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Formulas giving the impedance and admittance matrices of a system consisting of three underground single-core cables have already been proposed. By applying now classical methods, we deduce the wave-propagation modes in this system. The present paper differs from its predecessors in proposing: -a simplified method for working out propagation parameters (problem reduced to 2 dimensions instead of 6); -a definition of modes in the form of a "generalized concordia matrix" similar to what is done for overhead lines and having the advantage of greatly simplifying the calculation of the transient response of cables; - a definition and a method for calculating the impedances that characterize the modes. Furthermore, full-size tests, the results of which are given in the second part of this paper, have made it possible to check the validity of the hypothesis on which this study is based.

In this paper different methods are proposed for estimating the risk of failure on lines caused by switching overvoltages. These methods correspond to various degrees of simplification. They are compared in study of a single particular case in order to deduce which the most practical one that gives satisfactory results.