A dielectric strength test, commonly called a hipot test, dielectric withstand, or high potential, is a stress test of the insulation of a device under test (DUT). Such a test applies a voltage to the DUT that is much higher than normal operating voltage; typically 1000V AC plus twice the normal operating voltage. For a household appliance designed to operate at 120 or 240V AC, the test voltage is usually about 1250 to 1500V AC.

A DC hipot test can usually be substituted for an AC hipot test. The best voltage for a DC hipot is normally higher than the AC test voltage by a factor of 1.414. A product that would be tested at 1500V AC would be tested at 2121V DC.

For double-insulated products, the required test voltages may be much higher, such as 2500 VAC or even 4000 VAC for a 120 VAC power tool. The voltage is applied between the operating circuits and the chassis or ground – the parts of a product that a consumer might touch or come in contact with.

Refer to Figure 5 for typical AC hipot test setup. The setup for a DC hipot test would be identical.

The purpose of the test is to make sure consumers do not receive an electrical shock when they use the product. This typically is caused by a breakdown of the electrical insulation. The test also detects possible defects in design and workmanship that cause components and conductors to be too closely spaced. The danger is that air gaps between conductors or circuit components may become clogged with dust, dirt, and other contaminants over time in typical user environments. If the design spacing is inadequate, a shock hazard can occur after a period of use. By subjecting the product to a very high voltage, the hipot test overstresses the product to the point that arcing may occur if the spacing is too close. If the product passes the hipot test, it is very unlikely to cause an electrical shock in normal use.

Withstanding a very high voltage means that a large margin of protection exists for the consumer. Regulatory agencies usually require a stringent hipot test as a product “type test” before releasing the product for sale to the public and another less demanding test to be used on the production line. As a rule, testing laboratories consider the hipot test to be the most important safeguard for the consumer. They may accept “design” or “type” tests for other types of tests, but always require hipot tests for 100% of the units in a production line.

AC or DC Hipot?

The voltage used in a hipot test can be either AC or DC, depending on the requirements established by the regulatory testing agency. Both have their advantages and disadvantages. The typical rule of thumb used to select an AC or DC test is if the DUT is powered by AC, then use an AC test; if it is powered by DC, use a DC test.