Transient Earth Voltage (TEV) testing is the most widely used energised technique for partial discharge (PD) detection on air-insulated switchgear and RMU panels. A TEV sensor measures the transient voltage pulses that surface discharge and internal PD generate on the earthed metalwork of the panel. It is fast, non-invasive and requires no outage – which is why it is the backbone of switchgear inspection programmes.
What TEV detects
PD activity on or near the panel surface – surface discharge on insulators and busbars, tracking and poor-contact activity – produces pulses that travel on the panel’s earthed skin. The TEV sensor, held against the panel or mounted permanently, measures these pulses in dBmV. Higher readings indicate more active discharge; rising trends over time indicate developing defects.
Measurement points on a switchgear panel
- Cable compartment – the most common PD location in service, measured on the lower-front panel.
- Busbar chamber – measured on the upper section, above the breaker.
- Circuit breaker compartment – centre section, where internal discharge from the breaker assembly appears.
- Rear cable box and terminations – where accessible, to catch termination discharge.
Background correction: the critical step
TEV readings must be compared with the ambient background, not with a fixed absolute number. Before surveying a switchgear room:
- Measure background TEV levels away from the panels – for example at the room wall or door – to establish the room baseline.
- Compare each panel reading with the room background and with the panel’s own history.
- Re-measure at different load levels when possible, since some discharge is load-dependent.
A panel reading that is significantly above background, or that has risen since the last patrol, is the signal to investigate – not a single high number in isolation.
Interpreting readings
- Near background – no significant PD detected; record and move on.
- Moderately above background – repeat the survey sooner; correlate with ultrasonic checks on the same panel.
- Well above background, or rising trend – plan a detailed investigation, and consider permanent monitoring on that bay.
Combining TEV with ultrasonic measurement on the same panel improves confidence: TEV plus airborne ultrasound on a panel indicates surface discharge or tracking; TEV without ultrasound suggests internal discharge deeper in the panel.
Reporting and trending
Record readings per panel in a register with date and load. Compare across patrols to separate stable panels from progressing defects. This register is what turns periodic TEV surveys into a maintenance programme.
About HUWOR
Zhuhai Huawang Technology Co., Ltd. (HUWOR) manufactures handheld PD detectors with TEV and ultrasonic capability, dedicated TEV sensors and online monitoring systems for switchgear and RMU. Instruments are supplied worldwide with training support and OEM/ODM options. Contact the HUWOR sales team for instrument selection.