TEV vs Acoustic Camera for Partial Discharge Detection: Which Method First?
TEV probes and acoustic imaging cameras both detect partial discharge, but they answer different questions. TEV tells you that activity exists inside a switchgear panel; an acoustic camera tells you where a discharge is coming from in space. This guide explains when to use each, and why many teams use both.
What TEV Detection Measures
Transient Earth Voltage (TEV) is a pulse of voltage that travels along the surface of earthed metal when partial discharge occurs inside switchgear. A TEV probe placed on the panel surface detects these pulses and reports a magnitude in dBmV.
TEV is excellent for screening enclosed switchgear: you can walk down a row of panels and get a quick relative reading on each cubicle. It does not tell you exactly where inside the panel the discharge is, and it does not see discharges outside metal enclosures.
What Acoustic Imaging Measures
Partial discharge emits ultrasound in the 20 kHz to 100 kHz band. An acoustic imaging camera uses an array of microphones to detect that sound and overlay the source on a live video image, showing you the direction of the discharge in real time.
Acoustic imaging is strongest for locating the source in space — for example finding which bushing, insulator or termination on a transformer or open bus is emitting PD. It also works at distance, which makes it useful for live-line assets you cannot approach.
TEV vs Acoustic Camera: Head to Head
| Comparison | TEV probe | Acoustic imaging camera |
|---|---|---|
| What it detects | TEV pulses on earthed metal surface | Ultrasonic waves from the discharge |
| Best application | Enclosed switchgear screening | Source location at distance / open assets |
| Gives a location image? | No — magnitude only | Yes — visual overlay on live video |
| Typical cost | $ (low) | $$$ (higher) |
| Skill required | Basic — touch panel, read dB | Moderate — aim, interpret image |
| Works through metal enclosure? | Yes (surface measurement) | No — needs acoustic path |
Practical answer: TEV and ultrasonic contact probes are the fastest way to screen many panels. An acoustic camera is the fastest way to answer "which of those three bushings is actually discharging?" from a safe distance.
Best Method for Metalclad Switchgear
For a row of metalclad MV switchgear, TEV screening is the established first pass. A handheld TEV instrument such as the SC Series lets one engineer test dozens of panels in an hour. Panels with elevated readings can then be examined in more detail with ultrasonic contact probes or scheduled for internal inspection during an outage.
An acoustic camera adds limited value against a solidly enclosed panel because the ultrasound is trapped inside the metal — the camera cannot "see" through the enclosure.
Best Method for Open Substations and Terminations
For open bus, transformer bushings, insulators, cable terminations and pole-mounted equipment, the acoustic camera is the more effective tool. You can stand at a safe distance, scan the asset, and see the discharge source overlaid on the video image. This is where an acoustic imaging camera earns its cost.
Why Combined Screening Works Best
Utilities with comprehensive PD programmes use a layered approach:
- TEV + airborne ultrasonic contact screening for indoor switchgear — fast, low-cost, repeatable.
- Acoustic imaging for open assets and for confirming the spatial source of suspected activity.
- HFCT or UHF (where applicable) for permanent monitoring or detailed pattern analysis on critical assets.
Each layer answers a different question, and the layers are complementary rather than competing.
Unitech Tools PD Detection Products
| Product | Method | Best fit |
|---|---|---|
| SC Series | TEV + airborne ultrasonic | Switchgear screening, indoor rounds |
| HPD200 | TEV + non-contact ultrasound | Lightweight switchgear survey |
| HPD500 | AA + TEV + UHF + HFCT options | Multi-method PD diagnosis |
| GSW Series | Acoustic imaging camera | Visual source location at distance |
| SW136 / SW234 | Acoustic imaging (PD + gas + leak) | Multi-purpose acoustic imaging |
A Practical Screening Procedure
- Record load and baseline conditions before scanning.
- Screen switchgear rooms with a TEV instrument, noting dBmV per panel.
- Re-test elevated panels with airborne ultrasonic contact method to confirm.
- For open substation assets, scan with an acoustic imaging camera from a safe distance and capture images of any source.
- Classify severity, schedule follow-up and keep records for trend comparison.
Which PD tool does your programme need first?
Tell us whether you screen enclosed switchgear or open substation assets. We will recommend the TEV instrument, acoustic camera or combined kit that fits.