1. When conducting DC withstand voltage test, the cable must be fully discharged
The capacitance of power cables is very large. After DC withstand voltage test, the energy of residual charge is still relatively large, which directly affects the measurement of insulation resistance and absorption ratio. If the cable is not fully discharged before the insulation resistance test after the first DC withstand voltage test, the charging current and absorption current will be smaller than the first time, resulting in a false increase in insulation resistance and a decrease in absorption ratio.
In addition, conducting insulation resistance test immediately after DC withstand voltage test can produce false phenomena of decreased insulation resistance and increased absorption ratio. This is mainly caused by the opposite polarity of the wiring voltage of the megohmmeter used for measuring insulation resistance compared to the DC withstand voltage polarity. If the cable is not fully discharged after DC withstand voltage test and insulation resistance is measured immediately, the insulation resistance meter needs to output a lot of charge to neutralize the residual charge in the cable, causing a false decrease in insulation resistance. Since the DC withstand voltage test time is generally 5 minutes, the discharge time after the DC withstand voltage test should be greater than 5 minutes. The longer the cable, the longer the discharge time. After insulation resistance testing, the discharge time should be greater than the charging time.
2. Shielding must be applied during DC withstand voltage test
When conducting DC withstand voltage and DC leakage tests on power cables, due to the high test voltage, the leakage current of cables with good insulation is small, so the stray current caused by the equipment has a great impact on the test results. In order to eliminate the influence of stray current on the test results, the microammeter is connected to the high-voltage side, and the high-voltage lead and microammeter are shielded. This test wiring, because the microammeter is connected to the high-voltage circuit and the high-voltage lead and microammeter are shielded, can eliminate the influence of corona on the high-voltage lead and stray current from test equipment on the test results, and the accuracy of the test results is high. This wiring can be used for cables with or without insulation of the outer sheath to ground.
Under harsh environmental conditions, the surface leakage current of cables is large, making the test data unable to reflect the true insulation condition. Adding shielding at both ends of the cable to eliminate surface leakage current can completely eliminate the influence of surface leakage at both ends of the cable and measure the true leakage current data of the cable insulation.
