Walk into any plant room, substation, or HV cable chamber in the UK, and you’ll find something in common: insulation.
Miles of it. It’s there quietly doing the job of keeping electricity where it should be. But here’s the thing, insulation doesn’t always fail with a bang. Sometimes, it starts failing in whispers.
Those whispers have a name: partial discharge (PD). It’s the electrical equivalent of tiny stress fractures in steel, not enough to break it today, but if left unchecked, you’re on the road to trouble.
In the UK, where much of our electrical infrastructure is decades old, PD isn’t rare. It’s one of the top culprits behind unexpected outages, costly repairs, and in some cases, dangerous incidents.
This isn’t just an issue for power utilities. Construction firms, facility managers, industrial electricians, and anyone responsible for keeping HV or MV systems healthy need to know what PD is, how it develops, and why spotting it early matters. Let’s break it down.
Understanding Partial Discharge
In formal terms, partial discharge is a localised electrical breakdown within an insulation system.
Instead of electricity jumping the entire gap between conductors (that would be a full breakdown), PD occurs in a small part of the insulation inside a void, along a surface, or even in the surrounding air.
Why only “partial”? Because the discharge doesn’t bridge the main insulation completely. But don’t be fooled.
Those small discharges release energy, erode insulation, and over time, they can carve out a path for a catastrophic failure.
In UK systems, PD typically shows up for a few reasons:
- Manufacturing defects – tiny voids, inclusions, or uneven material density in cables, bushings, or resin cast components.
- Aging - insulation dries out, cracks, or loses mechanical strength after decades of serivce.
- Mechanical stress – pulling a cable too tightly, poor terminations, or bending beyond limits.
- Environmental Contamination - moisture ingress, salt deposits in coastal areas, dust in industrial setting.
One of the more insidious aspects of PD is that it often goes unnoticed until something forces a shutdown. By then, repairs can be significantly more expensive than preventative measures.
In UK power distribution, early PD detection is seen not as optional, but as a cornerstone of asset management.
The Science Behind Partial Discharge
Electrical insulation is designed to withstand high voltages without letting current leak across. In perfect form, the dielectric material (XLPE in cables, oil in transformers, epoxy in switchgear) has no weak points.
Reality? Perfection rarely exists. Inside an insulator, you might have a microscopic air bubble. The dielectric strength of air is much lower than the surrounding solid.
When the electric field across that tiny void exceeds the air’s breakdown voltage, an electrical discharge occurs inside the void.
The same thing can happen on the surface of insulation, say, along a slightly dirty bushing, where moisture and contaminants form a conductive film. Or in high-voltage equipment outdoors, where sharp points cause corona discharges into the air.
Engineers classify PD broadly into:
- Internal discharges – inside the insulation material.
- Surface discharges – along the surface of the insulation.
- Corona discharges – in gaseous surroundings, especially near sharp edges.
Each has its signature and impact, but all share a common problem: every discharge causes localised heating, chemical reactions, and erosion. Over time, these microscopic attacks add up to macroscopic failure.
Common Locations And Equipment Affected
PD doesn’t discriminate much, but in the UK, certain assets are more prone due to their design, age, or operating environment.
High-voltage power cables: Especially older XLPE cables installed in the ‘80s and ‘90s. Manufacturing back then didn’t have today’s clean-room precision, so voids are more likely.
Transformers: Cast resin types are vulnerable to internal PD in the resin insulation, while oil-filled units may develop PD in windings or bushings if moisture or particles are present.
Switchgear: Air-insulated (AIS) gear may suffer from surface PD due to dust and humidity, while gas-insulated (GIS) types can have internal PD from manufacturing defects or contamination introduced during maintenance.
Rotating machines: Generators and large motors in industrial plants are exposed to vibration and thermal cycling, which can degrade insulation over time.
Busbars and joints: Often overlooked, yet these connections can be hotspots for PD if not installed perfectly.
UK industry reports have documented real cases: PD in a hospital’s HV cable terminations leading to a 12-hour outage; PD inside offshore wind turbine switchgear causing unplanned maintenance delays; PD on railway traction equipment resulting in costly train cancellations.
These aren’t theoretical, they’re expensive, disruptive, and preventable with the right approach.
Why Partial Discharge Is A Critical Issue In The UK
The UK’s electrical infrastructure is a patchwork of modern installations and ageing assets. That mix creates fertile ground for PD problems.
From a financial perspective, PD-related failures often mean full component replacement rather than repair. The downtime alone can cost industrial operations tens of thousands of pounds per day.
On the safety front, PD can escalate to an arc flash or electrical fire. In confined plant rooms or switchgear chambers, that’s a serious hazard for maintenance teams.
Regulatory compliance is another driver. The Electricity at Work Regulations 1989 put a clear duty on employers to maintain electrical systems to prevent danger. Ignoring PD, or failing to detect it, could be seen as neglecting that duty.
There’s also the reliability factor. Whether it’s the National Grid, Network Rail, or offshore wind farms, PD-induced failures undermine service continuity and public confidence.
For these reasons, UK asset owners are increasingly integrating PD monitoring into their maintenance strategies, not just for compliance, but because the cost of prevention is far less than the cost of failure.
