According to industry estimates, 85% of medium-voltage equipment failures are caused by partial discharge (PD) activity. PD is defined as a breakdown that does not completely bridge the gap between two electrodes. It is a partial failure of the insulation that occurs because of contamination, flaws, voids, or irregularities inside or on the surface of the insulation. Common causes include poor manufacturing processes, improper design, poor workmanship, and adverse environmental conditions. These imperfections cause local stresses that exceed the capability of the insulation and generate small sparking events that produce signals with unique characteristics that can be detected non-intrusively while the equipment remains in normal operation.

Experience has shown that internal insulation defects, such as those within cable terminations, instrument transformers, or insulators, can only be detected using Transient Earth Voltage (TEV) sensors that pick up the radiated electromagnetic signals from the flaw while ultrasonic sensors work best for detecting surface problems. This is because they generate small airborne pressure waves that can be “heard” through air vents or equipment enclosure openings.

Partial discharges are rarely present on low-voltage equipment. They are much more likely to occur on medium-voltage equipment where the voltage stress on the insulation is much greater. Once PD is initiated, it will progress until complete failure occurs.