Partial Discharge (PD) Testing in MV Switchgear and Cables

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Partial discharge (PD) testing is a diagnostic measurement technique used to detect localized electrical breakdown occurring within voids, cavities, or along interfaces of the insulation system of medium-voltage switchgear, cables, and transformers, without causing a complete breakdown between conductors. Because PD activity is one of the earliest measurable symptoms of insulation degradation, PD testing is widely used both as a factory routine/type test and as an on-site condition-monitoring tool to predict failures before they cause an unplanned outage. This guide walks through the underlying physics and calculation approach (apparent charge in picocoulombs), a fully worked numerical example, a comparison of common PD test methods, selection criteria, typical pitfalls, and the governing IEC test standards. Utilities and industrial operators across the Middle East and Gulf region (including Egypt and Saudi Arabia), North Africa, the CIS, and Sub-Saharan Africa increasingly specify PD testing as part of commissioning and asset-management programs for MV networks, particularly where switchgear operates in high-humidity, high-dust, or high-ambient-temperature environments that accelerate insulation aging.

What causes partial discharge and how it is quantified

Partial discharge occurs when the local electric field inside a small gas-filled void, along a delamination, or at a sharp conductor edge exceeds the local breakdown strength of that medium — while the surrounding bulk insulation remains intact. Typical PD sources in MV equipment include:

• Internal voids in cast-resin insulation or cable extruded insulation formed during manufacturing or curing.
• Surface discharge (tracking) across a contaminated or moist insulator surface.
• Corona at sharp metallic points, floating shields, or poorly terminated cable stress cones.
• Interface discharge at cable joints, terminations, and bushing connections where two insulation systems meet imperfectly.

Because the actual discharge occurring inside the void cannot be measured directly, PD magnitude is expressed as apparent charge (q), measured in picocoulombs (pC), which is the charge that — if injected between the terminals of the test object — would produce the same change in terminal voltage as the actual internal discharge. This apparent charge is always smaller than the real discharge charge inside the void, but it is a repeatable, standardized, and comparable metric.

The basic detection circuit relies on a coupling capacitor (Ck) in parallel with the test object (Ca) and a measuring impedance (Zm) that converts the fast current pulse produced by the discharge into a voltage pulse that can be captured and calibrated. The measured apparent charge is derived from:

q = C_coupling × ΔV_measured / (calibration factor)

In practice, calibration is performed by injecting a known charge pulse (from a calibrator) into the test circuit before the actual PD test, so that the oscilloscope or PD detector reading in millivolts can be converted directly into picocoulombs for the real measurement. This calibration step is mandatory under IEC 60270 and is what allows PD readings from different laboratories or different test sessions to be compared meaningfully.

Worked example: converting a measured pulse into apparent charge

Suppose a calibration pulse of a known charge Q_cal = 100 pC is injected into the test circuit and produces a peak reading on the PD detector of 50 mV. This establishes the instrument’s calibration factor:

k = Q_cal / V_cal = 100 pC / 50 mV = 2 pC/mV

Now, during the actual energized test of an MV cable termination, the detector registers a repetitive discharge pulse with a peak reading of 35 mV. Applying the calibration factor derived above:

q_apparent = V_measured × k = 35 mV × 2 pC/mV = 70 pC

For example, if the acceptance criterion specified in the purchase specification or factory test procedure for this cable accessory is a maximum apparent charge of 100 pC at the specified test voltage, a measured value of 70 pC would be considered a pass, while a value above 100 pC would require investigation, rework, or rejection of the accessory before shipment or energization.

A second useful calculation is the PD inception voltage (PDIV) and PD extinction voltage (PDEV) — the voltage at which discharges first appear as the test voltage is raised, and the (usually lower) voltage at which they cease as the voltage is reduced. For a properly manufactured insulation system, the PDIV should be comfortably above the equipment’s rated phase-to-earth voltage, so that the discharge mechanism never becomes active under normal service conditions or slow transient overvoltages. The ratio PDIV/PDEV (sometimes with a required hysteresis margin) is also tracked as an indicator of insulation health — a large gap between the two values can indicate erosion of the void surface from repeated discharge activity.

PD test and monitoring methods compared

Each method has a different sensitivity, noise rejection capability, and suitability for factory versus on-site conditions. In practice, many asset owners combine an electrical IEC 60270 measurement at the factory acceptance stage with a non-intrusive method (UHF, TEV, or acoustic) for periodic on-site condition monitoring throughout the service life of the switchgear or cable system.

Selection criteria and common pitfalls

When specifying or interpreting PD testing, engineers should consider:

• Test voltage level — PD tests are typically performed at the equipment’s rated voltage and sometimes at an elevated voltage to verify the PDIV margin above service voltage; the acceptance criterion (maximum permissible apparent charge) must be tied to the specific test voltage used.
• Background noise — On-site electrical PD measurement is highly susceptible to corona from nearby overhead lines, switching transients, and radio-frequency interference; noise gating and synchronous multi-channel detection are used to separate genuine PD pulses from external noise.
• Sensor placement — For HFCT or UHF sensors, correct positioning relative to cable terminations, bushings, or GIS spacers is essential; incorrect placement can miss the discharge source entirely or produce misleading readings.
• Calibration traceability — Every electrical PD measurement session must begin with a documented calibration pulse injection so that pC readings remain comparable between sessions and between different test laboratories.
• Repeat measurements and trending — A single PD reading is a snapshot; trending apparent charge, PDIV, and pulse pattern over successive maintenance cycles is far more informative for predicting remaining insulation life than any single absolute value.
• Distinguishing PD types — Internal void discharge, surface discharge, and corona each produce characteristic phase-resolved PD (PRPD) patterns; correct interpretation of the pattern shape (not just the magnitude) is essential to correctly diagnose the fault mechanism and its urgency.

A common pitfall is treating PD testing as a simple pass/fail gate without considering trend direction — a switchgear compartment showing a stable low-level PD reading over years may be lower risk than one showing a rapidly rising trend even if the absolute pC value is still below the acceptance threshold. Another frequent error is applying a factory acceptance limit (derived under controlled, shielded IEC 60270 conditions) directly to noisy on-site UHF or TEV readings, which are not measured in the same units and are not directly comparable without site-specific baselining.

PD testing is closely linked to the overall dielectric design verification of medium-voltage switchgear and to the health assessment of protection and metering circuits; readers evaluating a complete MV protection strategy may also want to review our hub pages on medium voltage switchgear and [protection relays](/what-is-a-medium-voltage-protection-relay-mv-protection-relay/) for related selection criteria.

Governing standards

The reference standard for electrical partial discharge measurement on high-voltage and medium-voltage apparatus is IEC 60270, which defines the apparent charge concept, the calibration procedure, and the measuring circuit requirements used throughout this guide. Cable-specific PD testing during routine and after-installation testing is addressed within the relevant IEC 60502 series for power cables, while switchgear-specific PD acceptance criteria are typically embedded in the routine/type test requirements of the IEC 62271 switchgear series for the specific assembly type. On-site monitoring techniques such as UHF and TEV are covered by supplementary IEC technical specifications and guides rather than by IEC 60270 itself, since they do not measure apparent charge directly.

Related guides

See also our guides on gas-insulated switchgear, ring main units and metal-clad switchgear.