What is a VLF electrical test?

Very low frequency (VLF) diagnostic testing is the application of an AC sinusoidal waveform, generally applied at 0.1 Hz or lower, to assess the quality of electrical insulation in high capacitive loads, such as cables.
Very Low Frequency testing for Medium Voltage and High Voltage cables is a method for verifying the integrity of the cable insulation. VLF testing is a withstand test usually carried out at a frequency of between 0.1 Hz and 0.01 Hz rather than 50/60 Hz.