How to Detect Partial Discharge in Switchgear

Partial discharge (PD) in switchgear is one of the most common hidden faults that lead to insulation failure, unplanned shutdown, and even catastrophic arc flash or fire. Detecting partial discharge early is critical because it directly affects equipment reliability, safety, and lifetime.

How to detect partial discharge in switchgear is one of the most frequently asked questions by electrical and maintenance engineers—especially in medium-voltage systems where insulation stress is high. Traditional visual inspection is no longer enough. Modern plants now rely on advanced detection technologies, online monitoring, and predictive maintenance.

In this article, we will answer every real question engineers and plant managers ask about partial discharge—from what it is, why it happens, how to detect it, how to interpret results, and how to prevent or fix it.

1. What Is Partial Discharge in Switchgear?

Partial discharge is a localized electrical breakdown within insulation that does not completely bridge the electrodes. It is a sign that the insulation is deteriorating and may eventually fail.

Is partial discharge normal?

No. PD is not a normal condition. Even small PD activity indicates insulation weakness.

Is partial discharge always dangerous?

Yes—because PD gradually erodes insulation over time, leading to insulation breakdown and possible equipment failure or arc flash.

2. How Dangerous Is Partial Discharge?

Causes insulation carbonization
Generates heat and internal erosion
Produces ozone and nitric acid (chemical degradation)
Leads to short circuit or arc flash
Can cause fire or explosion
Reduces switchgear lifetime by 50% or more

3. What Causes Partial Discharge in Switchgear?

Insulation Defects (most common)

Manufacturing defects
Voids or air bubbles in insulation
Poor cable termination

Moisture or Humidity

Water lowers insulation resistance and increases PD risk.

Contamination

Dust, pollution, salt, and chemical vapors create tracking paths.

Aging and Thermal Stress

Insulation weakens over time, especially under high temperature cycles.

Mechanical Stress / Vibration

Cracks, loose connections, deformation = PD hotspots.

Poor Installation or Maintenance

Incorrect tightening, damaged insulation, poor cable bending.

4. Early Warning Signs of Partial Discharge

Many engineers ask: “How do I know if my switchgear has PD?”

Look for:

Hissing or crackling sound (ultrasonic)
Ozone or burnt smell
Visible tracking or carbon marks
Unusual heat spots
Nuisance tripping
Irregular voltage/current readings
Degraded insulation

But the problem: Often PD is invisible—so we must detect it using proper methods.

5. Where Does Partial Discharge Occur?

- Internal PD

Inside insulation voids or cavities.

- Surface PD (Tracking)

Along insulation surfaces due to contamination or moisture.

- Corona Discharge

At sharp edges in gas/air at high voltage.

- Cable Terminations and Joints

Most common PD locations.

- Busbars, Insulators, Connectors

Weak insulation or loose connections.

6. Why Is PD More Common in Medium Voltage Switchgear?

Higher electric field stress
More complex insulation systems
Larger equipment
Aging MV installations
Environmental exposure (dust, moisture)

7. How to Detect Partial Discharge (All Methods)

This is the most important section: “What is the best method to detect PD?”

1) Visual Inspection

Check for:

Burn marks
Dust accumulation
Cracks
Loose connections

Limitation: Only finds severe PD, not early-stage.

2) Ultrasonic Detection

PD generates ultrasonic (40 kHz+) signals
Handheld ultrasonic detector can “listen”
Works online (live equipment)

Benefits: Quick, non-invasive, detects small activity.

3) TEV (Transient Earth Voltage) Detection

Measures PD pulses traveling through metal enclosure
Only works on metal-clad switchgear

Most widely used method for switchgear.

4) UHF (Ultra High Frequency) Sensors

Detects electromagnetic waves (300 MHz – 3 GHz)
Very sensitive & accurate
Used in GIS and metal-enclosed switchgear

5) Infrared Thermography

Detects heat caused by PD or loose joints
Good for surface issues

Note: Not always reliable for internal PD.

6) Offline PD Testing

Requires shutdown.

Methods:

High Voltage (HV) test
Very Low Frequency (VLF)
Tan Delta
PD measurement in pC

Very accurate but needs outage.

7) Online Continuous PD Monitoring

Permanent sensors + monitoring system

Real-time data
Alarm when PD exceeds threshold
Trending over time
Supports predictive maintenance

Best solution for critical assets.

8) Chemical / Ozone Detection

PD creates ozone, NOx, gases.
Sensors or gas analysis can detect.

9) Combination of Methods

Most reliable = TEV + Ultrasonic + Thermal + UHF

8. Can Partial Discharge Be Detected Without Shutdown?

Yes. Use online PD testing:

Ultrasonic
TEV
UHF
Infrared
Online monitoring system

Only offline tests require outage.

9. Online vs Offline PD Detection – Which Is Better?

Factor

Online

Offline

Shutdown

Yes

Accuracy

Medium-High

Very High

Trending

Yes

Cost

Lower

Higher

Safety

High

Medium

Use

Routine monitoring

Deep diagnostics

Recommendation: Use online regularly, offline when problem occurs or during major shutdowns.

