Revisiting The Importance of Air Terminal Shapes for Lighting Protection Systems

Abstract:
In lightning protection systems (LPS), lightning rods play an important role as the initial conductive path for lightning to fol-low and safely discharge into the ground. Lightning rods were first invented by Benjamin Franklin in the 1750s, inspired by his ex-periments with electricity and lightning. Since then, lightning rods with sharp tips were supposed to be the best lightning rod shape. According to IEC 62305 shapes of vertical air terminals are not distinguished for LPS. However, about 25 years ago at Langmuir Lab, an extensive research project showed that in a pair rod competition over seven years, none of the sharp rods were struck by lightning, but the blunt rods took a total of 12 strikes. Since 2000, no additional work has been done of this type. Here we report ini-tial results with updated lightning instrumentation. In August of 2023 we measured currents in two lightning rods (one sharp and other blunt) from 12 nearby flashes. These 12 flashes occurred at distances ranging from to 2 to 20 km from the rods. Additionally, we acquired RF source locations with LMA instrumentation for each flash. The results show that sharp rods emit currents before blunt rods most of the time, presumably as a result of space charge screening of the sharp rods. Furthermore, positive CGs seem more likely to elicit a response from the lightning rods. This is consistent with the lower critical field to initiate positive streamer formation over that for negative streamers.