**5. Conclusions**

The time-current characteristic of overcurrent relay protection has to be carefully adjusted to the load characteristics, particularly for high-inertia drives. European Standard [4]-defined models are not suitable for some machines, nor for drives powered from weak (relatively high impedance) networks. Ventube fans and their drives are very important as their failure can cause methane explosions, or at least may force an evacuation of miners from the workplace, which leads to large economic losses. Although reliable startup of mining ventube drives is crucial for proper mine operation, no relevant relay protection characteristics exist. Nowadays, they are protected using relay characteristics far from their real startup characteristics. While such a solution enables a reliable start of the motor, it does not protect it from small but long-lasting overloads which can significantly degrade insulation characteristics and therefore shorten the life of the motor. The solution proposed in our paper can help

overcome this contradiction. In addition to the selection of several pre-defined standard characteristics, there should be a possibility of continuous change of the time-current characteristic parameters.

**Author Contributions:** Conceptualization, A.B.; formal analysis, S.B.; methodology, J.J.; software, A.H.; writing—original draft preparation, A.H.; supervision, J.J.; investigation, A.H. and J.J.; validation, J.J.; visualization, A.H.; funding acquisition, S.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** The APC was funded by the Silesian University of Technology, Faculty of Mining, Safety Engineering and Industrial Automation.

**Acknowledgments:** This work was supported by the Faculty of Mining, Safety Engineering and Industrial Automation of the Silesian University of Technology, Gliwice, Poland.

**Conflicts of Interest:** The authors declare no conflict of interest. The funder had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
