEEE 142 and IEEE 1100 Recommended Practices are suggested. Other recommended federal codes include military documents MIL HDBK 419A, Army PAM 385-64, NAVSEA OP 5, AFI 32-1065, NASA STD E0012E, MIL STD 188-124B, MIL STD 1542B, MIL STD 5087B, and UFC 3-570-01. The International Electrotechnical Commission’s IEC 62305 series for lightning protection is a comprehensive reference document for the lightning protection engineer. Adopted by many countries, IEC 62305 is a science-based document applicable to many design situations. Spanish-speaking mining operations should consult the Mexican lightning protection code NMX-J-549-ANCE. Beware of the French NF C17-102 and the Spanish UNE-21186 codes, which are produced by enthusiastic salesmen, not by objective engineers and scientists.
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 and notification, personnel education, maintenance, and employment of risk management principles are recommended.
Areas of vulnerability at typical mines include: security; communications and radio towers; IT operations; gated entrances; conveyors and crushers; drilling; loading; hauling; maintenance buildings; administrative buildings; laboratory buildings; cafeterias; weigh stations; compressor and pumping stations; guard crossings; exploration crew activities; diesel generator stations; as well as other locations specific to particular mine activities.