Partial Discharge Test Equipment: Types, Selection & IEC 60270 Guide [2026]

What Is Partial Discharge Test Equipment? (Quick Reference)

Partial discharge (PD) is a localised electrical discharge that partially bridges the insulation between two or more conductors of an electrical apparatus. The IEC 60270 standard defines PD as an important accepted diagnosis instrument of insulation damage for all types of high-voltage equipment. Unchecked, PD activity causes medium to long term erosion of insulation between high and low voltage potential conductors. Unacceptable condition leading to major power equipment failures.

A standard IEC 60270 partial discharge test apparatus is made up of five essential components working together to detect and measure the apparent charge of individual discharge pulses in picocoulombs (pC). Professional industrial partial discharge measurements are generally in the range of less than 1 pC for precision diagnostics in test environments, to tens or hundreds of thousands of pC for aged equipment field diagnostics.

The measuring impedance Z m is placed in series with the coupling capacitor, both in parallel with the device under test (DUT) . For maximum PD sensitivity, Zm can be placed in the grounding path of the DUT, although this comes with the risk of insulation breakdown during test in the unlikely event of failure. A bridge type impedance configuration depends on the highest possible immunity to noise and is more suitable where on-site testing environments are less predictable.

Conventional vs. Non-Conventional PD Detection: Which Approach for Heavy Assets?

The key decision for testing PD is the difference between conventional electrical detection methods that meet IEC 60270 (in which the equipment must be taken to shutdown) and non-conventional methods such as UHF, transient earth voltage (TEV) or acoustic emission. These latter methods tolerate techniques in energized service and can be called up once the equipment has been autopsied, identified as degrading and planning for more detailed partial discharge testing.

Use of conventional IEC 60270 electrical coupling, with coupling capacitor connected to the high-voltage (HV) terminal, is a frequent mistake made when testing energized in-service equipment. It is dangerous and unsound in test methodology to do. Conventional IEC 60270 partial discharge coupling requires that the power equipment be unplugged and isolated when applying high voltage; via contrast, non-conventional sensors work at ground potential and have no electrical coupling to the HV circuit.

In practice, both approaches are complementary. Factory acceptance testing and routine diagnostics rely on conventional IEC 60270 measurements for calibrated pC values. Ongoing condition monitoring between outages uses non-conventional sensors on energized equipment. Many utilities operate both in parallel: continuous UHF detection for early warning, with triggered conventional offline testing when sensor trends show deterioration.

What Is the Difference Between Online and Offline PD Testing?

Offline PD testing powers down the equipment and applies a known high voltage source, recording partial discharge with a galvanic coupling to the HV terminals as defined in IEC 60270. Users get calibrated apparent charge results in pC-consistent from test to test and against acceptance criteria. There is a scheduled outage.

Online PD testing employs non-conventional sensors (UHF, TEV, acoustic) that are permanently mounted to live, in-service equipment. There is no external source of voltage, nor electrical coupling required to the HV circuit. Users get semi-quantitative outputs (relative magnitude readings, PRPD patterns) rather than calibrated pC values. Use offline PD testing for acceptance examination and baseline trending; use online PD monitoring for live trending and fault prognostic.

PD Test Equipment for Power Transformers

Power transformer partial discharge testing is the most rigorous of conventional IEC 60270 applications. Theoil-paper insulation systems in transformers can experience PD activity within gas bubbles in oil, at voids in paper insulation or at protrusions near grounded metal conductors. Because the insulations are housed within grounded metal tanks, access to PD signals relies entirely upon how measurement coupling is achieved.

The power transformer PD test involves ramping from zero volts to the nominated test voltage (usually 1.5 Um/3 for induced voltage tests in IEC 60076-3), recording apparent charge levels at each step, and confirming PD activity below acceptance criterion at the test voltage. The simultaneous PRPD pattern allows event disambiguation from other electromagnetic signals.

What Equipment Is Needed to Test PD in a Power Transformer?

A complete transformer PD test setup requires five components: (1) a low-PD HV test transformer or resonant voltage source as the applied voltage source, (2) a coupling capacitor C_k rated at or above the full test voltage — or the bushing tap of a capacitively-graded bushing as the coupling point, (3) a measuring impedance Z_m (50 Ω), (4) an IEC 60270-compliant PD detector with sensitivity ≤1 pC for EHV transformers, and (5) a calibrated PD calibrator for circuit calibration before the test. The DEMIKS automatic partial discharge test system integrates all five functions with ≤1 pC sensitivity and supports both bushing tap and external coupling configurations. See also: partial discharge testing equipment and transformer test equipment.

PD Testing Equipment for High-Voltage Cables

High-voltage cable PD testing offers a unique problem relative to transformer partial discharge testing: the distributed capacitance of a long cable means that the test voltage source must supply continuing reactive current throughout the test. For long cable lengths, a standard power-frequency transformer cannot provide this reactive current – alternative voltage sources are required.

