Two measurements dominate insulation assessment: insulation resistance (IR) and partial discharge (PD). Both look at the health of insulation, but they measure fundamentally different properties, respond to different defect types, and give answers that can seem contradictory. Understanding the difference prevents a classic maintenance mistake – trusting a good IR reading while a PD defect grows unnoticed.
What insulation resistance measures
IR testing applies a DC voltage (typically 250 V – 10 kV depending on the equipment) and measures the leakage current through the insulation, reported as resistance in megohms or gigaohms. It detects:
- Moisture contamination and conductive pollution
- Gross insulation degradation
- Bulk defects that create a low-resistance path
IR is a coarse but vital screen: a low reading reliably indicates a problem. The measurement is typically taken with the equipment de-energised.
What partial discharge testing measures
PD testing detects the localised electrical discharge activity of small insulation defects – voids, delamination, tracking and poor interfaces – while the insulation is energised (or energised by a test source). It catches:
- Incipient internal voids that are invisible to IR testing
- Surface discharge and tracking under working stress
- Interface defects at terminations and joints
PD activity can be significant even when the bulk insulation resistance is still high – because a small local defect does not yet create a low-resistance path.
The key difference: stress levels and defect types
| Aspect | Insulation resistance | Partial discharge |
|---|---|---|
| Quantity measured | Bulk leakage resistance | Localised discharge activity |
| Voltage type | DC | AC (operating or test stress) |
| Typical condition | De-energised | Energised (online) or test source (offline) |
| Catches | Moisture, contamination, bulk degradation | Voids, surface discharge, interface defects |
| Blind to | Small local defects under AC stress | Uniform moisture ingress without discharge |
Why good IR does not mean no PD
A void inside an insulator, or a deteriorating interface, may be electrically invisible to a DC resistance measurement: the bulk path is still intact and dry. Under AC operating stress, however, that same defect discharges on every cycle, degrading further until it fails. PD testing is the measurement that sees this class of defect.
Combining them in practice
- IR testing for scheduled checks and after suspected moisture ingress – it catches the bulk problems cheaply.
- PD testing on critical assets and where operating experience says the defect class is likely – it catches the local problems IR misses.
- Both on the same asset for a complete picture: IR for bulk health, PD for local discharge activity.
About HUWOR
Zhuhai Huawang Technology Co., Ltd. (HUWOR) supplies PD detection instruments – handheld PD detectors, HFCT clamps, ultrasonic sensors and online monitoring systems – that complement routine insulation resistance testing in utility and industrial maintenance programmes, with OEM/ODM support and application guidance worldwide. Contact the HUWOR sales team for instrument selection.