The Complete Guide to Partial Discharge in Electrical Systems

Introduction

Partial discharge is an early warning of insulation deterioration in electrical equipment—a localized electrical discharge that only partially bridges an insulation defect before a full failure occurs. It can develop inside insulation, across contaminated surfaces, around high-voltage conductors, or at poorly installed connections. Although each discharge event is relatively small, repeated activity can gradually damage insulation until the equipment can no longer tolerate its operating voltage.

For maintenance managers, reliability engineers, electrical engineers, plant managers, operations directors, safety professionals, and asset management teams, partial discharge is an important condition indicator for switchgear, transformers, cables, motors, generators, bus systems, and other critical electrical assets. Detecting it early supports system reliability, gives teams time to investigate developing defects before they lead to insulation failure, arc flash, equipment damage, or unplanned downtime, and can help extend asset life through proactive maintenance.

However, partial discharge is not a single fault with one identifiable cause. Different discharge types produce different electrical, acoustic, electromagnetic, and chemical signatures. Effective testing therefore depends on selecting the correct detection method, understanding the operating environment, and trending results over time.

This guide explains what partial discharge is, what causes it, where it occurs, how it affects electrical equipment, and how it can be detected, monitored, prevented, and managed through online and offline testing, relevant standards, and a structured condition-based maintenance program, including how IRISS supports safer partial discharge detection.

What Is Partial Discharge?

Partial discharge, commonly shortened to PD, is a localized electrical discharge that only partially bridges the insulation between two conductors. Unlike a complete electrical breakdown, the discharge does not immediately create a continuous conductive path through the entire insulation system.

The activity usually develops where the electrical stress exceeds the strength of a small area within or around the insulation. This may occur inside gas filled voids, where a lower dielectric constant can increase local electric field intensity and partial discharge occurs when the applied voltage exceeds the local inception voltage in the void, along a contaminated surface, near a sharp conductor, or at an interface between different insulating materials.

Each individual discharge releases only a small amount of energy. The concern is the cumulative effect. Thousands or millions of discharge events can occur over time, progressively eroding insulation and producing heat, chemical changes, carbon deposits, mechanical damage, and electromagnetic interference.

IEC 60270:2025 provides standardised terminology, quantities, test circuits, and measurement methods for charge-based partial discharge measurements in electrical apparatus, components, and systems.

Partial discharge is therefore best understood as both:

• A symptom of an existing insulation defect
• A mechanism that can make the defect progressively worse

Its presence does not always mean that immediate failure is unavoidable. However, increasing discharge magnitude, changing patterns, or rapidly rising activity can indicate that insulation deterioration is accelerating.

What Causes Partial Discharge?

Partial discharge begins when the electric field in a localized area, under sufficient voltage stress, becomes strong enough to ionize the surrounding gas or stress the insulation beyond its local dielectric strength.

Common causes include:

• Air pockets or voids inside solid insulation
• Cracks, delamination, or defects introduced during the manufacturing stage
• Loose or poorly fitted cable terminations
• Sharp edges, burrs, or conductor protrusions
• Incorrect clearances between energized components
• Moisture inside or on the surface of insulation
• Dust, salt, chemicals, oil, and other contamination
• Thermal deterioration caused by overheating
• Mechanical vibration or movement
• Aging insulation
• Poor installation workmanship
• Damage caused during transportation or maintenance
• Excessive operating voltage or transient overvoltage
• Deteriorated stress-control materials
• Incorrectly installed joints and connectors

Voltage is not the only factor. Temperature, humidity, contamination, pressure, mechanical stress, and insulation geometry can all affect whether partial discharge begins and how quickly it develops.

For example, contamination may initially have little effect in a dry environment. When humidity increases, the contamination can absorb moisture and create a partially conductive path across the insulation surface. In high humidity, this can further worsen conditions on the insulator surface. This increases leakage current and electrical stress, allowing surface discharge to develop.

Similarly, an air-filled void inside solid insulation has a lower dielectric strength than the adjacent material. The electric field across the void can become high enough to ionize the trapped gas even though the surrounding insulation remains intact.

