VLF AC technology refers to Very Low Frequency alternating current, which is used in high-voltage testing equipment. A VLF hipot is an AC output tester with an output frequency of 0.1 Hz or lower, as opposed to the standard 50/60 Hz. The equipment in the image is a VLF AC hipot from the manufacturer High Voltage Inc. (HVI). These testers are used for performing AC over-voltage withstand tests on high-voltage apparatus like cables, switchgear, circuit breakers, and other electrical components.