Partial Discharge Readings: What Do dBmV and pC Mean?

Why the Numbers on a PD Detector Matter

A partial discharge detector does not print a clean "pass" or "fail". It prints a number — like -8 dBmV on a TEV sensor or 34 dB on the ultrasonic channel — and the value only means something in context. The same absolute reading can be a non-event in a noisy industrial switchroom and a serious alarm on a clean indoor panel.

That is why experienced inspectors treat PD readings as comparative and trending data, not absolute verdicts. The process is: establish a baseline for each asset, measure consistently with the same instrument and sensor placement, and watch how the number moves between visits. A jump of 6 dB or more between inspections is a stronger signal than any single reading, because it removes most of the environmental variables that distort one-off measurements.

Understanding the units themselves is the first step. TEV readings are reported in dBmV, ultrasonic readings in dB, and cable measurements in picocoulombs (pC). Each unit describes a different physical quantity, and mixing them up is the most common mistake in PD data interpretation.

TEV Readings in dBmV: What They Measure and Normal Ranges

dBmV is a logarithmic measure of the transient earth voltage amplitude picked up by the sensor, expressed relative to 1 mV. Because the scale is logarithmic, every +6 dB represents roughly a doubling of the apparent discharge level — which is why a 6 dB jump between surveys is treated as a significant change.

In practice, most TEV measurements on healthy MV switchgear fall between -20 dBmV and 0 dBmV, depending on the environment. Field experience across distribution utilities supports these rough reference bands:

These bands are guidance, not certification. Noise from outdoor corona, nearby switching, and poor sensor contact can inflate readings, while deep voids in encapsulated insulation can produce surprisingly low TEV values. Always record the measurement conditions — sensor position, panel door state, load level — alongside the number.

pC (Picocoulombs) and IEC 60270

Picocoulombs (pC) measure apparent charge: the quantity of charge that a discharge pulse would transfer into a calibration reference, a concept standardized in IEC 60270 for partial discharge measurements. One picocoulomb is one trillionth of a coulomb — an extremely small charge, which is precisely why PD activity is invisible to normal protection and metering equipment.

pC values are typically obtained with a high-frequency current transformer (HFCT) clamped around a cable screen, earth strap, or transformer neutral. The HFCT senses the current pulse of each discharge event, and the detector integrates it to a charge figure. This is the measurement of choice for cable networks: a healthy XLPE cable termination might sit below 100 pC at operating voltage, while an aging joint can rise into the hundreds or thousands of pC, a strong predictor of eventual failure.

The HPD500 includes HFCT input with pC sensitivity precisely for cable and transformer work, complementing its TEV, AE, and AA channels. In practice, utilities use pC for cable systems and dBmV/dB for switchgear, and the two unit families rarely appear in the same report.

Ultrasonic Readings in dB

The ultrasonic channel reports acoustic energy in dB on a scale that typically runs from 0 to 60 dB in the field. Because ultrasound is local, the reading depends strongly on distance and on whether the sensor is airborne (AA) or pressed to the surface (AE).

As a rough rule of thumb in a reasonably quiet substation:

• Below 20 dB: background level; nothing of concern.
• 20–30 dB: low-level activity; note it and continue.
• 30–40 dB: moderate; approach and localize the source.
• Above 40 dB: strong discharge or leak; investigate immediately.

Ultrasonic values are most valuable when used to confirm and localize a TEV flag. A panel at -8 dBmV that also gives 35 dB ultrasonic at the same bushing, with the signal peaking sharply as the probe approaches, is a genuine discharge source. A TEV flag with no ultrasonic correlate is often noise — the two-method cross-check is the fastest way to separate real defects from interference.

How to Interpret PD Readings: Reference Table

"The number gets you attention; the trend gets you a budget. We keep a panel-by-panel dBmV history and act when a reading moves more than 6 dB from its own baseline — that single rule has cut our switchgear failures by more than half in three years." — Electrical Maintenance Supervisor, Cement Plant, Peru

Reading PRPD/PRPS Patterns, Not Just Magnitude

Magnitude alone cannot tell you whether a discharge is dangerous. A stable internal void can sit at -12 dBmV for years, while a rapidly tracking surface discharge at the same level can fail within weeks. The phase-resolved pattern is what separates the two.

• Internal voids produce symmetric PRPD patterns straddling the zero crossings of both half-cycles — typically stable and slow to progress.
• Surface discharges appear in one half-cycle more than the other, often near the voltage peaks, and tend to grow with contamination and humidity.
• Corona shows tall, narrow spikes at the voltage peaks of one polarity, with a sharp onset threshold.

Because Latin American networks run at 60 Hz, a detector that locks to the mains frequency is essential — the phase position of each pulse is meaningless if the reference is wrong. The HPD500 renders PRPS and PRPD patterns plus Continuous, Flight, and Pulse Waveform views, so an engineer can classify the defect and judge urgency from the pattern shape, not just the level.

A Practical Trending Workflow with Unitech Tools Detectors

A defensible PD program needs four elements: a baseline, consistent measurements, documented trends, and a clear action threshold. A practical workflow looks like this:

1. Baseline survey: use the SC Series to sweep every MV panel, transformer, and cable termination, recording TEV in dBmV and ultrasonic in dB against a panel ID list.
2. Scheduled re-measurement: repeat the survey quarterly or half-yearly with the same instrument, same sensor placement, and same load conditions.
3. Trend analysis: flag any panel whose reading moves more than 6 dB from its own baseline, and any pC value that rises steadily across cable measurements.
4. Detailed diagnosis: bring the HPD500 to flagged assets, capture PRPD/PRPS patterns and HFCT pC data, and classify the defect type.
5. Maintenance decision: schedule corrective work based on severity and trend — not on a single number.

This workflow turns a handheld detector from a troubleshooting toy into a fleet-management tool. The instrument gives you the data; the discipline of reading, recording, and trending gives you the decision. For utilities and plants in Latin America looking to cut unplanned outages, that combination is the entire value proposition of online PD testing.