This International Standard describes procedures for modelling the dependability of a system and for using the model in order to calculate reliability and availability measures. The RBD modelling technique is intended to be applied primarily to systems without repair and where the order in which failures occur does not matter. For systems where the order of failures is to be taken into account or where repairs are to be carried out, other modelling techniques, such as Markov analysis, are more suitable. It should be noted that although the word “repair” is frequently used in this standard, the word “restore” is equally applicable. Note also that the words “item” and “block” are used extensively throughout this standard: in most instances interchangeably.