Types of Partial Discharge

Partial discharge is generally classified according to where and how the activity occurs.

Internal Discharge

Internal PD occurs inside solid or liquid insulation. It is commonly associated with voids, cracks, delamination, gas bubbles, interfaces between insulating materials, and small pinholes that can act as hidden initiation points in solid insulation.

Because the defect is hidden inside the insulation, it may not be visible during a routine inspection. Repeated discharge gradually damages the adjacent material and can eventually create an electrical tree through the insulation, a process called treeing that drives progressive deterioration, ultimately leading to complete dielectric failure.

Internal discharge is commonly associated with:

• Cables
• Cable joints
• Transformers
• Motor and generator windings
• Resin-insulated components
• Bushings
• Instrument transformers

Surface Discharge

Surface discharge travels across the surface of an insulating material rather than through its full thickness. It frequently develops when contamination, moisture, damaged coatings, or inadequate creepage distance creates a weak path across the surface and may intensify under greater stress.

Over time, surface discharge can create visible tracking, erosion, carbon deposits, and permanent conductive paths, and repeated activity can leave permanent chemical changes in the remaining insulation.

It is often found on:

• Cable terminations
• Bushings
• Insulators
• Switchgear supports
• Busbar insulation
• Contaminated outdoor equipment

Corona Discharge

Corona is a form of partial discharge that occurs in a gas surrounding a conductor, usually near a sharp point, rough surface, damaged conductor, or area with a highly concentrated electric field.

Corona does not initially bridge the insulation between conductors. It ionizes the air immediately surrounding the high-stress area.

Although corona can sometimes remain stable for an extended period, it may produce ozone, nitrogen compounds, ultraviolet radiation, audible noise, and insulation deterioration. It may also indicate poor equipment geometry or inadequate clearances.

Electrical Tracking

Tracking develops when repeated surface discharge creates a carbonized path across insulating material. Once the track becomes sufficiently conductive, current can flow more easily along the surface.

Tracking is particularly dangerous because it can progress from intermittent partial discharge to sustained arcing or complete insulation failure.

Arcing

Arcing is sometimes discussed alongside partial discharge, although it generally represents a more advanced and energetic electrical fault. An arc forms when current travels through an ionized path between conductors or between a conductor and ground.

Compared with early-stage corona or tracking, arcing typically produces greater energy, stronger acoustic activity, more heat, and a higher risk of immediate failure.

Understanding the distinction between these discharge modes helps maintenance teams assess fault severity and select the appropriate response.

Where Does Partial Discharge Occur in Electrical Systems?

Partial discharge can develop anywhere an insulation system is exposed to sufficient electrical stress. It is particularly common in high voltage electrical equipment and medium-voltage assets, although in deteriorated conditions it can also occur in systems operating at lower voltages.

Switchgear

Switchgear may develop PD around busbars, cable compartments, insulators, spouts, current transformers, voltage transformers, joints, and terminations. In the field, other methods may also be used to detect PD signals, and directional couplers detect signals from partial discharges at joints in switchgear.

Common contributing factors include contamination, inadequate clearances, loose connections, damaged insulation, and poor assembly.

Power Cables

Cable systems are vulnerable at joints, splices, terminations, stress cones, and areas where insulation has been damaged, including older or specialized designs that use paper windings.

Installation defects can create voids or uneven electrical stress that remain hidden until the cable is energized. In paper-based insulation, small defects can start a longer breakdown process before failure becomes visible. Partial discharge testing can help identify and locate these defects before they develop into cable failures.

Transformers

PD may occur within transformer windings, solid insulation, oil-paper interfaces, bushings, tap changers, lead exits, the transformer tank as a path for emitted signals, and areas affected by gas bubbles or contamination; as deterioration progresses, oil-paper insulation can undergo chemical transformation that raises electrical conductivity.

Because transformers are critical and expensive assets, PD activity is often assessed alongside dissolved gas analysis, thermal inspection, oil testing, and other diagnostic methods.

Motors and Generators

Rotating machines can experience discharge in stator winding insulation, slot sections, end windings, phase separations, and interfaces between coils and the grounded core.

