The IEEE Std 400.2-2013: IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF) (less than 1 Hz) was introduced to give an easy to interpret guide for conducting VLF withstand tests on shielded power cables rated 5 – 69 kV. Below is an overview of the recommended voltage levels that should be applied during installation, acceptance, and maintenance testing of medium voltage distribution cables depending on the cable system rating (phase to phase voltage). Generally, VLF withstand testing calls for testing the cables up to 3U0, where U0 is the rated phase to ground voltage.

VLF testing times should last between 15 and 60 minutes, depending on the age of the circuit and what type of test is conducted. For example, a minimum test time of 30 minutes is recommended for aged cable circuits. Extending the time to 60 minutes should be considered for particularly important circuits, such as feeder circuits. For installation and/or acceptance tests, the minimum recommended time is 60 minutes.

The times recommended for VLF withstand testing stem from studies conducted on tree growth rate on partial discharge defects in XLPE cable systems. According to IEEE Std 400: IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems, differences can be seen in the channel tree growth rate between power frequency, VLF sinusoidal, and VLF cosine-rectangular, and the fastest tree growth rate is achieved by applying VLF sinusoidal. A channel tree growth rate with a 3U0 test voltage at 0.1 Hz VLF sinusoidal on field-aged XLPE cables is 10.9 – 12.6 mm per hour (mm/h). A typical 15 kV medium voltage cable in USA has an insulation thickness of 0.22” (5.6 mm), and therefore, during a VLF test time of 30 minutes all defects within the cable should grow to failure. When comparing this figure to power frequency AC or 0.1 Hz VLF Cos-Rectangular, the tree growth rate is only 2.2 – 5.9 and 3.4 – 7.8 mm/h, respectively. As a result, 0.1 Hz VLF sinusoidal is the ideal frequency and waveform for cable withstand testing. Potential failures should happen during the actual test so that repairs can be made immediately. Failures in the cable during service result in higher costs for the utilities and are a nuisance for the power consumer.