Evaluation of Rolling Sphere Method Using Leader Potential Concept: A Case Study

Lightning protection design for a structure using Rolling Sphere Method is undertaken. In this case radius of the sphere is calculated using the ‘Leader Potential Concept’ for calculating striking distance. The purpose is to evaluate lightning protection performance based on the ‘Leader Potential Concept’. It is also demonstrated how low grounding resistance and bonding could be used to mitigate Ground Potential Rise (GPR) and secondary effects of lightning activities on electronic equipment. INTRODUCTION Lightning is a phenomenon that has often caused severe damage to life and property. Direct hits may cause for instance structural failure whereas indirect hits, through inductive or capacitive coupling, may affect the reliability and integrity of electronic equipment within the structure. Basically, a conventional method of lightning protection system consists of lightning rods exposed and placed at the highest levels of structures and connected through downward conductors to a grounding system. A design method is normally used to identify the most suitable locations for the lightning rods, based on the area of protection offered by each one. There are different methods of lightning protection systems. Examples of existing methods include geometrical constructions, such as the “Cone of Protection” and “Rolling Sphere Method” which is based on “Electrogeometric” models ( EGMs). In practice, the Rolling Sphere Method is widely used. The method recognizes that the attractive effect of the lightning rod is a function of a striking distance which is determined by amplitude of lightning current. This method is considered relatively simple and easy to apply but often result in over design. This is because calculated results for lightning strike to protected object using this method do not always agree with observed data [1]. Mazur et al proposed using Leader Potential Concept as a means of calculating striking distance [2]. The Leader Potential concept uses line charge model. The model assumes that lightning leader is equivalent to conducting wire within an ambient electric field of a Proceedings of The 2006 IJME – INTERTECH Conference 2 thundercloud. According to Mazur et al, striking distance is a function of both the leader potential and a constant electric field along the negative streamer head of the leader tip. Using striking distance formula by Mazur et al based on the ‘Leader Potential Concept’, a more reliable and economical protection system could be designed. The purpose of this paper is to evaluate lightning protection performance based on the ‘Leader Potential Concept’. It is also demonstrated how low grounding resistance and bonding could be used to mitigate Ground Potential Rise (GPR) and secondary effects of lightning activities on electronic equipment. Electrogeometric Model (EGM) Protection zone of a lightning protection system may be defined as the volume of space inside which an air termination provides protection against a direct lightning strike by attracting the strike to itself [3]. The commonly used engineering tool for determining zone of protection of lightning protection system is the ‘Electrogeometric Model’. This method recognizes that the attractive effect of the air terminal device is a function of a striking distance which is determined by the amplitude of lightning current. The striking distance is the length of the final jump of the stepped leader as its potential exceeds the breakdown resistance of the last gap of air to ground [4]. The EGM model assumes that point on a structure equidistant from the striking distance are likely to receive a lightning strike, whereas points further away are less likely to be struck. The Rolling Sphere Concept A sphere of radius equal to the striking distant is usually employed to visualize the likely stroke termination point, the so-called Rolling Sphere Method (RSM). Application of RSM involves rolling an imaginary sphere of a prescribed radius over the air termination network. The sphere rolls up and over (and is supported by) air terminal, shield wires, and other grounded metal objects intended for direct lightning protection. A piece of equipment is protected from a direct stroke if it remains below a curved surface of the sphere by virtue of the sphere’s being elevated by air terminals or other devices. Equipment that touches the sphere or penetrates its surface is not protected. The basic concept is shown in Figure 1.