The paper focuses on risk-based diagnostics of overhead transmission lines (OHTL). Each element of OHTL (towers, insulators, etc.) is subject to the progression of defects that can lead to technological failures. The probability of technological failures can be reduced by applying various diagnostic methods. In the framework of risk-based diagnostics approach, the risk is expressed as the expected total cost of eliminating technological failures and the cost of the selected scenario for their diagnosis. We provide a mathematical model that describes how the selected combination of diagnostic methods affects the risk of technological failures. We consider various scenarios that reduce the risk of a technological failure and discuss the best combination of diagnostic methods.