Spot Checks vs Online Partial Discharge Monitoring: Why 24/7 Visibility Matters

Partial discharge (PD) activity is one of the earliest indicators of insulation breakdown in high-voltage and medium-voltage electrical assets. However, PD activity can vary in activity levels based on environment and loading on the equiptment. Relying solely on periodic testing or inspections can leave critical events undetected if measured at the wrong time, increasing the risk of unexpected failure.

Understanding the difference between spot checks and online PD monitoring is key to managing electrical risk effectively.

The Limitations of Spot PD Testing

Spot PD tests are typically carried out on a calendar basis, during planned maintenance, commissioning activities, or investigations following a suspected issue. While valuable, they have clear limitations:

• PD activity may not be present at a suitable level at the time of testing
• Tests are often carried out under non-representative operating conditions
• Can be a admin / pen and paper task
• Trending and long-term degradation are difficult to assess

Measured externally in certain positions the signals can be reduced due to cubicle metal work and separations.  

External noise from LED lighting, machinery can impact readings  

As a result, assets can appear healthy during testing while deterioration continues unseen.

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Spot PD Testing vs Online PD Monitoring: A Practical Comparison

Examples of Missed Partial Discharge Events

PD activity is often influenced by load, temperature, humidity, and switching conditions. Commonly missed events include:

• PD occurring only during peak load periods or varying humidity / temperature fluctuations in the environment  
• Discharge triggered by switching operations or voltage fluctuations
• Progressive increase in PD intensity between inspection intervals

These events contribute to insulation degradation long before a failure becomes visible through spot testing.

What Technology Is Required for Online PD Monitoring

Online PD monitoring involves permanently installed sensors that detect and track discharge activity under normal operating conditions.

Typical components include:

• High-frequency current transformers (HFCTs) or coupling capacitors
• TEV, UHF and/or acoustic sensors depending on asset type
• Continuous data acquisition hardware
• Secure data transmission and analytics platforms or on-prem data processing
• Visual dashboards and alerting tools

Monitoring can be scaled based on asset criticality, voltage level, and risk profile.

The ROI of 24/7 Partial Discharge Visibility

While online PD monitoring requires investment, the return is closely tied to risk reduction.

Key benefits include:

• Early identification of insulation deterioration
• Reduced likelihood of catastrophic electrical failure
• Improved planning of outages and remediation work
• Avoidance of emergency repairs and extended downtime
• Greater confidence when extending asset life

Preventing a single failure in critical switchgear or transformers can justify the cost of monitoring many times over.

What to Do Next

1. Identify assets where failure would have the greatest operational or safety impact

2. Review historical PD testing results and failure history

3. Use temporary PD monitoring to validate risk where uncertainty exists

4. Deploy online PD monitoring for assets requiring continuous visibility

FAQs

Is spot PD testing still useful?
Yes. Spot testing remains valuable for diagnostics, but it should complement online monitoring for high-risk assets.

How long does it take to see meaningful PD data online?
In many cases, patterns and events become visible within weeks of installation.

Can online PD monitoring reduce outage frequency?
Yes. Early detection allows issues to be addressed during planned outages rather than emergency situations.

Conclusion

Spot PD testing and online PD monitoring serve different roles, but they are not interchangeable. While spot tests provide valuable snapshots, only online monitoring delivers continuous visibility into intermittent and evolving discharge activity. For organisations managing critical electrical assets, 24/7 PD insight significantly reduces uncertainty and risk.

Next Step

If you are exploring monitoring as a first step, working with a partner who understands both the technical and operational context can make the process more effective.

ACTENIQ helps teams plan, deploy and interpret monitoring solutions so that data leads to clear, confident decisions about risk, performance and next steps.

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