6.2 VLF Cable Testing & the DC/AC Voltage Conversion

Why VLF for Cables

A shielded power cable is a large cylindrical capacitor. The capacitive charging current at power frequency is I = 2πfCV — directly proportional to frequency. For a long 15 kV class feeder, the 60 Hz charging current at acceptance voltage would require a test set the size of a substation transformer. Dropping the frequency to 0.1 Hz cuts charging current by a factor of 600 (0.1/60), making a portable, field-practical AC withstand test possible.

This is why very-low-frequency (VLF) hipot has become the standard field dielectric test for medium- and high-voltage shielded cables. The test applies AC stress — which replicates service conditions far better than DC — at a frequency low enough to be practical.

VLF Waveforms and Frequency

IEEE 400.2 recognizes two VLF waveforms:

• Sinusoidal (0.1 Hz) — a pure sine wave; RMS = 0.707 × peak (assuming <5% harmonic distortion). Most modern VLF sets produce this.
• Cosine-rectangular (0.1 Hz) — alternates positive and negative half-cycles with a cosine shape; for this waveform RMS and peak are treated as equal, so the same numeric voltage produces a higher peak stress.

The standard test frequency is 0.1 Hz. Frequencies from 0.01 to 0.1 Hz are acceptable, but 0.1 Hz is the norm; lower frequencies extend the test duration per cycle and are used only when 0.1 Hz still draws too much current on very long cables.

VLF Test Duration (IEEE 400.2)

The 60-minute acceptance duration is not arbitrary — IEEE 400.2 research showed that cables with installation defects break down within the first minutes, while trees and aging-related weak spots often need the full duration to fail under VLF stress.

IEEE 400.2 VLF Test Voltages (Table 3, 2023)

Selected values from IEEE 400.2-2023 Table 3, phase-to-ground:

Maintenance voltage is approximately 75% of acceptance voltage. The 2024 revision expanded cable coverage to 138 kV and now references only RMS values (peak values removed for consistency).

The 62% DC/AC Conversion Factor (IEEE 95) — Not the Ground-Test 62% Rule

The 62% rule is a DC-to-AC conversion shortcut. (Note: this 62% rule is a DC-to-AC voltage conversion factor from IEEE 95 — it is unrelated to the 62% rule used in fall-of-potential ground-electrode resistance testing, covered in Section 10.1. Do not confuse the two.) IEEE Std 95 establishes the relationship V_DC = 1.7 × V_AC(rms) for rotating machinery acceptance tests. Taking the reciprocal:

V_AC(rms) ≈ V_DC / 1.7 ≈ 0.588 × V_DC ≈ 0.62 × V_DC

So the equivalent AC RMS value of a given DC test voltage is roughly 62% of the DC value. Equivalently, if you have an AC RMS test specification and want the DC equivalent, multiply by 1.7 (≈ 1/0.62).

Where the 62% rule applies

• Rotating machinery — directly from IEEE 95; the 1.7× factor was chosen as a conservative compromise for complete windings.
• General AC/DC hipot planning — useful for converting between published AC and DC test values when a single standard governs both.

Where the 62% rule does NOT apply

• Cables — the historical DC/AC ratio for cables is 2.0 to 3.0× depending on insulation type (rubber, paper, polyethylene), not 1.7×. The 62% rule under-predicts the DC equivalent for cables.
• XLPE cable in service — IEEE 400 and IEEE 400.2 no longer recommend DC hipot for extruded (XLPE/EPR) cables after 5 years in service. Space charge accumulates under DC stress and can damage the cable when voltage is removed. VLF is the preferred method.
• Laminated dielectric (PILC) cable — DC hipot is still accepted.

A common mistake is applying the 62% rule to a cable pressure test. The rule is a rotating-machinery conversion; cable DC voltages come from IEEE 400 / NETA ATS cable tables, not from a 1.7× multiplier.

MTS Frequency vs ATS Acceptance

ANSI/NETA ATS (Acceptance Testing Specification) governs new equipment acceptance — first commissioning after install or rewind. ANSI/NETA MTS (Maintenance Testing Specifications) governs in-service maintenance testing.

For cable, the practical differences:

• ATS — higher test voltages (acceptance column of IEEE 400.2 Table 3); 60-minute duration.
• MTS — maintenance voltage ≈ 75% of acceptance; 30-minute minimum (60 min for critical circuits). MTS-2023 added Tan Delta tables for XLPE and EPR and now prefers VLF for all MV/HV shielded cable dielectric tests.

MTS also defines frequency of maintenance intervals based on application criticality and environment — typically 1–3 years for cable feeders, shorter in harsh or critical-service environments.

Common VLF Traps

• Confusing cosine-rectangular peak with sinusoidal RMS — the two waveforms use different voltage scales for the same cable class (e.g., 35 kV class: 44 kV RMS sinusoidal vs 62 kV peak cosine-rectangular). Mixing them up over-tests or under-tests the cable.
• Shortening the 60-minute acceptance duration — IEEE 400.2 research showed aging-related trees can take the full duration to break down; cutting the test to 15 minutes can pass a defective cable.
• Applying the 62% rule to a cable DC test — the 62% factor is a rotating-machinery conversion; cable DC/AC ratios are 2.0–3.0× and IEEE 400 no longer recommends DC for in-service XLPE anyway.
• Ignoring MTS-2023's preference for VLF — DC withstand is allowed only for laminated dielectric or extruded cables under 5 years old; everything else should use VLF.