03 ago Hipot and VLF: What’s the Difference Between These Two Tests?
Hipot and VLF: What’s the Difference Between These Two Tests?
Understanding the difference between Hipot and VLF is essential for anyone working with electrical testing on medium- and high-voltage cables. After all, choosing the wrong test can compromise the cable’s lifespan — or even damage it during the test itself.
In this article, you’ll learn how each method works, why VLF gained ground over the past decades, and how to choose the right test for each situation. You’ll also find practical examples and the key technical standards involved.
What is the Hipot Test?
The Hipot (High Potential Test) is a high-voltage test used on cables, motors, generators, and transformers. Its purpose is to verify insulation integrity by subjecting the component to a voltage higher than its operating voltage.
This test simulates electrical stress conditions. As a result, it’s widely used in commissioning, predictive maintenance, and electrical diagnostics.
Conventional Hipot testing can be performed in two ways:
DC Hipot
DC Hipot applies a direct voltage to the cable and measures the resulting leakage current. It’s the most traditional method on the market.
However, it has an important limitation: it can create excessive stress on the insulation. This happens because the direct voltage polarizes the insulation’s polymers, forming accumulated charges that don’t reflect the cable’s real operating condition.
AC Hipot
AC Hipot is closer to the cable’s actual operating condition, since it applies alternating voltage at line frequency (50 or 60 Hz). On the other hand, it requires larger, more complex equipment with higher output power.
What is the VLF Test?
VLF (Very Low Frequency) is a more recent technology for electrical cable testing. It was introduced to the market in 1986 specifically to replace DC Hipot in tests on insulated cables.
Unlike conventional Hipot, VLF applies alternating voltage at a very low frequency — typically between 0.01 Hz and 0.1 Hz, with common values of 0.1 Hz, 0.05 Hz, or 0.02 Hz.
How the Ultra-Low Frequency Works
This reduced frequency is VLF’s main advantage. Since the applied waveform is sinusoidal and symmetrical, the insulation doesn’t accumulate long-lasting space charges, even in cables with XLPE insulation.
In other words, VLF can stress the insulation in a way similar to the cable’s normal operation, without the destructive effect typical of DC Hipot.
Hipot and VLF: The Main Differences
To make the comparison easier, here are the key points that set Hipot and VLF apart in practice:
- Waveform: Hipot uses direct current or alternating current at 50/60 Hz; VLF uses alternating current at ultra-low frequency (0.01 to 0.1 Hz);
- Impact on insulation: DC Hipot can be destructive; VLF is considered much safer for the insulation;
- Test duration: Hipot has a recommended maximum time of around 15 minutes, due to the risk of damage; VLF typically lasts between 30 and 60 minutes;
- Portability: modern VLF equipment is more compact, even for testing long cables;
- Complementary diagnostics: VLF allows additional measurements, such as tan delta and partial discharge, on the same equipment.
Why DC Hipot Fell Out of Favor
Over the years, several studies have shown that DC Hipot testing is destructive to modern polymeric insulation, such as XLPE and EPR.
This happens because direct voltage favors the growth of electrical trees within the insulation layer. As a result, the cable’s lifespan can be reduced, or it may even be damaged during the test itself.
For this reason, standards such as IEC 60502 no longer recommend DC testing for modern insulated cables.
The 4 Types of VLF Testing on Medium-Voltage Cables
VLF isn’t limited to a single type of test. In practice, there are four main applications:
- Withstand testing: a pass/fail test that applies voltage above the nominal level for a set period of time;
- Tan Delta: a non-destructive diagnostic test that measures insulation quality;
- Partial Discharge (PD): identifies specific points of electrical discharge within the insulation or in accessories, such as splices and terminations;
- MWT (Monitored Withstand Test): combines the withstand test with simultaneous monitoring of Tan Delta and/or PD.
The Role of the IEEE 400.2 Standard
VLF testing is standardized by IEEE Std 400.2, updated to its 2024 version. This revision brought relevant changes, including:
- An expanded voltage range, now covering cables up to 138 kV;
- Tan Delta stability calculations also required at lower voltages (0.5 U₀), enabling earlier detection of degradation;
- Clarifications on the relationship between Tan Delta and partial discharge during the Monitored Withstand Test.
Additionally, the standard defines important evaluation parameters, such as the Mean Tan Delta (MTD) and the voltage differential, known as “tip-up” — a sudden jump in this value usually indicates serious insulation defects.
Which Test Should You Choose: Hipot or VLF?
In practice, the choice depends on the purpose of the test and the type of installation. Here’s a quick overview:
- Choose Hipot for a quick assessment on a production line, before a piece of equipment goes into service;
- Choose VLF for commissioning and predictive maintenance of medium- and high-voltage cables in the field, especially when preserving the insulation is a priority;
- Combine VLF with Tan Delta and PD when you want to go beyond a simple pass/fail result and understand the insulation’s real condition over time.
As a result, more and more utilities, industries, and contractors have adopted VLF as the standard for medium- and high-voltage cables.
Frequently Asked Questions About Hipot and VLF
Does VLF completely replace Hipot?
Not necessarily. VLF has replaced DC Hipot in most applications on insulated cables, but AC Hipot still has relevant use in other contexts, such as production-line testing.
Can the VLF test damage the cable?
When performed correctly, VLF is considered non-destructive. This is actually one of the main reasons it became the reference standard for field testing.
How long does a VLF test take?
The IEEE 400.2 standard recommends, for example, a 30-minute withstand test for cables under maintenance, though this can vary depending on the type of test and the cable’s voltage class.
Conclusion
The difference between Hipot and VLF goes far beyond the applied frequency. It involves insulation safety, diagnostic depth, and result reliability throughout the cable’s service life.
While DC Hipot has lost ground due to its destructive potential, VLF has established itself as the more modern and safer alternative, especially when combined with Tan Delta and partial discharge testing.
Need to Run Hipot or VLF Testing on Your Installation?
EA Engenheiros Associados provides studies, commissioning, and maintenance for cables, substations, and electrical installations across Brazil. Talk to our team and find out which test is the best fit for your project
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