Thermal cycling, vibration, contamination, loose windings, and repetitive electrical stress can gradually weaken the insulation system.

In rotating machines, insulation health strongly affects asset life.

IEC 60034-27-2:2023 addresses online PD measurements on the stator winding insulation of non-converter-driven rotating machines rated at 3 kV and above. IEC TS 60034-27-6:2026 addresses online electrical detection and monitoring for motors and generators supplied by converters.

Bushings and Instrument Transformers

Voids, moisture, deterioration, and stress concentrations inside bushings and instrument transformers can produce internal discharge.

IEEE C57.160-2023 provides guidance for the electrical measurement of partial discharge within high-voltage bushings and instrument transformers using wide-band instruments.

Bus Systems

Bus ducts and enclosed bus systems may experience PD around joints, supports, insulation barriers, sharp edges, and contaminated surfaces.

These systems can be difficult to inspect because many critical components are enclosed. Permanently installed sensors or inspection ports can improve access to repeatable condition data. In enclosed bus systems, these sensors also support ongoing pd monitoring without increasing exposure during inspection.

Effects of Partial Discharge on Electrical Equipment

Partial discharge damages equipment through a combination of electrical, thermal, mechanical, and chemical processes that drive insulation degradation.

Possible effects include:

• Localized erosion of insulation
• Electrical treeing
• Surface tracking
• Carbonization
• Cracking and delamination
• Ozone and corrosive chemical formation
• Increased leakage current
• Reduced dielectric strength
• Damage to cable joints and terminations
• Winding insulation deterioration
• Increased risk of arcing
• Phase-to-phase or phase-to-ground faults
• Equipment failure
• Arc flash
• Fire
• Unplanned shutdowns

The rate of deterioration depends on the type of discharge, its location, operating voltage, insulation material, environmental conditions, and how long the activity continues. Repetitive discharge causes progressive chemical and mechanical damage, ultimately increasing the risk of machine failure.

A low apparent charge value is not automatically harmless. A relatively small but concentrated discharge in a vulnerable location may present a greater risk than a larger signal originating from a less critical source. This is why interpretation should consider discharge patterns, asset design, historical trends, operating conditions, and supporting inspection data rather than relying on one number alone.

Signs of Partial Discharge

Partial discharge may not produce obvious symptoms during its early stages. As activity develops, several warning signs may become noticeable.

Audible or Ultrasonic Noise

PD events can generate high-frequency sound. Some advanced faults may produce audible buzzing, crackling, hissing, or snapping, but early activity often occurs above the range of human hearing.

Ultrasound instruments use ultrasonic measurement to detect high-frequency sound from PD and convert it into an audible range that inspectors can hear and analyze.

Ozone Odor

Corona and other discharge activity can generate ozone, which has a sharp odor sometimes compared with chlorine or the smell following a thunderstorm.

Odor should never be used as the primary detection method, particularly around energized equipment. It may, however, support other findings.

White Powder or Chemical Residue

Chemical reactions caused by discharge can leave white, gray, or green deposits around affected components. These deposits may appear near terminations, conductors, insulation supports, and ventilation openings.

Surface Tracking

Dark lines, carbon deposits, cracks, burns, erosion, or branching patterns throughout insulation may indicate surface discharge or tracking.

Discoloration

Discharge-related heating and chemical activity can cause insulation to become discolored, brittle, or damaged.

Electromagnetic Interference

PD can generate radio-frequency interference and other electromagnetic signals. Unexplained interference may indicate electrical discharge activity, although other noise sources must be excluded.

Rising or Changing Test Results

An increase in partial discharge activity, including discharge magnitude, pulse count, repetition rate, or pattern complexity, may indicate that a defect is progressing.

Trending is therefore more valuable than relying on a single isolated test.

How Partial Discharge Is Detected

No single detection technology is suitable for every asset or discharge type. Different methods identify different physical effects created by the discharge.

Conventional Electrical Measurement

Conventional electrical testing measures apparent charge, commonly expressed in picocoulombs. The method typically uses coupling devices such as a coupling capacitor, measuring impedances, calibrators, filters, and specialized instruments.

