This type of lightning protection system consists of grounded masts or poles supporting a wire or wires suspended over the site in such a manner as to include site structures within the zone of protection defined by the masts and overhead wires.
It is useful to note that this type of system was primarily designed to protect electric utility company transmission and distribution lines. When utility power lines are erected, a ground (or static) wire is run above the phase conductors. The intent is to intercept any lightning strikes with the ground wire, convey it to the support structures and thence to ground. This theoretically prevents lightning from attaching to the phase conductors where the energy would be conveyed to customer facilities. The overhead wires have no effect on streamer formation, and therefore do not preferentially attract lightning away from the phase conductors. They merely “get in the way” of a direct strike, intercepting and conveying it to ground. This is pretty much the only type of lightning protection system that is practical for protecting transmission and distribution lines.
However, when this type of system in applied to other types of facilities, it can actually create problems. The catenary system is intended to intercept direct lightning strikes, keeping the heat and energy of the strike off the protected structures. However, most ignitions of oilfield facilities are not caused by direct lightning attachment, but are caused by secondary effect arcing. A catenary system is designed to intercept a direct lightning strike and convey it to ground immediately adjacent to the protected facility. It is this introduction of energy to ground that causes secondary effect arcing, the primary cause of ignition. In fact, if a catenary system performs exactly as designed and intercepts a direct strike, it maximizes the likelihood of secondary effect arcing across the tanks and appurtenances by bringing the lightning energy to ground near the base of the tank battery. The risk of high step and touch potentials is also increased and may endanger site personnel.
The catenary system also has no effect on the static charge on the stored product, does not provide bonding to miscellaneous masses of inductance on the tank, and does not provide purpose-designed air terminals on the tank or tank battery. As these are the most important components of lightning protection for production sites, the catenary system has little or no value in this application.
Part of the reason that catenary systems have been selected for use protecting production and storage sites is that their design and application was, until the 2017 revision, described in the National Fire Protection Association NFPA 780 standard in chapter 7, “Protection of Structures Containing Flammable Vapors, Flammable Gases, or Liquids That Can Give Off Flammable Vapors”. Inclusion of the catenary system in this section gave the impression that it was the preferred method of protection for these facilities. However, that was not the intent of the technical committee. This inclusion in chapter 7 was misleading in that the design and application of all other types of lightning protection systems are addressed in chapter 4, “General Requirements”. This has been rectified in the 2017 edition of NFPA 780, with the description of catenary systems moved from chapter 7 to chapter 4.