VA-TEK Engineering “Which VLF test set do I need for a 33 kV cable?” is the question buyers actually ask, and it has a definite answer: the set has to reach the test voltage the standard sets for that cable class, and it has to carry the cable’s capacitance at the frequency the test runs at. This page answers it class by class, for every voltage level a VLF set is used on, using the test-voltage table of IEEE 400.2-2024.
Key takeaways
- Voltage class picks the set’s rating; load at 0.1 Hz picks its range. Both have to be met, and a set that fails either is the wrong set.
- For IEEE 400.2-2024 acceptance levels, a 15 kV cable needs about 30 kV peak; a 25 kV cable about 45 kV peak; a 33 kV cable about 59 kV peak; a 46 kV cable about 81 kV peak.
- Above 46 kV the acceptance level passes the top of the VA-TEK range — a 66 kV acceptance test needs about 113 kV peak, which is beyond a 90 kV peak set. We say so rather than rounding up.
- tan delta and sheath testing are provided by the 34 kV and 45 kV class TD sets, which cover the 5–25 kV cable classes.
- Test voltage is written against U0, the phase-to-earth voltage — not the system voltage — and IEEE tables it in kV rms while sets are rated in kV peak.
1. Two questions decide the set, not one
A VLF test set is specified by two independent figures, and the second is the one buyers most often leave out.
The voltage class decides whether the set can reach the test voltage your cable class requires. That is a hard limit: a 60 kV peak set cannot be pushed to a 44 kV rms acceptance level that needs 62 kV peak, whatever the brochure implies.
The load rating at 0.1 Hz decides how much cable the set can charge at the frequency the test runs at. Long cable means a large capacitance, a large capacitance means more charging current, and the set’s current rating is what caps the length. A set with the right voltage and the wrong load rating will reach the level on a short sample and fail to regulate on the circuit you actually have to test.
Class is the first cut. This page resolves class; the load axis is covered under the buyer’s checklist and in how to choose a VLF test set.
2. Selection table by voltage class
Acceptance levels from IEEE 400.2-2024, sinusoidal waveform, with the VA-TEK set that reaches each one. The rms column is the standard’s figure; the peak column is what the set has to produce.
| Cable system voltage | Acceptance test voltage (kV rms) | Peak (kV) | VA-TEK set — withstand | VA-TEK set — with tan delta |
|---|---|---|---|---|
| 5 kV | 10 | 14 | VLF-30 | VLF-34TD |
| 8 kV | 13 | 18 | VLF-30 | VLF-34TD |
| 11 kV | 16 | 23 | VLF-30 | VLF-34TD |
| 15 kV | 21 | 30 | VLF-30 (30 kV peak) | VLF-34TD |
| 20 kV | 26 | 37 | VLF-40 | VLF-45TD |
| 22 kV | 28 | 40 | VLF-40 | VLF-45TD |
| 25 kV | 32 | 45 | VLF-50 | VLF-45TD (45 kV peak) |
| 28 kV | 36 | 51 | VLF-60 | — |
| 30 kV | 38 | 54 | VLF-60 | — |
| 33 kV | 42 | 59 | VLF-60 | — |
| 35 kV | 44 | 62 | VLF-80 | — |
| 46 kV | 57 | 81 | VLF-90 | — |
| 66 kV | 80 | 113 | Beyond range | — |
| 66 kV — maintenance only | 60 | 85 | VLF-90 | — |
| 69 kV and above | 84+ | 119+ | Beyond range | — |
Figures are IEEE 400.2-2024 Table 3 acceptance values, sinusoidal, phase to earth. Installation levels are slightly lower and maintenance levels lower again; a governing utility specification may call for more. Work the exact number with the VLF test voltage calculator.
3. Choosing, class by class
5, 8 and 11 kV cables — the small-class decision
These classes need a 30 kV peak set for a plain withstand (11 kV acceptance is 16 kV rms, about 23 kV peak), and the choice within them is about what else the test has to do. If the work is acceptance and maintenance of distribution cable, the VLF-34TD is the natural fit: it reaches every level in this band with margin, and it adds tan delta, DC testing and sheath testing with fault location in a single 22 kg unit. If the test is a plain withstand and a smaller footprint matters, the VLF-30 does the voltage job on its own.
15 kV cables — where the TD case is strongest
A 15 kV acceptance test is 21 kV rms, about 30 kV peak — the top of the small-class band, and the point at which the VLF-34TD’s 34 kV peak rating stops being a convenience and starts being the reason to choose it. The headroom between a 30 kV peak set sitting exactly on the requirement and a 34 kV peak set is what lets the same instrument go on to run a monitored withstand, which under the current edition is judged on tan delta and partial discharge rather than on survival alone.
20, 22 and 25 kV cables
The requirement moves up: 25 kV acceptance is 32 kV rms, about 45 kV peak. Here the VLF-45TD is the set whose 45 kV peak rating lands on the requirement, which is why it carries the 5–25 kV class rating and also carries the tan-delta and sheath-test capability. Where only a withstand is needed, the VLF-40 covers 20 and 22 kV and the VLF-50 covers 25 kV with margin — both from the shared 30–90 kV peak platform.
