TEV, UHF, HFCT and ultrasonic methods detect different physical signals produced by partial discharge: transient earth voltages on metal-clad switchgear, electromagnetic waves in the UHF band inside GIS, high-frequency currents in cable ground paths, and acoustic waves that travel through air or solid insulation. Choose the sensor by the asset, the access point and the question being answered, and treat these non-conventional methods as screening and location tools that complement, rather than replace, conventional measurement.
The Sensors Detect Different Physical Signals
Every partial discharge event releases energy in several forms: a fast electrical current pulse, electromagnetic radiation, acoustic pressure waves and, in some cases, light and gas. Each sensor family is tuned to one of these signals, and its sensitivity depends on how well that signal reaches the sensor through the asset’s construction. This is why a sensor that works well on one asset type can be blind on another.
It follows that no single sensor answers every question. The choice of method is an asset and access decision, not a ranking of technologies. Before comparing instruments, define the asset, the suspected discharge location, the available access point and whether the measurement is a screening pass, a confirmation or a location exercise.
The physical signal also sets the measurement quantity. TEV and UHF methods respond to the electromagnetic aspects of the event, HFCT responds to the conducted current pulse, and acoustic methods respond to the mechanical wave. Because these signals are attenuated and shaped differently by the asset, the same discharge produces different apparent magnitudes in different sensors, which is expected and must not be treated as an inconsistency.
TEV for Metal-Clad Switchgear Screening
Transient earth voltage (TEV) sensing measures the short voltage pulse that appears on the earthed metal enclosure when a discharge occurs inside metal-clad switchgear. The sensor is placed on the outside of the panel, which makes it a fast, non-invasive screening method for switchgear bays. TEV surveys are commonly used for routine scanning because they require no outage and cover many panels quickly.
TEV readings are relative indicators, not absolute discharge measurements. Their value depends on consistent sensor placement, background comparison and repetition over time, and a single elevated reading is a trigger for further investigation, not a diagnosis. TEV is generally not suited to open-air or non-enclosed assets, where the transient earth signal is not contained.
In a survey programme, TEV readings are typically expressed relative to a baseline and compared across panels and over time. The operator records the sensor position, the reading and the date, so a change from one round to the next is visible. A panel that rises consistently, or reads clearly higher than its neighbours with the same construction, is the candidate for the next investigation step.
HFCT for Cable and Ground-Path Measurements
High-frequency current transformers (HFCT) clamp around cable earth straps, cable cores or ground conductors and detect the high-frequency current pulses that flow when discharge occurs. Because they measure current in the conductive path, HFCT sensors are well suited to cable systems, cable terminations, joints and other assets where the discharge current returns through a measurable conductor.
The clamp position and the ground-path arrangement define what the sensor sees. An HFCT on the wrong conductor, or on a cable with multiple parallel earth paths, can miss the pulse or read a neighbouring signal. The method detects pulses in the kilohertz-to-megahertz range and is used both for periodic surveys and for continuous monitoring on critical circuits.
Because the measured quantity is the current pulse, the HFCT’s sensitivity and frequency response should be checked against the expected pulse spectrum of the cable system, and the clamp should be verified on a test pulse or on a known calibration source where the manufacturer provides one. The sensor position must be recorded and reused on every survey so the trend comparison remains valid.
UHF for GIS and Shielded High-Frequency Detection
Ultra-high-frequency (UHF) sensors detect the electromagnetic waves radiated by a discharge inside a shielded enclosure. In gas-insulated switchgear (GIS), the metallic enclosure and dielectric windows provide access points, and UHF sensors can be fitted at inspection covers or via coupling devices without opening the gas compartment. UHF detection is sensitive inside shielded volumes where the signal is contained and attenuated only by the insulation and barriers.
The shielding that makes GIS safe also makes other methods less effective, which is why UHF is a primary non-conventional method for GIS. The sensor’s location, the internal barriers and the propagation path affect what is detected, so a UHF reading should be interpreted together with the asset layout and, where possible, multiple sensor positions for comparison.
