AC vs DC Hipot Testing: When to Use Which and Why It Matters

Choosing between AC and DC hipot testing isn't just a checkbox on a test sheet - it directly determines whether you catch a defect or get lulled into a false sense of safety. Here's how to make the right call.

AC hipot testing applies a sinusoidal voltage at line frequency, typically 50 or 60 Hz, and stresses the insulation from both polarities on every cycle. That's the key advantage: it replicates how the insulation actually behaves in service. For any product that runs on AC mains power - appliances, switchgear, transformers, industrial control panels - the AC hipot test is what UL, IEC, and CSA standards typically require. The downside is capacitive charging current. Every AC hipot test produces a continuous reactive current component that flows through the insulation's natural capacitance, and on high-capacitance devices - long shielded cables, large motor windings, power supplies with big EMI filters - this current can swamp the resistive leakage reading and even exceed the tester's output capacity.

That's where DC hipot testing steps in. A DC test applies a steady voltage in one direction. There's an initial charging surge as the DUT's capacitance charges up, but once that settles after a few seconds, the only remaining current is resistive leakage through the insulation. The measurement is cleaner, and the tester itself can be smaller and lighter because it doesn't need to supply continuous reactive power. DC hipot is the go-to for field testing of cables, high-voltage DC equipment, and any situation where portability matters. But there's a catch: DC voltage stresses insulation in only one polarity and can create space charge accumulation inside extruded dielectrics like XLPE - which is why IEEE 400 explicitly recommends against DC testing on modern polymer-insulated power cables.