The UL101 is a proven technology for electrical inspection. Electrical discharge produces ultrasound, which is detected by the UL101. This discharge can be the result of arcing, tracking or corona (medium/high voltage). The ultrasound detected by the UL101 is converted down into the audible range and output through a headset. A distinct buzzing can indicate the presence of an electrical discharge and an early indication of potential hazard such as arcing or tracking (can sound like frying eggs).
The UL101 is used to sweep the cabinets before they are opened, a major advantage over infrared inspection. If there is any indication of intense ultrasound, steps can be taken to minimize the risks of arc flash.
The recording in image was taken of ultrasound in a transformer where no heat was present. The sound demonstrates the need to safely clean the transformer before it becomes damaged to the point that arcing or tracking occurs.