Why does a batch fail the hi-pot test at final inspection?

Main answer · Wecent Editorial Team

Short answer: A hi-pot failure means electricity crossed or leaked through the insulation between the mains side and the output side. Common causes are too little distance, dirt or flux residue, damaged insulation, moisture and a wrongly set test. Hold the batch, find the pattern, and do not accept a loosened limit unless the safety report supports it.

What the test checks

The hi-pot, or dielectric withstand, test applies a high voltage between the mains side and the user-accessible side for a short time. A good charger holds the voltage without breakdown or excessive leakage. It checks the insulation that protects users from the mains, so a failure is a safety signal, not a cosmetic one.

Common causes

• Distance and construction. Too little clearance or creepage between primary and secondary parts, a misplaced insulating barrier, or tape that has moved.
• Contamination. Flux residue, solder balls, dust, glue or metal particles that bridge the gap.
• Damaged insulation. A pinched wire, a transformer with damaged insulation, or a cut sleeve.
• Moisture. Units stored or tested in humid conditions, or boards that were not dried after cleaning.
• Assembly variation. Parts of the housing pressing on a lead, a component leaning toward another, or a different component supplier on one lot.
• Test setup. The wrong test voltage, ramp time or trip current, a poor contact on the fixture, or a probe touching the wrong point.

Where to start

1. Hold the batch and mark the failed units.
2. Re-test the failures on a second, checked tester to rule out the setup. Use the same settings that appear in the approved safety report.
3. Look for a pattern such as a production line, shift, date code, component lot, or position on the board.
4. Open some failed units and look for burn marks, residue, a lead near an edge or a displaced barrier.
5. Compare with a passing unit from the same batch.

Do not loosen the limit

The test settings come from the safety standard and the approved report. If the factory wants to change them, ask for a written reason and for the certification lab to agree. Raising the allowed leakage to pass a batch can hide a real safety problem.

What to ask for

• A written root-cause report with photos of the failed units.
• A corrective action, such as better cleaning, a changed barrier or tighter assembly control.
• A re-test of every unit in the batch, not just a sample, after the fix.

Check units already shipped

If earlier batches came from the same line, ask whether they passed with a smaller margin and whether any shipped before the issue was found.

Where this answer says to confirm with Wecent, the detail is not published on the product page and has to come from the datasheet or the factory. General facts come from the regulators, standards bodies or device makers named in the text.