How to Interpret Partial Discharge Test Results: Magnitude, Patterns and Pass/Fail
1. First question: is the signal actually PD?
The most common interpretive error is treating every measured pulse as discharge from the cable. Before interpreting magnitude, confirm the signal is PD.
| Signal type | Typical signature | How to distinguish |
|---|---|---|
| Real PD in the cable | Repeatable, phase-locked to the AC test voltage, stable from cycle to cycle | Appears only when test voltage is applied; pattern repeats consistently |
| External corona | High magnitude, erratic, dependent on nearby hardware and humidity | Often disappears when the test object is disconnected, or changes with weather |
| Switching noise (from VLF set) | Periodic, synchronised with the source | Present even with a dummy test object |
| Loose / poor connection | Intermittent bursts, unstable | Changes when connections are re-made |
| Ambient radio interference | Continuous, environment-dependent | Present before the test object is energised |
Practical rule: measure the background first, then energise. Only signals that appear with the test voltage and repeat consistently are candidates for PD.
2. Second question: where is the discharge?
Location determines severity and action. In field practice, most PD originates at accessories, not in the middle of an intact extrusion.
| Location | Typical implication | Usual action |
|---|---|---|
| Termination | Installation or contamination issue | Inspect, clean, re-assemble, retest |
| Joint | Workmanship, moisture ingress, or thermal aging | Plan repair/replacement before failure; monitor if not yet critical |
| Cable body (mid-span) | Insulation defect, water tree, mechanical damage | Highest concern — destructive defects, prioritise investigation |
| Test equipment / terminations | Artefact of the test setup | Fix the setup, not the asset |
Use PD mapping (time-of-flight/reflectometry) and correlate the reported distance with joint positions from as-built records. A PD result with a location can be scheduled; a PD result without one cannot — this is one of the most common mistakes in on-site PD testing.
3. Third question: magnitude, inception and extinction
Three numbers matter together:
- Magnitude (pC) — apparent charge. Higher is worse, but magnitude alone is a weak predictor: a stable 100 pC discharge may be less urgent than a rising 20 pC one.
- PDIV (partial discharge inception voltage) — the voltage at which discharge starts. A low PDIV relative to operating voltage means the defect is active in service.
- PDEV (partial discharge extinction voltage) — the voltage at which it stops. The margin between PDEV and operating voltage is a genuine severity indicator: if PDEV falls close to operating voltage, the defect is continuously active in service.
Record the PD–voltage curve, not a single value. The shape of that curve distinguishes a benign, stable discharge from one that grows rapidly with stress.
4. Fourth question: what is the trend?
PD is a precursor phenomenon — its diagnostic value comes from comparison.
| Observation | Interpretation |
|---|---|
| PD stable across tests, PDEV well above operating voltage | Monitor on normal cycle |
| PD magnitude rising between tests | Degradation in progress — shorten the survey interval |
| PDEV approaching operating voltage | Defect becoming continuously active — prioritise |
| PD appears where none was recorded at commissioning | New defect — investigate, check for recent work or damage |
| PD disappears after a repair | Confirms the diagnosis and the fix |
Always compare against the commissioning baseline (see the pre-energization checklist) and the previous test on the same circuit, using the same test method and voltage sequence. Comparing results from different methods (DC vs VLF, different bandwidths) is not valid.
5. How pass/fail criteria are set
There is no universal pC threshold that applies to every cable, voltage class and utility. Acceptance criteria come from three sources:
- Standards for the test method and the cable type (e.g. IEC 60270 for PD measurement, IEEE 400.3 for field PD testing of shielded cable systems, the relevant IEC cable standard)
- The utility's own specification for new assets at commissioning — see PD testing for commissioning
- The asset's own history — the baseline recorded when it was installed
Typical commissioning expectations for newly installed extruded cable are that no sustained PD attributable to the asset is present at the specified test level. In practice, what matters as much as the threshold is the record: the measured value, the pattern, the location, and the trend.
Interpretation quick-reference
| Question | Data to look at | Red flag |
|---|---|---|
| Is it PD? | PRPD pattern, background comparison | Signal present without test voltage |
| Where is it? | PD mapping + joint records | PD in cable body (mid-span) |
| How severe? | Magnitude + PDIV/PDEV | PDEV close to operating voltage |
| Is it changing? | Trend vs previous tests | Magnitude rising between surveys |
FAQ
Is 50 pC bad? It depends on location, voltage class and trend. 50 pC at a termination that has been stable for years is a different situation from 50 pC in the cable body that appeared since the last test. Interpret magnitude together with location and trend — never alone.
Can PD disappear on its own? Yes — PD can extinguish when stress changes, and a defect can enter a dormant phase. Disappearance in one test is not proof the defect is gone; that is why trends across multiple tests, and the PDEV margin, matter.
What is the difference between PDIV and PDEV? PDIV is the voltage at which discharge begins; PDEV is the voltage at which it stops. PDEV is always lower than PDIV. The gap between PDEV and the operating voltage tells you whether the discharge would be active in normal service.
Should I compare PD results from different instruments? Not directly. Apparent charge depends on the measurement circuit, bandwidth and calibration — which is why calibration practice matters as much as the instrument. Only compare like-for-like: same instrument family, same method, same voltage sequence, same coupling configuration.
Related reading
- Partial Discharge Testing of Power Cables — why PD predicts failure years in advance
- Common Mistakes in On-Site PD Testing — the errors that distort the result you are trying to read
- PD Testing for Commissioning — acceptance criteria at commissioning
- Partial Discharge on the Electricity Network — turning individual results into an asset-management programme