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Reply To: Wiring Riddle No.10 – Hipot test


    Any test that uses the test source voltage to be higher than the in-service operating voltage could be classified as destructive test because during testing the cable insulation will be subjected to a higher voltage than what it will see in service. Therefore, all hi-pot withstand tests would fall into this category. However, during a hi-pot test, if the voltage is applied in a steps and the leakage current is monitored, then the test may be classified as being nondestructive. The reasoning for this is that the test can be aborted before the insulation gets to a failure point since at every step of voltage application the leakage current is being monitored and evaluated before proceeding to the next step. An application of this test procedure is the step-voltage DC hi-pot withstand test. The same cannot be said of AC hi-pot withstand test since there is no way to evaluate the leakage current, therefore this test would be considered as go-no-go test and considered to be destructive. Insulation Resistance and DC Hi-Pot Testing In the past, insulation resistance and DC HV (hi-pot) tests have been used for acceptance (proof) and maintenance testing of cables. When testing cables with DC voltage, it should be understood that DC voltage creates within the cable insulation system an electrical field determined by the conductance and the geometry of the cable insulation system. However, the normal service voltage applied to cable is AC 60 Hz voltage, thus during normal service conditions the AC voltage creates an electrical field that is determined by the dielectric constant (capacitance) of the insulation system. Therefore the electric stress distribution with DC voltage will be different than with AC voltage. Further, conductivity is influenced by temperature to a greater extent rather than the dielectric constant, therefore comparative electric stress distribution under DC and AC voltages will be affected differently by changes in temperature in the insulation. The DC voltage tests are effective in detecting failures that are triggered by thermal mechanism. The value of the DC voltage diagnostic tests for extruded-type insulation are somewhat limited because failures under service AC voltage conditions are most likely to be caused by PDs in the voids of extruded insulation rather than by thermal mechanism. On the other hand, the DC voltage diagnostic tests are very meaningful for laminated-type insulation system where the failure is most likely to be triggered by thermal mechanism. The current trend is to minimize the use the DC hi-pot tests on extruded insulation for the reasons discussed above and because of potential adverse charging effects of DC hi-pot tests on extruded insulation. AC Hi-Pot Testing Cables and accessories may also be field tested with 60 Hz AC voltage, although this is normally not done because of the requirement for heavy, bulky, and expensive test equipment that may not be readily available or transportable to a field site. The most common field tests performed on cables are DC hi-pot or VLF tests, such as one-tenth of hertz frequency tests in lieu of AC hi-pot tests. However, if AC hi-pot acceptance and maintenance tests are to be conducted on cables, then it should be borne in mind that this test is not very practical in the field. Further, the AC hi-pot test can only be conducted as go-no-go test, and therefore it may cause extensive damage should the cable under test fails, i.e., a disruptive discharge through the insulation takes place during the test. On the other hand, AC hi-pot test has a distinct advantage over other test methods of stressing the insulation comparably to normal operating voltage. Further, this test replicates the factory test performed on the new cable. When performing the AC 60 Hz hi-pot test consideration should be given to the adequacy of the test equipment for successfully charging the test specimen. The AC test equipment should have adequate volt-ampere (VA) capacity to supply the required cable charging current requirements of the cable under test. The VA capacity of the AC hi-pot test equipment may be determined by the following formula. VA =2?fcE2 or kVA =2?fcE2