The article consists of two parts which make for an integral body. This article depicts the method of designating the critical damages in accordance with lean maintenance method. Author considered exemplary production system (serial-parallel) in which in time Δt appeared a damage on three different objects. Article depicts the mathematical model which enables determination of an indicator called “prioritized digit of the device”. In the developed model there were considered some parameters: production abilities of devices, existence of potential vicarious devices, position of damage in the production stream based on the capacity of operational buffers, time needed to remove the damages and influence of damages to the finalization of customers’ orders – CEF indicator.