Hipot Testing: Why the Voltage Should Ramp Instead of Snap

A hipot tester that jumps straight to 3 kV works fine on a resistive heater. Put the same setting in front of a long motor winding or a filtered power supply and the leakage meter pegs instantly, the relay drops out, and the product is condemned for a fault it does not have. The tester is not broken and the unit is not leaking. The voltage arrived too fast.

Capacitance turns a step into a surge of current

Every test object has stray capacitance between live parts and the enclosure. The charging current follows i = C × dV/dt. A step to several kilovolts produces a current spike that can reach into the milliamp range, well above typical 5 mA or 10 mA trip limits. A ramp spreads the same charge over seconds, so the capacitive current drops to a trickle by the time the full test voltage is reached.

This is why most safety standards ask for the voltage to be raised gradually, on the order of a few hundred volts per second, and then held for the specified time, often a second for production testing and longer for type tests. The ramp is not ceremony. It separates charging current from genuine leakage.

What the leakage reading does during a ramp

Settings worth checking on your own tester

Start the ramp from zero or from a low starting voltage below the working voltage of the device. Give the ramp enough time that the capacitive spike decays before the final voltage lands; a few seconds usually suffices for motor and transformer loads. Keep the hold time as the standard requires, and discharge the object after the test so the stored charge does not surprise the next operator. If trips still occur on units that pass a DC hipot or an insulation resistance test, the ramp time is the first setting to revisit, before anyone blames the product.