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The paper refers to thyristor ac to dc converters for industrial applications, nonreversing and reversing, in full-controlled three-phase bridge connection, as used for armature supply of dc motors and excitation of large ac generators. The possible causes of incorrect operation as well as the types of fault currents produced are investigated; the means available to interrupt or limit the value and the duration of these fault currents are recalled. Having in mind the performance required from the protective means in the various practical applications, a few different protection classes are defined. The criteria are then illustrated upon which selection of the protective means and their operating logic is based, in order to reach the targets put forward for each class of protection.