Comparison of standardised methods for implementation of direct-strike lightning protection

The protection of a building, structure, facility or site against direct lightning strikes is the first important step in providing a comprehensive lightning protection solution (LPS). This task is achieved with lightning rods or “air terminals” appropriately positioned or placed to provide a desired “interception efficiency” or level of protection. The number of air terminals required to provide this level of protection depends on the protection area afforded by each air terminal. The correct placement and number of air terminals is determined by the “design method” that is used. These methods are typically explained in lightning protection standards around the world.

Air terminal placement methods typically fall into one of five categories, namely:

      1. Pure geometrical constructions, such as the historic “cone of protection” or “protection angle” method (PAM).
      2. Faraday Cage concepts, in which a “meshwork” of conductors or air terminations is placed at set intervals over a structure.
      3. Electrogeometric models which use empirical relationships to relate striking distance to the peak current of a lightning stroke. The most common example is the “Rolling Sphere Method” (RSM), which is also partly a geometric construction.
      4. The “Early Streamer Emission” (ESE) method, which relies on a time advance of the upward streamer-leader system.
      5. Leader propagation models, where the physical mechanism of air breakdown are applied to the lightning scale and factors such as structure geometry, upward leaders, atmospheric parameters and leader propagation velocity are taken into account.


The aim of this paper is to make an objective, quantitative comparison of five different air terminal placement methods that are currently in use, under the same conditions. The five methods considered are the PAM, RSM, collection volume/field intensification method (CVM/FIM), leader inception theory (LIT) and ESE. The comparisons are made using case studies of typical structures.

The results are presented in the form of a spectrum – from most conservative to least conservative – in terms of the number of similar-height air terminals required to protect the structure. The results show that there are considerable differences between the methods. The reasons for these differences are explored in the paper. The mesh method was not included in this study as it does not provide interception of lightning strikes, rather it is a collector of lightning current once it strikes the structure

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