Investigation of service conditions with different phenomena for defects behavior in elements of OHL with covered conductors under operating voltages of 6-35 kV is provided. With aspect to the construction design of covered conductors and condition changes under climatic impacts backgrounds for defect structures formation in metal parts as well as in protection polymeric layers of covered conductors are found. Using offered models mathematical description of defect properties dynamically changing in time is given. Approved OHL construction elements lifetime models help to assess its technical condition in time under service factors impact. Introduction of offered models in practice presents the opportunity to improve the efficiency of service operation and provide the reliability of power supply.