Failures of high-voltage systems that recently occurred in Europe and USA resulted in the growth of an interest in problems concerning certainty of supply as well as security of exploitation of electrical power lines. This problem is also current in Poland. The serious failure in Warsaw in November 2004 proved the importance of this topic[1]. As a result of the fracture of two strain line insulators, 15% of power was shut off. About two hundred thousand inhabitants of five districts, and the underground and airport transportation systems were afflicted with blackouts for several hours. The main cause of a breakdown was ageing degradation of the material of insulators which had been in operation for over 30 years. This work presents a non-destructive ultrasonic investigation of a group of 15 line insulators of LP 75/17 type. These elements, made of C 120 type porcelain, have been in operation for about 30 years. The direct reason for this work was the mechanical breakdown of the line strain insulator on the 110/6 kV power engineering installation. On the basis of the measurements the cause of breakage was ascertained and an analysis was carried out of the quality of the tested insulators.
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