Can a Thermal Camera Detect Partial Discharge?

The Short Answer: Rarely, and Only Late

If you point a thermal camera at a switchgear panel that has active partial discharge (PD), you will usually see nothing unusual. PD releases energy in microscopic pulses inside insulation voids, surface defects, or floating components. The power involved is tiny — typically milliwatts scattered across a large surface — so the temperature rise is far below what an infrared detector can resolve.

Thermal imaging only becomes useful after PD has been burning for a long time, when the accumulated damage heats up a termination, a bushing, or a section of cable. By then the insulation is already degraded. This is why the industry standard answer is clear: thermal cameras do not detect partial discharge; PD detectors do.

Key principle: Thermal imaging finds the consequences of electrical faults that produce heat (loose connections, overloads, advanced PD damage). A PD detector finds the cause — the electrical activity itself — months or years before it becomes a thermal problem.

Why PD Is Invisible to Thermal Imaging

Partial discharge is a localized electrical breakdown of insulation that does not completely bridge the electrodes. It appears as corona, surface discharge, or internal void discharge, and its energy is released in nanosecond pulses at frequencies from hundreds of kHz to GHz. Three physical facts explain why thermography misses it:

• Very low power. A typical PD source dissipates microwatts to a few watts. Spread over a cable joint or bushing surface, that cannot raise temperature by even 0.1°C.
• Buried inside insulation. Internal PD happens inside solid insulation, where the heat must conduct outward through layers of dielectric — by which point it is indistinguishable from normal operating temperature.
• No steady-state signature. PD pulses are intermittent and follow the AC cycle. A thermal imager averages over seconds; the tiny, fluctuating heat signature is averaged away.

Compare this with a loose busbar connection, which can dissipate tens of watts continuously and shows up as a clear hot spot on a Hammer II or any thermal camera within seconds. Thermal imaging is superb for resistive heating faults; it is simply the wrong tool for detecting active PD.

When Thermal Imaging Does Catch PD Damage

There are three situations where thermography legitimately helps a PD program:

1. Advanced PD damage. After weeks or months of continuous discharge, the damaged area starts to heat. Tracking on a bushing, a carbonized cable termination, or a deteriorated stress cone will eventually produce a visible thermal anomaly — but this is late-stage damage, not early warning.
2. Companion faults. PD often coexists with resistive faults. A loose connection or corroded lug heats up while PD activity develops in the adjacent insulation. Thermography catches the resistive fault; PD detection catches the insulation problem.
3. Verification after repair. After a PD source has been located and repaired, thermal imaging confirms the associated hotspot has cooled to normal, proving the repair addressed the root cause.

So a thermal camera is a valuable member of the inspection team — but it must work alongside, not instead of, a PD detector.

How Dedicated PD Detectors Find the Activity

Dedicated instruments detect the electrical and acoustic signatures that PD emits directly. The two field methods used most often by utilities in Latin America are:

• TEV (Transient Earth Voltage). PD pulses in switchgear couple capacitively to the metal enclosure and travel as a surface voltage. A TEV sensor placed on the panel door measures this in dBmV. Instruments like the HPD200 and the HPD500 combine TEV with ultrasonic sensors in one handheld unit.
• Ultrasonic (40 kHz). PD, corona, and tracking all emit ultrasound in the 20–100 kHz band. A parabolic or contact ultrasonic sensor can hear this activity on switchgear, transformers, and overhead lines — even when the source is not visible.

Both methods are non-intrusive and, critically, do not require a power outage. The SC Series PD detector adds pattern analysis (PRPD/PRPS) so an engineer can distinguish internal PD from corona or external noise, which is the difference between a confident repair decision and a guess.

For locating the exact source once PD is confirmed, an acoustic imaging camera such as the GSW Series (136-microphone array) can visualize the discharge position in real time, even from several meters away.

Thermal vs PD Detection: Side-by-Side

Neither tool replaces the other. A complete condition assessment of medium-voltage equipment needs both: thermography for current-carrying integrity, PD detection for insulation integrity.

The Right Way: Combine Both in One Survey

Leading utilities run a combined survey that takes one technician pass but answers two questions:

1. Walk the switchgear row with a PD detector. Place the TEV sensor on each panel door, record dBmV, listen with the ultrasonic sensor, and log readings per panel. Flag any panel above your plant baseline.
2. Scan the same assets with a thermal camera. Capture thermograms of busbars, cable terminations, and connections. Flag any ΔT above 5°C relative to a similar healthy phase.
3. Cross-reference the two datasets. A panel with both high TEV and a warm termination is a high-priority repair. A warm connection with low TEV is a resistive fault. High TEV with a cool panel is pure insulation PD.
4. Locate and diagnose. Use PRPD pattern analysis on the HPD500 to classify the defect, and an acoustic camera to pinpoint the source.

This combined workflow is exactly what Unitech Tools recommends for substation and industrial maintenance programs — and it is why we supply thermal cameras, PD detectors, and acoustic cameras as a coordinated set rather than single instruments.

The Bottom Line

Can a thermal camera detect partial discharge? Not while it matters. PD produces almost no heat until the insulation is already damaged, so thermography gives you late confirmation, not early warning. Use a dedicated PD detector with TEV and ultrasonic sensors to find PD, use PRPD analysis to classify it, and use a thermal camera to catch the resistive faults that accompany it. Equipment that is both electrically and thermally healthy is equipment that does not fail. If your program only has a thermal camera today, adding a PD detector like the SC Series or HPD200 is the single biggest upgrade you can make to your early-warning capability.