In the DC withstand voltage of a power cable, the cable is equivalent to a capacitor and is charged by a high voltage. At this time, if the supply voltage drops or artificially reduces the test voltage, the high voltage charged by the cable is higher than the applied test voltage, and the cable will pass. The microampere meter and the test equipment are discharged to form a negative leakage current value. This phenomenon is not a problem, but pay attention to the voltage regulation performance of your test equipment and whether the power supply system is stable to prevent the test voltage from rising unexpectedly beyond the specified test voltage.
To withstand the pressure of the cable, it is best to disconnect other equipment connected to the cable, or to keep a few devices with higher withstand voltage values.
Check whether the microampere meter is shielded. If not, shield it.
Raise the test voltage by air to check the test equipment.