The paper deals with a problem of the identification of non-stationary operating conditions for belt conveyor condition monitoring working in underground copper ore mine reality. As a source data instantaneous output and instantaneous current consumed by electric motor have been used. These data have been taken from existing monitoring systems. It has been shown that in mixed, cyclic-continues transportation system, instantaneous load is a nonstationary process with wide range of variation. Before performing diagnosis, this variability has to be identified and described, for example by statistical analysis as proposed in the paper.