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Hipot Test | AC Versus DC Hipot Testing

The Hipot Test (Dielectric Voltage Withstand)

The Hipot test is the most common type of safety test. This test is designed to stress a product’s insulation beyond what it would encounter during normal use. The reasoning behind this test is that if the insulation can withstand high voltage for short period of time, it will be safe to use at nominal voltage throughout its useful life.

One of the main advantages of the Hipot test is its versatility. In addition to measuring leakage currents and detecting breakdowns, you can also use it to detect: 
  • Material and workmanship defects. 
  • Weak points in the insulation. 
  • Small gap spacing between conductors. Air is just an insulator and Hipot voltage will “jump” through the air across gaps that are too small. 
  • Condensation, dirt and contaminants in the insulation.
When running a Hipot test, high voltage is applied to the mains conductors (LINE and NEUTRAL). The Hipot return point is connected to the bare metal chassis of the fan. You’re essentially creating a capacitor, where the product insulation is the dielectric material. The Hipot tester measures the resulting leakage current fl owing through the insulation (represented as capacitors between mains and ground).

Leakage Current and Dielectric Breakdown

Any electrical device will produce small levels of leakage current due to the voltages and internal capacitance present within the product. Under normal circumstances this leakage current isn’t large enough to be perceived by the human body. Yet due to design flaws or manufacturing defects the insulation in a product can break down, resulting in excessive leakage current fl ow. This is exactly what a Hipot test is designed to check.

Dielectric breakdown is defined as the failure of insulation to prevent the fl ow of current. The best indication of dielectric breakdown is a leakage current measurement significantly higher than the nominal current measurement. The maximum leakage current is dependent upon the test voltage; therefore, the leakage current will vary depending upon the product being tested.


AC and DC Hipot Tests

AC and DC Hipot tests have advantages and disadvantages that become evident depending on the characteristics of the DUT. Below is list of advantages and disadvantages of both type of Hipot tests.

AC Hipot Test

Advantages 

  • Slow ramping of the test voltage isn’t necessary due to the changing polarity of the applied waveform.
  • It is unnecessary to discharge the DUT after AC testing. 
  • AC testing stresses the insulation alternately in both polarities. This makes it a more stringent Hipot test.
Disadvantages 

  • Measures only the total leakage current (from capacitive and resistive elements).
  • Requires a large Hipot transformer due to measuring the total leakage current.

DC Hipot Test

Advantages

  • The test can be performed at a much lower current level, saving power and with less risk to the test operator.
  • Leakage current measurement is a more accurate representation of the real current. This is due to the insulation capacitance charging after the ramp up cycle.
  • DC testing is the only option for some circuit components such as diodes and larger capacitance values
Disadvantages
  • Must ramp up the test voltage so inrush leakage does not exceed Hipot tester’s capability. 
  • Must discharge the DUT at the end of the test. 
  • Only stresses insulation in one polarity. Not as stressful of a Hipot test. 
  • Not always accepted by safety agencies.

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