VA Technology 
The output frequency and the vlf test duration are the two settings argued about at the cable termination. Together they decide whether the set can drive the circuit at all, and how long the crew waits beside it.
Frequency is the operator’s one real lever. Lowering it multiplies the capacitance a set can carry, which is what lets a portable instrument replace a trailer-mounted source on a long feeder — and it stretches every cycle in wall-clock time.
Why 0.1 Hz became the field frequency
A shielded cable is a capacitor, and charging current is proportional to frequency. Drop the test frequency far enough and a source weighing tens of kilograms does the work of a mains-frequency set that arrives on a trailer.
0.1 Hz is the balance point. The waveform still reverses polarity every half cycle, so the insulation is stressed as it is in service rather than accumulating the trapped space charge a DC proof test leaves behind. A split system — controller and high-voltage tank — then holds full test voltage on a run a 50 Hz source could never charge.
The same four settings, 0.1 to 0.01 Hz, span the VLF hipot family, from the rotating-machine-capable VLF-60 to the VLF-80 and VLF-90.
Frequency is a trade of reach against time
| Output frequency | VLF-30 to VLF-90 withstand units | VLF-34TD (34 kV peak) | VLF-45TD (45 kV peak) |
|---|---|---|---|
| 0.1 Hz | ≤ 0.5 µF | 1.0 µF | 0.5 µF |
| 0.05 Hz | ≤ 1.1 µF | 2.0 µF | 1.0 µF |
| 0.02 Hz | ≤ 2.2 µF | 5.0 µF | not specified |
| 0.01 Hz | ≤ 5.5 µF | not specified | 5.0 µF |
Each frequency carries the capacitance the set drives at full rated voltage, and the same unit gains eleven times the reach between 0.1 Hz and 0.01 Hz. The VLF-34TD follows the same shape against a lower ceiling, 1.0 µF at 0.1 Hz and 5.0 µF at 0.02 Hz for 5 to 15 kV cable.
None of it is free. One cycle takes ten seconds at 0.1 Hz and one hundred at 0.01 Hz, so a hold measured in minutes holds ten times as many polarity reversals at 0.1 Hz. The slower setting buys reach with crew time.
What the vlf test duration is for
A withstand test works by overstress: the cable is held above its service stress long enough that a weak point — a contaminated joint, a nick from the pull, a void — breaks down where it can be found.
What breaks it down is the number of times the stress reverses. Frequency and vlf test duration meet there: the frequency sets how many cycles fall inside the hold, so the same hold is more or less stress at each setting.
The duration is not calculated on site. It comes from the standard tables for the test category — acceptance, maintenance, or a monitored withstand — and from the cable owner’s specification, whichever is tighter. Published figures differ between sources, and the specification governs. Test levels themselves are derived in our VLF versus DC article.
Cumulative damage: a longer test is not safer
GB/T 3048.8-2025, the cable AC voltage test method effective 1 May 2026, warns about cumulative damage in extruded insulation and notes that long-duration VLF testing is less sensitive to some defect types. Neither statement supports the assumption that more test time buys more confidence.
VLF testing overstresses the insulation on purpose. A sound cable survives one application, but the stress is real, and repeated overstress accumulates as service ageing does. Stretching the hold for safety adds stress without adding information.
Extra cycles do not turn a marginal result into a diagnosis either. Where a cable is suspect, the productive step is a diagnostic measurement on the same excitation — a loss measurement read across stepped voltage — not a longer withstand. The tan delta article covers what that reads.
Frequently asked questions
Which output frequency should be selected?
Start with the frequency the specification names. Where it is open, use the fastest setting that leaves margin: compare the circuit capacitance against the load rating at that frequency.
How long should the vlf test duration be?
Whatever the applicable standard and the project specification require at the chosen test level; where the two differ, the specification governs. Get it in writing before the test.
Can a VLF test be repeated if the result is unclear?
Only with a reason. Every application adds stress, and GB/T 3048.8-2025 flags cumulative damage in extruded insulation. Read the current trace and the loss measurement before applying voltage again.
VA-TEK builds the VLF hipot family with the same four output frequencies across the range. Choosing a frequency and a vlf test duration is a project decision, and the standard, the specification and the cable’s history all have a vote. Send us the cable class, the route length and the specified duration and we will confirm which set holds that level.
Sources
- GB/T 3048.8-2025, Test methods for electrical properties of electric cables and wires — Part 8: AC voltage test, effective 1 May 2026: VLF annex; cumulative-damage warning for extruded insulation, and lower sensitivity of long-duration VLF to some defect types.
- IEEE 400.2-2024, IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF): test levels and durations from its tables for the applicable test category.
- IEC 60502-2, extruded-insulation power cables, 1 kV to 30 kV: in-situ AC testing.
- VA-TEK product data, VLF series load ratings and output frequencies (vendor data).
