Cable insulation quality should be regularly assessed to prevent interruption of power. Cable insulation is often damaged by natural weathering and thermal cycles. Insulation may also weaken as it ages. Testing helps you prevent starting problems from getting worse.
Measuring the Tan Delta or loss angle helps technicians understand the condition of the cable insulation. A VLF AC hipot and a Tan Delta bridge are the tools used for this test. This test can spot problems caused by water, trees, insulation cracks and contamination not normally seen through visual inspection.
To test for insulation problems, an AC test voltage from the VLF hipot is applied through the insulator. A cable acts like a capacitor where the insulation serves as the dielectric. Ideally, the applied current should be shifted 90 degrees when measured. Insulation damage can reduce resistivity and change the phase shift. Even minute problems can cause a change in the ideal phase shift of the test voltage.
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