Typical electrical components that can be inspected with ultrasound and infrared include switchgear, load interrupter switches, breakers, transformers, motor control centers, and terminal transition cabinets.
One electrical anomaly that ultrasound will detect is corona. Even though corona produces little to no heat, it does however produce ultrasonic emissions. If the inspector’s ultrasound instrument has on-board sound recording capability, the ultrasound emission from corona can be recorded and further analyzed for a correct diagnosis. A note of importance on corona is the fact that it is only present in voltage above 1000 volts. At 1000 volts and greater, air becomes a conductor and hence the ionization of air surrounding a connection can occur.
When the recorded ultrasound of corona is looked at in a spectrum analysis software, very prominent 60Hz harmonics can be noted. If the sound recording is done outside of North America, one would see very dominant 50Hz harmonics. Additionally, in between the 60Hz harmonics, you would see what is referred to as frequency content. Frequency content is basically harmonic activity between the more dominant harmonics. As the condition worsens, there will be a loss of the dominant 60Hz harmonics, and uniformity in the amplitude of the recorded ultrasound will decrease.
Tracking occurs when there is a low current pathway to ground across an insulator. Many will refer to tracking as “baby-arcing.” This event is common where there is severe breakdown of the insulating material and loose connections. Tracking can occur in low, mid, and high voltages and characterized as a steady buzzing sound with periodic “crackling” and “popping” sounds. Further damage is done when tracking is not corrected, and will rather quickly lead to arcing. Finally, arcing happens when there is a discharge to ground across an insulator.
