Tan Delta, also called Loss Angle or Dissipation Factor (DF) testing, is a diagnostic method of testing cables to determine the quality of the insulation. If the insulation of a cable is free from defects like trees, moisture and air pockets, etc., the cable approaches the properties of a perfect capacitor.

In a perfect capacitor, the voltage and current are phase shifted 90 degrees and the current through the insulation is capacitive. If there are impurities in the insulation, the resistance of the insulation decreases, resulting in an increase in resistive current through the insulation.

The cable becomes a less perfect capacitor and the phase shift will be less than 90 degrees. The extent to which the phase shift is less than 90 degrees is called the "loss angle," which indicates the level of insulation quality/reliability.

Cables with poor insulation have higher DF values than normal, and will exhibit higher changes in the tangent delta values with changes in applied voltage levels. Good cables have low individual TD values and low changes in tangent delta values with higher applied voltages levels.

In practice, a very low frequency AC Hi-pot is most often used as the voltage source to energize cable for tangent delta tests. Very low frequency is the preferred method over 60Hz for two reasons:

The increased load capability in field applications in which 60 hertz is too bulky and expensive, making it nearly impossible to test a cable of considerable length. At a typical VLF frequency of 0.1Hz, it takes 600 times less power to test the same cable compared to 60 Hz.

The magnitude of tan delta numbers increase as frequency decreases, resulting in easier measurements.

When performing tan delta, the cable to be tested must be de-energized and each end isolated. Test voltage is applied to the cable while the tan delta test set takes measurements.

The applied test voltage is raised in steps, with measurements first taken up to 1Uo, or normal line to ground operating voltage. If the tan delta numbers indicate good cable insulation, the test voltage is raised up to 1.5 – 2Uo.

The test itself can take less than twenty minutes, depending upon the settings of the instrument and the number of different test voltage levels used. It is only necessary to capture a few cycles of the voltage and current waveform to make the analysis.