VA Technology 
Choosing a vlf test set comes down to three questions, and they hurt in a fixed order. Get the voltage wrong and the instrument cannot test the cable at all. Get the load wrong and it stalls part-way up a long feeder. Get the third wrong and the test passes, but the record is thinner than the network asked for.
The VLF hipot family answers all three across eight models: six withstand units from 30 kV to 90 kV peak, and two diagnostics for 5–15 kV and 5–25 kV cable. All eight share the same four output frequencies, 0.1 Hz to 0.01 Hz.
Voltage: what the cable class demands
The first question belongs to the cable, not the instrument. The controlling figure is U0, the rated phase-to-earth voltage. Acceptance of new cable, joints and terminations sits at about 3U0; maintenance of aged cable at about 2U0. Our VLF versus DC article works through that derivation; the certified figure comes from the standard’s tables.
An 8.7/15 kV cable needs about 26 kV at acceptance and a 20.3/35 kV cable about 61 kV, both inside the 80 kV peak of the VLF-80 and the 90 kV peak of the VLF-90. Every rating here is peak, never multiplied by the square root of two.
Load and reach: how far one set goes
The second question sets the machine’s size. A VLF set drives a capacitor, so its charging current is proportional to capacitance and frequency. Dropping from 0.1 Hz to 0.01 Hz cuts that demand tenfold, which is what lets one tank hold full voltage on a much longer run.
How far a vlf test set reaches is decided by its load-rating ladder, not its voltage. A VLF-40 holds 0.5 µF at 0.1 Hz and 5.5 µF at 0.01 Hz, at the same 40 kV peak.
| Model | Peak output (kV) | Load at 0.1 Hz (µF) | Load at 0.01 Hz or 0.02 Hz (µF) | Weight and design |
|---|---|---|---|---|
| VLF-30 | 30 | ≤ 0.5 | ≤ 5.5 (0.01 Hz) | 4 + 20 kg, split |
| VLF-34TD | 34 | 1.0 | 5.0 (0.02 Hz) | 22 kg, single |
| VLF-40 | 40 | ≤ 0.5 | ≤ 5.5 (0.01 Hz) | 4 + 40 kg, split |
| VLF-45TD | 45 | 0.5 | 5.0 (0.01 Hz) | 23 kg, single |
| VLF-50 | 50 | ≤ 0.5 | ≤ 5.5 (0.01 Hz) | 4 + 40 kg, split |
| VLF-60 | 60 | ≤ 0.5 | ≤ 5.5 (0.01 Hz) | 4 + 50 kg, split |
| VLF-80 | 80 | ≤ 0.5 | ≤ 5.5 (0.01 Hz) | 4 + 50 kg, split |
| VLF-90 | 90 | ≤ 0.5 | ≤ 5.5 (0.01 Hz) | 4 + 55 kg, split |
On the largest units the ladder does not improve: the 80 kV and 90 kV sets drive no more capacitance than the 30 kV one. Where capacitance rather than voltage binds, the VLF-50 and the VLF-60 are positioned for long feeders and collector circuits; the VLF-60 also covers motor and generator windings.
Beyond a withstand: diagnostics and sheath testing
The third question is what the job has to produce besides a pass or fail. A withstand says the cable held, not how much margin is left.
Two models carry the extra front end. The VLF-34TD and the VLF-45TD add tan-delta diagnostics over a 5 nF to 10 µF load range at 1.0 × 10⁻⁴ accuracy, plus DC testing and sheath testing with sheath fault location: a 10 kV sheath test at 0.1 to 5.0 mA. The six withstand units carry neither, so a scope calling for tan delta should not be written around one. Weight follows the same split: a 4 kg controller plus a 20 to 55 kg tank on the withstand units; a single 22 kg or 23 kg case on the two diagnostics.
Frequently asked questions
Can one vlf test set cover every cable class?
A set covers any shielded cable whose specified test voltage falls inside its rating; the largest units here reach 90 kV peak. Capacitance is the second limit, so a long circuit can exceed a set’s reach even when the voltage fits.
Does a higher peak voltage mean more reach?
No. The load ladder is flat across the six withstand units, so the 90 kV set drives no more capacitance than the 30 kV one.
Two-piece or single unit?
The split sets put a 4 kg controller in one hand and the tank in the other, which suits manholes. The diagnostics are single cases.
Where the family stops
The largest units in the family are 90 kV peak, which puts 66 kV acceptance out of reach: a 38.1/66 kV cable has a U0 of about 38 kV and an acceptance level near 114 kV. The maintenance level for that class, about 76 kV, is reachable, and so is every class below it.
To match a vlf test set to a cable class, route length and diagnostic scope, send VA-TEK the cable schedule.
Sources
- IEEE 400.2-2024, Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF): acceptance and maintenance levels derived from U0.
- IEC 60502-2, power cables with extruded insulation from 1 kV to 30 kV: in-situ AC testing.
- VA-TEK product data, VLF family (vendor data): output voltages and unit weights; the load-rating ladder as tabulated; output current 0–30 mA on the withstand units, 0–14 mA on the VLF-34TD and VLF-45TD; tan delta accuracy 1.0 × 10⁻⁴ over 5 nF to 10 µF; sheath test 10 kV at 0.1–5.0 mA.
- U0 of the cable classes quoted (8.7/15 kV, 20.3/35 kV, 38.1/66 kV); levels about 3U0 and about 2U0, rounded to the nearest kV (vendor data).
