Partial discharge is one of the earliest indicators of insulation breakdown in high-voltage systems, yet it often remains undetected until failure occurs. Despite the well-understood consequences, such as equipment damage, downtime and safety risk, many facilities lack clarity on the underlying causes of partial discharge.
This guide explains what a partial discharge is, why it occurs and how different contributing factors interact. Understanding these causes allows facilities and operations teams to assess risks more accurately and prioritise targeted maintenance strategies.
What is a Partial Discharge?
A partial discharge is a localised electrical breakdown within the insulation that does not completely bridge the gap between conductors. It occurs when the electric field strength exceeds the insulation’s resistance at a specific weak point, even though the surrounding material remains intact.
These events generate heat, light, sound and chemical by-products that gradually degrade insulation. Over time, this phenomenon creates a self-reinforcing cycle where damage increases the likelihood of further discharge activity.
Because it occurs internally, partial discharge is not visible during standard inspections. It typically develops inside high-voltage assets, such as transformers, switchgear and cable systems, which is why specialised methods like partial discharge detection are required.
Primary Causes of Partial Discharge
Partial discharge rarely occurs in isolation. It is typically the result of one or more defects or environmental conditions affecting insulation performance. Understanding these causes of partial discharge helps you identify which assets carry higher risk and where inspection effort should be focused.
Manufacturing Defects and Material Flaws
Small voids or air pockets can become trapped in solid insulation materials like epoxy during manufacturing. These imperfections create localised weak points, in which the electric stress is higher.
Because these voids have lower dielectric strength than the surrounding solid insulation, they are more likely to break down under normal operating voltage. Similar issues can occur with gas bubbles in transformer oil or where insulation layers begin to separate.
Installation Errors
Improper installation is a common cause of early-stage partial discharge, particularly in cable terminations and joints. Small air gaps, uneven stress distribution or incorrect assembly can all create discharge initiation points.
Contaminants introduced during installation, such as dust, moisture or metallic particles, can compromise the insulation. Failure to follow manufacturer specifications or incorrect torque settings can also lead to long-term reliability issues.
Contamination and Moisture Intrusion
Surface contamination changes how insulation behaves under electrical stress. Dirt, oil or chemical residue can reduce surface resistance, allowing discharge activity to travel along insulation rather than remain contained.
Moisture increases this effect by lowering dielectric strength and promoting surface tracking. In exposed environments, moisture ingress through damaged sheaths or seals is a common contributing factor.
Thermal and Mechanical Stress
Electrical systems are subject to ongoing thermal and mechanical load. Temperature fluctuations cause materials to expand and contract, which can lead to cracking or the formation of internal voids.
Mechanical vibration, along with physical stress from installation or external impact, can weaken insulation layers over time. Where overheating occurs, insulation degrades more rapidly, increasing the likelihood of discharge.
Routine electrical thermal imaging helps identify these conditions before they affect performance.
Age-Related Deterioration
Over time, all insulation systems deteriorate. Continued exposure to electrical stress, changes in temperature and environmental conditions slowly weakens materials.
While ageing does not directly cause partial discharge, it does increase sensitivity to other contributing factors. Such conditions can result in visible damage, such as electrical treeing within solid insulation.
Design Weaknesses and Sharp Edges
Poor design can cause electric fields to build up in certain areas. Sharp edges, insufficient clearances or inappropriate material selections can all increase the likelihood of discharge.
Corona discharge, a specific form of partial discharge, often develops around exposed conductors in air where these design limitations exist. While modern standards address many of these issues, older installations may still present inherent risks.
How Different Types of Partial Discharge Develop
The various causes of partial discharge typically result in four main discharge types:
- Internal discharge occurs within insulation due to voids or delamination
- Surface discharge travels along contaminated or moist insulation surfaces
- Corona discharge develops in air around conductors with high electric field concentration
- Tracking discharge forms progressive conductive paths along insulation
Each type provides insight into the root cause and informs the appropriate response. In practice, identifying discharge type is a key step in electrical fault diagnostics.
The Progressive Nature of Partial Discharge Damage
Partial discharge doesn’t cause immediate failure. Instead, it develops over time, with each event gradually weakening the surrounding insulation.
In the early stages, activity can be intermittent and easy to miss. As the damage builds, it becomes more frequent and more severe. Chemical by-products such as ozone and nitric compounds further accelerate the breakdown process.
This is why early detection matters. Proactive monitoring, supported by structured preventative maintenance, makes it possible to address defects before they lead to critical failure.
Environmental and Operational Factors that Accelerate Causes
External conditions influence how quickly the causes of partial discharge develop. High ambient temperatures increase thermal stress, while humidity supports contamination and surface tracking. Load variation and voltage transients can also trigger discharge in systems that already contain defects.
Assets that operate continuously under high load are particularly exposed because sustained thermal stress accelerates insulation degradation.
Why Understanding Causes Matters for Your Facility
Understanding what causes partial discharge supports more effective risk management across electrical infrastructure.
- New assets can be assessed for manufacturing and installation quality
- Ageing assets can be monitored for progressive deterioration
- High-risk environments can be prioritised for more frequent inspection
- Load-critical infrastructure can be managed proactively
Services such as switchboard thermal imaging and broader thermal scanning programs provide practical visibility into these risks.
FAQs
Can partial discharge occur in new electrical equipment?
Yes. Manufacturing defects or installation errors can introduce weak points in insulation, allowing partial discharge to occur even in newly commissioned systems.
What's the difference between partial discharge and a complete electrical fault?
Partial discharge is a localised breakdown that does not fully bridge conductors. A complete fault involves full insulation failure and current flow, often resulting in immediate damage.
How quickly does partial discharge damage progress?
The rate varies. In some cases it develops over years, while in others it can lead to failure within months, depending on severity and operating conditions.
Which types of electrical equipment are most susceptible to partial discharge?
High-voltage equipment such as transformers, switchgear, cable terminations, generators and motors is most susceptible, as they operate under electrical stress, where even small defects can trigger partial discharge.
Assets with more complex insulation systems, including multiple layers or mixed materials, are particularly vulnerable, as these interfaces create more opportunities for weak points to develop.
For broader planning, see electrical thermal inspection.
Understanding the causes of partial discharge is only part of the process. Detection is what allows action to be taken before a failure occurs.
Thermal Scanners uses ultrasonic, thermal and TEV methods to assess high-voltage systems under operating conditions. Findings are delivered through clear, insurance-ready reports with prioritised recommendations.
To assess your assets or establish a monitoring program, get in touch with us today.