Reliability of the technological processes or reliability of devices used in different industries is an important part of designing safety critical systems. The failure of such systems leads to economic losses, health damage or environmental pollution. An important role in the development of safety critical systems is therefore the reliability analysis, the assessment of the risks associated with the use of the technical means and the consequent reduction of this risk. The actual level of risk considered tolerable will vary depending on a number of factors such as the level of human control over the circumstances, the voluntary or unintentional nature of the risk, the number of people at risk in each individual case, the degree of responsibility placed on safety and critical systems reflects the need for quality design and ensure of software safety. Various standards and methods are used to achieve the desired level of safety. One of the methods used for reliability analysis is the use of a block diagram of reliability.