28, 30 and 33 kV cables
A 33 kV acceptance test is 42 kV rms, about 59 kV peak — the class most often cited by buyers, and the one that sits above the diagnostic sets. The VLF-60 is the model that reaches it. Note what changes here: 60 kV peak is beyond the 45 kV peak ceiling of the TD sets, so a dielectric-loss diagnostic is not available in this band from the standard range. At this level the test is a withstand, and the set is specified for voltage and load.
35 and 46 kV cables
35 kV acceptance is 44 kV rms, about 62 kV peak, which needs the VLF-80 — a 60 kV peak set is 2 kV short. 46 kV acceptance is 57 kV rms, about 81 kV peak, which needs the VLF-90, the top of the range. These are the classes where headroom disappears and the load rating at 0.1 Hz does most of the work: a long 35 kV feeder is a large capacitance, and the set has to hold a regulated sine output at 0.1 Hz for the full 60-minute hold, not merely reach the voltage.
66 kV and above — the honest boundary
A 66 kV acceptance test is 80 kV rms, which is about 113 kV peak. The highest set in the VA-TEK range is the VLF-90 at 90 kV peak, so the range does not cover a 66 kV acceptance test, and we do not claim it does. Two things follow. A 66 kV maintenance test is 60 kV rms, about 85 kV peak, which the VLF-90 does reach — so a maintenance programme on 66 kV cable is within the range even though acceptance is not. And for acceptance, the honest answer is that the level needs a set rated 115 kV peak or above, which is a different class of equipment. Talk to us with the cable list and we will say plainly what is and is not available.
4. Within a class, the load rating decides the length
Voltage class gets you to the right instrument; the load rating at 0.1 Hz decides whether it will test the circuit in front of you. Charging current is proportional to frequency, capacitance and voltage, so at 0.1 Hz the current a set has to supply is a small fraction of what the same cable would draw at 50 Hz — which is the whole reason VLF testing is practical. But the cable’s capacitance still has to be charged, and the set’s current rating caps the length.
Two consequences worth stating. First, when two sets are quoted at the same voltage class, the load rating at 0.1 Hz is usually what separates them, and it is the figure most often omitted from a quotation. Second, a lower test frequency (0.05, 0.02, 0.01 Hz) extends reach on long cable at the cost of test time — which is why a set that offers the full 0.1 to 0.01 Hz span is more useful than one that offers 0.1 Hz alone. The trade-off is set out in VLF test duration and frequency.
5. Four mistakes that lead to the wrong set
- Sizing from the system voltage instead of U0. The test voltage is applied phase-to-earth. Confusing U with U0 inflates the target by about 73 percent and either over-buys or mis-states the procedure.
- Comparing rms to peak. IEEE tables the test voltage in kV rms; sets are rated in kV peak; a cosine-rectangular wave has its own relationship between the two. Compare like with like.
- Buying the voltage class and ignoring the load. A set that reaches the voltage but not the capacitance fails on the long circuit, which is usually the circuit that matters.
- Assuming one set covers every class. A 30 kV peak set that is right for 11 kV cable cannot run a 33 kV acceptance test. If the cable list spans classes, the set is sized to the top of that list, not the middle.
6. Frequently asked questions
What VLF test set do I need for a 33 kV cable?
For a 33 kV acceptance test, IEEE 400.2-2024 sets the level at 42 kV rms, about 59 kV peak, so the set must reach at least 59 kV peak. In the VA-TEK range that is the VLF-60. Confirm the load rating at 0.1 Hz against your longest circuit as well as the voltage.
Is a 30 kV VLF set enough for 15 kV cable?
For a plain withstand it sits exactly on the requirement — 15 kV acceptance is 21 kV rms, about 30 kV peak — leaving no margin. The VLF-34TD reaches the same level with headroom and adds tan delta and sheath testing, which is why it is the set recommended for the 5–15 kV band.
Which set covers the 5–25 kV range with diagnostics?
The VLF-34TD covers 5–15 kV and the VLF-45TD covers 5–25 kV. Both combine withstand with tan-delta diagnostics, DC testing and sheath testing with fault location in a single unit.
Can a VLF set test 66 kV cable?
It depends on the test. A 66 kV maintenance test is about 85 kV peak and is within the VLF-90’s range. A 66 kV acceptance test is about 113 kV peak and is not — that level needs a set rated 115 kV peak or above, which is beyond the current VA-TEK range.
Does a higher voltage class always cost more?
Within the family, yes — a higher rating means a larger high-voltage section. But the load rating at 0.1 Hz and the diagnostic options can move the price more than one step of voltage class, so judge cost from the specification, not the kV figure alone. See what a VLF hipot test set costs.
7. Where to go next
For the exact number for your class and test type, use the VLF test voltage calculator. For how the test voltage is chosen and why it is written against U0, see VLF test voltage selection. For the standard behind the table, see what IEEE 400.2-2024 requires. To verify a quotation, use the buyer’s checklist; the VA-TEK VLF range runs from 30 to 90 kV peak. Send your cable list and we will quote a configuration against it.
