It is important to realize the potential hazards of operating a facility without proper grounding (earthing) and lightning protection, especially a temporary and movable structure or set of structures.
Costly consequences that may result from lightning strikes to mission-critical facilities include equipment downtime, the costs associated with lightning related equipment damage, facility and equipment repair and replacement costs, danger to life and other consequences. Most importantly, the possibility of lost mission capability from equipment and instrumentation and personnel operational disruptions is unacceptable in many situations. Both facts and experience stress the requirement for three protection tiers encompassing comprehensive grounding/earthing, surge suppression and lightning protection systems for robust facility protection and uptime. These three tiers create the true and complete lightning protection system (LPS).
The interrelationship and interdependence of the three tiers starts with proper grounding, as neither the surge suppression nor lightning protection will function correctly without it. This is followed with surge suppression, where equipment is exposed to the secondary effects of lightning as well as internally generated transient voltages. Finally, direct strike lightning protection protects the physical structure and its contents. These three subsystems interlock into a robust and stable facility protection system.
Whether it is a metal structure at an oil/gas/petrochemical facility with attendant hazardous environment or a mission-critical military installation, electronics and personnel considerations demand overall direct and indirect lightning protection. A metal structure needs this protection, even though many metal structures are often generally viewed as a “faraday cage” and supposedly therefore immune from lightning. The reality is that there are many avenues for direct paths and indirect and induced paths for lightning energy to get into metal structures. In addition, lightning system codes do not allow the use of metal less than 3/16-inch in thickness as a component of an LPS.
Many metal structures are therefore non-compliant to code without additional LPS components. ALLTEC, Canton, N.C., is a consulting engineer and product solutions provider for many such applications, including mobile military shelter programs. We will discuss the external solution as the most visible and unique portion of the lightning protection three-tier system solution designed for the client.
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| Traditional Franklin rod air terminal rapid breakdown under a high voltage test. |
Science and experience show that Charge Dissipation Terminals (CDT), and the structures on which they are installed, are much less likely to sustain a direct lightning strike than unprotected structures or structures with traditional “Franklin Rod” lightning protection systems. ALLTEC engineers chose to design the metal shelter solution utilizing TerraStat Charge Dissipation Terminals. These are created to meet or exceed UL96A, NFPA 780, and IEC 62305-3 standards, while providing the enhanced performance offered by CDT technology. Facility designers, planners and owners often are not aware that UL96A, NFPA780, LPI175, IEC 62305 and other standards for lightning protection describe minimum requirements for lightning protection system design and installation.
Designs utilizing CDTs are created to meet and exceed those standards, offering enhanced protection. In addition, the CDTs are UL 96 listed air terminals and may be installed under the auspices of UL 96A “Master Label” lightning protection systems. CDTs incorporate electrodes that break down into corona before upward lightning “streamers” can form. These terminals lower accumulated static charges, consequently reducing or delaying upward streamer generation, thus mitigating the likelihood of a streamer connecting to a downward “leader” resulting in a direct lightning strike event.
Let’s review this technology. When a pointed, grounded conductor is placed in a high electric field (such as on a structure located in a thunderstorm), voltage effects at the point are increased greatly. Electrons from atmospheric atoms and molecules are stripped away and flow to the ground through the grounded conductor, leaving behind positive atmospheric ions around the point. This process is commonly referred to as the “corona” effect. Charge dissipation and the generation of space charge around the TerraStat CDT has been verified by emission current testing and measurement at an independent, high-voltage laboratory.
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TerraStat CDT dissipates charge buildup without breakdown under a high voltage test. |
The high-voltage laboratory testing validates the air-gap breakdown delay time (delta-T) of TerraStat CDT terminals relative to traditional Franklin rods. The testing confirmed that the CDT under the same applied voltage dissipates the buildup of visible corona through ionization/charge dissipation without the occurrence of a catastrophic lightning strike as with a Franklin rod.
Thus the metal structure’s natural ground-charge buildup preceding a lightning event is reduced and renders the structure less likely to be a source of streamer formation. In addition, the reduction of static and ground charge mitigates harm to sensitive electronic equipment under operation within the shelter, even without a lightning event.
It is worth noting that industry recommended practice for hazardous oil/gas/petrochemical facilities addresses this in API-2003, “Protection against Ignitions Arising out of Static, Lightning, and Stray Current: Appendix C.” This document describes CDTs as one of the lightning protection technologies used to mitigate the path of an incoming lightning stroke. Selection of CDTs should be based on thorough analysis that considers the probability of lightning strike at the site, the risk associated with it being directly struck by lightning and its installation.
In conclusion, metal structures and facilities are not inherently lightning protected and must incorporate a properly designed protection system to ensure maximum uptime and minimum personnel and property damage.
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Jim Grasty is the engineering manager and Doug Ritchie is vice president and general manager at ALLTEC, which develops and implements global clients’ lightning and surge risk-mitigation solutions and operates from facilities in Canton and Asheville, N.C. For more information, go to www.alltecglobal.com.