Hipot and Insulation Resistance Testing

Two Tests, Two Purposes

Insulation resistance and hipot tests are often mentioned together and they measure different things. The insulation resistance test applies a modest dc voltage and measures the resistance of the insulation, which detects contamination, moisture and a partial breakdown path. The hipot, or dielectric strength test, applies a much higher voltage for a defined period and looks for a breakdown, which is a more severe test of the same property. Both are safety related tests for a product that has a mains input or a high voltage section, and both have to be performed in a way that does not damage the product they are testing.

What Each Test Proves

The insulation resistance test proves that there is no significant leakage path between the circuits that must be isolated and the accessible parts. A low reading indicates contamination, absorbed moisture, a damaged insulator or a design with insufficient creepage. The hipot test proves that the insulation can withstand the voltage that could appear across it, including a transient, without breaking down. It is a go or no go test: the insulation either withstands the voltage or it does not, and a failure is usually a breakdown in the material or a flashover across a surface. Together they cover the leakage and the strength of the insulation, which is why standards require one or both.

Choosing the Voltage and the Limit

The test voltage and the duration come from the applicable standard, usually a multiple of the working voltage plus an allowance, applied for a specified period. The insulation resistance limit is also usually specified, commonly expressed in megohms at a given dc voltage. The limits should be taken from the standard rather than chosen for convenience, because a test at a lower voltage may not reveal a defect that the standard intends to detect, and a test at a higher voltage may damage the product. Where a routine production test is used, the standard often allows a reduced duration, provided the full test has been performed as a type test.

Testing Without Damaging the Product

The main risk of a hipot test is that it stresses the very insulation it is verifying. A repeated test at the full type test voltage degrades the insulation over time, which is why the standard allows a shorter routine test. Components that are connected across the isolation barrier may be exposed to the test voltage, and some of them, such as a filter capacitor or a semiconductor, cannot tolerate it. The practical solution is to isolate the component or to use a test voltage limited by what the components tolerate, and to document the decision. Where the product has a surge protection device, it will conduct during the test, and the test has to account for that.

Insulation Resistance and Its Sensitivity

The insulation resistance reading is very sensitive to conditions. Humidity affects it strongly, so the test result depends on the ambient and on whether the board has absorbed moisture. The reading also depends on the time for which the voltage is applied, because the insulation has a charging characteristic, and on the temperature. A reading taken immediately after the voltage is applied will be lower than one taken after the specified settling time. The test should therefore be performed under controlled conditions, with the specified settling time, and the result recorded with the ambient and the temperature so that a comparison between tests is meaningful.

Production and Type Testing

A type test is performed once on a representative sample to prove the design, and a routine test is performed on every unit. The type test uses the full voltage and duration; the routine test is a shorter version that catches a manufacturing defect such as a misassembled component, a contamination or a damaged insulator without degrading the product. The routine test should be designed to detect the defects that the process can produce, and it should be verified against known good and known bad samples so that its sensitivity is understood. A test that passes everything is worse than no test, because it creates confidence without evidence.

Safety and Equipment

A hipot test involves dangerous voltages and should be performed with proper guarding, interlocks and an operator who is trained in the procedure. The equipment should have a current limit that trips before the product is damaged, and the test area should be arranged so that the operator cannot touch the circuit under test. The instrument should be calibrated and its trip current verified, because a tester that does not trip at the specified current will damage products rather than detect defects. The records should identify the instrument, the voltage, the duration and the result for each unit, so that a failure can be investigated with the data at hand.

Routine Test in a Production Line

In production, a hipot or insulation test station is a bottleneck if it is slow, and it is a hazard if it is not respected. The station should be arranged so that the test is guarded, the unit is placed and removed without touching the conductors, and the result is recorded automatically against the serial number. Where the test is short, the cycle time can be absorbed into the line; where the test requires a settling period, the units should be staged so that the waiting occurs in parallel with other work. The station’s sensitivity should be checked periodically with a known bad sample, since a tester that has drifted is worse than no test at all.

FAQ

What is the difference between hipot and insulation resistance? Hipot applies a high voltage and looks for breakdown; insulation resistance applies a low voltage and measures leakage.

Why is the routine test shorter? Because a repeated full voltage test degrades the insulation the test is meant to verify.

Why is the insulation resistance reading variable? It depends on humidity, temperature and the time the voltage has been applied.

What if a component cannot take the test voltage? It must be isolated or the test voltage limited, and the decision documented.

How is the sensitivity of a routine test confirmed? By verifying it against known good and known bad samples.

Conclusion

Insulation resistance and hipot tests cover leakage and dielectric strength, and both must be applied at the specified voltage without damaging the product. Control the conditions and verify the sensitivity. Safety testing belongs to PCBA testing, the isolation design is part of PCB design and layout, and the assembly it verifies comes from PCB assembly. Safety test requirements for a new product are defined during prototype PCB assembly in 2026.