Short answer: A partial discharge (PD) test raises the voltage on a cable, holds it briefly, lowers it, and measures the tiny internal sparks that a defect gives off. Those sparks are reported as apparent charge in picocoulombs (pC). The cable passes if discharge stays below the declared sensitivity — 10 pC for cable, 5 pC for accessories — at the specified test voltage. It’s the most sensitive factory test for finding small insulation defects, which is why high-voltage cable leans on it.
The method itself lives in IEC 60270; the voltage levels and pass marks live in the cable standards IEC 60502-2, 60840 and 62067. Here’s how a test actually runs.
Table of Contents
What is partial discharge?
Partial discharge is a small electrical breakdown inside the insulation that doesn’t bridge the whole gap. Picture a tiny void, a contaminant, or a rough spot at a joint. The electric field there is higher than in the surrounding material, so it sparks across that small spot — but the rest of the insulation still holds. The discharge is “partial” because it only crosses part of the insulation.
On its own a single discharge is harmless. Repeated millions of times under service voltage, it slowly erodes the insulation and grows into an electrical tree, then a full breakdown. Catching it early is the whole point of the test.
Why the PD test matters
An AC withstand test tells you a cable survives a high voltage for a few minutes. It can’t see a small void — the cable passes the withstand test and fails in service two years later.
A PD test sees that void while it’s still small. It’s far more sensitive: it picks up discharges down to a few picocoulombs, well before they’re big enough to puncture anything. That’s why, as cables go up in voltage, the standards lean harder on PD detection and ease off the brute-force withstand voltage. The PD test is the fine filter; the withstand test is the coarse one.
The PD test, step by step
Step 1 — Build the measuring circuit
A standard PD circuit (per IEC 60270) has three parts beyond the test object:
- a coupling capacitor that gives the high-frequency discharge pulses a path,
- a coupling device that picks those pulses off the line, and
- a measuring instrument that turns them into a reading.
The cable under test sits across the same high-voltage supply.
Step 2 — Calibrate
You can’t measure the charge at the defect directly — it’s buried in the insulation. So instead the standard measures apparent charge: the charge that, injected at the cable terminals, produces the same reading as the real discharge.
To make that reading mean something in pC, you calibrate first. A calibrator injects a known charge (say 5 or 10 pC) into the complete circuit, and you set the instrument so its reading matches. Now every later reading is in real picocoulombs.
Step 3 — Check the background noise
Before energizing, you measure the noise floor with no test voltage applied. Electrical interference, corona on the test leads, and grounding problems all add noise. With good filtering and a clean setup you can get down to around 1 pC. The noise level has to sit well below the sensitivity you’re testing to, or you can’t trust a clean result.
Step 4 — Raise the voltage, hold, then lower it
This is the part that looks odd until you know why. The test doesn’t just sit at the measuring voltage. It goes higher first:
| Standard | Cable class | Raise to (hold 10 s) | Then measure at |
|---|---|---|---|
| IEC 60502-2 | MV | 2 U₀ | 1.73 U₀ |
| IEC 60840 | HV | 1.75 U₀ | 1.5 U₀ |
| IEC 62067 | EHV | 1.75 U₀ | 1.5 U₀ |
You raise above the measuring level to “switch on” any discharge sites, then drop back down to read them. That works because a defect needs a higher voltage to start discharging than to keep discharging (see inception vs extinction below). The brief over-voltage wakes up every weak spot; the lower measuring voltage is where good insulation goes quiet but a real defect keeps talking.
Step 5 — Read the apparent charge
At the measuring voltage you read the largest repeating discharge, in pC. For a three-core cable you test each core against the screen in turn.
Step 6 — Pass or fail
The criterion across all three standards is the same: no detectable discharge above the declared sensitivity at the measuring voltage.
| Standard | Sensitivity, cable | Sensitivity, accessories |
|---|---|---|
| IEC 60502-2 | 10 pC or better | — |
| IEC 60840 | 10 pC or better | 5 pC or better |
| IEC 62067 | 10 pC or better | 5 pC or better |
On the type test, the bar tightens to 5 pC, and PD is re-checked after the bending test and again after the heating cycles — because a defect often only shows up once the cable has been flexed and thermally aged.
What “apparent charge” and pC really mean
Apparent charge is not the actual charge bouncing across the void. That’s inaccessible. It’s the equivalent charge measured at the terminals — what you’d have to inject from outside to mimic the discharge. It’s reported in picocoulombs (pC), where 1 pC is a trillionth of a coulomb.
It’s a relative health indicator, not a physical map of the defect. A rising pC reading across tests, or any reading above the limit, tells you something is wrong inside — even though the number doesn’t tell you exactly what or where.
Inception and extinction voltage
Two related quantities come out of the same test:
- PD inception voltage (Uᵢ) — the lowest voltage at which discharge starts as you raise the voltage from a low level.
- PD extinction voltage (Uₑ) — the lowest voltage at which discharge stops as you lower the voltage.
Extinction voltage is normally lower than inception voltage. That gap is exactly why the test profile raises high and then drops back: you push past inception to start every weak site, then read at a level above extinction where any genuine defect is still active but clean insulation is silent.
FAQ
What is a partial discharge test on a cable?
It’s a factory or field test that applies voltage to a cable and measures small internal discharges from insulation defects. The result is given as apparent charge in picocoulombs (pC). A low, stable reading means healthy insulation.
What is a good PD test result?
A pass means no discharge above the declared sensitivity at the test voltage — 10 pC or better for cable, 5 pC or better for accessories under IEC 60840 and 62067. Lower is better; the goal is no detectable PD at all.
What does pC mean in PD testing?
pC is picocoulombs, the unit of apparent charge. It measures the size of each discharge as seen from the cable terminals, not the actual charge inside the defect.
Why is the PD test voltage raised then lowered?
Because a defect needs more voltage to start discharging than to keep going. The test raises the voltage to initiate discharge at every weak spot, then lowers it to a measuring level where good insulation is quiet and real defects still show.
What’s the difference between a PD test and an AC withstand test?
The withstand test is pass/fail at high voltage — it proves gross integrity. The PD test is sensitive and diagnostic — it finds small defects long before they’d fail a withstand test.
Key takeaways
- PD testing measures small internal discharges as apparent charge in pC, using the method in IEC 60270.
- The voltage profile raises above the measuring level, then drops back: 2 U₀ → 1.73 U₀ at MV, 1.75 U₀ → 1.5 U₀ at HV and EHV.
- Pass means no discharge above the declared sensitivity — 10 pC for cable, 5 pC for accessories.
- It’s the fine filter that catches what an AC withstand test can’t, which is why high-voltage cable depends on it.