IEC 60270:2025 defines standardized charge-based PD measurement techniques, including terminology, measurable quantities, frequencies, test circuits, and analogue and digital methods.

This method is widely used in laboratories, commissioning tests, factory acceptance testing, and controlled offline assessments.

Ultrasonic Detection

Partial discharge creates pressure waves that can be detected acoustically. Ultrasound instruments identify high-frequency sound associated with corona, tracking, and arcing.

Ultrasound is particularly useful for energized switchgear inspections because it can help technicians screen equipment without opening the enclosure. Signal characteristics can also help distinguish electrical discharge from mechanical or environmental noise.

Transient Earth Voltage

Transient earth voltage, or TEV, sensors detect high-frequency electromagnetic pulses that travel along the inside surface of metal-clad switchgear and appear briefly on the outside of the enclosure as tev signals, the measurable voltage spikes on surrounding metalwork caused by partial discharge.

These transient earth voltages make field screening convenient because they can be measured non-intrusively without direct electrical connection.

TEV testing is commonly used as a non-intrusive screening method for medium-voltage metal-clad switchgear.

High-Frequency Current Transformers

High-frequency current transformers, or HFCTs, detect high-frequency discharge pulses travelling through grounding conductors, cable screens, or bonding connections.

They are commonly used for cables, switchgear, transformers, motors, and other grounded systems.

Ultra-High-Frequency Detection

UHF sensors detect electromagnetic emissions created by partial discharge. This method is often applied to gas-insulated switchgear, transformers, and enclosed high-voltage equipment.

Acoustic Emission

Acoustic emission sensors can be mounted on transformer tanks or other equipment surfaces to detect mechanical waves generated by internal discharge.

Using multiple sensors may help locate the source through differences in signal arrival time.

Phase-Resolved Partial Discharge Analysis

Phase-resolved partial discharge, or PRPD, analysis maps discharge pulses against the phase position of the applied AC voltage.

Different defects often produce recognizable patterns. Experienced analysts can use these patterns to help distinguish internal void discharge, surface activity, corona, noise, and other conditions.

Electromagnetic and Acoustic Methods

IEC TS 62478:2016 addresses electromagnetic detection throughout HF, VHF, and UHF ranges, as well as acoustic PD measurements. It also considers sensor sensitivity, discharge location, calibration, and sensitivity checks.

The strongest diagnostic programs frequently combine more than one technology. For example, ultrasound may identify electrical discharge while infrared thermography determines whether a related thermal condition is also developing.

Online vs Offline Partial Discharge Testing

Partial discharge testing can be performed while equipment remains energized or while it is removed from normal service and connected to an external test source.

Online Partial Discharge Testing

Online testing is performed while the asset operates under its normal voltage, load, temperature, and environmental conditions, making it a practical method for detecting partial discharge under normal operating conditions.

Advantages include:

• No planned shutdown is required
• Equipment is tested under actual operating stress
• Intermittent or load-dependent activity may be detected
• Permanent sensors can support continuous monitoring
• Results can be trended over time
• Testing can be incorporated into routine inspection routes

Limitations include:

• Electrical and environmental noise may interfere with results
• The applied voltage cannot usually be controlled
• Some defects may not be active during the inspection
• Signal location and classification can be more difficult
• Results may not be directly comparable among different asset designs or sensor arrangements

Online testing is especially valuable for critical assets that cannot be easily removed from service.

Offline Partial Discharge Testing

Offline testing requires the asset to be isolated, de-energized, grounded, and connected to an external voltage source.

Advantages include:

• Test voltage can be controlled
• Electrical noise can be reduced
• Inception and extinction voltages may be determined
• Controlled diagnostic procedures can be performed, allowing detailed analysis of insulation condition during planned maintenance
• Calibration may be more straightforward
• Cable faults may be located more precisely

Limitations include:

• A planned shutdown is required
• An external test source and specialist equipment may be needed
• The test may not reproduce normal load and environmental conditions
• Incorrect test procedures can overstress aged insulation
• Testing can require more time and coordination

IEEE 400.3 recognizes both online and offline PD testing for installed shielded power cable systems. IEC TS 61934:2024 covers offline electrical measurement of PD in insulation systems subjected to repetitive voltage impulses from power electronic devices.

