Cable testing and commissioning: VLF, tan delta and partial discharge
Before a medium-voltage cable is energised it should be proven. A plain-English guide to VLF withstand, tan delta and partial discharge testing — and what each result tells you.
Al Sanaya Commissioning
·8 min read
A medium-voltage cable that has just been pulled, jointed and terminated looks finished — but installation is exactly where defects are introduced. A nicked screen, a contaminated joint or a void in a termination will not show up on a continuity check. It shows up months later as a fault. Commissioning tests prove the cable before it carries load.
Three tests do most of the work on MV cables: VLF withstand, tan delta and partial discharge. Here is what each one is for.
VLF withstand testing
A withstand test asks a simple question: can the cable hold a raised voltage for a set time without breaking down? Very low frequency (VLF) testing applies that stress as AC at 0.1 Hz.
The low frequency keeps the test set portable and avoids the space-charge problems that DC testing causes in modern XLPE insulation. If the cable survives the VLF withstand, gross installation defects have been ruled out. If it fails, a defect has been forced to break down under controlled conditions rather than in service.
Tan delta — the condition check
VLF withstand is pass/fail. Tan delta tells you about condition. By measuring the dissipation factor of the insulation at several voltage levels, it reveals how lossy — how aged — the insulation is overall.
A low, flat tan delta means healthy insulation. A high value, or one that rises as the test voltage increases, points to widespread ageing, moisture ingress or contamination. It is a whole-cable measurement, not a defect locator.
Partial discharge — finding the weak point
Where tan delta describes the whole cable, partial discharge (PD) testing pinpoints the weak spot. A PD is a tiny spark inside a void or defect, and each one slowly erodes the insulation around it. PD testing detects these discharges and, with time-domain analysis, locates them along the route.
On MV cables, PD problems most often sit at joints and terminations — the parts assembled by hand on site. Catching them early turns a future outage into a planned repair.
Putting the three together
- VLF withstand — proves the cable can hold rated stress (pass/fail)
- Tan delta — grades overall insulation condition and ageing
- Partial discharge — locates specific defects at joints and terminations
- Run as acceptance tests before energising, then periodically for diagnostics
Documentation and standards
Acceptance testing should follow the cable standard (IEC 60502 for the cable, with test voltages and durations to recognised commissioning practice) and be witnessed and recorded. The signed test pack becomes part of the handover and the baseline against which future diagnostic tests are compared.
How Al Sanaya can help
Our commissioning team carries out VLF, tan delta and partial discharge testing on MV cable systems across the GCC and MENA, supplies the witnessed test documentation, and feeds the results into a maintenance baseline for the asset's life.
Frequently asked questions
Why not just use a DC hipot test on modern cables?
DC testing can leave damaging space charge in modern XLPE insulation and may not reveal defects that only appear under AC stress. Very low frequency (VLF) testing applies an AC-equivalent stress at 0.1 Hz, which is far kinder to XLPE while still proving the cable.
What does a tan delta test measure?
Tan delta (dissipation factor) measures how lossy the insulation is. New, healthy insulation has a low, stable tan delta. A high or rising value — especially one that climbs with applied voltage — indicates aged, contaminated or water-affected insulation across the whole cable length.
What is partial discharge and why test for it?
Partial discharge (PD) is a small electrical spark inside a void or defect in the insulation. Each discharge erodes the insulation a little more. PD testing locates these defects so a weak joint or termination can be repaired before it fails in service.
When should cable testing be done?
Acceptance testing is done after installation and before energisation. Periodic diagnostic testing — tan delta and PD — is then repeated through the cable's life to track ageing, typically as part of a planned maintenance programme.
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