Lightning Rod Improvement Studies

Abstract

Although lightning rods have long been used to limit damage from lightning, there are currently no American standards for the shape and form of these devices. Following tradition, however, sharp-tipped Franklin rods are widely installed despite evidence that, on occasion, lightning strikes objects in their vicinity. In recent tests of various tip configurations to determine which were preferentially struck by lightning, several hemispherically tipped, blunt rods were struck but none of the nearby, sharper rods were ''hit'' by lightning.Measurements of the currents from the tips of lightning rods exposed to strong electric fields under negatively charged cloud bases show that the emissions consist of periodic ion charge bursts that act to reduce the strength of the local fields. After a burst of charge, no further emissions occur until that charge has moved away from the tip. Laboratory measurements of the emissions from a wide range of electrodes exposed to strong, normalpolarity thunderstorm electric fields show that positive ions are formed and move more readily over sharp-tipped electrodes than over blunter ones. From these findings, it appears that the electric field rates of intensification over sharp rods must be much greater than those over similarly exposed blunt rods for the initiation of upwardgoing leaders.Calculations of the relative strengths of the electric fields above similarly exposed sharp and blunt rods show that although the fields, prior to any emissions, are much stronger at the tip of a sharp rod, they decrease more rapidly with distance. As a result, at a few centimeters above the tip of a 20-mm-diameter blunt rod, the strength of the field is greater than that over an otherwise similar, sharper rod at the same height. Since the field strength at the tip of a sharpened rod tends to be limited by the easy formation of ions in the surrounding air, the field strengths over blunt rods can be much stronger than those at distances greater than 1 cm over sharper ones.The results of this study suggest that moderately blunt metal rods (with tip height-to-tip radius of curvature ratios of about 680:1) are better lightning strike receptors than are sharper rods or very blunt ones.

Attachment of lightning flashes to grounded structures

“…However, there is an optimum radius that is slightly more efficient (with an improvement of only 10% in the striking -inception -distance) than other rod radii to initiate upward leaders. This optimum radius agrees with the observations of competing rods in New Mexico [85,86] although the outstanding differences observed between sharp and blunt rods are attributed in Paper II to be caused by the setup of that experiment. x The initiation of upward connecting positive leaders is also strongly affected by the average velocity of the downward stepped leader.…”

“…This is partly due to the several experiments conducted at instrumented tall towers [44,61,[80][81][82][83][84][85][86] and to the use of rocket triggered lightning techniques [74][75][76], and partly due to the improvement in theoretical leader inception and propagation models [54][55][56][57][58][59]. From the experimental perspective, different experiments with natural and triggering lightning have provided valuable information about the physical properties of upward and downward leaders under natural conditions.…”

“…The model is validated by comparing its predictions with the initiation time of leaders in the laboratory under switching voltage waveforms and with an altitude rocket triggered lightning experiment [74]. Based on the predictions of the model, the effect of the tip radius on the efficiency of lightning rods as well as the field observations of competing lightning rods [85,86] are analyzed and discussed.…”

“…Although the testing of ESE terminals or lightning rods under natural conditions is the very best way to solve the controversy, it has obvious limitations [30]. Moreover, well-planned field experiments [85,86] devoted to evaluate the efficiency of lightning rods have been subject of questions [30]. Due to these reasons, the manufacturers of ESE devices keep indiscriminately using laboratory experiments to justify their claims [33,34].…”

Time dependent evaluation of the lightning upward connecting leader inception

“…As the lightning stepped leader approaches to ground, several bursts of streamers and aborted leaders are produced from grounded rods before the inception of the stable upward leader. This fact is not observed in laboratory experiments [1][2][3] but it has been found in field measurements associated with lightning flashes [22,23]. The charge associated with these streamer bursts and aborted leaders reduce the electric field in front of the rod.…”