Understanding How Tan Delta Testing Spots Cable Faults

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Tan Delta testing is the most common method of assessing the quality of insulation in cables. This test gives a more detailed assessment of cable insulation compared to high potential testing. With hipot testing, you only get a pass/fail result. Tan Delta testing can help you spot water trees, air pockets, moisture and contaminants in cables.

How does Tan Delta testing find faults? The central conduction and the insulator of the cable form basic capacitor. If an alternating current is passed through a capacitor, the current is shifted by 90 degrees. This information is used to find faults in cable insulation. If there are impurities in the insulator, these lower the resistance of the insulation. This allows resistive currents to flow which causes a lesser degree of phase shift. This is what you call “Loss Angle” or “Dissipation Factor”. To conduct the test, an AC hipot and Tan Delta Bridge is used. The hipot serves as the source of the test current while the bridge is the tool that measures the Loss Angle.

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