Ultrasound or thermal imaging for electrical equipment: which to choose

Thermal imaging and ultrasound find different electrical faults, so they are used together. According to SDT, an electrical fault usually only starts to generate heat at an advanced stage, once enough current flows along the discharge path, and only then does a thermal camera see it. Acoustic energy is produced at any stage of discharge, so ultrasound hears corona, tracking and arcing before heating begins.

What a thermal camera sees

Thermography shows surface temperature. It finds faults that already generate heat. SDT puts the limitation this way: heat does not accumulate until the situation has progressed to the point where enough current flows along the discharge path. SDT's conclusion: relying on infrared alone may result in a missed diagnosis.

What ultrasound hears

An electrical discharge emits ultrasound. According to SDT, a partial discharge produces a localised, directional acoustic signal in the 20–50 kHz range. SDT distinguishes three types of discharge:

Discharge type	SDT definition	Typical location
Corona	A breakdown of insulation in the air around high-voltage conductors	High-voltage conductors, busbars
Tracking	The creation of a conductive path along the surface of the insulator	Insulators, bushings; dirt, dust and moisture make it easier
Arcing	A continuous and intense discharge	Often loose or damaged contacts

Ultrasound is directional, so the source can be located quickly and from a safe distance. For switchgear, SDT notes another property: with an electrical defect, the inside of the panel is flooded with ultrasound, and ultrasound, being small in wavelength, passes through the tiniest air paths. How to tell corona from tracking and arcing by the recording is covered in Corona discharge at a substation: how to find it with ultrasound.

Comparing the methods

Question	Thermal camera	Ultrasound
Physical principle	Infrared radiation from a heated surface	Acoustic signal of the discharge, 20–50 kHz per SDT
When it sees the fault	Once the fault heats up: enough current flows along the discharge path	At any stage of discharge
What it finds	Heat	Corona, tracking, arcing
How findings are confirmed	With an ultrasound recording: is there a discharge	With a thermal camera: is there heat

Why the methods are combined: an SDT case

SDT describes a survey at a Gerdau Ameristeel plant. On 161/13.8 kV transformers, thermal images of phases A, B and C showed no noticeable hot spots. The ultrasound recording, however, gave clear evidence of early tracking on the B and C phase bushings. Events repeating at line frequency in the spectrum indicated nuisance corona as well, and noise between the peaks pointed to tracking.

The repair was simple: connections on all three phases were cleaned and re-tightened, after which the time signal improved significantly. Surveys are now done more often, and ultrasound monitoring of the transformers continues. SDT concludes that the ultrasound and infrared pairing suits detecting partial discharge on insulators, in MCC panels and on high-voltage lines.

This is one case described by a maker of ultrasonic instruments. The main lesson is that no heat on a thermogram does not mean no discharge.

How to set up an inspection route

1. Include both methods in the route: thermal imaging for heat, ultrasound for discharges.
2. With ultrasound, check panels at gaps, grilles and door seams, plus transformer bushings, insulators and busbars.
3. When a discharge is found, record the signal to identify the discharge type and to compare on later rounds.
4. Check the discharge with a thermal camera: is there heating yet?
5. Watch for dirt and moisture: according to SDT, they make tracking easier.
6. After repair, repeat the measurement and compare with the original recording.

More on partial discharge in Partial discharge in electrical equipment: ultrasonic detection.

Which instrument to choose

KEG TRK does not supply thermal cameras; the ultrasound part of the route is covered by SDT instruments.

• Rounds of panels, transformers and insulators with signal recording and analysis: SDT340.
• Surveying outdoor switchyards, busbars and high points with an image of the sound source: the SonaVu acoustic camera.

KEG TRK supplies SDT ultrasonic instruments and SonaVu acoustic cameras in Kazakhstan. Price on request.

Sources

• SDT Ultrasound Solutions. The Dynamic Duo: Ultrasound Inspection and Infrared Imaging, 2022.
• SDT Ultrasound Solutions. Using Ultrasound for Electric Power Reliability, 2022.
• SDT Ultrasound Solutions. The SDT340: Ultrasound for Electrical Integrity, 2025.