The paper presents an evaluation of MV/LV power transformer damage risk due to the impact of ambient temperature at their operation location. It features a presentation of the method of evaluating the power structures’ reliability in the conditions of the structures’ variable durability and exposure values. Based on perennial observations of ambient temperature and failure rate of MV/LV transformers, it was demonstrated that temperature is a factor that causes damage or is jointly responsible for the damage caused in all of the devices’ other failures.