VLF testing can be classified as a withstand or diagnostic test, meaning it may be performed as a proof test for acceptance or as a maintenance test for assessing the condition of cable. Unlike with DC voltage testing, very low frequency is not destructive to good insulation and does not lead to premature failures.

VLF testing is performed with an AC hi-pot at frequency ranging from 0.01 to 1 Hz. The most widely accepted test frequency is 0.1 Hz, however frequencies in the range of 0.0001–1 Hz may be useful for diagnosing cable systems that exceeds the limitations of the test system at 0.1 Hz.

The procedure for VLF testing is nearly identical to that of DC hi-pot testing and is also conducted as a go-no-go test. If the cable can withstand the applied voltage for the duration of the test, it is considered a pass.

Proper test voltage and time duration are critical for the success of the VLF test. If the applied test voltage used is too low and/or too short in duration, the risk of failure in service may increase following the test.

VLF testing is not only used for testing solid dielectric cable, any application requiring AC testing of high capacitance loads can be tested using very low frequency. The major application is for testing solid dielectric cable (per IEEE 400.2), followed by testing large rotating machinery (per IEEE 433-1974), and occasionally for testing large insulators, arrestors, etc.