The VLF withstand test applies to the cable a very low frequency AC voltage, commonly 0.1 Hz, higher than the operating voltage and held for a set time, to prove that it does not break down. The low frequency makes it possible to test long cables with portable equipment, because the reactive power demanded by the cable capacitance drops in proportion to the frequency. IEEE 400.2-2024 describes it for shielded cables rated 5 kV and above, with voltage tables for installation, acceptance and maintenance. It is a pass or fail test.
What the VLF test is
VLF stands for very low frequency: AC voltage below 1 Hz. In a VLF withstand test the cable voltage is raised above the operating voltage and held for a set time. If a serious defect exists (a badly made accessory, laying damage, a weak point in the insulation), the voltage drives it to break down during the test, with the cable de-energized, and not in service.
IEEE 400.2-2024, Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF) (less than 1 Hz), is the reference. It replaced the 2013 edition and covers the VLF withstand test and other field diagnostic tests on shielded cables rated 5 kV and above with extruded and laminated insulation. According to its abstract, the most widely used frequency is 0.1 Hz. The overview of all cable tests is in cable diagnostic testing.
Why 0.1 Hz
A power cable is a long capacitor: the conductor, the insulation and the shield form a capacitance that grows with length. To test it with AC at power frequency, the equipment must deliver a reactive power proportional to the frequency and to that capacitance. At 0.1 Hz the power needed is hundreds of times lower than at 60 Hz, and equipment that fits in a pickup truck can test medium-voltage circuits of considerable length. It is basic physics: capacitive reactance is inversely proportional to frequency.
Compared with DC voltage, VLF has the advantage of not leaving space charges trapped in the extruded insulation, which is the problem for which IEEE 400 discourages high DC on aged XLPE and EPR cables.
What IEEE 400.2 sets
- Three types of test: installation, acceptance and maintenance, each with its own recommended voltage level.
- Tables by cable voltage class: they give the recommended test voltage and the duration. We do not copy them: they are taken from the current edition of the standard.
- Criterion: the cable passes if it holds the voltage for the specified time without breaking down.
Some guides and manufacturers add monitoring during the test (tan delta or partial discharge at the same time) so that the test is not only pass or fail. The principle of loss measurement is in the guide on power factor and tan delta.
What IEC asks in the after-installation test
| Level | Cable standard | Insulation test after installation |
|---|---|---|
| MV (6 to 30 kV) | IEC 60502-2 | AC, including 0.1 Hz, or DC; plus the DC sheath test |
| HV (above 30 to 150 kV) | IEC 60840 | AC, generated in the field with resonant systems; plus the sheath test |
| EHV (above 150 to 500 kV) | IEC 62067 | Resonant AC and partial discharge measurement; plus the sheath test |
At high voltage VLF is not the usual source. CIGRE, in its Technical Brochure 841 (2021), analyzes the use of resonant, VLF and damped AC sources on high and extra-high voltage cables. Testing with resonant AC is explained in on-site withstand voltage test.
When it is done
- Sheath test after laying. Laying damage before accessories are made up
- Accessories installed. Joints and terminations per the manufacturer
- VLF acceptance test. Complete circuit, isolated at both ends
- Does it hold the voltage for the specified time? Yes: continue; No: Locate, repair and repeat.
- Reference record and energization
The sheath test in the first step checks the barrier against water before the accessories are made up. Before VLF, switchgear, voltage transformers, surge arresters and any connected equipment that must not receive the test voltage are isolated; the remote ends are marked and watched. When finished, the cable is discharged and grounded. Joints and terminations are the part that most often fails at acceptance. Field VLF equipment is described in the guides on the b2 HVA 60 and on the HVA 68, HVA 28TD and KPG families. For more detail on the high-potential test on medium-voltage switchgear and cables, read hi-pot testing of medium-voltage switchgear and cables.
What can distort the result
- Equipment capacitive load exceeded: a very long cable can exceed the capacity of the VLF equipment and force the test to be split.
- Dirty or wet terminations: they cause leakage or surface discharges that interrupt the test although the cable is not at fault.
- Connected equipment that was not isolated: a surge arrester or a VT in the circuit changes the load and can be damaged.
- Mixed cables: sections of paper-insulated and XLPE cable in the same circuit have different criteria.
What to do if the cable does not pass
The breakdown point is located with the methods of fault location (the fault usually remains a high-resistance one), the section or the accessory is repaired and the test is repeated. If the cable is new, the result and its location are the basis of the claim to the installer or the manufacturer.
What not to do
- Use high DC voltage on aged extruded cables.
- Apply the acceptance voltage to a cable in maintenance without considering its age and condition.
- Treat aging as diagnosed with a withstand test.
- Leave the cable undischarged and ungrounded at the end.
Cable testing with Tiza Group
Our team works with test equipment from recognized manufacturers in substations, industry and solar plants. If you must accept a medium-voltage cable, we help you plan and carry out the tests on your cable network. Field work is done in Colombia, or elsewhere on request. See the testing and commissioning service or write to us.
Related guides
- Power cable diagnostic tests
- On-site withstand voltage test
- Medium-voltage joints and terminations
- Medium-voltage collector network of a solar plant
- VLF tan delta on cables: reading and criteria
- On-site partial discharge on cables: IEEE 400.3
- Cable sheath test: DC voltage and leakage current
- Underground cable fault location: methods
- Hi-Pot Test on MV Switchgear and Cables
References
- IEEE Std 400.2-2024, Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF) (less than 1 Hz).
- IEEE Std 400-2023, Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems Rated 5 kV and Above.
- IEC 60502-2:2014, cables from 6 kV to 30 kV with extruded insulation.
- IEC 60840:2020 and IEC 62067:2022, high and extra-high voltage cables.
- CIGRE, Technical Brochure 841, After Laying Tests on AC and DC Cable Systems with New Technologies, 2021.
Frequently asked questions
Why is an XLPE cable not tested with DC voltage?
Because in aged extruded-insulation cables, high DC voltage leaves charges trapped in the insulation, especially where there are water trees, and when the cable is re-energized with AC those zones can fail. The IEEE 400 family discourages high DC on those cables and prefers VLF or AC.
What voltage and how long is applied in a VLF test?
IEEE 400.2 sets them in tables by the cable's voltage class and the type of test (installation, acceptance or maintenance), and the IEC cable standards set them for the after-installation test. They are taken from the current edition of the standard that the contract requires; values copied from another source should not be used.
If the cable passes the VLF test, is it in good condition?
Passing means the cable had no defects serious enough to break down during the test. It does not measure the degree of aging. For that it is complemented with VLF tan delta or with partial discharge testing.