VA-TEK Engineering Most VLF hipot test sets look alike on a brochure: a box, a voltage, a weight. The differences that decide whether a set does the job appear only when you line two of them up on the same basis. This is a 12-point checklist for specifying and verifying a VLF hipot test set before you buy — each point is something you can ask for on paper and confirm against the equipment.
Key takeaways
- Compare on a stated, shared basis: peak kV, µF at 0.1 Hz, and a named waveform.
- The two numbers buyers most often miss are the load rating at 0.1 Hz and the flashover trip time.
- Demand documents — a calibration certificate and test reports — not adjectives.
- A set is only as good as its service and spares when it eventually needs repair.
1. Why a checklist beats a datasheet
A datasheet lists what a supplier chose to list. A checklist forces the same questions across every candidate, so the answers can be compared. The twelve points below are grouped into what the set does, what it records, and what surrounds it — because a VLF set is bought for years, not for one test.
2. The 12-point checklist
1. Voltage basis, stated and measured the same way
Confirm that the kV figure is the peak of the output and that the measurement method matches the waveform. Two “60 kV” sets only compare if both are on the same basis.
2. Load rating at 0.1 Hz — not only at the lowest frequency
Ask for the µF at 0.1 Hz. That figure sets how long a cable the set charges at the frequency diagnostics run at, and it matters more than the headline voltage for real reach.
3. Waveform, named
Sine or cosine-rectangular, and if sine, its harmonic distortion. Sine is the basis for tan delta and partial discharge; cosine-rectangular is a withstand tool.
4. Frequency range and sweep
Which frequencies are available (0.1, 0.05, 0.02, 0.01 Hz) and whether the set sweeps automatically. Lower frequencies extend reach on long cable at the cost of test time.
5. Protection and trip behaviour
Flashover/breakdown trip time (should be in the tens of milliseconds), ground-continuity verification before energisation, closed-loop voltage regulation, and overvoltage/overcurrent protection.
6. Measurement chain and calibration
How the voltage is measured at the test object — divider, lead and instrument — and whether a traceable or accredited calibration certificate comes with it.
7. Diagnostics — and whether they localise
Is tan delta available? Partial discharge detection? And if PD, can it localise a defect along the cable, or only detect it? These are different capabilities at different prices.
8. Portability versus reach
Weight and piece count against the cable lengths you actually test. A set that covers the longest circuit carries more weight; a smaller set deploys faster but reaches less.
9. Records and logging
Does the set log voltage against time across the hold, or only a final value? A logged trace is what lets you defend a result months later — a single number cannot show drift.
10. Standards alignment
Which standard criteria the set is built to support — IEEE 400.2 for VLF, IEC 60060-3 for field test voltages — and how the test-voltage basis is expressed. See what IEEE 400.2-2024 requires.
11. Service, spares and training
Repair location and turnaround, availability of HV leads and connectors, and whether commissioning or operator training is included. For a first VLF set or a team moving off DC testing, training is not optional.
12. Evidence over adjectives
Every number above should be backed by a document: a datasheet with stated bases, a calibration certificate, a type-test or test report. Where a claim cannot be shown, treat it as marketing.
3. Red flags in a quote
- A voltage rating with no stated basis, or one that changes between documents.
- A load figure quoted only at the lowest frequency, with no 0.1 Hz number.
- Diagnostics claimed with no waveform specified — tan delta and PD need a sine source.
- No calibration certificate, or one that is neither traceable nor accredited.
- Compliance asserted by name only, with no reference to which criteria or basis.
- Support described in the future tense, with no named location or response process.
4. What to ask for on a demonstration
A demo is the cheapest place to test the claims. Ask the supplier to charge a known capacitance and show the voltage against time trace at 0.1 Hz; to trip the set deliberately and show the flashover trip time; and to run a tan delta or PD measurement if the unit claims it. Anything the set does on a bench, it should show on a demo.
5. Frequently asked questions
What is the single most important spec?
For most buyers it is the load rating at 0.1 Hz. It decides the maximum cable length you can test at the frequency diagnostics use, and it is the figure most often under-stated or omitted.
Do I need sine output, or is cosine-rectangular enough?
Cosine-rectangular is enough for a plain withstand test. If you want tan delta or partial discharge now or later, you need sine — those measurements are made on a sinusoidal source.
How long should the equipment last?
A VLF set is a long-lived instrument, so buy for the service life, not the first job. Weight the calibration intervals, spares availability and repair turnaround as heavily as the purchase price — they set the total cost over its life.
Is a bigger voltage class always safer?
Not automatically. Buying above your needs costs on every job and does not fix a weak load rating. Size the class to the cable you test most often and the load to your longest circuit.
6. Where to go next
Sizing is covered in how to choose a VLF test set, and cost in what a VLF hipot test set costs. For the test itself, see VLF test voltage selection and the VLF testing guide; the VLF test voltage calculator gives the exact level for a class. The VA-TEK VLF range lists voltage classes and load ratings from 30 to 90 kV peak, and you can request a configuration quote against your cable list.
