A hipot test asks a deceptively simple question: if this product's insulation is stressed well beyond its normal operating voltage, does it hold? The test itself comes in two genuinely different forms, AC and DC, and the choice isn't a matter of preference — it depends on what the insulation is made of and what failure mode the test needs to catch.
What a Hipot Test Actually Does
Hipot testing applies a voltage well above a product's rated operating voltage across its insulation, then measures leakage current. If leakage stays below a defined threshold, the insulation passes. The test voltage is deliberately well above normal operating conditions — commonly twice the rated voltage plus a fixed margin — to find weaknesses that normal operating voltage wouldn't reveal.
AC Hipot
AC hipot testing applies the test voltage as alternating current, most closely mimicking the actual electrical stress a product experiences during real operation. Its main practical limitation shows up with highly capacitive loads — long cables, large transformers — where charging and discharging capacitance twice per cycle draws substantial reactive current, requiring a much larger and more expensive tester.
DC Hipot
DC hipot testing applies a steady, non-alternating voltage. Because the test current settles to a small leakage current once capacitance is charged, DC hipot requires a much smaller, lower-power instrument than AC at the same test voltage — the main reason it's preferred for long cables and capacitive loads. However, DC voltage stresses insulation differently than the AC environment many products actually operate in, and for capacitive insulation systems, DC testing can fail to reveal weaknesses that AC testing would catch.
Choosing Between Them
| Factor | AC Hipot | DC Hipot |
|---|---|---|
| Represents real operating stress | Closely, for AC-powered products | Less directly for AC products |
| Equipment size/cost at high voltage | Larger — must supply charging current | Smaller — settles to leakage current only |
| Suitable for highly capacitive loads | Difficult — high reactive current demand | Well suited |
| Typical standard requirement | Often specifically mandated for AC products | Often used for cable, capacitor, DC-powered equipment |
The practical rule: AC hipot for products operating on AC supply, DC hipot for cable and other highly capacitive test objects. Where a product standard explicitly specifies one method, that requirement governs.
A Note on VLF Hipot
For medium-voltage cable testing specifically, very low frequency (VLF) hipot testing offers a middle path: AC voltage, but at a much lower frequency (typically 0.1Hz), dramatically reducing the reactive charging current needed while still testing with alternating polarity. This is covered in more detail in our VLF hipot testing guide.
Practical Test Considerations
Ramp rate matters. Most hipot test plans specify a defined ramp time to reach test voltage, a dwell period, and a controlled ramp-down, rather than an instant step change. The MEWOI AC/DC Hipot Tester Series covers both AC and DC hipot testing with programmable ramp and dwell controls.
Leakage current threshold, not just breakdown. Most hipot test plans also fail a unit whose leakage current exceeds a defined threshold well before catastrophic breakdown.
Operator and environment safety. Interlocked test enclosures and controlled access to the test area are standard practice given the voltages involved.