Neither approach is universally better. The appropriate choice depends on the asset, failure risk, outage availability, diagnostic objective, and testing standard, and scheduled offline work can reduce associated downtime and avoid production loss when repairs are coordinated during planned outages.

How to Prevent Partial Discharge

Partial discharge cannot always be eliminated, especially as insulation naturally ages. However, good design, installation, operation, and maintenance practices can greatly reduce the likelihood of PD developing.

Use Correct Equipment Design

Electrical systems ought to maintain appropriate clearances, creepage distances, conductor geometry, stress control, insulation thickness, and environmental protection.

Sharp edges, poorly formed conductors, inadequate stress relief, and uneven electrical fields should be avoided.

Control Contamination and Moisture

Keep electrical enclosures clean, dry, sealed, and properly ventilated where required.

Maintenance teams have to address:

• Water ingress
• Condensation
• Dust accumulation
• Salt contamination
• Chemical vapors
• Oil residue
• Damaged enclosure seals
• Failed heaters or ventilation systems

Follow Correct Installation Procedures

Cable joints, terminations, connectors, and insulating components must be installed according to the manufacturer’s instructions.

Small installation errors can create voids, trapped contamination, damaged insulation, or uneven stress that later develops into partial discharge.

Maintain Correct Torque and Mechanical Support

Loose connections, movement, and vibration can damage insulation or create abnormal electrical stress.

Torque requirements, support systems, cable bending radii, and vibration controls should be verified during installation and maintenance.

Prevent Overheating

Heat accelerates insulation aging and may cause cracking, embrittlement, separation, or loss of dielectric strength.

Infrared inspections, temperature sensors, visual indicators, and load monitoring can help identify thermal problems before they damage insulation.

Address Findings Early

Early-stage PD may sometimes be corrected through cleaning, drying, tightening, repairing, recoating, repositioning, or replacing affected components.

Delaying action allows the discharge to continue damaging the insulation.

Best Practices for Partial Discharge Monitoring

A successful monitoring program requires more than purchasing a detector and recording isolated readings.

Establish Baseline Measurements

Record baseline data when equipment is new, newly commissioned, recently serviced, or known to be in acceptable condition.

Without a baseline, it can be difficult to determine whether later results represent normal asset behavior or developing deterioration.

Use Repeatable Test Conditions

Where possible, collect measurements using consistent:

• Sensor positions
• Instrument settings
• Test routes
• Operating loads
• Environmental conditions
• Distances and angles
• Asset identification methods

Repeatability improves the value of trend analysis.

Trend Results Over Time

A single reading provides limited context. Trends can show whether activity is stable, intermittent, seasonal, load-dependent, or increasing.

Important trend variables may include:

• Signal magnitude
• Pulse count
• Repetition rate
• PRPD pattern
• Load
• Voltage
• Temperature
• Humidity
• Background noise
• Inspection date and time

Use Multiple Technologies

Combine PD data with infrared, visual, electrical, vibration, oil, and insulation test results where appropriate.

Different technologies reveal different failure mechanisms. Correlated findings give a stronger basis for maintenance decisions.

Separate Noise from Genuine Discharge

Variable-frequency drives, switching power supplies, radio transmitters, fluorescent lighting, nearby electrical assets, mechanical movement, and poor grounding can all create signals that resemble PD.

Noise identification should be part of every inspection and analysis process.

Prioritize Trends and Patterns

Absolute alarm thresholds may be useful, but they should not replace engineering judgment.

Identical readings can have different meanings depending on the asset type, sensor arrangement, insulation system, defect location, and previous history.

Document Findings Clearly

Inspection records should identify:

• Asset
• Measurement location
• Sensor type
• Instrument and settings
• Operating condition
• Environmental condition
• Signal level
• Discharge pattern
• Supporting photographs
• Audio recordings
• Recommended action
• Follow-up date

Define Escalation Procedures

The program should clearly state what happens when abnormal PD activity is detected. Some studies indicate PD-related insulation defects are responsible for roughly 80% of high-voltage failures.

