The paper describes the studies performed by a team of engineers from FURNAS and USP to determine the application conditions, the type and ratings of surge arresters to be installed on a few transmission lines operated by the first-named company in order to improve their performance for lightning strokes. A summary of the line-arrester application principles is included in the beginning of the paper, which subsequently presents the details of the calculations to define the line-arrester characteristics, their position along the lines and the line performance before and after the arresters' installation.