Dr. Albin J. Gasiewski, CTO, Enventa LLC, Boulder, Colorado and Professor, University of Colorado at Boulder – January 23, 2012
The development of standard practices for application and installation of lightning protection systems as detailed within the National Fire Prevention Association (NFPA) NFPA-780 document has followed time-tested procedures identified during over 250 years of practice by the building industry. While the phenomenon of lightning is not completely understood, these practices provide justifiable means of protecting life and property against lightning strikes. The basic approach in NFPA-780 is to mitigate lightning damage by implementing a means of initiating streamers (i.e., upward leaders) using grounded air terminals. The standard air terminal, also known as a Franklin rod, is a sharp-pointed heavy gauge metal conductor connected to ground by a similarly heavy gauge conductor. A streamer initiated by the air terminal propagates upwards to connect with the lightning channel leader so that the current in the lightning channel is safely shunted to ground rather than through a nearby structure. Preferential attraction of leaders and channeling of lightning current to ground has proven effective in mitigating damage caused by otherwise randomly located lightning strikes when the equipment is installed according to NFPA standards.
In spite of its completeness with regard to the use of air terminals, the NFPA does not address standards for application or installation of new technology aimed at reducing the likelihood of streamer formation at the outset, and thus for reducing the likelihood of leader attachment and strike current damage. Such “dissipation array system” (DAS®) or “charge transfer system” (CTS) technology is based on the hypothesis that production of positive space charge in the region around the DAS reduces near-surface electric field strength to levels below which streamer formation is likely. With no streamers emanating from the structure of concern the leader is more likely to connect to streamers originating from either other unprotected adjacent structures (both man-made and natural) or from any air terminals installed on these structures. The principle is consistent with Gauss’ electric field divergence law, which states (in one dimension) that the vertical growth of the electric flux density is proportional to the space charge density. If this space charge density is positive near the surface (which is the typical case during a negatively-charged leader approach) then the electric field increases to its peak strength near the leader tip from a surface value near the protected equipment that is lower than if the space charge were not present. As a result of this lower near-surface field strength there is less likelihood of streamer formation near the protected equipment, and hence less likelihood of leader-streamer connection. The principal of operation rests on the DAS being able to rapidly emit ample space charge during the time of leader approach. Accordingly, the DAS design attempts to maximize space charge emission, but without generating streamers of its own.
Due to the distinct physical mechanisms used to mitigate strike damage for the air terminal versus DAS, along with the more recent development of the DAS technology (and thus comparatively fewer case studies of its performance), the NFPA-780 document does not list standards for installation of DAS. Instead, it is believed that installation procedures proposed by the manufacturer of DAS hardware (Lightning Eliminators and Consultants, Inc.) comprise the current best practices regarding their usage. Significant empirical experience has been gained by LEC as a result of data gathered for numerous installations of DAS that suggests both good performance of DAS in averting strikes as well as judicious rules for placement of DAS hardware and procedures for best structural protection using DAS. These placement guidelines and procedures, while not embodied in NFPA-780, are reasonably well established by the manufacturer. However, since the principles of operation of DAS differ significantly from those of air terminals, the placement is guided more by space charge generation, drift, and dissipation physics than geometrical practices such as the rolling sphere method that is commonly used for air terminal placement.
Note that a technology related to the air terminal, an “Early Streamer Emitter’ (ESE) system, is based on the principle of initiating streamers from locations away from the device being protected so as to preferentially draw leaders away from structures and equipment and shunt them safely to ground elsewhere. The ESE thus operates on a distinctly different physical principle than the DAS, and one more closely related to that of the air terminal. However, since the physics of ESE operation is not well established there are similarly no standards for ESE installation defined in NFPA-780.