Possible responses should follow clear escalation paths so asset managers can act on abnormal findings before complete failure develops:

• Repeat the measurement
• Compare phases or identical assets
• Perform a second diagnostic test
• Increase inspection frequency
• Conduct an engineering review
• Plan an outage
• Repair or replace the affected component
• Remove the asset from service when failure risk is unacceptable

Standards and Guidelines for Partial Discharge Testing

Partial discharge testing should follow applicable international standards relevant to the asset and measurement technology.

Important documents include:

IEC 60270

IEC 60270:2025 is the primary international standard for charge-based measurement of partial discharge in electrical apparatus, components, and systems. It defines terminology, quantities, frequencies, test circuits, calibration considerations, and measurement methods.

IEC TS 62478

IEC TS 62478:2016 covers electromagnetic and acoustic PD measurements, including HF, VHF, UHF, and acoustic methods.

IEC 60034-27 Series

The IEC 60034-27 series addresses partial discharge testing and monitoring for rotating electrical machines. Different parts cover online and offline measurements, converter-fed machines, and specific insulation systems.

IEC TS 61934

IEC TS 61934:2024 applies to offline PD measurements on insulation systems exposed to repetitive voltage impulses from power electronic equipment.

IEEE 400.3

IEEE 400.3 provides guidance for partial discharge testing and location in installed shielded power cable systems. It recognizes both online and offline approaches.

IEEE C57.113

IEEE C57.113-2023 covers wide-band apparent-charge PD measurement in liquid-filled power transformers and shunt reactors during dielectric testing.

IEEE C57.160

IEEE C57.160-2023 addresses PD measurement in high-voltage bushings and instrument transformers.

IEEE C37.303

IEEE C37.303 provides field guidance for detecting, measuring, locating, trending, recording, and interpreting partial discharge in energized switchgear above 1,000 VAC.

The correct standard depends on the equipment, voltage class, insulation system, test method, and purpose of the assessment. Organizations should also follow equipment manufacturer recommendations, site procedures, and applicable electrical safety requirements.

How IRISS Supports Partial Discharge Detection

IRISS provides inspection solutions that help maintenance teams detect and trend partial discharge while reducing unnecessary interaction with energized equipment.

Ultrasound Inspection Devices

The IRISS Sonus Series allows technicians to detect high-frequency sound associated with corona, tracking, arcing, and other electrical anomalies. These instruments help inspectors locate suspicious activity and collect repeatable condition data without taking equipment offline.

Ultrasound is especially useful when discharge activity has not yet generated enough heat to appear during an infrared inspection. Combining ultrasound and infrared provides a wider view of electrical asset condition.

Partial Discharge Sensor Ports

IRISS partial discharge sensor ports provide a permanent, consistent inspection point through the equipment enclosure.

Instead of scanning panel seams or relying on inconsistent external angles, technicians can connect or position compatible ultrasound equipment at a defined location. This supports safer, faster, and more repeatable inspections.

VPDS Ultrasound Sensors

The VPDS Series provides an installed ultrasound sensing point for energized electrical assets. The sensor helps capture sound from inside the enclosure while allowing the panel to remain closed during inspection.

Combined Infrared and Ultrasound Access

IRISS offers inspection windows and access solutions that support multiple condition-monitoring technologies. Selected solutions combine infrared inspection access with ultrasound sensing, allowing teams to look for both thermal problems and discharge activity during the same inspection route.

This combined approach helps maintenance teams:

• Detect faults that may not produce heat
• Validate findings using more than one technology
• Standardize inspection locations
• Reduce unnecessary panel access
• Improve data repeatability
• Build stronger asset histories
• Prioritize maintenance according to actual condition

Partial discharge detection is most effective when it forms part of a documented condition-based maintenance program rather than being treated as an isolated test.

Frequently Asked Questions About Partial Discharge

What is partial discharge?

Partial discharge is a localized electrical discharge that only partially bridges the insulation between conductors. It typically occurs in voids, cracks, contaminated surfaces, gas spaces, or other areas where the local electrical field exceeds the insulation’s dielectric strength.

