VA Technology 
GB/T 3048.8-2025, the AC voltage test method for cables and wires, took effect on 1 May 2026, replacing the 2007 edition and adding annexes for very low frequency (VLF) and oscillating-wave testing — the first time either has sat inside China’s national standard. The cable after-laying test now has a domestic standard to point at, alongside IEEE 400.2 and IEC 60502-2 practice.
What the test has to prove
A cable that passed in the factory has since been coiled, pulled and jointed. The cable after-laying test measures it as installed: that the insulation still withstands its design stress, that no joint or termination was contaminated during assembly, and that the pull left no defect that will grow in service.
Very low frequency suits that job because it avoids the trapped space charge that DC leaves in extruded insulation: at 0.1 Hz the field still reverses each half cycle, while the charging current falls far enough for a portable set to hold voltage on kilometres of cable. Test levels derive from U0, the rated phase-to-earth voltage. IEEE 400.2 ties acceptance testing of new cable to about 3U0 and maintenance testing of aged cable to about 2U0, with the exact multiple and duration from the standard’s tables and the specification. Our VLF versus DC article works through that derivation.
The sequence, step by step
| Step | What it proves | What it catches |
|---|---|---|
| Insulation resistance and phase identification | The cable is whole and connected as drawn | Gross damage, crossed phases, water in a joint |
| Outer sheath test | The jacket survived the pull | Sheath damage that would let water reach the insulation |
| VLF withstand, 0.1 Hz, at the acceptance level | The installed insulation holds the test voltage for the set time | Joint and termination defects, assembly contamination |
| Tan delta and partial discharge on the same set | How much margin remains | Moisture in a joint, defects too small to flash over |
| Discharge, then ground | The cable is safe to touch | Stored charge in the cable capacitance |
The order matters: every step assumes the one before it held, and a check that runs out of turn may no longer reach what it was meant to inspect. Low-energy checks come first.
The discharge step earns more respect than it gets. A cable that has just held VLF test voltage is a charged capacitor, and the Fall 2026 NETA World article on residual voltage puts stored energy for 15 kV and 35 kV cable in the tens to hundreds of joules. Discharge through a resistor, then ground directly, and leave the ground in place for at least five time constants before anyone approaches the terminations.
Reading the result
A withstand test is binary, and a cable that survives it can still be carrying a defect. That is why the same set is used diagnostically: tan delta across the test, and partial discharge on the same excitation, convert a pass into a margin. Chinese practice now recognises both routes: GB/T 3048.8-2025 places oscillating-wave testing over a 20–500 Hz band and VLF at 0.01–1 Hz, and fixes neither test voltage nor duration, leaving both to the product standard and the specification.
The standard also states its own limits: oscillating-wave results may differ from those of a continuous AC test, and long-duration VLF is less sensitive to some defect types, with a cumulative-damage caveat for extruded insulation. That is the argument for reading a withstand result next to a diagnostic, not instead of one.
Frequently asked questions
Does the cable after-laying test replace the factory test?
No. The factory test is made on a drum, under controlled conditions, with calibrated equipment and a defined reference. The cable after-laying test measures the installation, and everything that happened after it left the works. A commissioning file needs both.
How long should the VLF withstand run?
The duration belongs to the standard and the specification, not the set. IEEE 400.2 defines the time at the acceptance level, and the site convention is to hold it for the full period while watching current and tan delta, rather than waiting for a flashover to end the test.
Can one set cover every step above?
Rarely in one pass. A VLF set is built for the insulation withstand and the diagnostics that follow it; the sheath test looks at the outer jacket at a lower voltage and often needs a different connection. The scope should name which instrument does which step.
VA-TEK builds the VLF hipot family for after-laying and maintenance work, from the VLF-80 and VLF-90 down to the portable units for distribution cable. Where a cable class needs an acceptance voltage the family cannot reach, we say so. Send us the cable class and the scope and we will match the set to it.
Sources
- GB/T 3048.8-2025, Test methods for electrical properties of electric cables and wires — Part 8: AC voltage test, published 5 October 2025, effective 1 May 2026; oscillating-wave and VLF annexes.
- IEEE 400.2-2024, field testing of shielded power cable systems using very low frequency: acceptance and maintenance levels from U0.
- IEC 60502-2, power cables with extruded insulation, 1 kV to 30 kV: in-situ AC test.
- NETA World, Fall 2026, Residual Voltage in Power Cables: A Little-Known Hazard in Field Testing, Baroni Jr, Bento, de Souza, Plantier: stored energy and discharge practice; those figures are its own worked cases, not VA-TEK measurements.
