Stop Damaging Medium Voltage Cables with HIPOT-DC Testing

It is essential to commission the elements of an electrical installation after project completion and before energization. This measure ensures the minimum performance requirements of electrical circuits and identifies potential critical defects that could lead to failures during operation. These steps align with best practices in electrical engineering and the recommendations of national and international standards. For many years, the commissioning of medium voltage insulated cables was carried out using Direct Current (DC) voltage tests, known as HIPOT-DC. However, with the introduction of polymeric insulations such as XLPE and EPR, replacing paper-insulated cables, this scenario has changed.

Today, the international engineering community recognizes that DC voltage tests or HIPOT-DC are harmful to XLPE and EPR-insulated cables. Numerous studies published by the Institute of Electrical and Electronics Engineers (IEEE), CIGRE, and other international forums highlight the disadvantages of applying DC voltage in the commissioning of new electrical installations.

In this post, we will highlight these studies and demonstrate best practices for commissioning new installations, following standards, guidelines, and industry best practices.

Why Can HIPOT-DC Damage Insulated Cables?

As observed, DC high-potential (HIPOT-DC) tests are harmful to insulated cables. A fundamental study has rigorously demonstrated the detrimental effects of HIPOT-DC testing on polymeric insulation, specifically XLPE. This study was presented at Jicable in 1995 (Effect of DC Testing on XLPE Insulated Cables by SRINIVAS N.N., Detroit Edison, Detroit, U.S.A., and BERNSTEIN B.S., EPRI, Washington, U.S.A.). It was commissioned and conducted jointly by the Detroit Edison Company and the Electric Power Research Institute (EPRI).

The authors demonstrated through multiple experiments how the failure rate increases when insulated cables are subjected to DC applied voltage tests. The study also highlights the inefficacy of DC voltage testing, as it fails to accelerate defects for the purpose of preventing future failures. Furthermore, it shows that these tests significantly reduce cable lifespan—by nearly four times.

First, some studies indicate that DC testing does not detect certain types of defects. Research further clarifies that these defects are not stressed under DC voltage. However, when the installations are put into operation under AC (60 Hz), these defects can rapidly evolve into failures. This explains why it is common to hear statements like: “We tested the cable with HIPOT-DC, and as soon as we put the circuit into operation, the cable failed.”

Two other issues identified in these studies are that DC voltage application promotes the accumulation of space charges, which can lead to future insulation failures, and that the high DC voltage applied causes polarization of the insulating material, further contributing to premature failures.

VLF (Very Low Frequency) as an Alternative to HIPOT-DC

Following the evidence of the harmful effects caused by HIPOT-DC, the international community sought alternatives to establish a new testing technique. The goal was to enable the application of a voltage test that would not damage the cable while still being feasible with portable equipment.

At industrial frequency (60 Hz), the equipment would be too large to achieve higher voltages. The alternative was to conduct the test using AC voltage but at a very low frequency (0.1 Hz), which is now known as the Very Low Frequency (VLF) test. Therefore, best practices for applied voltage testing recommend the use of alternating current (AC) voltage with a reduced frequency.

Since 2004, the IEEE 400.2 guide – Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF) (less than 1 Hz) has established guidelines for applied voltage testing in alternating current. These guidelines define the voltage levels to be considered, differentiating by AC voltage type (sinusoidal or cosine-rectangular) and according to the cable’s lifecycle stage.

The Importance of VLF Applied Voltage Testing During Commissioning

Testing insulated cables before energizing an operation ensures asset reliability during activities. However, given the issues caused by HIPOT-DC, how can this be done effectively? Applied voltage testing is based on the premise that the insulation must withstand a significantly higher voltage for a certain period than the voltage it will actually operate under.

An insulated cable behaves like a capacitor. As a result, the capacitive component becomes more significant in the test as the cable length and voltage class increase. The only variable that can be adjusted to reduce the power required from the testing equipment is the frequency. In other words, the lower the frequency, the less energy is needed, allowing for smaller testing equipment.

The development of Very Low Frequency (VLF) testing for cables arose from the need to replace DC testing. In the next post, we will discuss these tests in more detail, presenting important study data that validates the effectiveness of AC voltage testing at very low frequencies.