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Shielding failure has long been recognized as a possible mode of lightning flashover. This paper reports the development of a method for estimating the shielding failure number of transmission lines using the charge simulation method. The effects of towers, sags of conductors, and a perfectly conducting ground are represented in 3-D computation. In addition, the stepwise descending nature of a downward negative leader-streamer system is taken into account by using an appropriate progression model. Upward leader inception and propagation is also modeled utilizing critical equivalent streamer-length criterion as well as Rizk's formula. The results of the comparison between traditional and proposed techniques with different criteria are also presented.