Lightning air terminals - is shape important?

Abstract

Benjamin Franklin originally proposed the use of sharp pointed lightning rods as a way to prevent lightning strikes. Such rods do not prevent lightning strikes, but they prevent damage to a structure when it as struck by lightning. Conventional lightning protection systems consist of air terminals (lightning rods) to intercept a lightning discharge, downconductors to carry the current, and a grounding system to dissipate the current away from the protected structure. However, lightning protection systems do not prevent lightning, and the sharp points on lightning rods traditionally used in North America are not needed. To be effective, air terminals should be designed so that they are much more likely to be struck by lightning than objects on the structure they are protecting. Recent field studies indicate that a lightning rod with a blunt tip is more effective than a lightning rod with a sharp tip. Two non-conventional lightning protection systems are heavily marketed in North America - early streamer emission (ESE) air terminals and charge transfer systems (CTS). ESEs are claimed to have a much larger zone of protection than conventional air terminals. Proponents of CTS air terminals claim that corona current emitted from their arrays of sharp points can prevent lightning strikes to protected structures. Field studies of ESE air terminals show that their performance is similar to that of conventional sharp-pointed air terminals, and that they do not have the greatly enhanced zone of protection claimed for them. Field studies of charge transfer systems show that they do not prevent lightning strikes.