A hi-pot test was performed on every unit and a unit with damaged insulation still reached the market. The test had run at the right voltage for a duration short enough to pass a compromised barrier, and nothing in the record showed the difference. The test proves what its parameters and its construction allow it to prove, and nothing more.
Charger hi-pot test: what it is and what it is not
It is a dielectric strength check between the mains side and the output side, and it is not a general quality test.
The test applies a voltage across the isolation barrier for a defined period and looks for breakdown or leakage beyond a limit. It detects a barrier that is inadequate at the moment of the test: a pinched insulation, a component placed too close, a solder bridge. It does not detect a barrier that will fail later, a thermal weakness, or a design that is marginal by construction.
That distinction matters because the test is often cited as evidence of safety in general. It is one piece of evidence, and the construction review is what establishes whether the barrier is adequate in the first place.
What the test stresses, and why the construction decides the result
The voltage finds the weakest path, whatever it is.
Creepage and clearance distances, the insulation on the wire between the board and the output connector, the transformer’s internal construction and the separation between the mains terminals all sit inside the same barrier. Where those distances were designed with margin, the test is a confirmation; where they were reduced to fit an enclosure, the test becomes a lottery that some units pass and some do not.
Voltage, duration and the difference between type and routine testing
The same test name covers two different activities, and they are not interchangeable.
A type test is performed on representative samples to establish that the construction meets the standard, at a defined voltage and duration, and it is part of the safety file. A routine test is performed in production on every unit or on a sample, at a voltage and duration chosen to detect assembly defects, and it is a production control. Quoting the routine test as evidence of the type test, or setting the routine voltage so high that it stresses good units, are the two common errors.
Where hi-pot sits in the production flow
The position decides whether the test is meaningful and whether it is safe for the unit.
Placing the test after final assembly means the barrier is tested in the condition it will ship in, including the wire routing and the enclosure closing force. Placing it earlier, on the bare board, tests a different construction and misses the assembly defects the routine test exists to find.
The test also has to sit before packing rather than after, because a unit that fails has to be diverted while it is still accessible. Where the step is placed after packing, a failure means opening a packed unit, and the record of that has to show whether the unit was repacked or scrapped.
The misuse that damages units without finding faults
Over-voltage and long duration do not improve detection.
A voltage well above the requirement, or a duration applied repeatedly, subjects the insulation to stress it was not designed for and can create the failure the test is meant to find. A unit that is tested three times because it was reworked has been stressed three times. The parameters come from the standard and from the construction, not from a preference for stricter testing.
The records the test has to produce
Four fields make the result usable.
The unit or lot identity, the voltage applied, the duration and the measured result, together with the station that performed the test. Voltage and duration are recorded as figures rather than as a reference to a procedure, because the procedure changes and the record has to describe what was actually applied to those units.
The record is also what makes a comparison possible between batches: a shift in the leakage readings across a production run points to a process change long before a failure appears, and that comparison only exists if the values were kept.
| Question | Short answer | What it depends on | Common mistake |
|---|---|---|---|
| What does it detect? | A barrier that is inadequate now | The construction and the limits | Treating it as general quality evidence |
| Where is it applied? | Between mains and output | The isolation boundary of the design | Testing across the wrong points |
| What are the parameters? | Voltage and duration from the standard | Type or routine purpose | Reusing the type voltage in production |
| What does the record hold? | Unit, voltage, duration, result | The test station | A pass flag with no parameters |
How hi-pot relates to the safety file
One test among several that together describe the product.
The file contains the construction review, the creepage and clearance evidence, the temperature results and the fault condition assessments, alongside the dielectric strength results. A buyer reading the file should find that the type test was performed on a representative build of the shipped revision, with the voltage and duration stated. Where the file contains only a routine test result, the safety evidence is incomplete rather than the product unsafe.
What to ask a factory about its hi-pot step
Five questions, and the answers are short.
Which points the test is applied between; the voltage and duration used in production and where those figures come from; whether every unit is tested or a sample; what the station records per unit; and what happens to a unit that fails. A factory that answers all five is running a controlled step; one that answers only the first is running a ritual with a meter.
FAQ
Does a hi-pot test prove the charger is safe?
It proves that the isolation barrier withstood the applied voltage at that moment. Safety evidence also includes the construction review, creepage and clearance, thermal behaviour and the fault conditions assessed against the standard. It is one input among several. It proves the isolation barrier withstood the applied voltage at that moment, while the safety evidence also includes the construction review and the fault conditions.
Should every unit be hi-pot tested?
A routine test on every unit is common and detects assembly defects such as a pinched wire or a misplaced component. The parameters for a routine test differ from those of the type test, and setting them by copying the type test stresses good units. A routine test on every unit is common and detects assembly defects, and its parameters differ from the type test.
Can a failed unit be reworked and re-tested?
It can, and the rework has to address the barrier rather than the reading. Repeated testing at the same voltage stresses the insulation each time, so the number of re-tests belongs in the record and the cause belongs in the corrective action. The rework has to address the barrier rather than the reading, and repeated testing at the same voltage stresses the insulation each time.
What should the test record contain?
The unit or lot, the voltage, the duration, the measured result and the station. A pass flag without the parameters cannot be compared with another batch and cannot be used to answer a question about a specific unit. Record the unit or lot, the voltage, the duration, the measured result and the station, because a pass flag cannot answer a question about a specific unit.
Next step
Send the construction, the target markets and the test evidence you already hold to Wecent Group through the contact page. The reply states the points tested, the routine parameters for production, the record kept and how the step relates to the safety file. Related reading: our charger factory, charger manufacturing services and the factory floor. External references: IEC Webstore, IEC standards, UL standards and certification, TUV testing services, Intertek testing services and EU Low Voltage Directive.
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