VA-TEK Engineering Most VLF arguments on site are not about the test set. They are about which document the test has to satisfy. IEC 60502-2 is a cable standard that includes an after-installation test; IEEE 400.2 is a field-testing guide for installed cable systems. They overlap at 0.1 Hz, they answer different questions, and a specification that names both without saying which governs leaves the test engineer to choose — usually at the worst possible moment.
This article sets out what each document covers, where the two diverge in practice, and how to decide which one applies when a project specification cites both.
Key takeaways
- IEC 60502-2 is a cable product standard whose scope includes a post-installation electrical test for new 6–30 kV extruded-insulation cable — an acceptance act tied to a new circuit.
- IEEE 400.2 is an IEEE guide for field testing of installed shielded cable systems with VLF below 1 Hz — acceptance, maintenance and diagnostic testing on a system, not a product.
- They agree on the physics and often on the voltage level, but not on duration, criteria or the purpose of the test.
- A maintenance test on an aged circuit is governed by the field-testing guide, not by the product standard, in almost every case.
- When a specification cites both, name the governing document for the pass/fail decision and treat the other as reference.
1. What each document is actually for
IEC 60502-2 is part of the IEC 60502 family for extruded-insulation power cables. Part 1 covers 1–3 kV, part 2 covers rated voltages from 6 kV (Um 7.2 kV) up to 30 kV (Um 36 kV), and part 4 covers the accessories. Its post-installation test exists to confirm that a new circuit was assembled correctly: joints made, terminations fitted, sheath intact, insulation not damaged during laying. The test is an acceptance gate on a product that has just been installed.
IEEE 400.2 answers a different question. It is a guide for field testing of shielded power cable systems using very low frequency AC below 1 Hz, and it addresses acceptance testing after installation, maintenance testing of circuits already in service, and diagnostic measurements that go beyond a go/no-go result. The guide is written for the engineer deciding how to test an installed system, not for the factory deciding whether to ship a cable.
The distinction matters because the same 0.1 Hz waveform can be produced for two very different purposes. One document is asking whether this circuit is fit to energise for the first time. The other is asking what evidence is needed to keep operating a circuit that has already seen service, and what to do with the answer.
2. Scope, side by side
| Aspect | IEC 60502-2 | IEEE 400.2 |
|---|---|---|
| Document type | Cable product standard | Field-testing guide |
| Object tested | Newly installed extruded-insulation cable | Shielded cable system in the field |
| Voltage scope | 6–30 kV rated (Um 7.2–36 kV) | Shielded systems at distribution and transmission levels; the 2024 edition is reported to have extended the upper range |
| Purpose | After-installation acceptance | Acceptance, maintenance and diagnostics |
| VLF content | One permitted option among several AC methods | The subject of the document |
| Criteria | Withstand without breakdown | Withstand plus monitored-withstand and diagnostic criteria |
The asymmetry is the point. IEC 60502-2 mentions VLF as one route to an acceptance test; IEEE 400.2 is built around it. An engineer who has only read the cable standard will assume the 0.1 Hz test is a formality with a fixed duration, which is exactly what the guide does not say.
3. What the test levels look like
IEC 60502-2 clause 20 sets out the electrical tests after installation and permits several AC routes, including a 15-minute test at phase-to-phase voltage U and a 24-hour test at rated voltage U0. Among them is the option most field engineers use: 15 minutes at 3U0 at 0.1 Hz, applied between conductor and metallic screen. That is a short, high-level acceptance test with a fixed shape.
IEEE 400.2 tabulates acceptance voltages, maintenance voltages and durations by voltage class, and distinguishes a withstand test from a monitored withstand where tan delta or partial discharge is judged during the hold. The 2024 edition tightened the monitored-withstand criteria toward tan delta and partial discharge, and made other changes covered in IEEE 400.2-2024: what changed. The practical difference for a test engineer is that one document gives a single acceptance recipe, and the other gives a framework from which the applicable level has to be selected.
| Question | IEC 60502-2 route | IEEE 400.2 route |
|---|---|---|
| New circuit, acceptance | 15 min at 3U0, 0.1 Hz | Acceptance level selected by voltage class |
| Circuit already in service | Not the purpose of the document | Maintenance level, below acceptance |
| Diagnosis beyond pass/fail | Outside scope | Tan delta and partial discharge criteria |
| Judging during the hold | Withstand only | Monitored withstand permitted, with defined criteria |
For a 33 kV circuit the VLF withstand level is well within the output of a 90 kV peak test set, and the selection logic by voltage class is set out in choosing a VLF test set by voltage class. The arithmetic for the voltage itself is handled by the VLF test voltage calculator, which follows the voltage-class table rather than a single formula.
