As a result of a job commissioned by an external company, over the period of one year 3 measurements were performed on a steel-cord conveyor belt in one of Polish underground copper ore mines. The tests were performed with the DIAGBELT system for automatic belt diagnostics which was developed at Wroclaw University of Science and Technology. The non-invasive tests revealed that new damage developed in the belt core and that the already existing damage became more extensive. The factor of mean defect number per 1 meter of belt or the number of defects on a given belt section are not sufficient to effectively describe belt technical condition. Quantitative information should be extended with the total surface area of the defects. The analysis of the test results showed that estimations based on 3 measurements are insufficient and that verification should be performed after more measurements are obtained. In addition, the estimations for the complete belt loop are only approximate, as the damaged sections were cut out in the periods between the measurements and replaced with new, undamaged sections.