Ultrasonic detection of insulation degradation

The application of ultrasonic nondestructive testing techniques to the detection of insulation degradation has been investigated in a variety of polyester samples. Some samples contained inclusions such as needles and small steel ball-bearings. The pulse-echo ultrasonic mode was easily able to detect these inclusions by reflections from them. Reflections were also detected from electrical trees and from full breakdown channels in the polyester, between electrodes a few millimetres apart. Furthermore, internal mechanical strain, arising from the casting process, was detectable as an attenuation of the signal reflected from the backwall of the sample. A similar attenuation which arose from strain in the vicinity of barriers between two layers of material was also detectable. The success of this investigation suggests that such techniques may be useful in more practical systems involving multilayered insulation.