the period of usage required for a device to attain the operating limit. The operating period includes the effective operating time and all types of idle time, whether caused by maintenance and repair or by organizational or other reasons. Operating periods of devices of the same type may differ, since many unforeseen random factors come into play, for example, structural peculiarities in the device and factors relating to operating conditions. For this reason, a quantitative evaluation of operating period is carried out with the aid of probability indexes, such as average operating period (mathematical expectation of operating life) and the 7-percentage of operating life (the calendar period of usage during which the device will not reach its operational limit with a specified probability of 7 percent).
The rated operating period is the period of usage after which the device is either removed (and written off) or routed for an investigation of its technical condition with the aim of determining its suitability for further operation. If the device is operated continuously, its operating period coincides with its service life. In all other cases, the relationship between the operating period and service life of a device is determined by the intensity of operation.
V. N. FOMIN