The Aim of this paper is to evaluate the advantages of digital twin technology as compared with the conventional approaches to the design of a vital two-channel system. Methods. The system is described with a Markovian model. This model allows defining the quantitative safety characteristics if the system is affected by right-side failures. Results. The system’s primary quantitative safety indicators were identified as the mean time to wrong-side failure and mean time to right-side failure along with the quantitative relations of the prime and additional costs for a batch of products. Conclusion. Transforming the initial item into a system with digital twins allows significantly reducing the rate of wrong-side failures. This effect may be obtained not only with the use of digital twins, but also as the result of the system transitioning into the state of right-side failure in each event of discrepancy betwin the initial item and/or the digital twins. It has been established that the mean time to right-side failure under such conditions is not less than the mean time to failure of the initial item. That means that highly efficient measures for safety improvement allow maintaining the system dependability at a level not lower than that of the initial item. The introduction of digital twins into a system is a new, not yet tested way of ensuring system safety. The decision on the benefits of additional costs is taken by the customer and system developer together. At the same time, it must be taken into consideration that in case of large batches of manufactured technical systems, the effect of additional costs is reduced and the effect of significantly improved safety is maintained.

Aim. To harmonize the definitions of errors, faults, failures in the Russian and English languages. The Object of the paper is one of the most important subject matters of the dependability theory and functional safety. The Subject of the paper is the concepts and definitions of failures, errors, faults.Results of the research: analysis of the definitions of the concepts describing the dependability and functional safety of items in the Russian and international standards, such as GOST 27.002-2015, GOST R/IEC 61508-2012, IEC 60050, DIN 40041, as well as in publications by a number of authors. The analysis shows that failure is always associated with the loss of function, i.e., the ability to perform as required by all standards. It should be noted that wrong user expectation does qualify as failure. A failure should be distinguished from unintended functions. A fault is defined as a system’s inability to perform the required operation to the full extent that, under certain conditions, may escalate into a failure. An error as a discrepancy between a calculated, observed or measured value or condition and a true, specified or theoretically correct value or condition is a deviation that is present and, under certain conditions, would probably turn into a failure. A typical example is non-critical software errors. The so-called systematic failures are actually errors that can turn into critical errors (failures). Let us note that the definitions in the IEC 60050 international electrotechnical vocabulary can be used, as they show general agreement, which is not surprising for an international standard.