Shutdown for maintenance of generating equipment is crucial when planning repairs. In the analysis of this issue, it is necessary to understand the nature of the problem and find ways to solve it. It is necessary to define the concept of reliability in general and reliability of power systems in particular. The article examines electric power systems, their properties and elements. The relationship between functions and properties of energy systems is analyzed, and the main functions (associated with operation of generating equipment and its production) are described. The concept of function means that the object has a task that is being performed. In turn, fulfillment or non-fulfillment of the task is related to the concept of reliability. Reliability is a property of an object to perform a given function associated with its purpose, accounting for the given constraints