IEC 61124:2012 gives a number of optimized test plans, the corresponding operating characteristic curves and expected test times. In addition the algorithms for designing test plans using a spreadsheet program are also given, together with guidance on how to choose test plans. This standard specifies procedures to test whether an observed value of: failure rate, failure intensity, meantime to failure (MTTF), and mean operating time between failures (MTBF). The main changes with respect to the previous edition are as follows: - A number of new test plans have been added based on the Russian standard GOST R 27.402 [1], and it is intended to align the new edition of MIL-HDBK-781 [2] with this edition. Algorithms for optimizing test plans using a spreadsheet program are given and a number of optimized test plans are listed. Furthermore, emphasis is laid on the fact that the test should be repeated following design changes; - Discrepancies in test plans A, B as well as Annexes A and B that originated in IEC 60605-7 [3], now withdrawn, have been corrected so these test plans differ from those given in previous editions of IEC 61124. As requested by the National Committees, mathematical background material and spreadsheet program information has been moved to informative annexes. In addition, the symbol lists have been divided, so that some annexes have separate lists of symbols; - Guidance on how to choose test plans has been added as well as guidance on how to use spreadsheet programs to create them. Test plans A.1 to A.9 and B.1 to B.13 have been corrected; - Subcluses 8.1, 8.2, 8.3, Clause 9, Annex C, Clauses G.2, I.2, I.3 and Annex J are unchanged, except for updated terminology and references.