What causes partial discharge in electrical systems?

Partial discharge can be caused by insulation voids, manufacturing defects, contamination, moisture, poor cable terminations, sharp conductors, inadequate clearances, overheating, vibration, aging, and installation damage.

The immediate cause is excessive electrical stress within a localized area of the insulation system.

What are the different types of partial discharge?

The main types are internal discharge, surface discharge, and corona. Electrical tracking and arcing may develop as insulation damage becomes more advanced.

Each type occurs in a different location and produces different electrical, acoustic, and electromagnetic characteristics.

Where does partial discharge occur?

Partial discharge can occur in switchgear, cables, transformers, bushings, motors, generators, bus systems, joints, terminations, insulators, and other equipment containing electrically stressed insulation.

It often develops at voids, interfaces, sharp points, contaminated surfaces, or spots with damaged stress control.

Why is partial discharge important?

Partial discharge is important because it can indicate insulation deterioration before complete failure occurs.

Detecting and trending PD gives maintenance teams an opportunity to investigate developing problems, plan corrective work, and reduce the risk of equipment failure, arc flash, fire, and unplanned downtime.

How is partial discharge detected?

Partial discharge can be detected using electrical apparent-charge measurement, ultrasound, TEV sensors, HFCT sensors, UHF sensors, acoustic emission, and phase-resolved pattern analysis.

The correct method depends on the equipment, insulation system, discharge type, operating condition, and testing objective.

What is the difference between partial discharge and corona discharge?

Partial discharge is the more general term for localized electrical discharges that do not completely bridge the insulation between conductors.

Corona is a specific type of partial discharge that occurs in a gas surrounding a conductor, usually near a sharp point or area of concentrated electrical stress.

What is partial discharge monitoring?

Partial discharge monitoring is the repeated or continuous collection of PD data from electrical equipment.

The results are trended to identify changes in discharge magnitude, pulse activity, patterns, and operating conditions. Monitoring may use portable instruments, permanently installed sensors, or a combination of both.

How can partial discharge be prevented?

Partial discharge risk can be reduced through good insulation design, proper clearances, effective stress control, correct installation, moisture control, contamination management, temperature control, correct torque, and routine inspection.

Early corrective action is also important because existing PD activity can progressively damage insulation.

How harmful is partial discharge to electrical equipment?

Partial discharge can be highly harmful when it continues over time. Repeated activity erodes insulation, creates chemical and thermal damage, promotes tracking, and reduces dielectric strength.

The severity depends on the discharge type, location, energy, repetition rate, insulation system, and rate of change.

How do you perform a partial discharge test?

A partial discharge test typically involves selecting the appropriate sensor and measurement method, verifying instrument settings, establishing background noise, collecting data under documented conditions, analyzing discharge patterns, and comparing results with previous readings or similar assets.

Offline tests may also require isolation, grounding, an external voltage source, calibration, and specialized safety procedures. Testing should be performed by qualified personnel using an applicable standard and asset-specific procedure.

What is online partial discharge testing?

Online partial discharge testing is performed while electrical equipment remains energized and operating under normal voltage, load, temperature, and environmental conditions.

It allows maintenance teams to inspect equipment without a planned outage and can reveal load-dependent or intermittent defects. However, online measurements may be affected by background electrical noise and require careful interpretation.

Conclusion

Partial discharge is an early indicator of insulation weakness and an active contributor to insulation deterioration. It can develop inside insulation, across contaminated surfaces, around conductors, and at poorly installed joints or terminations.

Because the activity may begin long before a thermal anomaly or visible defect appears, partial discharge testing provides valuable information about faults that other inspection methods may miss.

An effective PD program combines suitable detection technologies, standardized inspection methods, baseline measurements, trend analysis, qualified interpretation, and clearly defined maintenance actions. Online and offline testing each have an important role, and the right approach depends on the asset, operating environment, and diagnostic objective.

By identifying partial discharge early and monitoring how it changes, maintenance teams can make better condition-based decisions, address insulation defects before they become critical, and improve the safety, reliability, and service life of electrical equipment.