T&D World Magazine

Cable Insulation Tester Offers High Ammeter Sensitivity

The LIpATEST is an insulation condition assessment test developed by Powertech Labs primarily for testing of insulation in power cables. In the LIpATEST, Powertech applies a negative, step-dc voltage to an insulation system configured as a guarded, three-terminal test electrode system. The LIpATEST measures the leakage current for one minute at each voltage step. The leakage current is measured with a sensitive, highly filtered, dc pico-ammeter, which is fiber optically coupled, to the data acquisition system. The test is computer controlled and the test duration is less than 10 minutes per phase. The equipment is suitcase-sized and easily transportable.

The LIpATEST is different from a conventional dc high potential, or “HiPot” test, because of the sensitivity of its ammeter used to measure the leakage current through the insulation. The normal HiPot tester’s ammeter has a sensitivity of about one μA (±1 x 10-6 A). The LIpATEST ammeter has a sensitivity of about 100 pA (±100 x 10-12 A). To obtain a leakage current reading above zero with a HiPot tester it is usually necessary to raise the dc voltage to well beyond the normal operating range of the insulation. With the LIpATEST unit Powertech does not raise the test voltage more than 1.5 times the normal peak operating range. This allows us to compare leakage currents, and the “tip-up” of the leakage current among various insulation systems within the normal range of operation. The LIpATEST measurement can only be performed on a de-energized cable.

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