4. Deciding which document governs
Three questions settle it in most cases.
- Is the circuit new or already in service? A new installation being energised for the first time is an acceptance act, and the cable standard is the natural reference where it applies. A circuit that has been operating for years is being maintained, which is the guide’s territory.
- What is the jurisdiction? IEC-based specifications and national adoptions tend to name the cable standard for after-laying tests; North American practice and many international project specifications name IEEE 400.2. Both are commonly cited in the same tender package.
- What has to be defended afterwards? If the deliverable is a pass/fail certificate for handover, follow the acceptance recipe. If the deliverable is a condition assessment that will be trended against future tests, the diagnostic framework is the better basis, and the method has to be recorded well enough to repeat — see withstand versus diagnostic testing.
5. When a specification cites both
This happens often, and it is not a drafting error. A typical specification names the cable standard for acceptance and the IEEE guide for maintenance, because the two phases of the asset life are different projects. The failure mode is leaving the governing document unstated for a particular test, which usually surfaces when a result is disputed.
The fix is procedural rather than technical: in the test plan, state which document governs the pass/fail decision for each test in the schedule, quote the level and duration from that document, and list the other as reference. Record the edition. IEC 60502-2 and IEEE 400.2 have both been revised, and an edition mismatch is enough to make two otherwise identical procedures incomparable.
6. Duration, criteria and the maintenance test
Duration is where the two documents separate most visibly. The IEC after-installation option is short and fixed. The IEEE framework treats duration as a variable tied to voltage class, cable age and the purpose of the test, with longer holds traded against lower levels. A maintenance test on an aged circuit is normally run below the acceptance level, which is a concept that has no counterpart in the cable standard because the cable standard is not describing maintenance.
This is also where the recording discipline differs. An acceptance certificate needs the level, duration and outcome. A diagnostic record needs the level, duration, temperature, capacitance, the measured loss behaviour and the previous result on the same circuit, because the value of the test lies in the comparison. A trend that has no baseline is a number without a meaning — the same argument that makes tan delta trending useful and a single tan delta figure decorative.
7. The 66 kV boundary, stated plainly
At the top of the medium-voltage range the two documents and the available equipment part company. A 66 kV class acceptance test at 3U0 requires a test voltage above the 90 kV peak ceiling of the VLF-30-90 range; the maintenance level at 2U0 is within it. That is a limit of this product line, not a limitation of the method, and it should be established at the specification stage rather than on site. Where a project requires the acceptance level at 66 kV, size the test set against the required voltage first, then confirm the waveform and the standard route.
Frequently asked questions
Does IEC 60502-2 require a VLF test?
No. It permits VLF as one of several AC options after installation. The choice among them belongs to the specification and to the test plan.
Can IEEE 400.2 be used for a new cable acceptance test?
Yes. The guide covers acceptance testing after installation as well as maintenance testing of circuits in service.
Why do the recommended durations differ between the two?
Because the objectives differ. The cable standard defines a fixed acceptance act on a new product; the guide defines a framework in which duration is selected with voltage class and purpose.
Which edition should a test plan quote?
The edition named in the project specification. If none is named, record the current edition you worked to and state it on the certificate.
Choosing the reference before the test, not after
The choice between these two documents is a specification decision with a technical consequence, and it is cheapest to make in the office. Once the governing document is named and the edition recorded, the rest of the procedure — level, duration, criteria and record — follows from it. The broader method background is in the VLF cable testing guide and in VLF versus DC testing.
The VLF-30-90 range covers acceptance and maintenance levels for MV cable up to the limits described above, and the VLF-45TD adds tan delta for monitored withstand work — see VLF-30-90 and VLF-45TD. For help matching a specification to a test set, send the voltage class and the governing standard through the contact form.
