PD Testing for Commissioning: What to Measure Before Your New Cable Goes Live
Every cable installer knows the feeling: the cable is in, the terminations are done, and the client wants it energized yesterday. The VLF withstand test passes. Everyone is relieved.
But the withstand test only proves the cable holds voltage today. It says nothing about the joint whose termination was assembled in dusty conditions, or the connector with a hairline void. Those defects show up in a partial discharge test — and the best possible time to find them is now, before energization, while the installer is still on site and the warranty still applies.
Why PD Testing Belongs in Commissioning
PD is the only routine commissioning test that detects active electrical discharge at defect sites — the direct precursor of future failure. A joint with high PD activity at commissioning will fail. The question is only when — and whether the failure happens during the warranty period with the installer on the hook, or years later with the utility paying.
The Commissioning PD Procedure
Step 1 — Test at the Right Voltage
PD activity often only appears at or above operating voltage. Test at a level that stresses the insulation realistically — commonly up to rated voltage or slightly above during the test sequence. If PD is measured during VLF excitation, this is often called "VLF + PD" testing.
Step 2 — Measure and Record the Baseline
Record the PD magnitude in picocoulombs (pC) at each test point, along with the phase-resolved pattern. A healthy new cable typically shows low or no PD. What matters is the record — this becomes the reference for every future test.
Step 3 — Locate the PD Source
If PD is detected, don't just note it — find it. Offline PD testing can localize the discharge site along the cable, typically pointing at a specific joint or termination. A joint that shows PD at commissioning can be re-made now, on the ground, in hours — versus a mid-service failure that costs days.
Step 4 — Document for the Asset File
File the baseline values, test conditions and the PD pattern. This is what makes future PD campaigns meaningful: "PD at joint 3 rose from 2 pC to 40 pC over five years" is an actionable finding. "PD was tested once" is not.
Interpreting Commissioning PD Results
- No significant PD → cable is healthy; baseline recorded
- PD at a joint or termination → re-make the joint now, before energization
- PD distributed along the cable → possible insulation issue; investigate before accepting
- Rising PD during the test → a defect that is growing; do not energize until resolved
Common Mistakes in Commissioning PD Testing
The Economic Case
A joint re-made at commissioning costs materials and an hour of crew time. The same joint failing in service costs an outage, excavation, a repair crew, and lost revenue. Every dollar spent on commissioning PD testing saves multiples at the first prevented failure — which is why PD testing for commissioning is standard practice in utilities that measure their reliability metrics.
How TriEdge Helps
TriEdge specializes in cable fault location and test systems; for PD testing services and equipment, consult a qualified PD testing provider or combine VLF + PD-capable test systems with our fault location stack for a complete commissioning and maintenance program:
- A10 / A10 Advanced TDR — commissioning baseline traces and future fault location
- Factory-direct pricing, 40–60% below equivalent Western brands
FAQ
What is PD testing for commissioning? Partial discharge testing performed before a new cable is energized, to detect active discharge sites at joints and terminations that could fail later.
Why do PD testing if the cable passed the withstand test? The withstand test proves today's integrity. PD testing reveals active defect sites that predict future failure — a cable can pass VLF and still have a joint that will fail in years.
How is PD measured? In picocoulombs (pC), using coupling capacitors (offline) or sensors, per IEC 60270 for conventional measurement.
What PD level is acceptable in a new cable? A healthy new cable should show low or no PD. Any significant PD at a joint or termination should be investigated and repaired before energization — small PD grows, it doesn't heal.
Can commissioning PD testing find WHERE the defect is? Yes — offline PD testing can localize the discharge site along the cable, typically identifying the specific joint or termination.