The hi-pot test can be applied either as a dielectric-absorption test or a step-voltage test. DC high potential tests are "go-no-go" tests. The cable is required to withstand the specified voltage for the specified time duration . These tests will normally reveal major problems due to improper field handling, improperly installed accessories, or mechanical damage. Recommended test voltages are given in standard tables for do and ac. To perform the do Hi-pot test the input voltage to the test set should be regulated. The current sensing circuits in test equipment shall measure only the leakage current associated with the cable under test and shall not include internal leakage of the test equipment. Record wet bulb and dry-bulb temperatures or relative humidity and temperature. The wet bulb temperature is defined as the temperature given by a thermometer bulb which is covered with an absorbent material (linen wet with distilled water) and exposed to the atmosphere so that evaporation will cool the water and the thermometer bulb. The dry-bulb temperature is defined as the temperature of the atmosphere given by an ordinary thermometer. Test each section of cable individually with all other conductors grounded.
All shields must also be grounded. Terminations shall be adequately corona suppressed by a guard ring, field reduction sphere, or other suitable methods as necessary. Precaution should be taken to ensure that the maximum test voltage does not exceed the limits for terminations specified in IEEE Standard 48 or manufacturers' specifications. When applied as a dielectric absorption test, the maximum voltage is applied gradually over a period of 60 to 90 seconds. The maximum voltage is then held for 5 minutes with leakage-current readings taken each minute. In cables, high ambient temperature or humidity at the ends of improperly prepared cables can raise the do leakage current to many times its normal value. When applied as a step-voltage test, the maximum voltage is applied in some equal increments, usually not less than eight, with each voltage step being held for an equal interval of time.
The time interval between steps should be long enough to allow the leakage current to reach stability, approximately 1 or 2 minutes. A leakage
current reading is taken at the end of each interval before the voltage is raised to the next level. A plot of test voltage versus leakage current or insulation resistance is drawn as the test progresses. A nonlinear increase in leakage current can indicate imminent failure, and the test should be discontinued. After the maximum test voltage is reached, a dielectric absorption test can be performed at that voltage, usually for 5 minute period.
how to do AC high-potential testing
Alternating-current high-potential tests are made at voltages above the normal system voltage for a short time, such a 1 minute. The test voltages to be used vary depending on whether the device or circuit is low or high voltage, a primary or control circuit, and whether it was tested at the factory or in the field. Manufacturers' instructions and the applicable standards should be consulted for the proper values.