A typical DC hipot test applies a voltage in gradual steps, commonly called ramping, pausing after each increase to allow the capacitance of the DUT to absorb a charge and stabilize. As shown in Figure 8, the current increases sharply after each increase in voltage as the capacitance charges, and then decreases to a low steady-state value. The time required for the charging current to decay after each step is called the stabilization time. Current that flows after the stabilization time has passed represents the leakage current through the insulation. If the voltage steps are too large, the sharp rise in charging current when the step is applied may exceed the high current limit, causing the test to fail prematurely. The magnitude and timing of the steps, therefore, should be carefully matched to the characteristics of the DUT.

By monitoring current flow as you gradually increase the applied voltage, waiting for the charging current to decay, and observing the leakage current (if any), you can detect a potential insulation breakdown before it occurs. If the leakage current suddenly starts to increase over the expected value, an insulation breakdown is likely to occur soon. Interrupting the test at this point can save the insulation from breakdown. The test fails but the product is not damaged and may be salvaged by visual inspection or some other means. Such a test, therefore, can be classed as “nondestructive”. If the product being tested does not have significant capacitance, there is little or no charging current, and the rate at which the voltage is gradually increased can be much more rapid.

Because a DC hipot test charges the capacitance of a DUT, the charge itself can present a hazard to testing personnel that must be removed after the test is over. Remove the stored capacitance by discharging the DUT to ground. Typically, the hipot tester automatically discharges the DUT for the same period of time the test voltage was applied.