Partial discharge testing is a non-intrusive way to find insulation weaknesses in high-voltage equipment before they turn into a fault. It picks up the small electrical discharges that quietly erode insulation inside switchgear, cables and transformers, often long before anything trips or fails. This guide covers what partial discharge is, what causes it, how the testing is actually done, and when your assets are due a survey.
What is partial discharge?
Partial discharge (PD) is a localised electrical discharge that only partially bridges the insulation between two conductors. It does not flash over completely, so the equipment keeps running, but every discharge chips away at the insulation. Left alone, the damage compounds until the insulation gives way, usually under load and usually without warning.
PD activity is measured as apparent charge, expressed in picocoulombs (pC), under the international standard IEC 60270. Apparent charge is not the actual charge at the discharge site, which cannot be measured directly. It is the charge that would give the same reading in the test circuit, so it is a consistent, comparable number to trend over time. In condition monitoring, a rising pC reading is the warning sign that matters.
Why does it matter so much? Because partial discharge is progressive. A discharge that is harmless today widens the void or extends the tracking path a little further, which raises the local stress, which drives more discharge. Given enough time that feedback loop ends in a full insulation breakdown, and in metal-clad switchgear that can mean an internal arcing fault, an unplanned outage and a serious arc flash risk to anyone nearby. Catching PD early is a safety measure as much as a reliability one.
What causes partial discharge, and the three types
Partial discharge is caused by high electrical stress concentrating on a small defect in or around the insulation. That defect might be a void left in casting, a crack, a contaminated surface, moisture ingress, or a sharp metallic point. Wherever the local field exceeds what the insulation can withstand, small discharges begin, and they tend to get worse as the insulation degrades.
Engineers usually group PD into three types by where it happens.
|
Type |
Where it occurs |
Typical cause |
|
Internal (void) |
Inside solid or cast insulation |
Gas-filled voids, cracks, or delamination within the dielectric |
|
Surface |
Along an insulation surface |
Contamination, moisture, or tracking across a boundary |
|
Corona |
In air around a conductor |
Sharp points or edges at high potential ionising the surrounding air |
Internal discharge is often the most serious, because it works away out of sight until the insulation is close to failure. Surface and corona activity are frequently easier to detect early, which is exactly why a structured survey is worth doing.
How partial discharge testing works: the main detection methods
Partial discharge testing works by detecting the electrical, electromagnetic or acoustic signals that each discharge gives off. No single method sees everything, so a good survey combines a few. The common techniques are set out below.
|
Method |
What it detects |
Best suited to |
|
Transient earth voltage (TEV) |
Small voltage pulses on the metal enclosure |
Internal PD in metal-clad MV/HV switchgear |
|
Ultrasonic (acoustic) |
High-frequency sound from surface and corona discharge |
Surface tracking and air discharges you can hear at vents and joints |
|
UHF |
Ultra-high-frequency electromagnetic emissions |
Gas-insulated switchgear and transformers |
|
HFCT |
PD current pulses on cable earth connections |
Cables and cable terminations |
Handheld instruments make this practical in the field. The UltraTEV Plus² from EA Technology, for example, combines TEV and ultrasonic sensors in one unit, so an engineer can identify internal, surface and corona activity on a walk round without taking the plant out of service. You can read more about the technology on the EA Technology site.
Online vs offline partial discharge testing
The big practical choice is whether you test the equipment live or take it out of service. Both have their place, and the right one depends on what you are trying to learn.
Online testing is done with the plant energised and in normal operation. It is non-intrusive, needs no outage, and shows PD under real working voltage and load, which is why it suits routine condition monitoring and switchgear surveys. Offline testing energises the asset from a separate test supply at a controlled voltage, often using very low frequency (VLF) for cables. It is more sensitive and more diagnostic, and it lets you raise the voltage in steps to find the PD inception point, but it needs an outage and more setup.
As a rule of thumb, online testing is for screening a fleet and catching problems early. Offline testing is for investigating a suspect asset in detail, or for commissioning tests on new cables before they go into service.
Which assets need PD testing, and how often
Any medium or high-voltage asset that relies on insulation is a candidate for partial discharge testing. In practice, the assets that repay it most are metal-clad switchgear, MV and HV cables and their terminations, and transformers, because a failure in any of those is costly, dangerous and disruptive.
- Switchgear: a routine TEV and ultrasonic survey, typically annually, catches developing insulation faults in ring main units and panels.
- Cables: HFCT or offline VLF testing checks joints and terminations, and is worth doing at commissioning and after any repair.
- Transformers: UHF and acoustic methods, often alongside insulating oil analysis, track internal condition.
There is no single legal interval that fits every site. The sensible approach is risk-based: test critical and older assets more often, and let the trend in your readings tell you when something needs a closer look. Working on or near energised equipment must always follow a safe system of work, in line with HSE electrical safety guidance and the relevant Irish rules.
Equipment and getting a survey done
You have two routes: run partial discharge testing in-house, or bring in a survey team. Buying an instrument such as the UltraTEV Plus² makes sense if you have a lot of switchgear and want to screen it regularly. It puts TEV and ultrasonic detection in your own engineers' hands, and the readings build a baseline you can trend year on year.
If you would rather not build that capability, a partial discharge survey gives you the same insight without the outlay, carried out by people who do it every week and interpret the results for you. If you are buying the kit, it pays to get your team trained too; partial discharge training covers how to take reliable readings and, just as importantly, how to read them.
FAQs
Is partial discharge testing done live or does the equipment need to be switched off?
Most routine surveys are done online, with the equipment energised, using TEV and ultrasonic detection. That is the whole appeal: you get condition data with no outage. Offline testing, done with the asset de-energised and fed from a test supply, is reserved for detailed diagnostics and commissioning, where the extra sensitivity is worth the outage.
What is a normal partial discharge reading?
There is no universal pass or fail number, because it depends on the asset, its age and its design. What matters more is the trend. A steady low reading is usually fine; a reading that climbs over successive surveys is the signal to investigate, whatever the absolute pC figure.
How often should switchgear be tested for partial discharge?
An annual survey suits most medium-voltage switchgear, with older or critical assets tested more often. Base the interval on risk and on what your previous readings show, rather than a fixed calendar date alone.
Partial discharge testing is one of the most cost-effective ways to protect high-voltage plant, because it warns you while there is still time to act. If you want your switchgear, cables or transformers surveyed, or you are weighing up buying your own detector, speak to our team about a partial discharge survey or the right instrument for your site.