Doctoral thesis presents the main problems of mixed medium voltage (MV) lines and possible solutions of these problems. Performed research of cable lines outer polyethylene (PE) jacket ageing. For this purpose experiments were made at RTU Institute of Polymer Materials with a differential scanning calorimeter (DSC). Developed neutral selection algorithm for MV mixed (overhead and cable) networks, by which is possible easy and quickly select the optimal neutral grounding type. Designed the algorithm for optimal line type selection which is based on economic considerations. It allows quickly and easily compare and evaluate different types of line construction, maintenance and repairs costs,), and depending on whether- companies internal or external- funds the line is constructed and power line environment (on a stretch, in the forest or under asphalt. Developed method for touch voltage calculation in case of earth fault in medium voltage networks. Doctoral thesis written in Latvian language, it contains 4 chapters, conclusions and recommendations for further work, list of references. Total amount is 110 pages, which include 64 figures and 13 tables. List of references includes 85 literature sources.