Hipot

The Hi-Pot test
The Hi-Pot test, or "Dielectric Strength" or "Insulation Breakdown" test, is intended to verify that creepage distances, clearance distances and insulation levels around live parts, are sufficient to prevent excessive leakage currents causing a hazard to the user. As another "core" electrical safety test, this test is also a requirement of many National, European and International standards.

In application, ‘live parts’ refers to both the Line (Phase) and Neutral supply paths. For test purposes these are joined together to form one test point, with the other test point being the Earth (protective conductor) for Class 1 products, or outer surfaces of Class 2 products. Any function switches on the product should be turned ON to include all parts of the supply path. Typical ‘type test’ requirements are for a test voltage of between 1000 and 1500V, for Class 1 products (2500-4200V, for Class 2 products) to be applied for a 1 minute period and for the product to withstand a leakage current level of up to 100mA. For Production Line testing, where high throughput is a major consideration, the test period can be reduced to a few seconds by increasing voltage levels by 10%, and at the same time lowering the acceptable leakage current to less than 5mA. This has the added benefit that it makes the test safer for the operator.
Most HiPot test requirements stipulate that the test voltage should be from an a.c. source, with a substantially sinusoidal waveform. With the increase in sensitive components within many of todays products, alternative methods are sometimes called for. These can range from the application of d.c. voltages (usually at 1.4 times the stated a.c. voltage) to the soft-start advantages offered by more sophisticated testers that use programmable ramp profiles for raising and lowering the test voltage. This minimises the rapid rates of potential change that are the main cause of sensitive component failures, not the actual high voltage.

It is important to remember that HiPot testing can be hazardous if improperly applied, and common sense precautions should be taken, such as the use of safety enclosures, well defined test areas and adequate operator training.

To test or not to test
The view that HiPot testing is a destructive test is often raised in discussion. This largely originates from its use in type testing laboratories where high voltages are sometimes applied for long durations, with the intention of testing products to destruction. However, in terms of production and routine re-testing applications, there is little or no evidence to suggest that the short period HiPot test degrades a product, even after repeated testing. There are many reasons for HiPot test failures, most of which lead back to such things as manufacturing errors, inappropriate materials and designs that fall short of the exacting standards required in today’s safety conscious environment. The HiPot test doesn't cause the problems, it merely provides the means to find them.