Advantages of DC hipot test compare to AC test

DC/AC Hipot and Insulation Test

The primary distinction between AC and DC hipot tests lies in their methodologies. Can DC hipot also be regarded as an insulation resistance test? Currently, we are installing a 36kV direct buried line, which is being evaluated using a 10kV DC insulation test. Another team is now requesting a HIPOT test, although it remains unclear whether it will be AC or DC. Based on our previous experience, such tests are typically applied to bare copper and conductors without any insulation.

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Benefits of DC Hipot Tests

One significant advantage of using a DC test voltage is that the leakage current trip can be set to a much lower level compared to an AC test voltage. This allows manufacturers to filter out products with marginal insulation that would otherwise pass unnoticed during an AC test.

Using a DC hipot tester may require a safe-discharge device since capacitors within the circuit can become highly charged. It is a best practice to ensure that any product is discharged before handling it, regardless of the voltage type involved in the testing.

DC hipot testing applies voltage gradually. By monitoring current flow as the voltage increases, an operator can detect potential insulation breakdowns before they occur. A minor downside of the DC hipot tester is that generating DC test voltages is more challenging, which may result in a slightly higher cost compared to AC testers.

The primary advantage of DC testing is that it does not produce harmful discharges as readily as AC does. It can be applied at higher levels without risking damage to good insulation. This capability allows for a more effective "sweeping out" of localized defects.

Local defects create simpler series circuit pathways, which can be more easily carbonized or reduced in resistance by DC leakage current than by AC. As the fault path resistance decreases, the leakage current increases, leading to a snowball effect that typically results in visible dielectric punctures. Since DC testing is free of capacitive division, it is also more effective in identifying mechanical damage and areas within the dielectric that exhibit lower resistance.

Conducting high-voltage DC testing on XLPE insulated cables tends to polarize the insulation, potentially leading to capacitive space charges forming in tiny anomalies within the insulation. These space charges can develop into water trees that will eventually seek a path to earth. While the ideal testing frequency is 50Hz/60Hz, the equipment required for this type of test is often bulky and heavy, especially for longer cable lengths due to the charging current needed. A Very Low Frequency (VLF) test can serve as a compromise, providing a true sine wave AC test at low frequency without the destructive impact of DC tests.

Contact us to discuss your requirements for the DC hipot test kit for electrical power cables. Our knowledgeable sales team is ready to assist you in identifying the options that best suit your needs.

Comparison of AC and DC Hi Pot Testing

DC hipot testing assesses insulation resistance by applying a high DC voltage to cables while measuring leakage current, which can potentially damage older cables during the process. In contrast, AC hipot testing applies a high AC voltage and measures a higher leakage current as cables act like capacitors, allowing AC current to flow more readily through capacitors than DC. While AC hipot testing requires less voltage, it demands more power due to low capacitive reactance at 60 Hz. Consequently, DC hipot testing is often preferred over AC testing for cables, as it avoids complications related to cable capacitance that could skew insulation resistance measurements.

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Hello,

High voltage testing...
We evaluate our products (power supplies) as follows:
Test input to ground, output to ground, and input to output if necessary, particularly when galvanic isolation exists between the two.
Here, ground refers to the housing (earthing point). Typically, we perform this testing with DC voltage, which has the advantage of charging any capacitors against ground (EMC!). If we were to conduct testing with AC, it would result in an unwanted constant current flow.
The inputs and outputs are connected together to protect internal components in the event of a breakdown.
The polarity does not have a significant role in this configuration for DC tests, but generally, the negative is connected to ground while the positive connects to the inputs or outputs.
There was, however, a prior incident where a diode was connected in series at the input for reverse polarity protection. In this case, the negative connected to the input while the positive connected to the ground.
As a result, when there was a breakdown against ground and the reverse voltage of the diode was lower than the test voltage, it malfunctioned during the breakdown. This issue would not have occurred had the positive been connected to the input.
Since then, the positive has consistently connected to the input and the negative to ground.
In the event of a high-voltage breakdown, several possible causes could include:
- Test voltage accidentally set too high
- Tripping current set too low for DC, particularly relevant with charging EMC capacitors against ground
- Damaged insulation in installed cables
- Insufficient insulation between potentials
- Presence of metal chip debris in the housing
- Wire residues within the housing or between potentials due to improper cable insulation
- Excessive humidity during testing (creepage distance: 1mm/V)
- Rarely, defective components in the system itself.

Repetitions after a failed high voltage test should ideally be conducted with reduced test voltages, provided this does not fall below minimum requirements. The rationale is that components experience stress during a high voltage test, which should not be frequently applied to avoid shortening their service life.

For more information, please visit DC hipot test kit for motors.