10. What Sensors/Tools Are Used for PD Detection?

Handheld detectors

Ultrasonic guns
TEV meters
IR cameras

Portable and low cost.

Portable PD Test Kits

For offline measurement
High accuracy

Permanent PD Monitoring Systems

Multiple sensors (UHF, TEV, HFCT)
24/7 monitoring
SCADA / DCS integration
Alarm and data logging

Best for critical equipment or high-risk industries.

11. Step-by-Step: How to Perform a PD Test

Inspect visually
Choose test method (online/offline)
Place sensors correctly (cable terminations, busbars, covers)
Collect PD signals over time
Filter noise (important!)
Analyze patterns and magnitude
Identify source location
Classify severity
Record results and trend over time
Plan maintenance action

12. How to Interpret PD Results

Most engineers ask:
“What PD level is acceptable?”

Depends on standard (IEC/IEEE)
Depends on voltage level
Measured in dB, pC, TEV, or mV

General guideline:

0 – Low: Acceptable, monitor
Medium: Attention needed, schedule maintenance
High: Urgent action required

Always compare with baseline or manufacturer data.

13. How Often Should PD Be Tested?

Critical MV switchgear: every 6–12 months (offline) + continuous online
Less critical: annually
After major maintenance or installation: immediately

14. What Is a Partial Discharge Monitoring System (PDMS)?

A PDMS is a fixed system that continuously measures PD activity using sensors installed inside the switchgear.

Key Features:

Real-time alerts
Trending and history analysis
Threshold-based alarms
Integration with SCADA / DCS
Supports predictive maintenance
Can combine ultrasonic, TEV, UHF, HFCT

Helps detect faults months before failure.

15. Can PD Monitoring Be Integrated with SCADA?

Yes. Modern systems provide:

Modbus, IEC 61850, OPC
Alarm to control room
Automatic logging
Visual dashboard
Predictive analytics

This is the future of switchgear reliability.

16. Is PD Testing Expensive?

Handheld test: low cost
Offline test: medium cost
PD monitoring system: high initial cost, BUT cheaper than unplanned downtime.

1 hour of plant shutdown = $$$ more than PD system cost.

17. Is It Cheaper to Repair or Replace Switchgear with PD?

Early detection → simple repair / cleaning
Late detection → insulation failure → full replacement

Prevention always cheaper than failure.

18. What Happens If PD Is Ignored?

Accelerated insulation breakdown
Short circuit / arc flash
Fire or explosion
Equipment damage
Worker injury
Long downtime
High repair cost
Insurance claim issues

Real case: Many transformer/switchgear failures started with undetected PD.

19. How to Fix Partial Discharge

- Minor PD:

Clean contamination
Dry moisture
Tighten loose parts
Improve ventilation

- Medium PD:

Repair insulation
Apply insulating coating
Re-terminate cables
Replace bushings

- Severe PD:

Replace component or entire switchgear

20. How to Prevent Partial Discharge

Use high-quality insulation
Proper installation practices
Control humidity and dust
Perform regular PD testing
Implement online monitoring
Follow maintenance schedule
Avoid overvoltage and stress

21. Standards for PD Testing

Most common:

IEC 60270 – PD measurement
IEC 62271 – High-voltage switchgear
IEEE 400 – Cable testing
IEEE 1656 – Field PD testing

Some industries (oil & gas, utilities) require PD testing by regulation or insurance.

22. Which Industries Have Highest PD Risk?

Cement plants (dust + vibration)
Water treatment plants (humidity)
Oil & Gas (critical reliability)
Petrochemicals (toxic gases)
Data centers (zero downtime)
Mining (harsh environment)
Pulp & Paper (moisture + heat)

23. Indoor vs Outdoor Switchgear

Indoor	Outdoor
Better protection	Exposed to weather
Less moisture	High humidity
Cleaner	Dust, contamination
Lower PD risk	Higher PD risk

24. Why Old Switchgear Has More PD

Aging insulation
Worn seals
Poor previous maintenance
Old design (no PD monitoring)
Thermal stress history

25. Advanced Technology for PD Detection

- AI and Machine Learning

Predicts failure before alarm.

- IoT sensors

Wireless monitoring.

- Cloud analytics

Remote access, dashboards.

- Combining multiple methods

TEV + Ultrasonic + UHF + Thermal = maximum accuracy.

Conclusion

Partial discharge is one of the most common and dangerous hidden faults in switchgear. It starts small and silent, but over time it destroys insulation, causes equipment failure, and can lead to arc flash or fire.

The good news?
With modern detection methods—ultrasonic, TEV, UHF, infrared, offline testing, and especially online monitoring systems—you can detect PD early, trend it over time, and take action before a failure happens.

Detecting and monitoring partial discharge =
✅ Higher reliability
✅ Longer equipment life
✅ Lower maintenance cost
✅ Zero unplanned downtime
✅ Improved safety

Engineers who invest in PD detection move from reactive maintenance to predictive and proactive maintenance—and that is the key to modern industrial success.