Very Low Frequency, (VLF) cable testing is the cutting-edge technology for proof testing of fully operating medium voltage cables. VLF cable testing makes use of AC voltages in a frequency range of 0.01 Hz to 0.1 Hz, allowing for cables lengths up to 10 miles. Unlike DC hipot proof testing, VLF allows for lower voltage levels, minimizing cable damage. This technology works with XLPE, PE, EPR, and PILC cable type insulation. Adding the Tan-Delta module to the testing regiment allows for metrics to be correlated to cables, not just PASS/FAIL. Tan-Delta data can be archived for comparative analysis with future maintenance testing to monitor cable degradation. VLF Tan-Delta data provides maintenance planners information to rate cables systems to aid in determining long-term cable replacement priorities.