The tan delta test checks insulation quality in medium and high voltage cables. It measures the resistance in the cable as current flows through it to determine the level of phase shift and insulation condition. It can uncover water tree problems, but does not provide information about the location of a problem. Technicians must take precautions to protect themselves from electric shock.
The tan delta test is a procedure a utility can use to check the quality of insulation in medium and high voltage cables. A technician can use this test in a diagnostic evaluation of a malfunctioning cable. It can also be part of a routine maintenance check to determine if the insulation is still in good shape. The test requires some special equipment to measure the resistance in the cable as current flows through it.
When a cable is in good condition, the voltage and current go out of phase and operate at a 90 degree angle to each other instead of pedaling in unison. As the insulation degrades, the level of phase shift decreases. In the tan delta test, a technician observes the change in angle, known as the delta. The test measures the tangent of the corner 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 shafts are water deposits that form inside cables when exposed to moisture. Utilities use insulation and forced air to keep cables as dry as possible, but when the insulation starts to crack, moisture can seep in. Eventually, those lines will need to be replaced, because they can fail or pose a safety hazard. A normal tan delta test can determine which lines should be prioritized for replacement and which ones can wait.
This test does not provide information about the location of a problem. It just shows that the insulation somewhere between two points is starting to break down. A technician can manually inspect a cable for obvious physical signs of damage or can perform other tests to learn more about the situation. Tan delta test equipment manufacturers typically make a variety of other products for diagnosing problems in electrical wiring.
When performing the tan delta test and working with medium and high voltage lines in general, technicians must take precautions to protect themselves from electric shock. This is especially important when service personnel are responding to customer reports of a problem such as a bowed line. If there is something wrong with the line, there is a greater chance that it could misbehave and not wearing protective gear and handling the line with care could be dangerous.
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