Detecting Partial Discharge
The good news? PD can be detected, often long before it leads to failure. The challenge is picking the right method for the situation.
Offline detection (equipment powered down):
- IEC 60270 PD testing – Measures discharge magnitude in picoCoulombs using sensitive detectors. Very accurate, but requires an outage.
- Very Low Frequency (VLF) testing – Applies a low-frequency AC voltage to stress insulation without damaging it, while monitoring for PD.
- Oscillating Wave Test Systems (OWTS) – Creates a decaying oscillating voltage and measures PD response.
Online detection (equipment energised):
- Ultrasonic/acoustic testing – PD emits ultrasonic noise; portable sensors can pick it up.
- High-frequency current transformers (HFCT) – Clamped around earth connections to sense PD pulses.
- Transient Earth Voltage (TEV) detection – Common in UK switchgear inspections, measuring PD-induced voltage pulses on the metal casing.
- UHF sensors – Used in GIS to detect PD signals in the ultra-high-frequency range.
Thermal imaging can also help, though it’s more for spotting secondary heating effects than PD itself.
Each method has trade-offs. Offline testing is more thorough but disrupts operations. Online testing allows condition monitoring without downtime, though it can be more prone to interference.
In practice, UK asset managers often combine both periodic offline testing during planned outages and regular online monitoring as part of routine inspections.
This layered approach catches both emerging and fast-developing PD issues.
Analysing And Interpreting PD Test Results
Detection is only half the battle. Understanding what the readings mean is critical.
PD measurements are typically expressed in picoCoulombs (pC), indicating the apparent charge per discharge event.
Higher values generally suggest more severe activity, but context matters. A 50 pC reading in one piece of equipment might be negligible, while in another, it could signal imminent failure.
Phase-resolved partial discharge (PRPD) analysis adds another layer by showing when discharges occur relative to the AC cycle. Different PD types produce distinct patterns, helping pinpoint the fault location and cause.
Trending over time is essential. A stable PD level might be acceptable for years; a sudden upward trend means it’s time for action.
And a note of caution, electrical noise can mimic PD signals. Skilled technicians use filtering, pattern recognition, and cross-verification to avoid costly misdiagnosis.
Preventing And Mitigating Partial Discharge
Prevention starts at installation. In the UK, good site practice is the first defence against PD:
- Keep equipment clean during assembly, dust and moisture are prime PD enablers.
- Follow the manufacturer's torque settings on terminations and busbar joints.
- Respect minimum bending radii for cables.
For existing assets, regular inspection is key. That means scheduled PD testing and visual checks for signs of tracking or corrosion.
Environmental control helps too, keeping humidity down in switchgear rooms and sealing outdoor terminations against water ingress.
Material choice matters. High-quality XLPE cables, properly specified bushings, and stress-graded insulation components may cost more up front, but pay off in reliability.
Where PD is detected, mitigation might involve:
- Replacing or re-terminating faulty components.
- Applying insulating boots or coatings.
- Improving ventilation to reduce condensation.
The guiding principle? Small interventions now prevent big failures later. The UK industry has countless examples where early PD remediation added years to an asset’s service life.
Standards, Regulations, And UK Industry Best Practice
Partial discharge detection and control are governed by a framework of international and UK-specific standards.
- IEC 60270 – The primary standard for PD measurement methods.
- BS EN 60034 – Rotating electrical machines, covering insulation testing.
- BS EN 60502 – Power cables and their accessories.
In terms of legislation, the Electricity at Work Regulations 1989 remain the main legal reference. Regulation 4(2) explicitly requires electrical systems to be maintained to prevent danger, which includes addressing PD risks.
The Health & Safety Executive (HSE) also publishes guidance on electrical maintenance, stressing condition monitoring as part of safe systems of work.
Industry bodies like the Institution of Engineering and Technology (IET), the Electrical Contractors’ Association (ECA), and certification bodies like NICEIC all incorporate PD awareness into their training and best practice guidance.
For contractors and asset managers, aligning with these standards isn’t just about compliance, it’s about ensuring the credibility and reliability of their work in a competitive UK market.
Future Trends In PD Detection And Management
Technology is making PD monitoring more accessible and accurate.
Smart sensors are now built into some new switchgear and transformers, continuously feeding data to control systems.
AI-driven analytics can spot PD patterns humans might miss, turning raw readings into actionable maintenance alerts.
There’s also growing interest in integrating PD monitoring into broader predictive maintenance platforms, so it sits alongside thermal, vibration, and other condition data.
As the UK transitions to more renewable energy and EV charging infrastructure, PD management will only become more important.
The more complex and decentralised the grid becomes, the greater the need for continuous insulation health monitoring.
Conclusion
Partial discharge might start small, but in the UK’s high-voltage world, it’s rarely harmless for long.
Understanding how it forms, where it hides, and how to detect it early can save thousands in repairs, prevent dangerous incidents, and keep essential services running.
For tradespeople looking to sharpen their PD knowledge without the risk of real-world faults, simulation tools like Tradefox offer a safe way to practise diagnosis and electrical skills before applying them on site.
The bottom line? Treat PD like rust in steel. By the time you can see it, the damage underneath might be far worse. Proactive monitoring and maintenance aren’t optional extras, they’re the best insurance policy an electrical system can have.
