Testing
After-installation test voltages for MV extruded cables
- where
- a.c. test voltage for 5 min, equal to the system line voltage U (kV rms)
- rated voltage to earth (kV)
- a.c. test voltage for 24 h (kV rms)
- 0.1 Hz VLF test voltage (kV rms)
- VLF peak for a sinusoidal waveform (kV)
Explain this equation
Lists the withstand test voltages commonly applied to a new extruded MV cable system after installation, from its rated voltage to earth U₀.
U_ac,5 min is the system line voltage, √3 × U₀, applied for 5 minutes. U_ac,24 h is the operating voltage U₀ held for 24 hours. U_VLF is a 0.1 Hz test at three times U₀, and U_VLF,peak its peak for a sinusoidal waveform.
The multiples are set by standards and practice rather than physics: high enough to break down gross installation defects in joints and terminations, but not to age sound insulation. √2 converts rms to peak for a sine wave.
For an 11 kV cable (U₀ = 6.35 kV) this gives 11 kV for 5 min a.c., 19 kV rms VLF and about 27 kV peak. For 6.6 kV cable (U₀ = 3.8 kV) VLF is 11.4 kV rms.
Always follow the level and duration in the contract or network operator specification. Check whether a VLF set reads rms or peak, and use reduced levels on service-aged cable.
Withstand voltages for commissioning tests on new extruded (XLPE/EPR) MV cable systems, from the rated U0.
Enter your values to see the result.
Good to know
IEC 60502-2 (tests after installation) gives an a.c. test at the system phase-to-phase voltage U (= √3·U0 ≈ 1.73U0) for 5 min, or at U0 for 24 h; a d.c. test at 4U0 for 15 min is also listed but is discouraged for aged XLPE. VLF at 0.1 Hz sinusoidal, 3U0 for typically 15–60 min, is widespread European/UK utility practice (CENELEC HD 620 after-laying tests and network operator specifications). IEEE 400.2 instead tabulates VLF levels by cable voltage class (for example the 15 kV class: about 18 kV rms installation, 20 kV acceptance, 16 kV maintenance in the 2004/2013 tables), with 30 min recommended for acceptance; check the current edition. Always use the test level in your contract or network operator specification and the accessory manufacturer's limits. Example U0 values: 3.8 kV (6.6 kV system), 6.35 kV (11 kV), 12.7 kV (22 kV), 19 kV (UK 19/33 kV cables to BS 7870, Um 36 kV, commonly tested on the same basis although IEC 60502-2 itself stops at 18/30 kV).
Common mistake
Applying VLF levels to cosine-rectangular test sets without checking whether the specification is in rms or peak, or retesting old/service-aged cable at new-cable levels.
Limits
Commissioning tests on new extruded MV cable systems only. Maintenance tests use lower levels (IEEE 400.2 maintenance column, or about 75–80 % of acceptance in many utility specs). Paper-insulated and oil-filled cables follow different (often d.c.) practice.
Sources
- IEC 60502-2:2014 Power cables with extruded insulation and their accessories for rated voltages from 1 kV (Um = 1,2 kV) up to 30 kV (Um = 36 kV) – Part 2: Cables for rated voltages from 6 kV up to 30 kV
- IEEE 400.2-2024 Guide for field testing of shielded power cable systems using very low frequency (VLF)
- IEC 60060-3:2006 High-voltage test techniques – Definitions and requirements for on-site testing
- Moore, G.F. (ed.) Electric Cables Handbook, 3rd edn, Blackwell Science / Wiley
For competent high-voltage engineers. Results are calculations from the published methods shown and depend entirely on the data you enter. They are not a substitute for a system study, manufacturer data, the applicable standards and Engineering Recommendations, or your organisation’s safety rules. Never use a result to justify working on or near live HV equipment.