What Is the Tan Delta Test?

Mary McMahon

The tan delta test is a procedure a utility can use to check on the quality of insulation in medium to high voltage cable. A technician can use this test in a diagnostic evaluation of a cable that is not performing properly. It can also be part of a routine maintenance check to determine if the insulation is still in good condition. The test requires some special equipment to measure the resistance in the cable while current passes through it.


When a cable is in good condition, the voltage and current undergo a phase shift and operate at 90 degree angles to each other rather than cycling in unison. As insulation degrades, the level of phase shift decreases. In the tan delta test, a technician looks at the change in angle, known as delta. The test measures the tangent of the angle delta to determine how much insulation is left, and what kind of condition it is in. A score of zero indicates a cable in perfect shape.

One of the most common problems this test can uncover is a phenomenon called a water tree. Water trees are deposits of water that form inside cables when they are exposed to moisture. Utilities use insulation as well as forced air to keep cables as dry as possible, but when the insulation starts to break down, moisture can penetrate. Eventually, such lines will need replacement, because they can fail or pose a safety hazard. A regular tan delta test can determine which lines should be prioritized for replacement, and which can wait.

This test does not provide information about the location of a problem. It only shows that insulation somewhere between two points is starting to break down. A technician can manually inspect a cable to look for obvious physical signs of damage, or may pursue other testing to find out more about the situation. Manufacturers of tan delta test equipment typically make an array of other products for the diagnosis of problems in electrical cables.

When performing the tan delta test and working with medium and high voltage lines in general, technicians need to exercise precautions to protect themselves from electrical shock. This is especially important when service personnel respond to customer reports of a problem like an arcing line. If something is wrong with the line, there is an increased chance that it may behave abnormally, and failure to wear protective gear and handle the line with care could be dangerous.

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