Signal propagation inside GIS is influenced by spacers, disconnectors and the geometry of the compartments, and a discharge on one side of a barrier may be attenuated before it reaches a sensor on the other side. A well-planned UHF programme uses the available access points to divide the installation into zones, so an indication can be associated with a compartment rather than only with the whole bay.
Ultrasonic and Acoustic Methods for Location
Ultrasonic sensors detect the acoustic pressure waves produced by a discharge or a loose contact. In air, the signal travels from the source to a handheld sensor or an array; in solid insulation or enclosed equipment, contact sensors pick up structure-borne vibrations. Acoustic methods are particularly useful for locating a source once screening has identified an affected asset, because the direction and time-of-flight of the sound help pinpoint the origin.
Acoustic sensitivity is affected by distance, intervening barriers and background noise, and airborne ultrasonic signals attenuate quickly. A strong acoustic signal often indicates a surface discharge, tracking or a mechanical issue such as a loose connection, which is why the acoustic result must be correlated with electrical measurements before drawing a discharge conclusion.
Many instruments combine an airborne sensor and a contact sensor. The airborne sensor scans open surfaces, while the contact sensor couples to accessible metal parts to pick up structure-borne vibration. Using both on the same asset increases the chance of locating a source that is hidden from one path, and the comparison between the two signals adds another discrimination dimension against noise.
Combining Sensors to Reduce False Conclusions
Each method has blind spots, so the strongest field investigations combine two or more independent signals. A typical workflow is a TEV or HFCT screening pass, followed by a confirmatory measurement with a second sensor family, and then acoustic or multi-sensor location on the affected asset. Agreement between independent signals raises confidence; disagreement triggers a review of the setup before any conclusion.
| Method | Signal detected | Typical assets | Main strength | Key limitation |
|---|---|---|---|---|
| TEV | Transient earth voltage | Metal-clad switchgear | Fast, non-invasive screening | Relative indicator; enclosure-dependent |
| HFCT | High-frequency current pulse | Cables, terminations, ground paths | Clamp-on, no outage | Depends on ground-path and clamp position |
| UHF | Electromagnetic waves | GIS and shielded volumes | Sensitive inside shielded enclosures | Requires access point and propagation path |
| Ultrasonic | Acoustic pressure waves | Airborne and contact location | Pinpoints source direction and position | Attenuates quickly; background noise |
All four methods belong to the non-conventional family covered by IEC TS 62478, which addresses partial discharge measurement by electromagnetic and acoustic methods, while IEC 60270 covers conventional electrical measurement with defined quantities and calibration. Conventional measurement provides quantified, comparable values; non-conventional methods provide sensitivity, location and access where conventional coupling is impractical. The investigation plan should state which question each method is answering.
A complete investigation often moves from screening to confirmation to location: a TEV or HFCT screening pass identifies the suspect asset, a second sensor family confirms that the signal is real and internal, and an acoustic or multi-sensor arrangement locates the source for inspection or repair. Each step uses the method’s strength and records the evidence in a form the next step can use.
Frequently Asked Questions
Which method is best for switchgear?
TEV is the common first-line screening method for metal-clad switchgear because it is fast and non-invasive. Elevated or changing TEV readings are followed up with ultrasonic or other sensors to confirm and locate the source before any conclusion is drawn.
Which method works for cables?
HFCT sensors clamped around earth straps or ground conductors are the typical non-conventional choice for cable systems, especially at terminations and joints. The clamp position and the ground-path arrangement must be checked so the sensor sees the discharge current.
Which method is used inside GIS?
UHF sensors are the primary non-conventional method for GIS because the shielded enclosure contains the electromagnetic signal. Sensor access points, internal barriers and propagation paths must be considered when interpreting the readings.
For the method-selection framework across assets and outage conditions, refer to the partial discharge testing guide on the product page. When you need to compare sensor-based systems for your asset mix, review the available PD test equipment and request a technical proposal with your access points and test programme.
Whichever combination of methods you choose, document the sensor positions, the baseline and the repetition interval, because the trend over time is the strongest evidence the non-conventional methods can produce.