Online vs Offline PD Testing: Which Method is Right for You?

November 27, 2025

Online and offline PD testing depends on maintenance goals, equipment criticality, and the severity of the suspected fault. In many cases, a combination of both methods offers the most comprehensive insight to ensure electrical asset reliability and safety. Partial discharge occurs when a small electrical breakdown occurs inside a tiny air gap or defect inside the insulation. These repetitive discharges eventually damage the insulation material, weakening it to the point where the damage may have advanced and the equipment fails, demanding costly repairs or replacements or even safety threats. This is why Partial Discharge (PD) Testing is so important, as it gives a clear idea of the insulation’s condition and helps identify early signs of stress before major issues develop.

The information gathered from PD testing is valuable for understanding how well critical electrical assets will perform over time. A variety of equipment can be assessed using PD testing, including:

Motors and generators
Switchgear assemblies
Air-insulated components such as insulators, surge arresters, and bushings
Power cables, paper-insulated or extruded

Among the types of tests generally done to detect these tiny discharges. We have Online and offline PD testing, which vary in their purpose and how they are used, depending on the equipment sensitivity.

Online testing is done for regular, ongoing, and continuous PD monitoring. Online PD testing is ideal for routine screening and for in-service monitoring without downtime. While Offline testing is done for detailed diagnostics and precise localisation during planned maintenance, it often requires the machinery under test to be de-energised to conduct the monitoring.

Online PD tests are quicker and lower-cost, but are comparatively less sensitive to minor faults and more susceptible to interference. Offline PD tests are rather more sensitive to minor faults and ideal for detailed and thorough analysis, for instance, to determine inception and extinction voltages. A major concern for offline tests is that they require a full outage, are time-consuming, and may have to incur a high cost when compared to online PD testing.

Feature	Online PD Testing	Offline PD Testing
Equipment power	Energized	De-energized
Downtime	Not requires	Required, require planned outages
Primary purpose	For continuous monitoring and routine screening	Detailed diagnosis and accurate localisation
Speed	Readings can be taken quickly	Cumbersome and slower, could take hours
Cost	Medium, due to remote monitoring using sensors	Higher, due to longer testing time, labour cost, and production loss
Skill level	Basic skills required	Highly skilled operators are performing Offline tests
Accuracy	Prone to interference, less precision and minimal focus on minute defects	Accurate and sensitive monitoring under controlled conditions
Data provided	Real-time operational data received	inception/extinction voltages, thorough defect analysis

What is Partial Discharge?

Partial discharge is considered a major cause of long-term degradation and defects in medium-voltage (MV), High-Voltage (HV), and extra-high-voltage (EHV) equipment. Partial discharge is mostly a small, localised electrical spark occurring between electrodes, or within insulation of electrical assets when insulation is deteriorated or weakened.

PD is generally developed in voids, cracks, weak joints, or in deteriorated cable insulations, switchgears, transformers, and rotating machines. Monitoring PD levels is crucial and helps identify developing faults. It is critical to locate these faults before they become critical. This acts as a key element of predictive maintenance and asset reliability strategies.

Why PD Testing?

PD testing helps find insulation defects at a very early stage. This allows the maintenance team and operators to take preventive actions before a failure occurs. It significantly reduces the chances of unexpected outages, improves safety, and extends asset life. By duly tracking the faults in electrical assets, PD testing improves the reliability of high-voltage systems, reducing the maintenance costs by focusing on targeted repairs instead of full replacements.

PD testing also supports compliance with international safety standards and offers documented evidence of an asset’s condition. Strengthening the industry's reliability, workplace safety and operational continuity.

Types of PD testing Online and Offline

Partial Discharge testing can be broadly classified into online PD testing and offline PD testing, each for different working conditions and maintenance schedules or strategies. Online PD testing is done remotely when the equipment is operational and energised, running under normal conditions to capture real-time insulation behaviour.

Offline PD testing is conducted when the equipment is isolated, de-energised, and tested using a controlled external voltage source. Online methods help detect live conditions and defects, whereas offline testing provides an in-depth and accurate diagnosis. Both these types use special sensors such as TEV, HFCT, UHF, acoustic and coupling capacitors. Most of the industries use a combination of these tests and sensors to ensure maximum reliability and accurate assessment.

Online Vs Offline PD Testing

Parameter	Online PD testing	Offline PD testing
Equipment status	Tested when energised	Tested when isolated and de-energised
Downtime	nil	yes
Sensitivity	Moderate to high	Very high
Fault detection	Good	Excellent
Stress level	Actual operating voltage	Adjustable high-volt injection
Ideal for	Routine PD monitoring	Root cause analysis and commissioning

When Do We Choose Online or Offline PD Testing?

Online PD testing is ideal when operations cannot be interrupted and real-time insulation conditions need to be evaluated. It is used for routine health monitoring, early fault detection, and to know the PD trends and PD activity over months or years. Offline PD testing is chosen when detailed diagnostics are required or when commissioning new equipment.

It is useful for confirming defects found during online tests, checking insulation integrity after repairs, or validating asset health before energization. Industries typically start with online testing and move to offline testing if abnormalities appear.

Online PD Testing Tool

Online PD testing uses the most advanced sensors and portable devices specifically designed to detect discharges without interrupting power or operations. Common tools found in the market include HFCT sensors for cables, TEV sensors for switchgear, UHF sensors for GIS, acoustic probes and various other handheld PD detectors. These tools measure discharge level and magnitude, pulse frequency, and noise interference.

Modern online PD analyzers provide industries with real-time feed, data logging, and remote monitoring options. They help identify insulation degradation, loose connections, corona discharge, or internal voids in live equipment. With the help of these tools, technicians can perform quick checks and schedule preventive maintenance promptly and effectively.

Online PD testing with Technomax

Technomax offers an all-inclusive range of online Partial Discharge testing solutions specifically focused on high-voltage assets in industrial environments. These systems include a combination of the most advanced sensors, easy cloud-based analytics, and portable or handheld devices that capture PD activity without interrupting operations.

With continuous monitoring facilities, Technomax’s PD testing and monitoring tools help track insulation health, detect emerging weak points, and support data-oriented maintenance decisions. The technology enables early detection of faults in cables, transformers, switchgear, and terminations.