On-site cable PD testing often exposes workmanship flaws in joints and terminations rather than manufacturing flaws inherent in the cable body. Industry experience indicates that improperly installed joints – insufficient conductor ferrule compression, contaminated insulation interfaces, voids trapped within the joint – constitute the most common causes of PD-provoking defects in the field. Testing each joint or termination independently by terminating each end separately pinpoints the specific shorted location.

DEMIKS supplies both a VLF high-voltage generator and a variable frequency resonance system suitable for cable PD testing. For cable fault location following a PD detection, see DEMIKS’s cable fault locating system.

PD Testing for Gas-Insulated Switchgear (GIS) and Switchgear Panels

Gas-insulated switchgear – arguably the most hard-to-test equipment for traditional PD measuring techniques – incorporates a grounded metal enclosure that proves as effective as a Faraday cage in blocking the sensitive internal SF6 gas insulation from external electrical coupling. Attempting to use an industry-standard IEC 60270 coupling capacitor on the grounded GIS enclosure produces no measurable PD. Here: Metal construction of the GIS prevents external PD signals from being coupled onto the testing measurement circuit; – no signal is produced.

The five common modes of PD in GIS include: (1) free moving conductive particles on the enclosure floor, (2) protruding particles on the housing inner wall, (3) protrusions on the conductor, (4) particles or contamination on the insulating spacers, and (5) spacer insulation with delaminations or void defects. In addition, UHF sensors mounted at ground potentials permit energized operation during diagnostics and give an increase in obtained signal to noise ratio inherent in that the detection frequency range of 200 MHz-2.5 GHz is well above the range of most power system electrical noise. See also: DEMIKS switchgear test equipment.

Core PD Test Equipment Components and Technical Specifications

Seven technical parameters can be used to assess the suitability of a commercial partial discharge detector system for a particular application. The specification values shown below are typical of the range achieved by professional IEC 60270-grade partial discharge detector systems in use for industrial power apparatus testing.

The calibration of the measuring system is an important step to be done prior to every partial discharge test, since it provides the scale factor to convert the instrument reading into the true reading in terms of the apparent charge.

DEMIKS’s partial discharge detector achieves ≤1 pC background noise sensitivity with full PRPD pattern display — meeting the EHV transformer specification row above. For a deeper comparison of commercial analyzers, see our companion article on the top features to look for in a PD analyzer.

IEC 60270:2025 — The Updated Standard and What It Means for Your Equipment

On June 5, 2025, IEC published Edition 4 of IEC 60270 — the first full revision of the partial discharge measurement standard since the original 2000 edition with its 2015 amendment. The new title — High-voltage test techniques – Charge-based measurement of partial discharges — signals a substantive shift, not a cosmetic update. Most published guides on PD testing equipment still reference the 2015 version.

Here we will explain what has changed and what equipment it will impact in 2025-26.

Both IEC 60076-3 (power transformers) and IEC 62271-203 (GIS above 52 kV) reference IEC 60270:2025. Any new procurement contract for high-voltage power assets should specify IEC 60270:2025 compliance rather than the superseded 2015 version.

How to Choose the Right PD Test Equipment — Decision Framework

The best way to compare specific test options is to start with the asset type, then narrow down to detection method, then to key specifications, and finally to the equipment tier required. The matrix below summarizes the five main asset types most commonly tested at industrial high-voltage assets in 2025. For procurement factors beyond technical specs, see our article on how to choose the best partial discharge test equipment.

Industry Trends in PD Testing Technology (2025–2026)

3 major trends are affecting how utilities and industrial operators are approaching partial discharge test equipment purchase decisions in 2025 and 2026.

1. IEC 60270: 2025 is initiating equipment audits and replacements. the first major update of IEC 60270 since 2000 (June 2025 publication)- mandated the addition of normative calibrator requirements (Annex A) which older calibrators may not meet without recertification. Procurement audit teams at test laboratories and utility inspection agencies reviewing this revision are now auditing their existing partial discharge test fleets. Equipment manufacturers report higher enquires for calibrators and detectors since the revision was published.

2. The global market for online partial discharge monitoring systems was valued around USD 1.24 billion in 2025 and anticipated to reach USD 2.87 billion by 2032with a Compound Annual Growth Rate of 11.02%. The global driving factors are aging HV grid infrastructure in North America and Europe, increased use of HVDC systems integrating DC-compatible PD detection solutions and changing maintenance schedule from planned outages to on-line power asset management in power sector. Leading European Union published transmission system operators have begun prior structural changeover to permanent UHF sensor implementation from 2023, based on new CIGRE D1 study group suggestions;

3. AI-assisted PRPD pattern classification is a commercial product. Phase-resolved PD pattern analysis has long-sufferedfrom requiring highly-skilled engineers to interpret fingerprint patterns providing a qualitative identification of whether a pattern is indicative of a void discharge, corona, or surface tracking. Automation of defect-type identification from PRPD data by AI-assisted pattern classification software became available as a research project in the 2020-2022 timeframe and entered commercial availability from 2024-2025. This evolution enables a natural integration of digital processing guidance updates incorporated into IEC 60270:2025 Annex E and other emerging regulation for an AI-assisted PD analysis workflow.

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