When high-voltage insulation starts to fail, it rarely stays silent. Long before a bus flashes over, the ionised air and micro-discharges around a defect emit sound at frequencies far above human hearing, along with radio-frequency noise. Ultrasonic electrical inspection is the practice of listening for those signals with an airborne ultrasound instrument to find corona, surface tracking, and arcing inside switchgear and around high-voltage assets. This guide explains what those three fault types sound like, how the instrument detects them through enclosure gaps, and why the technique complements infrared thermography rather than replacing it.
Ultrasonic electrical inspection in one line: Ultrasonic electrical inspection is a condition-monitoring technique that uses an airborne ultrasound instrument to detect the high-frequency sound emitted by corona, surface tracking, and arcing in energized electrical equipment. Because these partial-discharge phenomena radiate ultrasound before they cause visible heat or a flashover, scanning switchgear and terminations lets a technician find and rank developing insulation faults from outside the enclosure without shutting the equipment down.
Three distinct electrical fault mechanisms produce ultrasound, and learning to tell them apart is much of what ultrasonic electrical inspection is about. Corona is the ionisation of air around a high-voltage conductor where the electric field is strong enough to break the air down but no solid path has yet formed. It tends to appear on medium and high-voltage systems, typically above a few thousand volts, and through headphones it comes across as a steady, even hiss. Corona itself is often low in energy, but it produces ozone and nitric acid that slowly attack insulation, so a persistent corona site is a defect in the making rather than an immediate emergency.
Surface tracking is the next stage. Here a conductive path begins to form across the face of an insulator, usually helped along by contamination, moisture, or a manufacturing flaw, and the discharge follows that path in bursts. Instead of the smooth hiss of corona, tracking sounds ragged and erratic, with a crackling, popping quality that rises and falls. Arcing is the most severe of the three: a sustained or repeated discharge across a gap, carrying real current, which sounds harsh and buzzing and often the loudest of the signatures. Arcing is close to failure and warrants prompt attention.
Because each mechanism has its own character, an experienced inspector uses the sound quality, not just the loudness, to judge what is happening inside an enclosure. Many instruments can also demodulate the ultrasonic signal down into the audible range and record it, so the waveform and its frequency content can be reviewed later or compared against reference samples. Corona, tracking, and arcing also radiate radio-frequency energy, which is why some survey programmes pair ultrasound with transient earth voltage or RF sensing to confirm and locate partial discharge inside metal-clad switchgear.
An airborne ultrasound instrument is essentially a highly directional microphone tuned to a narrow band well above human hearing, commonly centred around forty kilohertz, with electronics that shift that ultrasound down into audible tones the technician hears through headphones while watching a level on a display. Because the sensor is directional, the inspector sweeps it slowly across a panel and homes in on the loudest bearing, which points toward the source. A parabolic or long-range attachment extends the reach so terminations, insulators, and bushings can be checked from a safe distance without opening anything live.
The key insight for switchgear is that ultrasound travels through air, so the instrument does not need line of sight to the discharge itself. Sound leaks out through any air path in the enclosure: door seams, ventilation louvres, cable-entry gaps, and inspection windows. Scanning along these openings while the gear is energized lets the discharge betray itself even though the fault is buried inside a metal cabinet. This makes the survey non-intrusive and safe, since the technician stays outside the arc-flash boundary and never has to expose live parts to take a reading.
Detectability does depend on there being an air path and on the discharge being strong enough to overcome background noise, so a quiet, well-sealed, dry cubicle can mask a small defect. For that reason ultrasound is often combined with contact and RF techniques on enclosed switchgear, and surveys are ideally repeated under similar humidity because damp conditions amplify tracking. The practical output of a scan is a set of located, characterised, and severity-ranked findings that feed the same maintenance backlog as any other inspection, each tagged with the sound signature and level that justified it.
Ultrasonic electrical inspection and infrared thermography find different failure modes, which is why mature electrical maintenance programmes run both. Thermography sees heat, so it excels at resistive faults such as a loose or corroded connection or an overloaded conductor that runs hot under load. Ultrasound hears discharge, so it excels at insulation faults such as corona and tracking that may emit almost no heat until the very end. A high-resistance joint that a thermal camera lights up brightly might be silent to ultrasound, while an early corona site that ultrasound flags clearly might show no thermal signature at all. Used together, the two close each other's blind spots.
There is also a timing argument. Corona and tracking are progressive, so ultrasound often detects an insulation problem earlier in its life than thermography detects the heat of its eventual breakdown. Catching a fault at the corona stage buys weeks or months of planning time compared with catching it once it is hot enough to be obviously failing. In practice a survey route covers the same substations, motor control centres, and switchrooms with both instruments, and the findings are merged so that each asset carries a combined electrical health picture rather than two disconnected reports.
Feeding those findings into a central system is where cloud SCADA and monitoring platforms fit. A survey produces located, ranked defects that belong in the same asset history as vibration readings, oil results, and thermographic anomalies, so a reliability team can see the whole story for a switchboard in one place. In distributed oil and gas and utility operations, where switchgear is scattered across remote sites, a platform such as Merobix lets ultrasonic findings be logged against the specific asset, trended over successive surveys, and turned into work orders alongside live process alarms. That keeps a corona or tracking finding from a routine walkdown from being lost between spreadsheets and ties it back to the equipment it threatens.
Thermography detects heat, so it finds resistive problems such as loose connections and overloaded conductors that run hot. Ultrasonic inspection detects the high-frequency sound of corona, tracking, and arcing, which are insulation faults that often emit little heat until they are close to failure. The two are complementary: many electrical programmes run both because each catches faults the other can miss.
Yes, as long as there is an air path for sound to escape. Ultrasound leaks out through door seams, ventilation louvres, cable entries, and inspection windows, so scanning along these openings can reveal a discharge inside a metal enclosure without opening it. Very well-sealed or quiet cubicles can mask weak signals, which is why ultrasound is often paired with radio-frequency or transient earth voltage sensing on enclosed gear.
It is, and that is one of its main advantages. Corona, tracking, and arcing only occur when the equipment is under voltage, so the survey has to be live to detect them. Because the instrument reads airborne sound from outside the enclosure, the technician stays outside the arc-flash boundary and never exposes live parts, making it a safe, non-intrusive check.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.